Geology Dictionary Illustrated: A–D

Geology dictionary illustrated: A reference covering geological terms from A to D, with definitions focused on paleontology, historical geology, and Earth science, combining tectonics and stratigraphy with fossil records and deep-time perspectives on Earth’s evolution.

Alphabetical index of geological terms

This section provides a reference list of essential geological concepts frequently used in Earth sciences, 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 Geology Dictionary Illustrated

Geology Dictionary Illustrated—This reference guide combines rigorous geological definitions with a paleontological and historical perspective. It covers rocks, minerals, structures, and various earth processes. The guide places every term within the context of Earth’s 4.5-billion-year history. The resource also highlights the fossil record that documents this timeline. Fossils are much more than mere curiosities. They serve as chronological markers and environmental indicators. These remains have witnessed mass extinctions and major evolutionary radiation. Each entry includes an illustration from a real geological or paleontological example. These images ground the terminology in our planet’s physical and biological record.

Keywords: geological dictionary, paleontology, Earth science, historical geology, fossil record, basalt, banded iron formation, chert, diagenesis, continental drift

A

ablation

Ablation — in glaciology, the loss of mass from a glacier, floating ice, or snow cover through melting, evaporation, erosion, or the calving of icebergs. In geomorphology, ablation also refers to the removal of rock or soil by weathering and erosion (wind, water). In paleogeography, glacial ablation deposits (ablation till, moraines) serve as key markers of ancient glaciations. These deposits often contain reworked fossils and paleosols to help reconstruct glacier margins and climatic fluctuations. Examples: moraines of the last Weichselian glaciation in Europe and the Laurentide Ice Sheet in North America.

abrasion

Abrasion — in geology, the mechanical breakdown of rocks by waves, currents, wind, ice, or gravity. The process wears and rounds particles into coastal sediments. These deposits contain reworked fossils that serve as markers for ancient shorelines and sea levels. In high-energy zones, abrasion causes fragmentation and destruction of fossils. In paleogeography, abrasion helps reconstruct ancient coastlines: rounded shells and fragmented fossils in conglomerates indicate strong wave action. Examples: Devonian fossils (brachiopods, trilobites, corals) in limestone pebbles along the Michigan shoreline (USA); Jurassic coastal deposits of South Wales with abrasion surfaces and encrusting organisms.

absolute age

Absolute age — the age of a geological event, fossil, or rock expressed in years. Geologists most commonly determine absolute age using radiometric methods. Constant radioactive decay and specific isotope ratios reveal the exact time of rock formation. Dendrochronology and amino acid analysis are also applied. Absolute age enables precise correlation of fossil appearances and major biostratigraphic events with the geochronological timescale, reconstructing Earth’s history. Examples: the Cambrian explosion (538–521 Ma, U-Pb dating of volcanic layers with trilobites and early animals) and the Permian–Triassic boundary (252 Ma, zircon dating, coinciding with the mass extinction).

accretion

Accretion — in plate tectonics, the process through which island arcs, terranes, or plate fragments attach to larger landmasses along convergent (collisional) plate boundaries. This mechanism represents a primary driver of continental growth. In geology, the term also refers to the gradual expansion of land along riverbanks through deposition of sedimentary material (alluvium) by river currents or floods. Exotic faunal assemblages and paleomagnetic data record terrane accretion from the Phanerozoic onwards (particularly during the Mesozoic–Cenozoic). These records reveal how continental margins added fragments with distinct paleogeography and biota. Example: the Cordillera of North America. Synonym: tectonic accretion.

achnelith

Achneliths — in volcanology, are a type of pyroclastic fragment consisting of small, glassy volcanic bombs with spherical, dumbbell-shaped, or teardrop forms. They form during mild explosive eruptions (Strombolian or Hawaiian type), when extremely fluid magma is ejected into the air and rapidly solidifies in flight. Depending on the cooling conditions, they acquire rounded or nearly perfectly spherical shapes and sometimes irregular polygonal outlines. Their size ranges from a few millimeters to several tens of centimeters. An achnelith consists of a porous, glassy core surrounded by a thin, smooth shell of black obsidian. Synonyms: achnelite, pumiceous achnelith.

acid rock

Acid rocks — igneous rocks containing more than 63% (according to some authors, more than 66%) SiO₂. They typically also contain feldspar and about 20% free quartz. These rocks are often light-colored, less dense, and characteristic of the continental crust. In historical geology, they frequently indicate late stages of continental development and acidic granite formation. Examples: granite, rhyolite, granodiorite, pegmatite, obsidian, and adamellite. Synonym: acidic rock.

aeolian deposition

Aeolian deposition — refers to the accumulation of sand and loess transported and deposited by the wind. These deposits characterize coastal zones, deserts, and steppes with strong winds and low humidity. In these areas, sparse vegetation cannot anchor the loose material. They form dunes, loess sequences, and sand plains. In the geological record, Pleistocene loess is particularly important. It contains remains of mammoth fauna, pollen, and buried paleosols that reflect the climatic changes of glacial periods. Examples: The Loess Plateau of China and stratigraphic sections in Eastern Europe containing the bones of mammoths and woolly rhinoceroses, along with a rich pollen record. Synonym: eolian deposition.

agate

Agate — a translucent variety of chalcedony with banded coloration, composed of SiO₂. Occurs mainly in effusive rocks as amygdales, nodules, and veins; typically fills cavities in rocks and hydrothermal voids. Commonly obtained from placer deposits. Researchers often employ agates as chronological indicators for late hydrothermal processes and the cooling of volcanic sequences. Color: gray, light gray, milky white, gray‑blue, green, brown, pink, blue, or black.

agglomerate

Agglomerate — a pyroclastic igneous rock composed of large (more than 64 mm) angular fragments of minerals ejected during eruptions and firmly cemented together in place. Poorly sorted and retaining sharp edges due to minimal transport. Records ancient explosive eruptions, aiding in the reconstruction of volcanic history and paleoenvironments. Rarely contains captured or reworked fossils but may include plant remains or marine fossils from underlying strata. Examples: Oligocene–Miocene agglomerates of the Panama Canal with reworked marine fossils; Jurassic volcanic agglomerates with incorporated plant remains.

aggregate

Aggregate — in geology, a mass of mineral fragments or rock particles such as sand, gravel, or crushed stone, or their mixture. Some rocks consist of aggregates of crystals, for example, dolomite, rock gypsum, and lapis lazuli. In the context of historical geology, aggregates serve as indicators of depositional conditions, stages of diagenesis, and rock reworking. Aggregates, especially in conglomerates and breccias, often contain reworked fossils, allowing reconstruction of ancient fluvial, coastal, and glacial environments. Examples: Pleistocene gravel aggregates with bones of mammoth fauna in European river terraces; Carboniferous sand-gravel aggregates with plant remains and spores in coal-bearing sequences.

aleurite

Aleurite — a loose fine‑grained sedimentary rock composed mainly of mineral grains (quartz, feldspar, mica, and others) measuring 0.01–0.1 mm. Occupies an intermediate position between clay and sand. Typically forms in river valleys and floodplains, less commonly in glacial zones. Often used synonymously with “silt.” Through lithification, aleurite transforms into siltstone. Aleurites frequently preserve delicate traces of biological activity (ichnofossils), plant remains, and microfauna.

