Geological Dictionary Paleontology Q–Z: an illustrated reference with definitions highlighting connections between stratigraphy, tectonics, and the fossil record, tracing major geological processes and biological events across deep time and Earth’s evolving environments.
Alphabetical index of geological terms
The following Geological Dictionary Paleontology is an organized list of key geological terms widely employed in Earth science, paleontology, and sedimentology.
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
Introduction to the Geological Dictionary of Paleontology
Geological Dictionary Paleontology Q–Z—this final section completes the illustrated reference guide with terms ranging from quartzite and regression to stratigraphy, till, and watershed. Definitions trace the advance and retreat of ancient seas, the fossilization of Pleistocene megafauna, and the chemical processes that preserve organisms across geological time. Stromatolites and microbialites carry the record of life back to the Archean; rifts, unconformities, and transgressions connect individual terms to the grand narrative of Earth’s history. Every entry includes a photograph or diagram from a real geological or paleontological example.
Keywords: geology terminology, geological vocabulary, Earth science terminology, sedimentary geology terms, geological processes, geological dictionary, paleontology
Q
quartzite
Right: Ridge-Forming Quartzite: An outcrop of Catoctin Mountain, which is part of the Blue Ridge Mountains. Chimney Rock is mostly made of quartzite, a metamorphic mineral. © Alex Demas
Quartzite — metamorphic rock composed mainly of quartz (SiO₂, >90%). Strong and difficult to work with, usually white, gray, or reddish. It forms through recrystallization of quartz sandstones under regional metamorphism; more rarely, from certain acidic igneous rocks (quartz porphyries). It may contain minor impurities of mica, feldspar, chlorite, and others. Quartzites occur from the Paleoproterozoic (~1.7–2.5 Ga) as metamorphosed ancient sandstones; weakly metamorphosed or weathered varieties sometimes preserve ichnofossils and relic traces. Examples: grooves and tracks in Proterozoic quartzites; Eocene burrows Thalassinoides in weathered quartzites of the Mount Barren Group, Australia; Cambrian quartzites with ichnofossils.
R
ravine
Right: Echo Ravine, Free State province, South Africa. © Free State Tourism
Ravine — a small, narrow, deep depression with steep slopes formed by gully–valley erosion. It is smaller than a gorge or canyon but larger than a gully; depth usually exceeds 5–10 meters. Flowing water, especially temporary streams or mountain brooks, cuts it out. Ravines often expose sedimentary layers with fossils useful for studying fauna and stratigraphy. Examples: Babi Yar (Ukraine), Ravenna Park (United States), Taishaku Valley (Japan).
recrystallization
Recrystallization — in geology, a metamorphic process of restructuring mineral grains in deformed rocks in the solid state (without melting) under elevated pressure and temperature. Grains grow or change shape, or new stable crystals form (e.g., limestone → marble, sandstone → quartzite). In fossil-bearing rocks, recrystallization destroys microstructures of fossils (microfossils, fine details of shells and skeletons, organic remains), which are important for paleontology.
reducing atmosphere
Right: Before life began on the planet, Earth’s atmosphere was largely made up of nitrogen and carbon dioxide gases.
Reducing atmosphere — an atmosphere lacking oxidizing gases such as molecular oxygen, ozone, and halogens. Carbon dioxide and nitrogen predominated, methane also played a noticeable role, and water vapor and small amounts of hydrogen were present. In Earth’s early stages (Hadean–early Archean, up to ~3.8 Ga), the atmosphere was strongly reducing; during most of the Archean (3.8–2.7 Ga), it became weakly reducing. Such an environment favored abiogenic synthesis of complex organic compounds and the origin of life. The absence of oxygen ensured preservation of primitive microfossils (e.g., stromatolites).
regolith
Right: Lunar regolith landscape. Geophone Rock, lunar surface photography during Apollo 17. © NASA
Regolith — a loose, unconsolidated layer of clastic material (ranging from dust to boulders) covering bedrock. On Earth it includes rock fragments, volcanic ash, glacial, alluvial, and aeolian deposits, soil, and organic matter. On other bodies of the Solar System, it forms through meteorite impacts, space weathering, and solar wind. Regolith is known from the Archean on Earth and from ~4.4 Ga on other planets. In paleontology, it is valuable as a host for microfossils and remains of Quaternary fauna. Examples: loess sections of China and Europe with mammoth fauna; cave deposits with Neanderthal bones; Gray Fossil Site (Pliocene, USA). Synonym: rhegolith.
regression
Right: A series of storm beach ridges at Turakirae Head and along the shoreline of Palliser Bay. The marine regression here is caused by the successive uplift of the southern end of the Remutaka Range. © L. Homer / GNS Science
Regression — retreat of the sea from the land due to uplift of the continent or relative (eustatic) lowering of sea level (often during glacial epochs). Throughout geological history, regressions alternated with transgressions, usually coinciding with mountain-building episodes. Periods of significant lowering of global sea level were comparatively rare and short-lived (e.g., in the Cryogenian, Permian, Pleistocene). Regression produces continental and coastal facies with fossils and exposes shelf bioherms and reefs. Regressions repeatedly caused mass extinctions of marine biota due to the reduction of shelf zones. Examples: Late Permian regression and the Permian–Triassic extinction; Late Cretaceous regression preceding the K–Pg extinction; Pleistocene regressions that exposed shelves and opened migration routes for terrestrial fauna.
reticulite
Right: Reticulite. Mauna Loa Summit, Hawaii, United States of America. © e-Rocks Ltd. | Caliban
Reticulite — extremely porous (95–98%), fragile basaltic pumice in which bubble walls have burst and merged, leaving a three‑dimensional network of the thinnest interconnected glass threads. It is the lightest known rock. Reticulite forms during intense degassing in powerful Hawaiian‑type fountain eruptions. Related Hawaiian deposits sometimes record traces of birds and insects. Synonym: thread‑lace scoria.
rhyolite
Right: Rhyolite specimens found in Cumbria. © Northern Geological Supplies Limited.
Rhyolite — acidic effusive rock with a glassy or fine‑grained groundmass and porphyritic inclusions of quartz, sanidine, plagioclase, biotite (less commonly amphibole or pyroxene); volcanic equivalent of granite. SiO₂ content: 69–78%. Forms through rapid cooling of viscous lava. Light in color (white, gray, pink). Glassy varieties include obsidian, perlite, pumice, pitchstone. Rhyolitic tuffs alternate with sediments and contain microfossils (radiolarians, conodonts, pollen), allowing precise correlation of sections with biostratigraphic horizons. Examples: Permian–Triassic rhyolitic tuffs in formations of the western United States and eastern Kunlun; Miocene tuffs of the Monterey Formation, California; Quaternary rhyolites of Yellowstone. Synonym: liparite.
ridge
Right: Crib Goch, Snowdon, North Wales. © BaldHiker (Paul Taylor)
Ridge — a long (usually more than 10 km) narrow mountain uplift, generally over 500 m high, with a sharp crest, steep slopes, and a distinct watershed line. It may have a tectonic, erosional, sedimentary (accumulative), or volcanic origin. Ridges expose various sedimentary–metamorphic complexes, where erosion reveals horizons with macro‑ and microfossils (ammonites, brachiopods, belemnites, foraminifera, etc.). Examples: Dinosaur Ridge, Colorado—Jurassic dinosaurs and their tracks; Guadalupe Mountains, Texas—Permian Capitan reef with brachiopods, trilobites, and ammonites; Fossil Mountain, Utah—Cambrian trilobites and graptolites. Synonym: mountain ridge.
rift
Right: Rifting episode in Afar Depression in the Main Ethiopian Rift. BGS © UKRI.
Rift — a linearly elongated zone of lithospheric extension (hundreds to thousands of km long, tens of km wide); a system of grabens and horsts with normal faults. Accompanied by seismic activity and basaltic volcanism. Continental rifts can lead to continental breakup and the birth of oceans. Syn‑rift lake and lagoon deposits often contain rich fossil assemblages with exceptional preservation. Examples: Cretaceous rifts with the Jehol Biota (China, ~133–120 Ma—feathered dinosaurs, birds, insects); East African Rift—archive of hominid evolution (Olduvai Gorge, Hadar with “Lucy”).
rift valley
Right: The Rift Valley in East Africa. © Shutterstock
Rift valley — a linear lowland or basin formed by subsidence of the crust between parallel normal faults in a continental rift zone. Often contains rift lakes. Rift valleys are classic sites of accumulation of fine lake and lagoon deposits with exceptional fossil preservation. The most famous and richest Lagerstätten are associated with them: modern lakes Tanganyika, Turkana, Baikal; ancient formations—Newark Supergroup, Santana Formation, Jehol Biota, Green River Formation.
river mouth
Right: Drone view of the mouth of the Tamarindo River in Costa Rica. © Marcel Freitez
River mouth — the place where a river flows into the sea, a lake, or another river. It may take the form of a normal mouth, an estuary, or a delta. Slowing of the current causes deposition of alluvium, forming deltas, sand spits, bars, and islands. Mouth deposits often contain abundant fossils (mollusks, brachiopods, fish, ostracods, more rarely, plants and vertebrates); they form taphocoenoses and concentration Lagerstätten. Examples: Mississippi Delta, Thames Estuary.
S
sabkha
Right: The Great Sand Dunes area is the San Luis Lakes State Wildlife Area. © NPS/Patrick Myers
Sabkha — coastal muddy–sandy–clayey supratidal plain in arid zones, slightly above normal tide level, where evaporites (gypsum, anhydrite) precipitate within the sediment. Fossil indicators include evaporites, mud cracks, nodular levels, and tepees. The geological record of the UK and Ireland (Permian–Triassic) includes clastic sabkhas such as the Mercia Mudstone Group and Sherwood Sandstone. Paleontological significance lies in microbial mats (stromatolites, microbial laminites) and rare microfossils (diatoms, ostracods in transitional facies). Example: Sebkhat El Melah (Tunisia). Synonyms: sebkha, sabkhah, sebkhat.
sandstone
Right: Sandstone outcrop showing sediment erosion and pronounced horizontal bedding characteristic of ancient marine or fluvial deposits. © PPD
Sandstone — clastic sedimentary rock composed of sand grains (0.0625–2 mm) cemented by clay, carbonate (calcite, dolomite), siliceous (opal, chalcedony, quartz), ferruginous, or other material. Forms through erosion of rocks, transport, and diagenesis. Quartz predominates, and feldspar is less common; color ranges from white or gray to yellow, red, brown, or greenish. In paleontology, sandstones are valuable as host rocks for ichnofossils, shells, fish teeth, vertebrate bones, and plant remains in coastal, deltaic, fluvial, and aeolian facies. Examples: Navajo Sandstone—Jurassic dinosaur tracks; Old Red Sandstone—Devonian fishes and early land plants; Coconino Sandstone—Permian synapsid tracks.
saprolite
Right: A saprolite developed in metamorphosed diorite, where some of the original Precambrian gneissic layering is preserved. North of Kalbarri, Yilgarn Craton, Western Australia. © Geological Digressions
Saprolite — soft, friable rock formed in situ through chemical weathering: it retains relict structure but loses density. Typically clayey or kaolinitic, enriched in iron and aluminum oxides; color is red, brown, or ochre, less commonly white or gray. Saprolites rarely contain macrofossils but often preserve plant remains, spores, pollen, and microfossils in Late Cenozoic tropical and subtropical deposits. Examples: Late Cretaceous–Paleogene kaolinitic saprolites of Australia and Africa; Pleistocene saprolites with pollen in western North Carolina, USA; Neogene profiles of South America. Synonyms: rotten rock, decomposed rock.
sapropel
Right: Sapropel. Outcrops of the steep coastline of the Staldzene cliff in Latvia. © Aivars Gulbis
Sapropel — unconsolidated, jelly‑like sedimentary mud (silt) composed of remains of aquatic organisms (algae, bacteria, phytoplankton), plants, and detritus decomposing anaerobically in shallow lakes, less commonly lagoons and seas. Forms under low O₂ conditions; during diagenesis it transforms into sapropelic rock (sapropelite). Sapropels preserve microfossils (diatoms, spores, pollen), invertebrate remains, plant fragments, and rare traces of bioturbation—mainly in Holocene and Pleistocene deposits.
schist
Right: Outcrop of schist with the penetrative foliation. Southern Alps, New Zealand. © Marli Miller.
Schist — medium‑ to coarse‑grained metamorphic rock with pronounced schistosity from oriented micas (muscovite, biotite, chlorite), quartz, feldspar, garnet, amphiboles, staurolite. Forms under medium‑ to high‑grade regional metamorphism (greenschist–amphibolite facies) from clayey sedimentary, volcanic, or granulitic rocks. Serves as markers of orogenies (Caledonian, Hercynian, Alpine). Low‑grade varieties preserve deformed fossils of protoliths. Examples: graptolites and trilobites in Caledonian schists of Scotland and Wales; brachiopods in mica schists of Mount Clough, New Hampshire; deformed crinoids in calci‑silicate schists.
scoria
Middle: Scoria (Holocene, 1200s A.D.; crater of Stampar Volcano, Reykjanes Peninsula, Iceland). © James St. John
Right: Pico de Fogo summit cone (Cape Verde): Pico Grande and lava flow. © Ji-Elle
Scoria — highly porous (vesicular) pyroclastic fragments (bombs, lapilli) or lava crusts of mafic (basaltic, basaltic‑andesitic) magma. Large pores; denser than pumice, sinks in water; colors black, dark brown, red (from iron oxidation). Indicator of Strombolian/Hawaiian volcanism and flood basalts (Siberian Traps). Fossils are extremely rare—only charred plant remains or traces of scorched organisms. Synonym: cinder.
sedimentary rock
Right: Sedimentary rock cliffs. Appalachian Mountains, Zion National Park. © Leene
Sedimentary rocks — rocks formed from compacted and cemented deposits of aquatic environments (oceans, seas, lakes, rivers), less commonly, glacial or aeolian settings. They are divided into clastic, chemical (from solutions), and organogenic types. They cover about 75–80% of Earth’s land surface and ~90% of the sedimentary cover of platforms, contain a significant portion of mineral resources, and serve as the main source of fossils, ichnofossils, and biomarkers. Sedimentary rocks are known from ~3.8 billion years ago (Isua, Greenland) and are key to stratigraphy, paleogeography, and paleoclimatic reconstructions. Examples: stromatolites 3.48 billion years old, Australia; Permian and Jurassic deposits with vertebrates and plants.
sedimentation
Right: Schematic diagram showing how sediment is routed through the system. © Geological Digressions
Sedimentation — settling and accumulation of dispersed particles (products of weathering, denudation, biogenic material, or chemical precipitates) transported by water, ice, wind, or gravity, forming layered deposits. Lithification transforms them into sedimentary rocks. Sedimentation records facies conditions, ancient biocoenoses, and events of mass deposition, for example, anoxic episodes in black shales that create fossil-rich horizons. Examples: Solnhofen Limestone—Jurassic Archaeopteryx and marine reptiles; Green River Formation—Eocene fishes and insects.
shale
Right: Gray shale outcrop. © James St. John.
Shale — soft, brittle, fine‑grained (<0.06 mm) layered clastic sedimentary rock. Composed of clay minerals (kaolin, illite, montmorillonite), quartz, calcite, and organic matter; cemented during diagenesis. Splits into thin laminae (<1 cm) due to fissility. In paleontology, shale serves as a primary source of graptolites, conodonts, trilobites, ammonites, and pyritized fishes in black shales, reflecting conditions of deep‑water anoxic basins. Shale has formed major Konservat‑Lagerstätten since the Cambrian, providing exceptional preservation of soft tissues. Examples: Chengjiang—Cambrian explosion of life; Posidonia Shale—Jurassic ichthyosaurs, fishes, and ammonites; Hunsrück Slate—Devonian soft‑bodied organisms.
shell rock
Right: Vertical high rock made of white shell rock near the Black Sea coast near Sozopol, Bulgaria. © Oleg Kovtun
Shell rock — porous organogenic sedimentary rock composed of cemented whole and fragmented shells, marine fossils (>50% calcium carbonate) with calcite cement and lime silt in the matrix. Forms in shallow‑water settings (shore zone, lagoons, shelf). A soft, porous stone of light yellow, rusty yellow, or light brown color. In paleontology, shell rock is an important source of mollusks, brachiopods, echinoderms, foraminifera, bryozoans, used to reconstruct biogenic communities and marine facies. Examples: Pleistocene Anastasia Formation, Florida, with millions of mollusk shells; Late Cretaceous coquina in France; Devonian and Jurassic shelly limestones with brachiopods and ammonites. Synonyms: shellrock, shelly limestone, fossiliferous limestone, shell‑rich sandstone, coquina.
shield
B: Canadian Shield rock in Georgian Bay, Ontario. © No_Election8476
C: The map of the Canadian Shield is shown as an example. © xerushmerchant
Shield — in geology, an extensive stable part of a platform where Precambrian (Archean–Proterozoic, 3.5–0.54 billion years) crystalline or metamorphic basement crops out, with little or no sedimentary cover. Exposed portion of a craton (as opposed to a platform). In historical geology, a marker of the oldest continental crust. In paleontology, shields are valuable for stromatolites, microfossils, and Ediacaran multicellular forms preserved in metasediments. Examples: Canadian, Baltic, Indian, Australian shields.
shield volcano
Right: Shield volcano Mauna Kea, viewed from the northern slope of Mauna Loa (cinder cones in the foreground), shows off its broad shield shape. © USGS
Shield volcano — central volcano with a broad, gently sloping shield‑like form (slope 3–8°), wide base, and low dome. Forms through repeated eruptions of fluid basaltic lava spreading tens of kilometers. Eruptions are mainly effusive, with ~90% of material as lava flows and minimal pyroclastics. Develop over hundreds of thousands to millions of years, often with a summit caldera or chain of craters. In the context of historical geology, they reflect mantle plumes and intraplate volcanism. Fossils are rare: charred plants, spores, pollen in sedimentary interbeds between flows. Examples: Mauna Loa and Kīlauea (Hawaii), Erta Ale (Ethiopia), Olympus Mons on Mars.