aleurolite

Aleurolite — a fine‑grained sedimentary rock composed of more than half particles measuring 0.004–0.063 mm. Its main constituents are quartz, mica, and clay minerals. Forms through cementation of loose sediments (loess, sandy loams, loams) and usually displays reddish, brown, or gray coloration. Texturally occupies an intermediate position between sandstones and shales. Important for reconstructing paleoenvironments of river floodplains, lakes, and coastal zones, often containing well‑preserved fossils and traces of biological activity. Examples: Devonian aleurolites with plant remains; Permian Wandrawandian Siltstone (Australia) with brachiopods and bivalves. Synonym: siltstone.

alkaline rock

Alkaline rock — a rare type of rock characterized by high contents of Na₂O and K₂O relative to other oxides and a deficiency of Al₂O or SiO₂. Therefore, feldspathoids, alkali pyroxenes, or alkali amphiboles occur instead of quartz. This rock is typical of intraplate and rift settings from the Late Precambrian and Phanerozoic. It is associated with alkaline magmatism and contains significant amounts of rare-earth elements. Includes syenites, phonolites, alkali basalts, and tephrites.

allochthon

Allochthon — redeposited sedimentary material transported from its place of origin. This material originates from a distant locality. It includes organic remains, fossils, minerals, and rock blocks. In paleontology, allochthonous fossils indicate transport and redeposition. It helps to distinguish autochthonous communities from mixed ones and to reconstruct pathways of sediment transport in ancient basins. Examples: Eocene lacustrine deposits of North America with allochthonous plant remains and insects; Cambrian conglomerates with redeposited trilobites and brachiopods from shallow‑water zones. Synonym: allochthone.

allogenic

Allogenic — a mineral or rock component formed in a distant locality, including material derived from pre‑existing rocks and transported to the site of present deposition. Examples: xenolith in igneous rock, pebble in conglomerate, detrital mineral in a placer deposit, and particles in detritus. Such components serve as indicators of sediment source areas, transport pathways, and depositional environments. They help scientists reconstruct paleontologies and facies from past geological settings. The term refers to individual constituents rather than entire formations. Synonyms: allothigene, allothogenic.

alluvial fan

Alluvial fan — in sedimentary geology, a fan‑shaped accumulation of loose clastic material (boulders, pebbles, sand, and clay) formed where mountain streams emerge onto plains or valley terraces and flow velocity decreases sharply. Commonly develops at the foot of mountains. They also occur underwater in coastal zones, river mouths, and submarine canyons at the base of continental slopes. Ancient alluvial fans often contain redeposited fossils and plant remains, providing evidence for the paleorelief and climate of foreland regions. Examples: Miocene fans of the Siwalik Formation (Himalayas) with mammal bones; Permian alluvial fans of Europe with redeposited Glossopteris flora. Synonyms: detrital cone, talus fan.

alluvial plain

Alluvial plain — in sedimentary geology, a broad, flat, gently sloping plain formed through prolonged deposition of alluvium by streams descending from mountainous regions. Typically characterized by landforms such as winding river channels, terraces, and meanders. Alluvial plains cover extensive areas where river floodplains shifted over geological time. They serve as important indicators of paleohydrological conditions, long‑term sediment accumulation, and facies changes in the lower parts of river systems. Examples: Pleistocene alluvial plains of Siberia and Alaska with mammoth fauna remains; Late Jurassic Morrison Formation (USA) with alluvial plain deposits containing dinosaur fauna and plants. Synonym: aggraded valley plain.

alluvium

Alluvium — in sedimentary geology, loose deposits laid down by permanent or temporary water flows (such as floods) in river and lake valleys. Composed of fragments of various sizes (boulders, pebbles, gravel, sand, sandy loam, clay, and silt). River deltas consist entirely of alluvial deposits. Sometimes deposits of lake origin are termed “lacustrine,” while those formed by rivers are termed “fluvial.” Alluvium frequently contains rich assemblages of redeposited fossils, mammal bones, plant remains, and archaeological finds. Example: Pleistocene alluvium of the Nile River with remains of ancient settlements and fauna.

amphibolite

Amphibolite — one of the most widespread metamorphic rocks, composed of hornblende (amphibole) and plagioclase, with very little quartz. Forms through metamorphism of mafic igneous rocks and marly or compositionally similar sedimentary rocks. Typically a crystalline, medium‑grained rock of grayish‑green to dark green‑black color, often mottled. Amphibolites are common in ancient Precambrian metamorphic complexes and provide evidence for Precambrian and Phanerozoic orogenesis. However, due to high temperatures and pressures, fossils are rarely preserved.

andesite

Andesite — an effusive volcanic rock composed mainly of sodium plagioclase and one or more mafic minerals (amphibole, pyroxene, biotite, muscovite, and hornblende). Color ranges from light gray to dark gray. Forms from relatively viscous lava and typically occurs in island volcanic arcs. Dense, fine‑grained matrix, often porous; silica constitutes more than half of the composition, with the remainder consisting of ferromagnesian minerals. Andesites build lava flows, dikes, and small intrusive bodies. A characteristic rock of subduction zones, it serves as an indicator of ancient convergent plate boundaries. It also contributes to the reconstruction of Mesozoic and Cenozoic orogenesis.

andesite-basalt

Andesite-basalt — a fine-grained effusive rock intermediate in chemical and mineral composition between andesite and basalt, with a silica content of approximately 50–55%. Andesite-basalts are characteristic of volcanic arcs and oceanic island systems, where they reflect a transition toward more silicic magmatism. Color dark grey to greenish-grey or black. Synonyms: basaltic andesite, andesibasalt.

angular unconformity

Angular unconformity — in structural geology, a situation where two adjacent rock complexes are not parallel and are inclined at different angles. This reflects a break in sedimentation. During this time, the underlying layers were disturbed by deformation and folding. As a result, the strata above and below the unconformity surface have differing dips. Angular unconformities allow reconstruction of tectonic events, erosion, and the evolution of life through changes in facies and fossil assemblages. Example: The classic Hutton’s unconformity at Siccar Point (Scotland), between Silurian and Devonian strata. Synonyms: clinounconformity, structural unconformity.

anorthosite

Anorthosite — an intrusive mafic rock of the gabbro family composed of more than 90% plagioclase, with only minor amounts of mafic (dark-colored) minerals. It forms through the flotation of plagioclase crystals to the upper part of a mafic magma chamber, where they accumulate and crystallize. On Earth, anorthosites occur mainly in ancient cratons (e.g., Ukraine, Canada, Finland) and are typically older than 1 billion years. On the Moon, they make up the bulk of the light-colored highlands, reflecting the crystallization of a global magma ocean about 4.5 billion years ago. Researchers have also identified them in comets and chondrites. Examples: Proterozoic massifs of the Ukrainian Shield (e.g., the Korosten Pluton, ~1.8–1.74 Ga) and lunar ferroan anorthosites from the highlands (Apollo samples, ~4.3–4.5 Ga). Synonym: plagioclasite.

anticline

Anticline — in structural geology, a geological fold with an upward-convex, arch-like shape, often visible at the Earth’s surface in exposed rock strata. Its core contains the oldest layers. Anticlines reflect zones of tectonic compression in fold belts of Paleozoic and younger ages. They often serve as traps for oil and gas. Structurally, they are the opposite of synclines. Examples: the Willow Creek Anticline (Montana, USA), containing nests and bones of Maiasaura in the Two Medicine Formation; the Codos Anticline (Spain), with fossils from the Ediacaran–Cambrian boundary.

apatite

Apatite — a group of phosphate minerals with a vitreous luster, typically greenish, bluish, yellow-green, or pink in color. It occurs as an accessory mineral in most igneous and metamorphic rocks. Biogenic microcrystals of apatite form part of the teeth and bones of vertebrates and are also found in invertebrates, plants, and bacteria. Due to its resistance to diagenesis, apatite preserves its morphology and isotopic composition well. This makes it a valuable source of information on ancient life and paleoecology from the Cambrian to the Holocene. Varieties: fluorapatite, chlorapatite, hydroxylapatite, francolite, strontioapatite. Synonym: calcium phosphate.

aragonite

Aragonite — a natural polymorph of calcium carbonate (CaCO₃), typically white, yellowish, or gray, and denser and harder than calcite. It occurs in gypsum and iron ore deposits, in hot spring precipitates, and in marine sediments. Aragonite forms part of the exoskeletons of corals and the nacreous layer of mollusk shells, where it contributes to pearl formation. Its good preservation or transformation into calcite allows reconstruction of “aragonite seas,” paleotemperatures, and shallow-marine facies. Examples: Jurassic ammonites and belemnites from England and Germany, as well as ammolite from the nacreous layer of ammonites in Late Cretaceous deposits of Canada and the United States. Varieties: sprudelstein, conchite, nicholsonite, tarnowitzite, alstonite, witherite, strontianite, and cerussite.

arenite

Arenite — a clastic sedimentary rock composed of quartz and rock fragments with a medium grain size of 0.063–2 mm, i.e., a type of sandstone. It forms through the erosion of preexisting rocks or by turbidite redeposition of sands and typically has well-sorted, rounded grains. Arenites often contain fossil remains. These enable the reconstruction of ancient depositional environments, ranging from desert dunes and river valleys to shallow-marine beaches and deltas. Examples: ichnofossils in the Coconino Sandstone, shells of brachiopods and mollusks in the Dakota Sandstone, and carbonized plant remains in sandstones preserving leaves and stems.

argillite

Argillite — a hard, compact sedimentary rock formed from dehydrated and indurated clay minerals with admixtures of quartz, mica, and feldspar particles. It shows less distinct layering than mudstone or shale. Argillite represents a sedimentary rock (e.g., claystone or siltstone) that has undergone the effects of elevated temperatures and pressures. Its color ranges from bluish-gray and black to slate-like, light, or whitish. Geologists sometimes refer to platy argillites as clay slates. Argillites often preserve fine lamination and microfossils; for example, black argillite sequences of the upper Ediacaran and lower Cambrian contain Vendotaenia antiqua and entire assemblages of microscopic algae and organic microstructures (e.g., in the Kotlin Formation). Synonyms: argillyte, zebra stone, khailite.

ash

Ash — in volcanology, fine fragments of lava or rock ejected into the atmosphere during a volcanic eruption. It is an unconsolidated pyroclast of variable shape with an average diameter of less than 2 mm. Wind can transport volcanic ash over long distances. Its consolidated equivalent forms ash tuff. Volcanic ash often creates thin marker horizons that preserve fossil remains exceptionally well and allow precise dating of strata. Examples: ash deposits that buried forests in Yellowstone, as well as ash layers in dinosaur-bearing sediments of North America. Synonyms: ash grain, volcanic dust.

asthenosphere

Asthenosphere — a relatively ductile and hot layer of the upper mantle (with temperatures close to the melting point), located beneath the colder and more rigid lithosphere. Magma can form within it. It lies at depths of approximately 100–120 km beneath continents and 50–60 km beneath oceans. The asthenosphere has a thickness of about 100 km.

attrition

Attrition — the process of wearing down pebbles and rock fragments through abrasion, involving grinding, breakage, and rounding during transport under the action of wind, ice, currents, or waves.

aulacogen

Aulacogen — a rift zone, i.e., a deep, narrow, and elongated trough (graben) in the basement of an ancient platform, filled with a platform sedimentary cover and bounded by faults. It represents a broad depression formed by an ancient failed rift arm that later underwent subsidence and erosion and became filled with sediments. Its formation relates to the cessation of divergence between two tectonic plates. Aulacogens can extend for hundreds of kilometers in length and tens of kilometers in width. It is also defined as a tectonic depression on a craton, bounded by faults. Examples: the Mississippi Embayment; the Cambay Basin (India). Synonym: rift zone.

autochthon

Autochthon — a fossil or rock material that formed in the place where it is currently found and has not been transported. It often includes biogenic limestones, coal, and evaporites. Autochthonous assemblages, such as coral reefs in shallow carbonate sequences of the Carboniferous or plant and animal communities in coal-bearing successions of the Carboniferous and Permian, reflect original living communities and depositional environments (e.g., shallow carbonate basins and swamp facies). Examples: Carboniferous autochthonous coral reefs in the Moscow Region and the Donbas; Permian coal-bearing autochthonous Glossopteris floras of Gondwana (Australia and India).

B

badland

Badlands — an arid landscape where soft sedimentary rocks and clay-rich soils undergo intense wind and water erosion. This process produces a highly dissected, uneven topography with short, steep slopes and sparse vegetation, characterized by ravines, gullies, buttes, and hoodoos. Badlands provide excellent exposure of fossil-rich strata. Example: Badlands National Park (USA), where erosion reveals Late Eocene and Oligocene deposits containing thousands of skeletons of ancient mammals.

banded iron formation

Banded iron formation (BIF) — a common type of banded iron ore deposit. It is composed of alternating layers of quartz, chalcedony, or other silica minerals and iron oxides (magnetite, hematite, and siderite). It forms during the Archean and Proterozoic eons. BIFs form when volcanic activity dissolves ferrous iron in seawater. In the upper ocean, the iron reacts with molecular oxygen. The reaction oxidizes the iron to ferric iron. It then precipitates as insoluble sediment on the seafloor. Silica then accumulates until dissolved iron builds up again in the water, and the cycle repeats. Banded iron formations serve as key indicators of the gradual accumulation of oxygen in the atmosphere and oceans. Therefore, they provide important evidence for the evolution of the early biosphere.

bank

Bank — in oceanography and geology, an isolated shoal or elevation on the floor of a river or sea where the depth is significantly shallower than the surrounding area, typically near sea level. It can also refer to a barrier island along a coastline composed of sand that separates an inner lagoon or strait from the open ocean. Banks often preserve rich shallow-marine fossil assemblages—corals, mollusks, foraminifera, and reef structures—allowing reconstruction of ancient shallow-water environments and sea-level fluctuations. Examples: Devonian coral and stromatoporoid banks in the Timan–Pechora Basin; Cretaceous banks with rudists and large foraminifera in the Mediterranean region; the Bahama Banks.

barite

Barite — a mineral composed of barium sulfate (BaSO₄), also known as heavy spar. It often contains strontium and calcium as impurities. Its origin is mainly hydrothermal, and it sometimes forms independent ore veins. Barite is colorless and sometimes transparent but more commonly white or gray; impurities may give it yellow, red, bluish, light brown, or other hues. It has a pearly or vitreous luster. In sedimentary rocks, barite forms concretions and “barite roses,” often in association with fossils or in organic-rich layers. Barite’s sulfur and oxygen isotopic composition reconstructs paleotemperatures, anoxic episodes, and ancient ocean conditions from the Paleozoic to the Cenozoic. Examples: barite concretions containing ammonites in Jurassic deposits of Germany and England.