siderite
Right: A cluster of lustrous brown rhombohedral siderite crystals that formed from a pyrite-rich matrix, collected from the Julcani Mine in Huancavelica, Peru. © FossilEra
Siderite — iron carbonate (“iron spar”) (FeCO₃) with admixtures of Mg, Mn, Ca. Forms in hydrothermal veins, sedimentary deposits (oolites, concretions), and metasomatites. Color ranges from yellow‑brown to black, with vitreous luster. Valuable iron ore (up to 48% Fe). In paleontology, siderite produces concretions (Mazon Creek, Montceau‑les‑Mines) with 3D fossils (ammonites, graptolites, crinoids, plants) or replaces them, often together with pyrite. Synonyms: bemmelenite, chalybite, spathic iron, sparry iron, iron spar, siderose.
silica
Right: The volcanic rocks of the Sai Kung Geopark in Hong Kong were formed when dense volcanic ash and lava cooled following massive eruptions approximately 140 million years ago. They are composed of about 50% silica. © gionnixxx
Silica — silicon dioxide (SiO₂), colorless crystals of high hardness, making up ~59% of Earth’s crust. Forms magmatically, hydrothermally, sedimentarily, and metamorphically. Occurs in nature as quartz, chalcedony, opal, stishovite, and others. In paleontology, silicification ensures the preservation of diatoms, sponges, radiolarians, plants, and carbonate shells; in the Precambrian, stromatolites and microbial mats, with opalized fossils, are often found. Examples: stromatolites and microfossils of the Apex Chert and Dresser Formation (Australia); Devonian brachiopods and crinoids at Falls of the Ohio; Jurassic opalized belemnites.
silicate
Silicate — the most widespread silicon–oxygen rock‑forming minerals built on SiO₄ tetrahedra (~90% of Earth’s crust). Form magmatically, metamorphically, and through weathering; constitute the basis of rocks and ores. Include quartz, feldspars, micas, amphiboles, pyroxenes, olivine, clays, garnets. In paleontology, silicates occur in the silicification of shells and skeletons (microquartz, chalcedony) and serve as facies indicators in shales and siliceous sequences. Biogenic silicates (diatoms, radiolarians, sponges) form thick deposits in Phanerozoic marine and lacustrine basins. Examples: Devonian silicified brachiopods and crinoids; Cenozoic diatomaceous earths.
silicification
Right: Chalcedonized gastropods—these snail shells have been replaced by translucent, microcrystalline quartz (SiO₂). This variety of quartz is called chalcedony. Chalcedonization is a variety of silicification. Cretaceous. © The Ohio State University
Silicification — metasomatic replacement of rocks and fossils by silica (opal, chalcedony, quartz) through pore filling and dissolution of original minerals. Occurs during catagenesis, diagenesis, and hydrothermal processes. In paleontology, silicification preserves 3D structures: petrified wood (cells, growth rings), pseudomorphs of shells (mollusks, brachiopods, echinoderms), and biogenic skeletons (diatoms, sponges). In rare cases, it preserves soft tissues: Lagerstätten such as the Rhynie Chert (Devonian, Scotland) and Gunflint Chert (Precambrian, USA). Synonym: silification.
sill
B, C: The Purcell Sill intrusion in the Proterozoic Belt Supergroup, Glacier National Park, Montana—its dark sill is exposed on a near-vertical cliff and produces white contact-metamorphic haloes where the surrounding rock is bleached. © Marli Miller.
Sill — tabular intrusion of mafic composition, parallel to the bedding of host rocks, at depths of 0.5–3 km (thickness 10–600 m, extent up to 100+ km). Forms through horizontal emplacement of magma. Characteristic of platform covers and flood basalts (Siberian, Deccan, Paraná), as well as rift zones. In historical geology, sills serve as markers of intraplate magmatism and mantle plumes. Sills always form later than the host rocks, and geologists use this age difference for stratigraphic dating. In paleontology, they cause contact metamorphism of fossil‑bearing sediments (hornfels rocks) and thermal effects leading to destruction/carbonization of fossils in the contact zone, rarely producing pseudomorphs.
silt
Right: Silt over varved clay on the Clam River (Wisconsin), USA. © University of Wisconsin–Madison
Silt — sedimentary material 0.002–0.063 mm in size (between clay and sand), composed of quartz, feldspar, mica. Deposits in deltas, lakes, floodplains, shelves as bottom sediments or remain in suspension. In paleontology, silt serves as the matrix for microfossils (diatoms, spores, pollen) and laminated shales with ichnofossils; indicator of calm, anaerobic water bodies. Examples: Hunsrück Slate (Devonian, pyritized arthropods); Mazon Creek (Carboniferous concretions, USA); Beecher’s Trilobite Bed (New York, USA, Ordovician)—silty shales with pyritized trilobites.
siltstone
Siltstone — consolidated silt with the texture and composition of shale but lacking its fine fissility. See aleurolite. Synonym: siltite.
slate
Right: Slate from Alta, Norway, breaking into “platy” fragments due to the strong fissility of the rock. © Siim Sepp.
Slate — fine‑grained metamorphic rock of low metamorphic grade with distinct slaty cleavage, splitting into thin tiles. Forms from clay‑rich sedimentary rocks (shale, argillite), often retaining their relict structures. Mineral composition: quartz, muscovite (sericite), illite, chlorite; may also include biotite, epidote, pyrite, graphite, and others. Colors: gray, green, black, or violet. In paleontology, slates typically preserve flat impressions of hard parts: trilobite exoskeletons, brachiopod shells (Devonian and early Paleozoic), two‑dimensional imprints of mollusk shells (Devonian and Carboniferous), as well as plant detritus and thermally resistant microfossils (conodonts, chitinozoans, etc.).
spar
Right: Native barium sulfate mineral, or heavy spar, is often used in drilling muds. Source: Reverso
Spar — historical name for various transparent or translucent non‑metallic minerals with perfect cleavage, easily splitting into smooth fragments. Typically show vitreous luster and light coloration. Spars include several silicates (feldspar), carbonates (calcareous spar or calcite, Iceland spar), barite (heavy spar), fluorite (fluorspar), siderite (iron spar), and satin spar. In a narrower sense, the term often refers to feldspars and calcite. Many spars form in hydrothermal veins, caves, or as secondary minerals in sedimentary rocks from aqueous solutions. In fossiliferous limestones, sparitic cement fills pores or replaces the remains of organisms. Synonym: spath.
spreading
Spreading — in plate tectonics, the process of lithospheric plates moving apart on the ocean floor under mantle dynamics (including convection currents), mainly along mid‑ocean ridges. Magma rising from the mantle solidifies in the rift zone of the ridge, pushing plates sideways at rates from less than 1 to 15–18 cm/year (sometimes higher). Spreading also occurs in back‑arc basins and marginal sea troughs, where young oceanic or transitional crust forms. This type of plate motion on Earth began no later than 3.5–3.0 billion years ago (Archean). Synonym: sea‑floor spreading.
spur of a mountain range
Right: A small offshoot of the mighty Mýrdalsjökull icecap in the distance. © dominiqueghijselinck
Spur of a mountain range — shorter and lower secondary ridge projecting sideways at an angle from the main range, descending toward its periphery. Typically forms when a major river cuts into the broad slope of the main ridge, less often due to local neotectonic uplifts. Spurs often have gentle slopes bordered by steeper ones, giving the side profile of the range a “feathered” appearance. They reflect stages of geomorphological and tectonic evolution of the range and serve as markers of zones of active erosion and sediment transport along its margins.
storm surge
Right: Storm surge at Port William, Dumfries and Galloway. © David Baird/Geograph
Storm surge — abnormal rise of water level in the coastal zone caused by strong winds, leading to intense coastal flooding and erosion. A storm surge can be abrupt, resulting in scouring and destruction of soils and changes to the shoreline. In the paleontological record, such events usually appear as sharp facies disturbances: destruction and burial of benthic communities, redeposition of shell material, and sudden changes in sedimentation conditions. Examples: Devonian tempestites with mass burials of trilobites and brachiopods; Cretaceous storm beds with redeposited mollusks; modern analogs—shell beds after hurricanes.
stratification
Right: Horizontal strata exposed in the Quebrada de Cafayate (Argentina). © travelwayoflife
Stratification — in geology, the layering or division into separate strata (beds, layers) of sedimentary and volcanic sequences under changing physical‑chemical, hydrodynamic, and biological conditions of sedimentation. Layers are separated by stratification planes, along which the composition, texture, or structure of the rock changes. Stratification records shifts in depositional environments and faunas, allowing reconstruction of geological event sequences, relative ages of rocks, and paleoecological conditions. Examples: Devonian and Silurian graptolitic shales of Central Europe and North America, where layering clearly reflects rhythmic marine deposits; Mesozoic marine chalk and marl sections (e.g., Crimea), where stratification helps correlate layers with planktonic foraminifera. Synonym: bedding.
stratigraphy
Right: Part of William Smith’s Table of Strata shown in comparison with modern stratigraphic sequences and sea-level curves, modified from John W. Snedden and Chengjie Liu (2010) (Exxon)
Stratigraphy — branch of historical geology that studies the layers of rocks forming Earth’s crust, their properties and spatial distribution, the chronological sequence of their formation, and their relative ages. Stratigraphy is closely linked to paleontology: biostratigraphy uses fossil remains of organisms to determine ages and correlate sections, thereby creating the geochronological framework for all geological research. Classic stratigraphic sections: Cincinnati Arch—Ordovician series with brachiopods and trilobites; Grand Canyon Supergroup—from Precambrian to Permian; Morrison Formation—Jurassic bone‑bearing layers with dinosaurs.
stratovolcano
Right: Semeru Volcano in Indonesia is an active stratovolcano. © Mangiwau
Stratovolcano — cone‑shaped volcano with steep slopes, built of alternating layers of lava and pyroclastics. The most common type of central volcano, reaching 3–4 km in height. Eruptions usually begin explosively; lava is viscous and thick, most often andesitic or dacitic in composition. Associated with subduction zones, its layered structure reflects successive eruptions. Tephra and paleosol interbeds often contain remains of fauna and flora used for biostratigraphy and paleoenvironmental reconstruction. Examples: Fuji (Japan), Klyuchevskaya Sopka (Kamchatka), Mount St. Helens (USA), and Pinatubo (Philippines). Synonym: composite volcano.
strike-slip fault
Right: 3D model of a strike-slip fault using the Piqiang Fault in China as an example. © USGS/NASA | themapsguy
Strike‑slip fault — fault in which blocks of Earth’s crust move mainly horizontally, parallel to the strike of the fault plane, along a vertical or subvertical surface. Forms under tectonic stresses, during orogeny, and in earthquakes. Displaced rocks and fossil remains allow reconstruction of plate kinematics and paleogeography. Examples: the long‑lived San Andreas Fault in California (~1200–1300 km, right-lateral displacement) and the Great Glen Fault in Scotland. Synonyms: lateral fault, transcurrent fault, wrench fault, transform fault (in the case of a lithospheric plate boundary).
Strombolian-type eruption
Middle: Ash rising from Stromboli after the eruption. © BBC.
Right: The major eruption of Stromboli volcano in November 1920. A large part of the pyroclastic density current was the result of the collapse of the crater rim. © Adriano Di Pietro.
Strombolian‑type eruption — moderately explosive central‑vent volcanic eruption where gas‑charged portions of lava (usually basalt or basaltic andesite) burst in rhythmic blasts from an open crater. Volcanic bombs, scoria, and blocks of incandescent pyroclastics hurl into the air, soaring hundreds of meters, while the magma chamber lies close to the surface. Characteristic lava flows form short tongues on the slopes. Strombolian eruptions feature loud sound effects and bright glowing from lava ejections. Such eruptions can continue for years. The name derives from the volcano on Stromboli Island in the Lipari Islands, Mediterranean Sea. Examples: Izalco (El Salvador), Yasur (Vanuatu).
subaerial
Subaerial — processes, deposits, and structures that form on the land surface in direct contact with the atmosphere (as opposed to subaqueous). Examples include erosion, aeolian deposits, weathering, regolith, and paleosol formation. In paleontology, the term refers to burial conditions of terrestrial remains and ichnofossils (tracks, burrows, roots, rhizoliths). Subaerial environments became especially significant from the Silurian–Devonian, when land vegetation spread. Examples: Devonian paleosols of Canada with rhizoliths of early plants; Oligocene paleosols of the White River Formation, USA—mammal burrows and tracks; Pleistocene paleosols with megafaunal bones and pollen.
subaqueous
Right: Subaquatic eruption of a volcano off the coast of Nuku’Alofa, Tonga, in March 2009. © Dana Stephenson
Subaqueous — processes, deposits, or features that form or exist underwater, on the bottoms of water bodies (seas, oceans, lakes, rivers, lagoons), in contrast to subaerial (terrestrial) conditions. Examples include turbidity currents, submarine volcanic eruptions, subaquatic gravitational currents, formation of subaqueous soils, lagoonal, shelf, and deep-sea sediments. In paleontology, the term refers to the depositional conditions of aquatic biogenic remains and ichnofossils: skeletons and shells of aquatic organisms, bioherms, stromatolites, microbial mats, and subaqueous swim traces. Examples: Ordovician graptolitic shales of Wales; Devonian reef complexes of Australia; Carboniferous brachiopod banks of the Moscow region. Synonym: subaquatic.
subcontinent
Right: The term “subcontinent” usually refers to the Indian subcontinent. For a long time, it existed in isolation as a fully fledged continent and only later, as a result of tectonic plate movement, collided with Eurasia. This map was created using digital elevation data (DEM) obtained during NASA’s Shuttle Radar Topography Mission.
Subcontinent — large landmass, often a major peninsula, separated from a continent by geographic barriers (mountains, seas) or tectonic boundaries (distinct plates). Subcontinents are smaller than recognized continents but possess independent geological development, showing evidence of continental drift and plate collisions. In paleontology, they represent unique paleobiogeographic provinces with endemic faunas and floras, including relict Paleozoic/Mesozoic taxa (e.g., Gondwanan fauna of the Indian subcontinent before its collision with Eurasia). Examples: Indian Subcontinent, Arabian Peninsula, Iberian Massif.
subduction
Right: Subduction zones: areas where one plate dives beneath another, causing earthquakes and volcanic activity.
Subduction — process at convergent boundaries of lithospheric plates, where denser oceanic crust moves beneath continental or other oceanic crust (at rates of 2–10 cm/year), forming deep‑sea trenches and volcanic arcs. The result includes crustal descent into the mantle, active volcanism (stratovolcanoes), seismicity, and dehydration and metamorphism of the slab at depths of 100–150 km. Subduction complexes and accretionary terranes in orogens contain distinctive fossils (Gondwanan/Tethyan faunal and floral assemblages). In historical geology, subduction complexes and accretionary terranes of orogens serve as paleontological markers (e.g., Gondwanan/Tethyan assemblages of pelagic mollusks and radiolarians). Examples: Pacific Ring of Fire, Andes, Japanese Islands, Indonesian Arc.
subglacial-type eruption
Right: Iceland Neovolcanic Rift Volcanic Province. Grímsvötn. © Smithsonian Institution
Subglacial‑type eruption — volcanic eruption beneath a glacier with intense interaction between magma and ice, snow, or meltwater. Contact with ice produces pillow lava and glassy breccia (hyaloclastite). Melted ice forms a lake, and activity shifts to the Surtseyan type. After the water evaporates, lava flows become effusive, creating tuyas—volcanoes with flat tops and steep sides composed of hyaloclastite. Tuyas preserve glacial and postglacial Pleistocene and Holocene fossils. Examples: Tuya Butte (British Columbia, Canada), Hjorleifshofdi, Eyjafjallajökull, Grímsvötn, and Vatnajökull (Iceland).
submarine-type eruption
Right: West Mata erupted in 2009 about 1,219 meters beneath the surface of the Pacific Ocean, between Fiji, Tonga, and Samoa. The orange glow of magma is visible beneath the plume of volcanic gases. © NOAA and NSF
Submarine‑type eruption — volcanic eruption in subaqueous conditions that forms underwater mountains (seamounts), which may break the surface to create volcanic islands. At mid‑ocean ridges, basaltic effusives dominate (pillow lava, sheet flows); in subduction zones, viscous calc‑alkaline lavas occur with volcaniclastic deposits and hyaloclastite. Eruptions may take the Surtseyan type in shallow water. In paleontology, subaqueous deposits contain microfossils (radiolarians, foraminifera). In ancient ophiolites, such rocks serve as indicators of paleodepths and paleo‑oceanic environments. Examples: Kamaʻehuakanaloa (Loihi), Bowie Seamount, Davidson Seamount, and Axial Seamount.
suite
Right: St. Peter Suite in the southwestern Gawler Craton, South Australia. Paleoproterozoic. © Pawley et al.