barrier island

Barrier islands — in geography, a dynamic coastal landform consisting of long, narrow, sandy islands formed by waves and tidal currents parallel to a continental shoreline and separated from it by a lagoon or bay. They typically occur in chains that can extend for hundreds of kilometers and shift under the influence of storms. Ancient barrier islands are often preserved in the geological record as sandy arenites containing rich shallow-marine fossil assemblages—mollusk shells, brachiopods, foraminifera, and ichnofossils—helping reconstruct sea-level fluctuations and coastal evolution during the Pleistocene and Holocene. Examples: Long Beach (New Jersey, USA), Lido (Venice, Italy), Fraser Island (Australia).

basalt

Basalt — the most widespread fine-grained igneous rock of mafic composition that erupts onto the Earth’s surface. It consists mainly of plagioclase, pyroxene, and augite and often contains olivine and nepheline. Its silica content ranges from 44% to 53%. Basalt is typically dark to dark-gray in color. It forms the oceanic crust and is characteristic of rifting and other types of magmatism. Basalts are important indicators of the development of oceanic crust and continental plateaus, and geologists widely use them to reconstruct the paleogeography of ancient oceans and Phanerozoic supercontinents.

basement rock

Basement rock — a thick foundation of ancient metamorphic and igneous rocks that forms the core of continental crust. Basement rocks lie beneath the sedimentary platform and above the mantle, extending downward to the Mohorovičić discontinuity (Moho). They can reach thicknesses of 30–50 km. These crystalline continental rocks have repeatedly undergone deformation, metamorphism, partial melting, and magmatism. Examples: the Vishnu Basement Rocks in the Grand Canyon (USA, ~1.84–1.375 Ga) and the rocks of the Ukrainian Shield (~>2.5–1.7 Ga).

basic rock

Basic rocks — igneous rocks with a silica content between 45 and 55%. Basic rocks, also called mafic rocks, are typically dark in color and are rich in iron-, magnesium-, and calcium-bearing minerals. Typical examples include basalt and gabbro, as well as minerals such as pyroxene, olivine, hornblende, and biotite. Basic rocks play an important role in the formation of continental crust: Archean and Proterozoic greenstone belts, as well as large basaltic plateaus (e.g., the Deccan Traps), reflect large-scale magmatic events associated with mantle plumes and plate tectonics. Synonym: mafic rock.

basin

Basin — in geography and geomorphology, a basin is an area that is lower in elevation than the surrounding terrain. In geology, the term “basin” may refer to a large region where sedimentary rocks accumulate, a continuous area of sedimentary mineral resources, or a drainage area from which surface waters flow toward a single outlet, such as a river mouth or another body of water (a drainage basin). Sedimentary basins, such as the Michigan Basin or the Anadarko Basin, act as repositories of thick sedimentary sequences and fossil assemblages, allowing reconstruction of paleoecology, facies distribution, and stages of tectonic evolution. Examples: the Williston Basin, the Mississippi River drainage basin, the Clear Creek drainage basin, the Appalachian Basin.

batholith

Batholith — a large, irregularly shaped igneous intrusion with a surface exposure of at least 100 km², formed by slow crystallization at considerable depth. Batholiths often take the form of a giant lens. They consist predominantly of granitoid rocks that intrude sedimentary sequences in fold belts, and later erosion reveals them by removing the overlying material. Example: Much of the Sierra Nevada mountain range in California forms part of a giant batholith approximately 480 km long, formed between 120 and 85 Ma. Synonym: batholite.

bedding

Bedding — layers formed in sedimentary rocks as a result of changes in the rate and conditions of sediment deposition. The boundaries of the layers are called bedding planes. It is also a layered structure in igneous or metamorphic rock. Bedding makes it possible to reconstruct the sequence and conditions of ancient sediment accumulation. In sedimentary strata, it often preserves fossil remains, traces of biological activity, and seasonal rhythms (for example, annual layers in evaporites).

bedrock

Bedrock — the solid rock lying beneath loose deposits or at the base of soil. Bedrock consists of igneous, sedimentary, or metamorphic rocks. Geologists often call the exposed part of bedrock an outcrop. Outcrops of sedimentary bedrock layers contain important fossil discoveries: for example, Cambrian–Ordovician carbonate bedrock in the Appalachians contains trilobites and brachiopods, while the Burgess Shale outcrops (Canada) yield a unique Cambrian fauna that reflects the early stages of life’s evolution.

bentonite

Bentonite — a soft or greasy clay mineral of the smectite group, formed millions of years ago through the chemical alteration of glassy volcanic ash upon contact with water. It has absorbent properties because it swells in the presence of water. Bentonite layers serve as excellent marker horizons for stratigraphy and precise dating. They are often associated with fossils, for example, in the Morrison and Two Medicine formations (USA), where they help date fossil-bearing layers with dinosaurs.

biogenic deposit

Biogenic deposits — sedimentary deposits and rocks formed predominantly (more than 30–50%) from the products of organisms’ activity or the accumulation of their dead remains (skeletons, shells, carapaces, organic matter). Such rocks include coal, peat, oil shale, shell limestone, limestones, crinoidal limestone, chalk, diatomite, radiolarite, and others. Biogenic deposits represent a crucial archive of the evolution of life and climatic changes. Examples: chalk strata composed of coccolithophorid and foraminiferal remains (chalk of Western Europe); Carboniferous crinoidal limestone and coals with autochthonous flora; Silurian–Devonian reef limestones formed from stromatoporoids and corals. Synonyms: biogenic sediments, organogenic deposits/rocks, biogenic sedimentary rocks.

biostratigraphy

Biostratigraphy — a branch of stratigraphy that studies the relative age and subdivision of sedimentary rocks based on their index fossils, grouping layers into biostratigraphic units (zones, horizons, biocomplexes). It provides the foundation for paleontological correlation of sections and for reconstructing stages of life’s evolution and paleogeographic environments in geological time. Examples: Olenellus trilobite zone—a classic biostratigraphic indicator of the Early Cambrian in North America; Perisphinctes ammonite zone for the Late Jurassic; Globigerinoides foraminiferal zone for the Miocene.

biotite

Biotite — a variety of dark magnesium–iron mica, common in igneous rocks (especially granites) as shiny black crystals and occurring as a product of metamorphism in gneisses and schists. It is a complex silicate of iron and magnesium with potassium and hydroxyl. Its color ranges from dark green and brown to black. Because of its high potassium content, biotite is widely used in geochronology: potassium–argon and argon–argon methods allow dating of igneous and metamorphic events from the Archean to the Cenozoic. Examples include K–Ar dating of volcanic layers with Oligocene mammals (Chadronian, USA) and Triassic–Jurassic basalts associated with mass extinction. Synonyms: black mica, iron mica, magnesia mica.

bomb

Bomb — in volcanology, this term refers to pyroclastic fragments of extrusive rock or lava, predominantly basaltic in composition, ejected from a volcano during an eruption. Their diameter ranges from 64 mm to 6 m. A rounded or spindle-shaped form indicates that they were wholly or partly molten during formation or subsequent transport. Volcanic bombs record precise moments of ancient eruptions and serve as material for paleomagnetic dating. Examples: large bombs from Azuma (Japan), dated using paleomagnetic methods; bombs from Holocene eruptions (8300 BC), used to reconstruct Quaternary volcanism. Synonyms: black mica, iron mica, magnesia mica.

boulder clay

Boulder clay — an unsorted mix of clay, sand, pebbles, and boulders that glacial ice sheets deposited over the last 2.6 million years. It shows no layering and extremely poor sorting of material, marking Late Quaternary glaciations. Coastal exposures often reveal reworked Jurassic and Cretaceous marine shells (ammonites, belemnites, and bivalves) within boulder clay, even though the clay itself rarely contains its own fossils. These help trace glacial paths and Pleistocene paleogeography. Examples: Jurassic ammonites and belemnites in Holderness deposits (Eastern England); Cretaceous ammonites and bivalves in Norfolk tills (United Kingdom). Synonym: drift clay.

breccia

Breccia — a rock composed of large angular fragments (1 cm or more) of pre‑existing rock, embedded in a fine‑grained matrix and cemented by ash or mineral solutions. It forms during volcanic eruptions, rockfalls, talus accumulations, submarine landslides, or tectonic ruptures. Unlike conglomerate, breccia contains almost no rounded fragments. Breccias often record catastrophic events such as tectonic faults, collapses, or tsunamis. Sometimes they preserve reworked fossils—for example, shell fragments in tectonic breccias of the Alps or fossilized wood remains in Mesozoic collapse breccias. Synonym: rubblerock.

C

calcite

Calcite — the crystalline form of calcium carbonate (CaCO₃). It occurs in igneous, hydrothermal, metamorphic, and sedimentary rocks; it is the main component of limestone, chalk, marl, and marble. Its color ranges from colorless and white to shades of gray, yellow, and blue. Geologists know transparent calcite as Iceland spar. Calcite is the principal biomineral in fossil skeletons and shells of many organisms (corals, mollusks, echinoderms, foraminifera, some sponges, algae, and others) from the Cambrian to the present. It preserves the morphology of organisms exceptionally well, for example, in the chalk shoals of Europe or in Permian reef limestones, and calcite isotopes provide a basis for reconstructing the temperature and chemistry of ancient oceans. Synonym: calcspar.

caldera

Caldera — a circular or oval depression of volcanic origin with steep walls and a more or less flat floor. It forms when the summit of a volcano, and sometimes the surrounding area, subsides and collapses. A caldera differs from a volcanic crater by its much larger size—10–20 km in diameter and hundreds of meters deep. Examples: Yellowstone Caldera, Krakatoa Caldera, and Santorini.

caliche

Caliche — a hard calcareous crust that forms when calcium carbonate cements sand and gravel in the surface layer of stony soils in arid regions. Caliche often buries and preserves fossil remains of plants, roots, and traces of Pleistocene and Holocene animals. In some regions (for example, the southwestern USA and Argentina), it contains entire paleosols with fossilized root systems that reflect ancient arid climates. Synonyms: calcrete, hardpan, kankar.

canyon

Canyon — a long, deep cleft between rocks or escarpments with very steep, often vertical slopes and a narrow floor, usually entirely occupied by a river channel. The longest is the Grand Canyon of Greenland, while the most famous is the Grand Canyon of the Colorado River. Canyons expose thick sequences of sedimentary rocks that reflect the paleogeography and evolution of marine and continental basins; their walls, for example, in Paleozoic limestones and sandstones, preserve fossil marine organisms and serve as stratigraphic markers.

carbonate

Carbonate — minerals that are salts of carbonic acid (calcite, aragonite, siderite, magnesite, malachite). Carbonates form sedimentary rocks (limestone, chalk, dolomite, marl, and others) and metamorphic rocks (marble and others), accounting for more than 20% of all sedimentary rocks on Earth. Carbonate rocks are the primary source of the Phanerozoic paleontological record. They preserve fossil shells, skeletons, and entire reef structures exceptionally well. Examples: Cambrian archaeocyathid reefs, Permian reefs of Tethys, and Cretaceous chalk deposits of Europe with abundant fauna.

catagenesis

Catagenesis — in geology and geochemistry, one of the stages of lithogenesis, involving processes that transform sedimentary rocks and kerogen after their initial formation from sediments through diagenesis but before their conversion into metamorphic rocks. The main factors driving catagenesis include elevated temperature, high pressure, groundwater, and subsurface solutions. Sometimes the term “late stage of diagenesis” is used. Catagenesis causes the thermal maturation of organic matter and affects fossil preservation, contributing to the formation of petroleum from kerogen and altering biomolecules in fossil remains. Examples: catagenetic maturation of kerogen in Cambrian and Silurian shales.

cement

Cement — a constituent part of rock, the material that binds individual fragments and fills pores or gaps between them. The term applies only to clastic, sedimentary, cataclastic rocks and ores. The most common cements are silica (quartz, opal, chalcedony), carbonates (calcite, dolomite, siderite), and various iron oxides. Others include clay minerals, barite, gypsum, anhydrite, and pyrite. The type of cement strongly influences fossil preservation: carbonate cement conserves shells and bones very well, while siliceous cement often replaces organic remains, creating excellent pseudomorphs. Examples: siliceous sandstones with Cambrian trilobites or Devonian brachiopods.

chalcedony

Chalcedony — a translucent mineral, a fine‑fibrous variety of quartz. It forms from post-volcanic hydrothermal solutions in basalts, andesites, and more rarely rhyolites, together with zeolites, amethyst, and calcite. It also occurs in weathering crusts and during catagenesis of carbonate deposits. Its color ranges from white to honey yellow and bluish. The term “chalcedony” refers to an entire group of minerals—agate, onyx, chrysoprase, carnelian, sard, heliotrope, and others. Chalcedony often replaces organic remains, creating beautiful pseudomorphs and fossils. Classic examples include silicified petrified trees and trunks in the deserts of the USA and Argentina, as well as chalcedony replacements of mollusk shells and corals in sedimentary rocks.

charnockite

Charnockite — an ancient Precambrian intrusive rock belonging to the family of low‑alkali granites, with plagioclase and potassium feldspar in nearly equal proportions and quartz content ranging from 20 to 50%. Among dark minerals, it contains hypersthene and, less commonly, garnet, diopside, hornblende, and biotite. It forms under high temperature and pressure. Its color is dark greenish‑gray, or more rarely, light gray with a bluish tint. Charnockites are widespread in Precambrian complexes (for example, the Anabar and Baltic Shields and the Ukrainian Shield), recording high‑temperature metamorphism and the collisional structure of ancient cratons.

chemical weathering

Chemical weathering — the breakdown and transformation of rocks through chemical reactions with air, water, and dissolved acids (hydrolysis, hydration, oxidation, dissolution, carbonation, etc.). Chemical weathering plays a key role in fossil preservation: it dissolves carbonate shells in acidic soils but promotes the replacement of organic matter by silica or iron oxides, creating excellent fossils. Examples: silicified trilobites or replaced tree trunks in paleosols.

chert

Chert — a hard and dense sedimentary rock consisting predominantly of very fine quartz crystals (SiO₂). Often contains admixtures of clayey material, hematite, or organic matter. Chert occurs mainly in carbonate sedimentary rocks, in the form of nodules or layers. This mineral has a matte or semi-vitreous luster and a conchoidal fracture. The terms “siliceous shale” and “flinty shale” refer to layered varieties with clay admixtures, while hornstone generally refers to metamorphic rocks. Chert in carbonate sequences (for example, Devonian and Pennsylvanian limestones) commonly preserves microfossils, conodonts, and silicified sponge spicules, serving as an important archive of Paleozoic marine ecosystems and diagenetic history.

cinder cone

Cinder cone — a conical landform with steep slopes, built of loose pyroclastic fragments such as volcanic ash, lapilli, clinkers, or scoria, usually of basaltic or andesitic composition, formed around a volcanic vent by explosive eruptions or fountains of gas‑rich lava. Cinders are lava fragments about 1 centimeter in diameter. Cinder cones record relatively recent episodes of volcanic activity and help reconstruct the history of Holocene and Quaternary volcanism. Their deposits often bury soils and plant remains, preserving them beneath the surface. Examples: Parícutin (Mexico); Sunset Crater (Arizona). Synonyms: scoria cone, spatter cone.