Suite — in geology, a stratigraphic unit that groups a rock sequence (sedimentary, volcanogenic, metamorphic, or their alternations) sharing common age, lithological, facies, petrographic, or paleontological characteristics (such as distinctive fossil remains). It differs from overlying and underlying subdivisions by clear boundaries. The term applies mainly to intrusive, deformed, and volcanogenic complexes lacking stratification but also to sedimentary suites in local stratigraphic schemes. Examples: Tashtyp Suite of the Devonian in the Minusinsk Basin with tabulate corals, crinoids, and fish; Krivoluk Suite of the Vendian in the Southern Urals.
sulfate
Sulfate — class of minerals, salts of sulfuric acid (H₂SO₄) with the anion SO₄²⁻. Crystalline substances, often colorless or pale‑toned, with variable solubility in water. They form in evaporitic, oxidative (from sulfides), hydrothermal, and metamorphic environments. Sulfates are typical in sedimentary rocks, especially evaporites, where traces of biological activity may occur (microbial mats, stromatolites, crawling traces) and occasionally even rare macrofossils within gypsum deposits. Examples: Neoproterozoic gypsum sequences of Australia and China; Permian evaporites of the Castile Formation, USA; Miocene gypsum deposits of the Mediterranean. Synonym: sulphate.
sulfide
Sulfide — class of minerals, natural compounds of metals and some nonmetals with sulfur, salts of hydrosulfuric acid (H₂S). They often display metallic luster. Their origin is mainly hydrothermal and magmatic, sometimes sedimentary (in reducing environments, such as hydrogen sulfide basins). Paleontological aspect: sulphides serve as important indicators of anoxic (reducing) conditions in sedimentary basins, which promote preservation of organic matter and pyritization of biogenic remains. Examples: pyrite, chalcopyrite, galena, sphalerite, and pyrrhotite. Synonym: sulphide.
sulfite
Right: Scotlandite: PbSO₃. Clara Mine, Oberwolfach, Ortenaukreis, Freiburg Region, Baden-Württemberg, Germany. © Carsten Slotta
Sulfite — salts of the weak and unstable sulfurous acid (H₂SO₃). Many sulfites dissolve in water and act as strong reducing agents. They are extremely rare in nature due to their instability and tendency to oxidize into sulfates. Sulfites occur mainly in hydrothermal veins (scotlandite), volcanic gases, and fumaroles. They are virtually absent in sediments and paleontological deposits because of oxidation. Examples: normal salts (sodium sulfite, Na₂SO₃), acid salts (sodium hydrogen sulfite, NaHSO₃); natural mineral—scotlandite (PbSO₃). Synonym: sulphite.
superterrane
Right: Terrane and superterrane affiliations: Angayucham (red), Insular (orange), Intermontane (purple), Guerrero (yellow), Western Jurassic belt (dark orange)—which is composed of the Western Klamaths, basement of the Great Valley, and northwest Sierra Nevada—North America (dark blue), Farallon (green), and Kula (pink). © Clennett et al.
Superterrane — a large composite geological block formed by the accretion of terranes (volcanic arcs, microcontinents, oceanic plateaus) with different geological histories. Major faults bound these extensive blocks (hundreds of kilometers across), and they usually weld themselves onto continental margins. They play a key role in the growth of continental crust. Superterranes contain ancient faunas that help date the accretion (for example, Triassic faunas and floras in Wrangellia). Examples: the Intermontane Superterrane, the Insular Superterrane (Canadian Cordillera), the Hunic Superterrane (Europe and Asia), and the Sandur Superterrane (Precambrian India).
supralittoral
Right: A classic diagram of the vertical zonation of a rocky shore. © James W. Huffman
Supralittoral — the coastal zone above the spring high tide, sprayed by surf and storm surges but usually not submerged. It borders the littoral zone below and forms evaporites and stromatolites. It also shows diagenetic changes in carbonates, including dolomitization and cementation. Paleontological aspect: under periodically exposed conditions, it preserves ancient microbial mats and biostromes as indicators of arid/semi-arid environments and the evolution of microbial communities before the appearance of metazoans. Examples: Mesoproterozoic microbial mats in supralittoral facies of Lushan, China; Neoproterozoic mats in Australia and Namibia; Devonian microbial structures of the Belcher Islands, Canada. Synonyms: supratidal, splash zone, spray zone.
surf
Right: Sea wave during a storm in the Mediterranean Sea. © bearfotos
Surf — in geology, refers to waves that run up onto the shore and break in shallow water. Wave steepness increases sharply because the lower part slows down against the seabed while the upper part retains its speed. The crests topple forward, dispersing into foam and spray. The energy of the surf erodes rocks (abrasion), redistributes sediments, and shapes coastal relief (abrasional platforms, beaches, coastal ridges, and bars). Surf activity leaves traces of waves such as oscillation ripple marks (wave‑formed ripples), interference ripples, and swash/backwash marks—indicators of ancient coastal zones and water depths. Synonyms: surf zone, breaker zone.
Surtseyan-type eruption
Right: Underwater eruption of the Hunga Tonga Hunga Ha’apai volcano on the Pacific island of Tonga. © Sanya Ruggiero
Surtseyan-type eruption — an explosive interaction of water and mafic magma (mostly basaltic, less commonly andesitic) in shallow water. Water instantly flashes into steam, while magma fragments into ash and lapilli, forming tuff rings and dark jets of vapor. These eruptions resemble Strombolian activity but are more explosive because of the presence of water. They typically occur on volcanic islands and in crater lakes. Tuff deposits often contain fossils of marine fauna (corals, brachiopods, mollusks, etc.). Later, biogenic reefs and biostromes develop on these substrates, allowing reconstruction of ancient coastal environments. Examples: Surtsey (Iceland), Ukinrek Maars (Alaska), Capelinhos (Azores), Ferdinandea (Graham Island, Mediterranean Sea), Hunga Tonga (Tongan archipelago). Synonyms: shallow-water phreatomagmatic eruption, hydromagmatic, hydrovolcanic.
syenite
Right: Quartz alkali feldspar syenite from Estonia. © Sandatlas
Syenite — a light-colored, coarse-grained intrusive rock of intermediate composition that lacks quartz. It contains a predominance of alkali feldspars (orthoclase, microcline, perthite) over plagioclase, along with biotite, pyroxene, amphibole, and occasionally fayalite. Syenites are typical of continental rift zones and complex intrusive bodies. Their color ranges from pink to gray or greenish. Syenites in rift zones occur alongside sedimentary basins that contain rich faunal and floral assemblages, as well as biogenic reefs and biostromes. Hydrothermal dolomitization enhances fossil preservation. The syenites themselves, however, contain almost no fossils.
syncline
Right: This is a rounded, upright, subhorizontal, open, upward-facing, synformal syncline. Bramber, Nova Scotia. © John Waldron and Morgan Snyde
Syncline — in structural geology, a fold bent downward (U‑ or V‑shaped in cross‑section). It curves in such a way that the youngest rocks lie in the center of the fold, while the oldest rocks occur along the margins. Synclines are typical of fold mountain belts and form during crustal compression. They often preserve thick, continuous stratigraphic sequences that allow scientists to study the long‑term evolution of fauna and flora. Forms: brachysyncline (oval), synclinorium (linear). Examples: the Paris Basin—a Cretaceous–Paleogene syncline rich in marine and terrestrial fauna; the Great Donbas Syncline with Carboniferous plant remains; the Jurassic syncline in the Alps with ammonites.
T
takyr
Right: Cracked earth at the Dead Sea. © Willem van de Poll
Takyr — a clay‑silt plain, the floor of an ephemeral lake in desert and semi‑desert zones of Central Asia. When it dries, it develops a dense crust broken by desiccation cracks into polygons. Most takyrs are saline. They can cover several square kilometers and form in flat depressions after seasonal rains. Takyrs serve as indicators of arid conditions; their fine‑grained sediments may contain fossil remains of microbial mats, algae, lichens, or microfauna. Analogs: playa (USA), kavir (Iran), sabkha (Middle East). Synonym: takir.
talus
Right: Talus cones developed at the base of mountain cliffs, Bow Lake, Banff National Park, Alberta. © Andy Fyon
Talus — an accumulation of angular rock fragments at the base of steep rocky slopes, cliffs, and canyons, formed by rockfalls and mechanical weathering (freeze–thaw cycles). The deposits consist of coarse, unsorted fragments with a slope angle of 32–40°. Taluses serve as indicators of periglacial conditions during the Pleistocene. Interclast sediments within these formations sometimes contain vertebrate bones, including herbivorous mammals, and other paleontological remains. Many taluses are relict features, reflecting colder Pleistocene environments. Examples: taluses in Alaska and Yukon with mammoth and bison bones; cave-talus deposits in the Alps with remains of cave bears; relict taluses in the Carpathians and Caucasus. Synonym: scree.
tektite
Middle: Libyan Desert Glass, a type of meteorite impactite found along the Egyptian and Libyan borders. © FossilEra
Right: Dark teardrop-shaped tektite collected in the impact glass strewn field, Atacama Desert, Chile. © Lepnardo de la Torre
Tektites — small (from millimeters to centimeters) melted pieces of natural glass, usually black, greenish, brown, or gray in color. They form when hypervelocity meteorite impacts eject molten droplets of terrestrial rock, which then rapidly cool. Tektites differ from volcanic glass by their high silica content, extremely low water content, and often aerodynamic shapes. They serve as stratigraphic markers of major impact events, creating extensive strewn fields that allow correlation of Quaternary deposits with fossil microfauna or pollen. Examples: australites (~790 thousand years), moldavites (~15 million years), bediasites, billitonites.