cirque

Cirque — a chair-shaped depression (basin) resembling an amphitheater, with steep, cliff-like slopes. Cirques form in two main ways: glacially, through prolonged firn action feeding mountain glaciers, or via basin development after landslides on steep slopes of clay-rich rocks. Cirques serve as key indicators of Pleistocene glaciations. Lakes and peat bogs filling cirques often preserve pollen, plant macrofossils, and insect remains that record Holocene climate shifts. Examples: peat bogs in Alpine and Carpathian cirques with pollen records of Holocene climate fluctuations; lake sediments in Rocky Mountain cirques (USA) containing late-glacial fauna and flora remains. Synonym: corrie.

clastic rock

Clastic rock — a general term for fragmental sedimentary rocks. It consists of cemented, redeposited, broken fragments of rocks, minerals, organic remains, and similar materials. The fragments usually show angular shapes and lack size sorting. Clastic rocks often contain redeposited fossils and ichnofossils. For example, conglomerates and breccias frequently include fragments with mollusk shells, brachiopods, or plant remains. Examples: breccia, conglomerate. Synonym: fragmental rock.

clay

Clay — a fine‑grained natural soil composed of hydrated aluminum phyllosilicates (kaolinite, smectite, illite, chlorite, and others). It is plastic when wet, and its particles are smaller than silt (<2 μm). Clay minerals form through hydrothermal activity or prolonged chemical weathering of silicate rocks. Clays often preserve paleobiota and paleocommunities: Holocene and Pleistocene deposits in deltas and lagoons (such as the Dnieper‑Donets Basin and the Black Sea region), as well as ancient layers in Paleozoic and Mesozoic marine basins, mark quiet‑water conditions with well‑preserved shells.

clay slate

Clay slate (black) — an argillaceous rock with slaty cleavage, typically dark gray, black, or, less commonly, reddish or greenish. It consists of very fine clay minerals (hydromicas, chlorite, and others) mixed with quartz and other mineral grains. Essentially, it represents compacted and metamorphosed clay, found in some of the oldest geological deposits. Clay slate often contains well‑preserved imprints of organisms; a classic example is the Burgess Shale (Canada), where fine‑grained texture and rapid burial preserved the soft bodies of Cambrian fauna. Synonym: argillaceous shale.

cliff

Cliff — a high escarpment or steep slope of bedrock along a lake or sea shore, formed mainly by the action of waves or other exogenous processes (including erosion and weathering). Over time, these forces cause the cliff to retreat inland, creating a wave‑cut notch and a wave‑cut platform at its base. Cliffs expose continuous sections of sedimentary and volcanic strata, serving as important fossil sites; for example, the Cretaceous chalk cliffs of the Normandy and southern English coasts record the Late Cretaceous paleofauna.

collision

Collision — in plate tectonics, the collision and interaction of two continental lithospheric plates, which causes folding and thickening of the Earth’s crust and the rise of fold mountain chains. Collisions mark key stages in Earth’s history—the closing of oceans and the formation of supercontinents, accompanied by the uplift and erosion of mountains, leading to the accumulation of rich marine and terrestrial fossils in foreland and intermontane basins. Classic examples include the Alps and Himalayas, formed during the closure of the Tethys Ocean and the collision of India with Eurasia.

colluvium

Colluvium — loose, unconsolidated clastic material consisting of a mixture of rocks and stones of various sizes, accumulated on mountain slopes or at their bases under the influence of gravity (scree, rockfalls, landslides) and partial washout. It lacks clear stratification. Modern examples include blocky and rubble‑block talus deposits on mountain slopes. Fossil colluvium often contains redeposited organic remains, bones, and pollen, allowing reconstruction of ancient slope processes and Pleistocene climate. Synonym: colluvial deposits.

concretion

Concretions — solid, compact mineral formations that differ in composition from the surrounding medium and form through the precipitation of dispersed components of the host sediment, such as silica (chert), calcite, dolomite, iron oxide, pyrite, or gypsum. Mineral cement fills the porosity (i.e., the spaces between sediment grains). Concretions most often show rounded shapes, and iron‑manganese concretions on the ocean floor stand out as well‑known examples. Concretions may preserve fossils in their centers—bones, shells, plants, or even soft tissues. Classic examples include concretions with fish and ammonites in Cretaceous deposits or with trilobites and soft‑bodied organisms in Cambrian shales.

conformity

Conformity — the layering of rocks in which the surfaces of strata are usually nearly parallel to each other and sediment accumulation occurred continuously without interruptions. Such layers contain an unbroken paleontological record, allowing precise tracking of evolutionary changes. The presence of conformity indicates relatively stable environmental conditions during the period of deposition. Example: continuous marine sections of the Cretaceous period in Western Europe (from the Albian to the Maastrichtian). Synonym: concordant bedding.

conglomerate

Conglomerate — any coarse‑grained (>2 mm) cemented clastic sedimentary rock composed mainly of rounded fragments of various rocks (cobbles, pebbles, or boulders). The cement binding the fragments usually consists of fine particles of calcite, silica, or iron oxide. Conglomerates form through the action of waves, erosion, transport, and redeposition of older rocks. They often contain redeposited fossils and fossil‑bearing fragments, helping reconstruct ancient river, beach, and delta environments. Examples: Carboniferous limestone conglomerates with brachiopods and crinoids that reflect reworking of shallow‑water facies; basal Cretaceous conglomerates with shell fragments of ammonites and brachiopods.

continental crust

Continental crust — a layer of igneous, metamorphic, and sedimentary rocks that forms the continents and continental shelves. It has lower density compared to oceanic crust and the mantle, so its buoyancy pushes it upward, with most of it lying above sea level. The thickness of the continental crust ranges from 25 to 70 km. The continental crust preserves ancient crystalline shields, sedimentary basins, and fossil‑rich Phanerozoic strata—from Paleozoic coal forests and swamps to Cenozoic river, delta, and lake deposits.

continental drift

Continental drift — the movement of continental masses driven by plate tectonics. Continents diverge when the seafloor spreads along mid‑ocean ridges, and they converge when intervening oceanic lithosphere sinks into the mantle at subduction zones. Continental drift shaped the evolution of biogeographic provinces and climate zones; for example, the divergence of Gondwana and Laurasia in the Late Paleozoic/Mesozoic explains the similarity of fossil faunas and floras on opposite shores of the Atlantic.

continental margin

Continental margin — a transitional zone of the ocean floor that separates the thin, denser oceanic crust from the thicker, lighter continental crust. Active margins form along convergent plate boundaries, where oceanic lithosphere sinks beneath the continent, accompanied by subduction, magmatism, and folding (for example, the western coast of South America). Passive margins lie within plates, away from plate boundaries; they show low seismicity and volcanism, yet they accumulate thick sedimentary sequences rich in marine fauna. Examples: Cretaceous deposits of the Atlantic coast of the USA; Permian reefs of the Tethys.

convergent boundary

Convergent boundary — in plate tectonics, a zone where two lithospheric plates meet, with one plate descending beneath the other (subduction) or undergoing direct collision (collision). These boundaries show high geological activity: intense earthquakes, volcanism, and mountain building. Terrigenous and volcanogenic deposits at convergent boundaries often preserve rich fossil assemblages, which serve as markers for biostratigraphy and paleobiogeography. They allow reconstruction of ancient ocean basins, faunal migrations, and stages of ocean closure (for example, the Tethys).

core sample

Core sample — a cylindrical column of rock extracted from a borehole during drilling. It represents a continuous sequence of subsurface layers. Geologists collect cores to study rock formations; they provide evidence of past climate changes, types of fauna and flora that existed in earlier geological epochs, and the sedimentary structure of the Earth’s crust. Synonym: core.

craton

Craton — a Precambrian shield plus an ancient continental platform. It represents the rigid, stable core of a continent, composed of crystalline basement rocks. Cratons form the main part of modern continents, occupying the interior, least tectonically active regions of plates, and since the Archean, they have undergone almost no folding deformation, surviving multiple cycles of supercontinent assembly and breakup. On craton platforms, thin but continuous sedimentary sequences preserve key paleontological records—from Archean stromatolites to Early Cambrian fauna—enabling studies of life’s evolution and climate over more than 3 billion years. Examples: Indian Craton, Kaapvaal Craton (South Africa), Canadian Craton.

crushed stone

Crushed stone — a granular clastic rock composed of sharp‑angled, unrounded fragments usually 5–100 mm in size. This material is produced by crushing hard rocks such as granite, basalt, or limestone into fragments of specified dimensions. In nature, similar materials occur in weathering crusts and talus deposits. In natural screes and ancient breccias, crushed‑stone‑like material sometimes contains redeposited fossils. Particularly notable are limestone gravels with fragments of brachiopod shells, corals, or trilobites, which help reconstruct ancient collapse and slope processes. Synonym: angular rock.

crystalline rock

Crystalline rock — a rock composed of interlocking crystals. Almost all igneous and metamorphic rocks are crystalline, unlike sedimentary rocks. They form when molten rock cools or when recrystallization occurs under heat and pressure; evaporite deposits form when solutions evaporate. Examples: granite, basalt, gneiss, and others. Crystalline rocks rarely contain fossils because they form under high temperatures and pressures. However, they play a crucial role in understanding Earth’s ancient history—especially Precambrian crystalline shields, which radioisotopic methods date at up to 4 billion years.

D

dacite

Dacite — a fine‑grained volcanic igneous rock of acidic composition with high silica content. It consists of sodium plagioclase, quartz, and minor amounts of biotite, amphibole, hornblende, or pyroxene. Dacite forms lava flows, domes, dikes, laccoliths, and sometimes massive intrusions in the central parts of volcanoes. It played an important role in the formation of continental crust, especially during the Archean. In the Phanerozoic, dacite complexes of volcanic arcs help reconstruct subduction zones and stages of continental growth (for example, in the Andes and the Cascade Range). Synonym: quartz andesite.

deflation

Deflation — the process in which wind blows away fine‑grained particles (dust, sand, silt, snow), transports them, and shapes them through aeolian abrasion. It occurs most actively in deserts and semi‑deserts, where a layer of larger stones and gravel remains—the so‑called desert pavement. Extreme forms include dust storms, deflation basins, and stony deserts (hamadas, kyrs). Deflation actively removes fine sediments, exposing and damaging small fossils, but it also creates deflation surfaces (winnowed surfaces) with concentrated larger remains. Example: In the Gobi Desert (Cretaceous period), wind deflation and sandstorms contributed to the burial of dinosaurs, including specimens with “mummified” skin such as Edmontosaurus.

delta

Delta — a lowland at the mouth of a river, composed of fluvial sediments and divided by a branching network of islands, distributaries, and channels. It forms as a result of deposition of sedimentary material carried by the river when it enters a more tranquil basin. Deltaic deposits, owing to rapid burial, often preserve a rich paleontological record (mollusks, ostracods, plants, fossil soils). Examples: Hell Creek Formation (Late Cretaceous, USA)—fluvial deltas with exceptionally diverse dinosaur fauna (Triceratops, Tyrannosaurus, Edmontosaurus); Carboniferous deltas of Europe and the USA—cyclic deposits with abundant plant remains and insects.

deluvium

Deluvium — an accumulation of loose weathering products of bedrock and soil horizons, deposited on slopes and along the foot of mountains and uplands. Rainwater and meltwater wash material down, while gravity, frost creep, and soil flow also contribute to its formation. Deluvial deposits often contain reworked bone and plant remains. Examples: Pleistocene deluvial aprons in the foothills of the Altai and Central Asia, with mixed bones of large mammals (mammoths, bison, horses), deluvium in karst regions with concentrations of Pleistocene megafaunal bones.

denudation

Denudation — the process of removing and transporting weathering products and eroded rock material from uplands into lowlands. It gradually smooths the relief, exposing deeper layers. Gravity, running water, wind, moving glaciers, and other slope processes drive denudation. By stripping upper layers, it creates stratigraphic unconformities and exposes ancient fossil-bearing rocks but may also destroy smaller remains. Examples: Denudation in the Grand Canyon (USA) has revealed Paleozoic strata rich in marine fauna; in the Alps and the Urals, denudation has exposed Mesozoic and Paleozoic deposits containing ammonites and plant remains.

depression

Depression — in tectonics, a region of crustal downwarping or a lowered part of the Earth’s surface formed through rifting, faulting, or subsidence. In a broader sense, it refers to any depressed landform (for example, an interior basin or a karst sinkhole). Tectonic depressions create sedimentary basins with thick accumulations, preserving a rich paleontological record and reflecting the evolution of climate and fauna. Examples: Caspian Depression—marine and continental deposits with Pleistocene mammals and Akchagyl fauna; Dnieper-Donets rift basin—Carboniferous coal-bearing sequences with abundant plant remains and marine fauna; volcano-tectonic depressions— the Danakil Depression and Lake Toba in Sumatra.

diagenesis

Diagenesis — a complex of chemical, physical, and biochemical processes that transform loose sediments into sedimentary rocks under low pressures and temperatures. It includes compaction, cementation, recrystallization, and the formation of new minerals. Diagenesis strongly affects fossil preservation and determines the type of fossilization—from permineralization and replacement to carbonization and dissolution. Early diagenesis can provide exceptional preservation of soft tissues, but it may also destroy parts of the paleontological record. Examples: Burgess Shale (Middle Cambrian, Canada)—carbonization and pyritization of soft-bodied organisms; Solnhofen (Late Jurassic, Germany)—fine lamination and phosphatization of insects and Archaeopteryx.

diamictite

Diamictite — a sedimentary rock composed of a mixture of unsorted clasts of various types and sizes, including blocks, boulders, pebbles, and sandy particles, dispersed within a finer-grained matrix (silt, sand) resembling cement. Glacial (tillite), volcanic, marine, tectonic, erosional, and extraterrestrial processes can generate diamictite; in its unconsolidated form, geologists call it diamicton. Diamictite is usually poor in fossils but occasionally contains reworked remains. Examples: Carboniferous diamictite of Oman with crustaceans and copepods; Ediacaran diamictites (Elatina Formation, Australia)—markers of Cryogenian glaciation preceding the emergence of the Ediacaran biota. Synonym: mixtite.

diapir

Diapir — an intrusion in which a more ductile material (salt, clays, or magma) penetrates into overlying brittle rocks, piercing and uplifting their layers. The resulting structures may be dyke-like, drop-shaped, or mushroom-shaped. Geologists most often apply the term to salt domes and mud diapirs, which create local unconformities and exposures, rather than to magmatic intrusions. Examples: the salt diapirs of the Zagros (Iran) and the Flinders Ranges (Australia), containing inclusions of Ediacaran sediments, as well as the salt domes of Texas and Louisiana, where the uplift of strata facilitates the study of Mesozoic and Cenozoic fauna in hydrocarbon traps.