tephra
Right: Huge tephra deposit in the Veiðivötn region, part of the Bardarbunga volcanic system, Iceland. © djalin
Tephra — unconsolidated pyroclastic fragments and particles of volcanic glass, regardless of size, expelled by volcanic explosions, gases, or lava fountains. During diagenesis and lithification, tephra deposits transform into tuffs. Tephra horizons serve as important stratigraphic markers for dating and correlating Quaternary deposits. They often contain pollen, spores, and remains of small fauna, recording the moment of volcanic catastrophe and enabling precise tephrochronological correlation of paleontological and paleoecological records. Examples: Laacher See tephra ~12.9 thousand years ago (Europe); Dawson tephra ~30 thousand years ago (Yukon).
tephrite
Right: Tephrite. Roca Negra volcano, Olot, Garrotxa, Girona, Catalonia. © Peter Seroka
Tephrite — a fine‑grained effusive volcanic rock of basic composition, similar to alkali basalt but enriched in feldspathoids (nepheline, leucite), calcic plagioclase, pyroxene (augite), and magnetite; it contains very little olivine. Its color is gray to dark gray. Tephrite is characteristic of continental rift zones. Tephrite horizons serve as stratigraphic markers of Cenozoic magmatism and allow correlation of deposits with paleontological assemblages, including fossils of primates and early hominids in rift‑lake basins. Examples: volcanic series of the East African Rift in Olduvai Gorge and Afar, Ethiopia, which date remains of Australopithecus and early Homo; Miocene–Pliocene tephrites of the Turkana Basin.
terrane
Right: Terrane, Western North America, and the eastern Pacific. The collision and accretion of the Stikinia and Wrangellia terranes of the North American Cordillera and their subsequent displacement northward along large right-lateral strike-slip faults. © C.R. Scotese & B.A. van der Pluijm
Terrane — a fragment of continental crust bounded by faults, transported over long distances, and accreted to another plate or continental margin. It possesses its own geological history with distinctive stratigraphic, magmatic, metamorphic, and structural features. Terranes may represent microcontinents, island arcs, or accretionary complexes. A major fault defines the suture zone between a terrane and the crust, displaying clear evidence of subduction or collision. Terranes record the history of drift and accretion, preserving unique paleontological assemblages, paleomagnetic data, and biogeographic evidence of their distant origin. Examples: Avalonia with Cambrian trilobites (Paradoxides) of African type in New England; Wrangellia with Late Triassic carbonates.
terrigenous deposit
Right: Terrigenous-carbonate sediments in the Late Cambrian complex of Eastern Tuva. The terrigenous layers are reddish-brown in color, while the limestones are gray. © fed_zhimulev
Terrigenous deposits — clastic sediments of continental origin, formed by weathering and erosion of land rocks. They include coarse‑clastic, sandy, siltstone, and clay varieties. Transported mainly by rivers, these sediments accumulate on land, in lakes, and in seas along continental margins. They predominate on shelves and the upper slope, covering about 20–25% of the ocean floor. Typically, they contain 80–95% mineral particles and 5–15% biogenic admixtures (in pelagic zones less than 5%). These deposits may include allochthonous fossils and autochthonous benthic fauna. Examples: Devonian Old Red Sandstone with fish and plants; Carboniferous coal‑bearing strata with flora and insects; Cretaceous deposits of Western Siberia with dinosaurs and marine fauna.
thalassocraton
Thalassocraton — a tectonically stable region of the ocean floor with abyssal plains, analogous to a craton in the oceanic lithosphere. It consists of oceanic crust about 6–7 km thick, overlain by a thin sedimentary cover (10–300 m). Flat or gently rolling abyssal plains shape the relief. Geologists used this term before the rise of plate tectonics, but now they consider it obsolete. Earlier, thalassocratons also included submerged continental blocks (microcontinents), such as the Rockall Plateau. Sometimes the concept is contrasted with hedreocratons—stable continental cratons.
tholeiitic basalt
Right: Tholeiitic basalt lava flows of Mesoproterozoic North Shore Volcanic Group, North Shore, Lake Superior. © John Goodge.
Tholeiitic basalt — a widespread subalkaline volcanic rock (SiO₂ 48–52%) with low alkali content and a high Fe/Mg ratio. It is usually fine‑grained and composed of labradorite, augite, orthopyroxene, and magnetite, sometimes including olivine (0–10%). Volcanic glass is also present, and hornblende is rare. Tholeiite reflects processes of partial melting in the upper mantle in regions of magmatism. It dominates in oceanic crust, in flood basalt provinces (Deccan, Siberian Traps, Columbia River Plateau), rift zones, and back‑arc basins. Researchers have also identified tholeiitic basalts in lunar maria and on Venus. They mark large igneous provinces, which often connect to mass extinctions. Synonyms: tholeiite, tholeite, tholeyite, subalkaline basalt.
thrombolite
Right: Thrombolites Walking Trail, Flower’s Cove, Great Northern Peninsula, Newfoundland and Labrador, Canada. © Christopher Mitchelmore
The inset on the left shows the difference in the structure of a layered stromatolite and a clotted thrombolite.
Thrombolite — carbonate microbialites formed by cyanobacterial photosynthetic communities that, in shallow waters, trapped, stabilized, and cemented sediment grains. They resemble stromatolites but display a clotted texture; they build biostromes or bioherms. Cyanobacterial mats that produced thrombolites and stromatolites generated the oxygen atmosphere necessary for the development of life. Thrombolites were widespread from the Proterozoic to the Early Ordovician (decline after the Guttenberg Carbon Isotope Excursion). Today, such bioherms are rare. In a historical‑geological context, they mark the transition from the “microbial world” of the Precambrian to the “skeletal” Phanerozoic. Example: Lake Clifton in Western Australia.
tidal flat
Right: Brewster Tidal Flats on Cape Cod. © Tom’s Cape Cod
Tidal flats — extensive, nearly horizontal coastal zones of the littoral, flooded by tides once or twice daily, with sandy‑silty deposits showing flaser, wavy, and lenticular bedding. They form under the influence of tidal currents, rivers, and wind. Mudflats may also become exposed when wind drives water away from shallow areas into the sea. They are rich in bioturbation and benthic life (crabs, mollusks, worms). Typical trace fossils include Skolithos, Arenicolites, and Cruziana. Such ichnofossils mark transitional (coastal‑marine) facies in the Paleozoic–Mesozoic. Examples: Wadden Sea (Netherlands), Bay of Fundy (Canada), coast of the Yellow Sea. Synonym: mudflat.
till
Right: The surface of the ground in this area adjacent to Sherman Glacier is covered with a coarse glacial till. Chugach Mountains, Chugach National Forest, Alaska. © USGS Education
Till — an unsorted mixture of rock fragments, clay, and sand deposited directly by a glacier in the melting zone, without significant reworking by meltwater. It is a non-stratified and heterogeneous mixture of angular and striated fragments of various sizes. Main types: lodgement till, deformation till, meltout till. It forms moraines. In interglacial layers, it preserves pollen, plant remains, and bones of mammoth fauna. Examples: till of the Wisconsinan glaciation (North America) with mammoth and mastodon bones; Middle Pleistocene till of the East European Platform with woolly rhinoceros and reindeer. Synonyms: glacial till, boulder clay.
tillite
Right: Glacially smoothed & striated tillite outcrop (Gowganda Formation, Paleoproterozoic, ~2.3 Ga; Straight Lake West roadcut, north of Temagami, Ontario, Canada). © James St. John
Tillite — lithified glacial till: an unsorted mixture of clay, sand, gravel, and striated boulders, reaching hundreds of meters in thickness. Key marker of global glaciations: Paleoproterozoic (Huronian, ~2.4–2.2 Ga); late Neoproterozoic (Cryogenian: Sturtian 720–660 Ma, Marinoan ~650–635 Ma); Permo-Carboniferous Gondwana glaciation; and Pleistocene (often not yet fully lithified). Paleomagnetic data indicate deposition at tropical latitudes (Svalbard, Greenland). Tillites are frequently overlain by cap carbonates, reflecting rapid warming following deglaciation. Example: Cryogenian deposits of Snowball Earth. Synonym: glacial diamictite.
tonalite
Right: Tonalite (or Granodiorite). Intrusive igneous rock of intermediate to felsic composition. This sample displays plagioclase feldspar (white), quartz (gray), and biotite and magnetite (black). Terra Blanca Mountains alluvial fan gravel deposit, San Diego County, California. © MiraCosta College
Tonalite — a coarse-grained, felsic intrusive granitoid rock in which almost all feldspar (>90%) is sodium plagioclase, with quartz content >20%. The mafic minerals include hornblende and biotite and, more rarely, pyroxene; accessory minerals are apatite, zircon, titanite, and sometimes magnetite, ilmenite, and epidote. Its color is gray or greenish-gray. Tonalites are typical of tonalite–trondhjemite–granodiorite (TTG) complexes of the Archean (>3.0 Ga). These “gray gneisses” represent the Earth’s oldest continental crust, providing essential data for reconstructing supercontinents and early life environments. In the Phanerozoic, they occur during the early stages of orogeny. Varieties: trondhjemite, plagiogranite.