diastrophism

Diastrophism — large-scale, slow processes of deformation and displacement affecting extensive portions of the Earth’s crust, including folding, faulting, and volcanism. Diastrophism results from the action of epeirogenic (vertical) and/or orogenic (mountain-building) movements, reflecting the long-term evolution of tectonic structures. It produces stratigraphic unconformities, drives changes in sea level, climate, and habitats, and thereby influences the development of flora and fauna as well as the preservation of the paleontological record. Examples: the late Proterozoic “Grand Canyon revolution,” characterized by thick conglomerates, and the Appalachian revolution (late Paleozoic), associated with aridization and the origin of reptiles.

 dike

Dike — a narrow, linear intrusive body formed by the injection of magma (endogenous dikes) or plastic sedimentary material (exogenous, “Neptunian” dikes) into fractures cutting across older strata. In the landscape, it often appears as a narrow wall or ridge when composed of rocks more resistant than the surrounding deposits. Dikes record tectonic stages and stratigraphic breaks. Neptunian dikes incorporate mixed marine and terrestrial remains together with fragments of the host rocks. Examples: the Rhaetian Neptunian dikes at Holwell (England), containing remains of sharks and the earliest Mesozoic mammals, and Jurassic Neptunian dikes with mixed insular biota. Synonym: dyke.

diluviun

Diluvium — an obsolete term for Pleistocene coarse clastic unconsolidated deposits associated with catastrophic outbursts of ice-dammed lakes (glaciolacustrine) and the breaching of glacial dams at the end of glacial epochs. These sediments differ sharply from the more slowly formed alluvium; characteristic landforms include giant current ripples, terrace-like ridges, and branching flow traces. Diluvial deposits are typically poor in fossils but may contain reworked fragments of Pleistocene megafauna. Examples: the giant Altai deposits (outbursts of the Chuya–Kuray lakes) with rare bones of mammoths and bison; Late Pleistocene gravel bars of the Mississippi Valley containing remains of Mammuthus columbi and Arctodus simus.

diorite

Diorite — an intrusive igneous rock of intermediate composition, occurring in dikes, laccoliths, and large plutonic bodies. Its principal minerals are sodic plagioclase (andesine, oligoclase) and amphibole (hornblende), with pyroxene or biotite present less commonly. The color is typically dark gray to greenish-brown. Diorite forms in subduction zones and orogenic settings, reflecting the growth of continental crust and tectonic activity. Examples: the diorites of the Andes and the Urals, associated with Mesozoic–Cenozoic subduction; the Permian diorites of eastern China (Daheishan), formed during the closure of the Paleo-Asian Ocean.

disconformity

Disconformity — a type of unconformity in which sedimentary layers above and below the erosional surface remain generally parallel. It reflects a break in sedimentation or an interval of erosion, creating a stratigraphic gap (hiatus) and often accompanied by changes in fauna and flora as well as in depositional regimes. Examples: a disconformity in the Paleozoic strata of the Grand Canyon (between the Supai Formation and the Redwall Limestone), where interruptions in marine faunas are recorded; regional unconformities at the Cretaceous–Paleogene boundary associated with the extinction of dinosaurs. Synonyms: parallel unconformity, erosional unconformity, nonangular unconformity.

dislocation

Dislocation — a disturbance of the original position of rock layers and bodies caused by tectonic, magmatic, or exogenic deformation. It manifests as the folding of originally horizontal strata into folds, their displacement along fractures, and the repositioning of intrusive and other bodies. Dislocations disrupt stratigraphic continuity, complicating correlation and biostratigraphy. Examples: the fold belts of the Alps and the Urals, where tectonic forces displace Paleozoic and Mesozoic strata containing ammonites and brachiopods; faults in the San Andreas Fault zone that juxtapose Pleistocene and Holocene bone-bearing layers.

divergent boundary

Divergent boundary — a tectonic boundary where lithospheric plates move apart. On continents, these zones form rifts; in oceans, they form mid-ocean ridges where new crust develops through seafloor spreading and basaltic volcanism. Divergent boundaries drive the separation of flora and fauna when landmasses split, create barriers for terrestrial biota, and generate new marine habitats. Examples: the East African Rift, which has yielded rich hominin finds (Lucy, Turkana Boy) and records the evolution of African mammals; the breakup of Pangaea along the Mid-Atlantic Ridge, which led to the separation of Gondwanan faunas (Mesosaurus, Lystrosaurus).

dolerite

Dolerite — a fine- to medium-grained mafic intrusive igneous rock composed of plagioclase (commonly labradorite), pyroxene, olivine, and titanomagnetite. It is typically dark gray with a greenish tint. Dolerite occurs widely in dikes, sills, flood-basalt provinces (traps), oceanic crust, and volcanic islands. It represents a key component of large igneous provinces (LIPs) associated with rifting and the breakup of supercontinents. Examples: the Karoo–Ferrar dolerites (~183 Ma, Jurassic), linked to the breakup of Gondwana; the Siberian Traps (doleritic sills and dikes, ~252 Ma), associated with the end-Permian mass extinction. Synonym: diabase.

dolomite

Dolomite — a sedimentary carbonate rock and mineral composed of calcium and magnesium carbonate (CaMg(CO₃)₂). It is typically white, colorless, or yellowish and brittle. Dolomite forms through the replacement of limestones by magnesium-rich seawater during diagenesis and occurs in hydrothermal deposits. Dolomitization often reduces the preservation quality of fossils, although many dolomites retain primary textures and fossils such as trilobites, brachiopods, corals, and stromatolites. Examples: the Middle Triassic dolomites of the Dolomites (Cassian Formation) with rich reef faunas (corals, ammonites, mollusks); Silurian dolomites of Ohio containing trilobites and gastropods; Precambrian dolomites of Glacier (USA) with stromatolites. Synonyms: bitter spar, pearl spar, magnesian spar, rhomb spar.

dormant volcano

Dormant volcano — a volcano that has not erupted in historical time but still shows signs of potential activity. It may remain inactive for thousands of years and then suddenly reactivate. Such volcanoes preserve thick successions of Pleistocene and Holocene deposits that record changes in climate, vegetation, and fauna between eruptions. Examples: Mount Kilimanjaro, with glacial cores and lacustrine sediments containing pollen and megafaunal remains; Diamond Peak (Oregon) and other Cascade volcanoes with tephra layers marking late-glacial ecological shifts.

downwelling

Downwelling — in geology and hydrodynamics, the process by which denser, usually colder, fluid layers sink beneath less dense layers. In oceans, it involves the sinking of surface waters into deeper zones, producing oligotrophic conditions. In tectonic and mantle geodynamics, the term describes the descending branches of convection, where cold and dense material sinks into the mantle, driving plate subduction and dynamic subsidence of continents. Example: subduction-related downwelling beneath the eastern margin of North America (remnants of the Farallon slab), which contributed to Cenozoic subsidence and marine transgressions.

Geology Dictionary illustrated: This reference covers terms from A to D, combining precise definitions with paleontological and historical geology context. Each entry highlights the deep interconnection between geological processes and the history of life on Earth, situating terms within the broader narrative of 4.5 billion years of planetary evolution.

Related sections of the Geological Dictionary:

Geological Dictionary A–D

Geological Dictionary E–K

Geological Dictionary L–P

Geological Dictionary Q–Z

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