transgression
Right: Transgression. Hypothetical change in the coastline in the event of a 70-meter rise in sea level. © National Geographic
Transgression — the rise of sea level and its advance over land due to tectonic subsidence, eustatic rise, or glacial melting. The shoreline retreats inland, flooding vast territories; deep-water sediments overlie shallow-water deposits, reflecting shifts in paleoecoystems. Transgressions drive faunal migrations, changes in benthic communities, and accumulation of sediments with rich fossil assemblages. Examples: the Late Devonian transgression of Eurasia and North America; the Early Cretaceous transgression of the Western Interior Seaway (USA); the Late Jurassic boreal transgression of Western Siberia.
travertine
Right: Travertine terraces “Cotton Mountain” in Pamukkale (Turkey). Layers of travertine are formed in small pools of mineral-laden water. © UseNaturalStone.org
Travertine — a dense or moderately porous, often banded calcium carbonate rock (calcite, aragonite) that is chemically precipitated from groundwater, surface water, or hydrothermal waters. It forms stalactites and other speleothems in karst caves and fills fractures and veins. It often contains fossils of cyanobacteria, algae, and mosses, which reflect paleoclimate and paleohydrological conditions. The color is usually light—yellowish-gray, cream, or brownish-gray. Notable occurrences include Pamukkale in Turkey, Mammoth Hot Springs in Yellowstone, and Tivoli in Italy. Synonyms: calcareous sinter, calc-tufa, calc-sinter.
tuff
Right: Rhyolitic volcanic tuff in the Wall Campground, Mojave Desert. © geologicallyspeaking
Tuff — a light, porous pyroclastic rock composed of consolidated products of explosive volcanic eruptions: ash, sand, lapilli, and bombs, compacted and cemented by opal, zeolites, and calcite. It often contains volcanic glass. The average grain size ranges from 1/16 to 2 mm; it is the equivalent of volcanic sandstone. Siliceous tuff is a chemogenic opaline rock deposited by warm or hot springs (geyserite). Tuff layers help date volcanic events and can preserve fossils (leaves, insects, traces, and even soft tissues) in fine-grained tuff. Example: the exceptionally rich fossil assemblage of the Florissant Formation in Colorado (early Eocene). Synonyms: volcanic tuff, ash tuff, pyroclastic rock.
turbidite
Right: Turbidite sequence in the Spanish Pyrenees. Lighter and tougher layers are composed of sand and silt; darker and softer layers are mostly mudstone. © Sandatlas
Turbidite — a rhythmically bedded deep-water deposit formed by turbidity currents, characterized by a graded alternation of sands, silts, and pelites (the Bouma sequence). They originate from landslides, earthquakes, or storms at the shelf edge and move downslope to the abyssal plains. Modern turbidites form at depths greater than 2,000 m, while fossil turbidites also appear in shallower paleo-environments, sometimes as shallow as 200–300 m. Scientists have recognized turbidites since the Cambrian, and they spread widely from the Ordovician to the Carboniferous, where they formed thick flysch sequences with ichnofossils such as Nereites and Zoophycos and rare benthic remains. Examples: Cambrian–Ordovician turbidites of the Cloridorme Formation (Quebec), the Devonian flysch of the Rhenish Slate Mountains, and Cretaceous turbidites of the Pyrenees.
U
ultramafic rock
Right: Layered ultramafic rocks in the Unst ophiolite of Shetland. © Oliver Strimpel
Ultramafic rocks — a group of igneous and metamorphic rocks of the upper mantle with silica content <45% SiO₂ and high concentrations of magnesium and iron. They consist of more than 90% mafic minerals—olivine, pyroxenes, and amphibole—and contain virtually no feldspars or feldspathoids. Varieties include dunites, peridotites (harzburgite, wehrlite), pyroxenites, and others. They are typically dark green to black and often undergo serpentinization. Ultramafic rocks occur widely in ophiolites—fragments of ancient oceanic lithosphere. They rarely contain fossils, but they commonly occur alongside ophiolites that include fossil-bearing sedimentary rocks. Ophiolite belts, such as the Ural Mountains, Caucasus Mountains, and Alps, mark ancient zones of subduction and collision. Synonyms: ultrabasic rock, peridotite.
ultra-Plinian-type eruption
Right: The 1991 Ultra-Plinian eruption of Mount Pinatubo in the Philippines was the second largest eruption of the 20th century. Taken from inside the former US Clark Air Base in Angeles, located in the province of Pampanga. © Arlan Naeg/AFP
Ultra-Plinian-type eruption — the most powerful class of volcanic eruptions, exceeding Plinian eruptions in both intensity and eruption rate. It produces extremely high eruption columns (>41–45 km) and can generate large calderas through the collapse of the roofs of magma chambers. These eruptions release enormous volumes of rhyolitic (less commonly dacitic) tephra, pumice, and dense pyroclastic flows (ignimbrites) that blanket vast areas. Ultra-Plinian-type eruptions often qualify as supervolcanic eruptions. Heavy ash fall rapidly buries ecosystems, preserving plant impressions, trace fossils, and sometimes even soft tissues (similar to tuffs). Their global climatic effects (a volcanic winter) can influence mass extinctions or major reorganizations of biodiversity. Examples: Hatepe (Lake Taupo, New Zealand), Lake Toba (Sumatra), and eruptions of the Yellowstone Caldera.
unconformity
Unconformity — a significant break in the geological record that separates rocks of different ages. It represents an eroded or long-exposed surface of older rocks upon which younger layers rest unconformably. Unconformities form as a result of erosion, tectonic uplift, deformation of rocks, or a lack of sedimentation, marking a stratigraphic gap (hiatus). Their surfaces may preserve paleosols, weathering crusts, or horizons with reworked fossils. Examples: the Great Unconformity in the Grand Canyon (Precambrian–Cambrian); the Late Ordovician–Silurian unconformity in Europe; the pre‑Jurassic unconformity in Western Siberia. Synonyms: discordant bedding, stratigraphic discontinuity, stratigraphic break.
Types:
V
vein
Left: Veins containing layer-parallel and perpendicular branches. (Sestri Liguria, Italy). © Koehn et al.
Right: Tree-like branching vein (Isle of Kerrera, Scotland, UK). © Koehn et al.
Vein — in geology, a mineral body filling a crack or fault in rock, differing in composition from the surrounding host rock. Vein formation usually occurs during tectonic movements and volcanism, when mineralized hydrothermal solutions fill fractures, or through metasomatic replacement. This is how deposits of ores such as gold, silver, copper, lead, and zinc typically form. Geologists call small veins veinlets. Types of veins include hydrothermal, pegmatite, and sedimentary. By cutting across ancient rocks and fossils, veins help date events and record stages of geological and hydrothermal activity. Examples include quartz gold-bearing veins of Colorado and vein deposits of the Urals (gold, copper, rare metals). Synonyms: mineral vein, hydrothermal vein.
volcanic crater
Right: Koko Crater, Hawaii, on the eastern side of Maunalua Bay along the southeastern side of the same island, Oʻahu, in Hawaiʻi. © Eric Tessmer.
Volcanic crater — a bowl- or funnel-shaped depression located at the summit or on the slope (a lateral crater) of a volcanic cone. At its bottom, one or more vents usually open, through which lava and pyroclastic materials reach the surface. Craters form mainly during explosive eruptions; inside a crater, a new volcanic cone may develop, and after eruptions cease, a crater lake may appear. Crater diameters range from tens of meters to several kilometers, while depths vary from a few meters to several hundred meters. Phreatic eruptions create maars. In the geological record, craters and their associated deposits serve as important indicators of paleovolcanism and help date volcanic events.
volcanic gas
Right: A gas plume rises from the lava dome on the south side of the Poás summit crater lake in this February 2000 view from the NW. © Federico Chavarria Kopper
Volcanic gases — primarily water vapor, sulfur dioxide, and carbon dioxide—dissolve in magma and escape during degassing in eruptions and fumarolic activity. They may also include hydrogen sulfide, halogen hydrides (such as hydrogen chloride and hydrogen fluoride), carbon monoxide, and other components. SO₂ causes short-term climatic cooling through sulfate aerosols, while CO₂ contributes to long-term warming. Large emissions of volcanic gases drive mass extinctions, trigger oceanic anoxia, and cause global changes in sedimentation, leaving traces in isotopic anomalies and abrupt shifts in the paleontological record.
volcaniclastic rock
Right: This volcaniclastic rock is composed of ash and other pyroclastic fragments.
Volcaniclastic rocks — clastic rocks ejected during a volcanic eruption in solid or partially molten form. They consist predominantly (>25%) of fragments of volcanic origin: pyroclastic, lavoclastic, hyaloclastic, or epiclastic (reworked). These rocks include scoria, pumice, ash, and admixtures of older lava fragments and non-volcanic material. Volcaniclastic deposits often contribute significantly to sedimentary basins, forming characteristic marker horizons in the stratigraphic record and providing a basis for studying paleovolcanic events. Synonym: volcanic debris.
Vulcanian-type eruption
Right: Vulcanian eruption in 1888 from the Fossa cone on the island of Vulcano. Photo from the collection of Maurice and Katia Krafft.
Vulcanian-type eruption — an explosive eruption of a central volcano with viscous andesitic, dacitic, or rhyolitic lava containing high amounts of gas. The lava usually has an intermediate composition and a temperature of about 900 °C. Because of its high viscosity, gases cannot escape freely; they accumulate energy and periodically burst out, breaking the lava crust and central dome. These eruptions produce short but powerful explosions (lasting minutes to hours), hurling large lava blocks, lapilli, bombs, and dark ash, and then often shift into brief effusive phases that rebuild the dome. Eruption columns can reach about 20 km in height, and eruptions recur after long periods of dormancy. The name derives from Vulcano (Aeolian Islands). Examples: Tavurvur (Papua New Guinea), Santiaguito and Fuego (Guatemala), Anak Krakatau (Indonesia), Klyuchevskoy, Avachinsky, and Bezymianny (Kamchatka). Synonyms: Vulcano-type eruption, paroxysmal eruption.
W
watershed
Right: A watershed boundary is sometimes referred to as the ridgeline or watershed divide. Source: The Watershed Project
Watershed — the boundary between basins of water bodies or watercourses, where runoff begins to flow into different river systems or bodies of water. On plains, it runs along low interfluves and can be difficult to distinguish; in mountains, it coincides with the crest of a ridge. The term may also mean drainage basin. Watersheds act as barriers to the migration of organisms, regulate the transport of sedimentary material, and help reconstruct tectonic and climatic changes. Examples: the Ural watershed in the Permian–Triassic, which separated European and Siberian faunas, and the Andean watersheds in the Miocene, which controlled the migration of South American mammals.
weathering
Right: Torcal de Antequera Natural Park in Spain, Andalusia, Province of Malaga. The bizarrely shaped rocks have been eroded over millions of years. Weathering shapes this limestone landscape. Premium/UIG/Universal Images Group/Getty Images
Weathering — a continuous process of breaking down and chemically altering rocks and minerals at Earth’s surface. Weathering can be physical (mechanical), chemical, or biological. Do not confuse weathering with the action of wind alone, because it represents a complex phenomenon. Weathering occurs in situ and does not involve material transport, which distinguishes it from erosion. It serves as a crucial preparatory stage in soil formation and in the development of loose weathering crust, provides a source of sedimentary material, and influences the preservation of paleontological remains as well as the reconstruction of paleoclimatic information.
weathering crust
Right: Weathered crust in the area of Lagerniy Sad Park on the right bank of the Tomi River. © A. Yalaltdinova
Weathering crust — the upper altered part of bedrock that forms in situ as a result of various types of weathering. It represents a loose, heterogeneous layer (soil, eluvium, clay minerals, laterite, bauxites, etc.). Products that remain at the site of their formation are called residual weathering crust (eluvium). The thickness and degree of weathering depend on climate, relief, tectonic processes, and the composition and strength of the parent rocks. Regolith includes the weathering crust. In the geological record, ancient weathering crusts and paleosols serve as indicators of paleoclimate, paleorelief, and tectonic stability. They also influence the burial conditions and preservation of paleontological remains.
X
xenocryst
Right: Euhedral zircon Zr(SiO₄) xenocryst in basalt. Hillside xenocryst locality, Vernon Co., New South Wales, Australia. © Matthew Goodwin
Xenocryst — an individual foreign crystal enclosed in an igneous rock that did not crystallize from the magma itself but was captured from the surrounding rock or from a xenolith during the ascent of the melt. Xenocrysts are usually older than the host rock. Examples include quartz in basalt, zircon or garnet xenocrysts in some acidic lavas, and diamonds in kimberlite. Xenocrysts provide information about the composition, conditions, and age of rocks in the deep crust and upper mantle. Xenocrysts (especially zircons) allow geologists to date very ancient geological events. Synonym: accidental crystal (obsolete).
xenolith
Right: Mantle peridotite xenolith. Interior crater, Lanzarote (Canary Islands). © viperbsg62
Xenolith — a fragment of foreign rock enclosed within a rock of another type, often found inside igneous rocks (volcanic or plutonic). It is frequently a piece of deep-seated rock that magma captures during its ascent or intrusion and carries to the Earth’s surface. A xenolith has a composition and origin different from the host rock. Xenoliths provide important information about the structure and composition of otherwise inaccessible parts of the deep crust and upper mantle. Sometimes crustal xenoliths preserve metamorphosed sedimentary rocks with relics of ancient fauna or flora. Synonyms: inclusion, accidental enclave, accidental inclusion.
xeric
Right: Cyperus conglomeratus growing on the dunes of the magnificent Sharqiya Sands in Oman. © Paul Alfrey
Xeric — in geology and geography, a term describing conditions of extremely low humidity, where evaporation exceeds precipitation. Such conditions characterize deserts, semi-deserts, and xeric shrublands, and they produce distinctive soils (often with carbonate or gypsum horizons), drive physical weathering, and support sparse vegetation composed of xerophytes. Deposits from these environments often contain arid paleosols and rare, fragmentary skeletal remains (organic material preserves poorly). Plant remains and pollen from drought-resistant species serve as tools for reconstructing paleoaridity. Synonyms: arid (partially), drought-affected.
xerochore
Right: The Karakum Desert is one of the largest deserts in Central Asia. Source: Alpagama.
Xerochore — a climatic zone of extremely arid and waterless (hyperarid) deserts, where evaporation greatly exceeds precipitation and rainless periods can last for years. These zones support biotas that are poor in species diversity but highly specialized, consisting of xerophytes and xerotolerant organisms. Characteristic features include aeolian deposits, arid paleosols, evaporites, and rare, often fragmentary terrestrial remains such as vertebrate skeletons, along with pollen from drought-resistant plants. Xerochores are important for reconstructing periods of climate aridization, the expansion of desert zones, and associated biotic migrations in the geological past. Synonyms: hyperarid zone, desert zone.
Y
yardang
Right: Yardang landform in Dun Huang, Yadan National Geopark, Gobi Desert, Gansu Province, China. © Eric PK
Yardang — a narrow, elongated ridge carved by wind erosion in arid regions, most often in clay, siltstone, or sandstone formations. It has a streamlined shape resembling an inverted ship’s hull, aligns parallel to prevailing winds, and typically occurs in groups. When yardangs expose ancient deposits, they sometimes reveal fragmentary terrestrial fossil remains or traces of animal activity, though such finds are rare and incomplete due to intense weathering. Arid-type paleosols and aeolian deposits testify to the prolonged influence of arid climates. The orientation of yardangs provides evidence of ancient wind directions and helps reconstruct arid paleoenvironments.
Z
zeolite
Right: Greek Zeolite, Thrace. Source: zeolife
Zeolites — a group of hydrated framework aluminosilicates of alkali and alkaline earth metals with a porous structure. They form during low-temperature alteration of certain volcanogenic materials and within hydrothermal systems (lakes, marine basins, hydrothermal fields). Zeolites can release and reabsorb water depending on temperature and humidity. Their distribution reflects ancient volcanism, hydrothermal activity, changes in paleo-salinity and alkalinity of water bodies, as well as paleoclimatic events. In some cases, zeolites provide unusual preservation of soft tissues or delicate structures (such as feathers) when rapid replacement of organic remains by fine-crystalline zeolite prevents complete destruction. Varieties include clinoptilolite, chabazite, phillipsite, analcime, and mordenite.
zircon
Right: Zircon (ZrSiO₄) usually contains hafnium impurities and comes in a wide range of colors. © Martin Heigan.
Zircon — a durable silicate mineral, ZrSiO₄, often containing hafnium and other elements and found in a wide range of colors. Chemically and thermally stable, zircon is widely used for U–Pb dating because of its uranium content. Crystals dated to 4.4 billion years (Jack Hills) represent the oldest known materials on Earth and serve as key evidence for studying the planet’s earliest geological history. They indicate crust formation through interactions between magma and water around ~4.4–4.1 billion years ago, during the transition from the Hadean to the early Archean. Graphite inclusions with light carbon isotopic compositions suggest the possible existence of an early biosphere, though direct evidence of early life remains lacking. Synonyms (obsolete): zirconite, hyacinth, jacinth.
The geological dictionary paleontology Q–Z completes this illustrated alphabetical reference, tracing Earth’s history from quartzites to watersheds. Definitions offer a consistent paleontological and historical geology perspective, connecting individual terms to the grand narrative of planetary and biological evolution.
Related sections of the Geological Dictionary:
