Geological Dictionary Terminology: E–K

Geological Dictionary Terminology E–K: an illustrated guide with definitions emphasizing paleontology, historical geology, and Earth science, connecting rock-forming processes, eruption types, and glacial deposits to the fossil record and major events in Earth history.

Alphabetical index of geological terms

In this geological dictionary terminology, you will find a catalog of fundamental geological terms applied in studies of Earth science, paleontology, and sedimentary geology.

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 Geological Dictionary Terminology

Geological dictionary Terminology E–K—this section of the illustrated reference guide presents key concepts in Earth sciences, paleontology, and historical geology. From eruption types and flood basalts to glacial deposits and greenstone belts, these terms span some of the most dynamic phenomena in geology. The fossil record runs throughout: entries illuminate mechanisms of preservation, tools of biostratigraphy, and exceptional windows into ancient life. Each definition grounds terminology in specific geological formations and time periods, and includes an illustration from a real geological or paleontological example.

Keywords: geological dictionary, paleontology, Earth science, eruption types, flood basalt, glacial deposits, greenstone belt, Konservat-Lagerstätten

E

Earth’s core

Earth’s core — the deepest internal sphere of the Earth, with a radius of 3,480 km. It consists of a solid, extremely dense inner core (1,220–1,300 km in radius) and an outer layer of molten material about 2,200 km thick with temperatures of 5,000–6,000 °C, which generates the planet’s magnetic field. The core is likely composed of an iron–nickel alloy (approximately 90% iron and 10% nickel). It begins at a depth of about 2,900 km and accounts for roughly 16% of Earth’s volume and 31–32% of its mass. High pressure maintains iron and nickel in a solid state within the inner core. The Earth’s core formed through planetary differentiation (“iron catastrophe”) about 4.5–4.54 billion years ago, when dense molten materials sank toward the center of the protoplanet.

Earth’s crust

Earth’s crust — the outer shell of the Earth and the upper part of the lithosphere, comprising continental and oceanic crust. It consists of igneous, sedimentary, and metamorphic rocks. The crust reaches about 7–8 km in thickness beneath the oceans, 30–50 km beneath continents, and up to 70 km beneath mountain belts; the Mohorovičić discontinuity separates it from the mantle. Basalt dominates the oceanic crust, whereas granitic rocks make up most of the continental crust, and sedimentary rocks provide an important source of paleontological data. The crust formed ~4.5 billion years ago during the cooling of a magma ocean. Its oldest fragments include zircons from Jack Hills (4.4 Ga) and the Acasta gneiss (about 4.03 Ga). Stabilization of the continental crust occurred in the Archean through differentiation and early tectonic processes.

effusion

Effusion — a relatively quiet, non-explosive outpouring of fluid lava that forms lava flows and sheets. Effusive rocks form when magma solidifies at the Earth’s surface and include andesite, basalt, dacite, rhyolite (liparite), komatiite, and others. Large-scale effusions (for example, the Siberian Traps) have contributed to biotic crises in Earth’s history. Effusive volcanism dominated the early Earth: during the Archean (3.5–2.5 billion years ago), ultra-hot, low-viscosity komatiitic lavas (up to 1600 °C) erupted extensively. Classic examples include the Barberton Greenstone Belt (South Africa) and the Pilbara Craton (Australia).

eluvium

Eluvium — loose, unsorted products of rock weathering that remain in place without transport. It forms weathering crusts and serves as the basis for soils. Eluvium develops on horizontal or gently sloping surfaces and shows neither sorting nor stratification. Weathering crusts (eluvium) have formed throughout geological history, with especially thick developments in the Precambrian, Mesozoic, and Paleogene. Ancient eluvium may persist beneath sedimentary cover, contain paleosols with fossil traces of life, and associate with the formation of mineral deposits such as kaolin, bauxite, and nickel ores.

endogenic process

Endogenic process — large-scale geological phenomena driven by the Earth’s internal thermal energy (radioactive decay, gravitational differentiation, and residual heat from accretion). They include plate tectonics, volcanism, magmatism, metamorphism, and seismic activity. These processes shape the relief, form the Earth’s crust, and drive the cycling of matter within the lithosphere. They have operated since the formation of the Earth (~4.54 billion years ago), causing planetary differentiation and the development of the core, mantle, and early crust. In the Archean, intense magmatism and komatiitic volcanism predominated, while modern plate tectonics became established around 3–2.5 billion years ago. Endogenic processes created conditions for the origin of life and continue to sustain the biosphere through volcanism and tectonics. Synonim: endogenous.

epeirogeny

Epeirogenesis — slow vertical movements of large blocks of the Earth’s crust that cause the uplift or subsidence of continents without significant deformation. These movements form plateaus and basins, and influence sea level and sedimentation. Epeirogenesis characterizes stable cratons since the Archean and has controlled cycles of marine transgressions and regressions, producing thick sedimentary sequences (for example, the Sauk–Tippecanoe cycles in North America). Examples: the uplift of the Karnataka Plateau (India); the subsidence of the Russian Platform during the Mesozoic; and long-term vertical movements of the Kaapvaal Craton (South Africa).

epicontinental sea

Epicontinental sea — shallow-water areas of continental platforms within the shelf zone, connected to the ocean and formed during marine transgressions. Such seas were widespread in the geological past: during the Oligocene–Miocene in western Patagonia (La Cascada Formation), in the Jurassic of Siberia, and globally in the Late Cretaceous. The Eromanga Sea in Australia represents a shallow inland sea that hosted marine reptiles such as Kronosaurus, plesiosaurs, and ichthyosaurs. Modern examples include the Black Sea, the Caspian Sea, and the Baltic Sea. Synonyms: shelf sea (depth up to 300 m), epeiric sea.

epigenesis

Epigenesis — the alteration of sedimentary rocks after diagenesis under the influence of aqueous solutions, pressure, and temperature. It may proceed in a progressive or regressive manner and involves recrystallization, replacement, and compaction of rocks. Epigenesis became especially active from the Proterozoic onward, particularly in the Phanerozoic during the subsidence of sedimentary basins. It controlled secondary mineralization and often improved or reduced fossil preservation through cementation and replacement. Example: carbonate concretions in Devonian and Cretaceous deposits, where epigenetic cementation enabled exceptional preservation of soft tissues and marine reptiles.

erg

Erg — a broad, flat desert area covered by wind-blown sand, with sparse or absent vegetation. Ergs occur in arid regions and undergo long-term wind erosion and deposition. Modern “sand seas” contain sand accumulations ranging from a few centimeters to about 40 meters in thickness. Some Mesozoic and Paleozoic sand seas reached average thicknesses of several hundred meters. Examples: Rub’ al Khali (Arabian Peninsula), Issaouane Erg (Algeria). Synonyms: ergh, reg, sand sea, dune sea.

erosion

Erosion — a complex process of rock destruction and transport by mechanical (abrasion, corrasion), wind (deflation), and chemical (corrosion) action. The main agents include rivers, wind in arid regions, and wave action along coasts. Erosion directly impacts how fossils are exposed and preserved. While it uncovers dinosaur bones in the cliffs of the Badlands, it also selectively destroys soft remains while sparing tougher forms such as ammonites and trilobites. It has operated since the Archean, producing unconformity surfaces and supplying sediment for sedimentary rocks. The largest global erosional event occurred ~700 million years ago during the Neoproterozoic “Snowball Earth,” when erosion removed up to 5 km of crust and helped set the stage for the Cambrian explosion of life.

erratic

Erratic — a wandering block or boulder of bedrock transported by an ancient glacier far from its place of origin. The occurrence of erratic boulders and blocks allows reconstruction of the centers and directions of ancient (Late Cenozoic) glaciations and glacial moraines with Quaternary faunal remains. Example: granite boulders from Scandinavia scattered across northern Germany and Poland during the Weichselian (Würm) glaciation; these sometimes contain Ordovician–Silurian fossils carried by ice over distances of hundreds of kilometers.

eruption type

Eruption types — range from simple steam-driven explosions and quiet lava effusions to catastrophic events that eject vast volumes of material into high ash columns. Eruption types reflect the evolution of volcanism and major Phanerozoic events: ultra-Plinian eruptions (e.g., Toba, Yellowstone) have caused global “volcanic winters” and mass extinctions; flood-basalt eruptions (Hawaiian, Icelandic-style in broader usage) have formed large igneous provinces associated with biotic crises (e.g., the Siberian and Deccan Traps); submarine eruptions have produced ophiolite complexes. Individual volcanoes may exhibit one or several eruption types even within a single phase of activity.

• Magmatic eruption
• Hawaiian-type eruption
• Icelandic-type eruption
• Katmai-type eruption (eruption of pyroclastic flows)
• Peléan-type eruption
• Phreatic-type eruption (Bandaian-type)
• Phreatomagmatic-type eruption
• Plinian-type eruption (Vesuvian-type)
• Strombolian-type eruption
• Subglacial-type eruption
• Submarine-type eruption
• Surtseyan-type eruption
• Ultra-Plinian-type eruption
• Vulcanian-type eruption

estuary

Estuary — a narrow, funnel-shaped embayment at a river mouth connected to the sea. It forms when currents or tides remove river-borne sediments and when the adjacent coastal sea has relatively deep waters. In contrast to a delta, an estuary typically has a single main channel. Estuaries have existed since the Late Silurian (about 423 million years ago), when ostracods first colonized brackish environments. During the Devonian, estuarine deposits recorded a gradual transition from marine to coastal ecosystems and documented early arthropod adaptations to fluctuating salinity conditions. Example: the Bluesky Formation (Lower Cretaceous, Canada), an ancient wave-dominated estuary preserving evidence of colonization of marginal marine environments. Modern estuaries include the Thames, Elbe, Hudson, Yenisei, and Amur rivers.

evaporite

Evaporites — chemical sediments that precipitate on the floors of basins during evaporation and solution supersaturation, mainly salts and certain limestones. They form in marine embayments, lagoons, lakes, and salt marshes; individual sequences may reach hundreds of meters in thickness. The most common minerals include halite, gypsum, anhydrite, and sylvite. The oldest evaporites (Paleoproterozoic, ~2 billion years ago, Karelia) relate to the Great Oxidation Event and increased sulfate concentrations in the oceans. In the Phanerozoic, evaporites serve as indicators of arid climates, while the largest deposits formed in the Permian (Zechstein, Muschelkalk, and Keuper basins in Europe).

exogenous process

Exogenic process — a geological process that occurs at the Earth’s surface and in the upper parts of the crust (including weathering, erosion, glacial activity, slope processes, and others). Solar radiation, gravity, and biological activity mainly drive these processes, which involve the breakdown of rocks, the transport of weathering products, and their accumulation as sediments. Burial of organism remains promotes fossilization; for example, permineralization in sandstones or the formation of limestones from the shells of marine organisms in carbonate sediments.

exotic terrane

Exotic terrane — a fault-bounded block of the Earth’s crust that has migrated from its place of origin and undergone significant tectonic transport relative to the stable craton to which it was later accreted. It therefore has a separate and fundamentally different geological history. The paleontological record of exotic terranes also often differs sharply from that of surrounding rocks, providing evidence of their distant provenance. For example, Cambrian trilobites Paradoxides in the Avalonia terrane near Boston show typical “African” affinities, and Permian fusulinids in the North American Cordillera have the closest analogues in Japan and Southeast Asia.

extrusive rock

Extrusive rock — an igneous rock formed by the solidification of erupting magma at or near the Earth’s surface (as lava flows, pyroclastic deposits, and tuffs). Rapid cooling under low pressure produces fine-grained, porphyritic, or glassy textures. Extrusive rocks include obsidian, rhyolite, andesite, pumice, and scoria. Rapid burial by volcanic ash often enables exceptional preservation of organisms, including soft tissues. Examples: dinosaurs and early birds of the Jehol Biota (northeastern China, ~125 Ma); marine organisms from Herefordshire (Silurian, ~425 Ma); plant and tree impressions preserved in tuffs. Extrusive rocks also help refine the dating of sedimentary sequences and fossil assemblages.

F

facies

Facies — a homogeneous unit or assemblage of minerals in igneous, metamorphic, or sedimentary rocks formed under the same specific physicochemical conditions. In igneous and metamorphic complexes, facies reflect the set of minerals and their associations that crystallized under a given temperature, pressure, and environmental composition. Facies may be marine, continental, lagoonal, aeolian, volcanic, glacial, etc.; in metamorphic sequences, geologists distinguish facies according to pressure–temperature regimes (e.g., blueschist facies, amphibolite facies, and others).

Characteristic fossil assemblages define biofacies and reflect a particular environment of deposition; see also ichnofacies.

fault

Fault — a tectonic disruption in which the rupture of a rock mass is accompanied by displacement of its blocks relative to one another along the fracture surface. Faults may develop over long periods and extend to great depths, cutting through the lithosphere. Earthquakes commonly occur in active fault zones. Faults disrupt stratigraphic sequences and offset fossil assemblages, complicating layer correlation. However, they provide key evidence for reconstructing tectonic history, as seen in major displacements along the Alpine Fault in New Zealand or the San Andreas Fault, where faunal assemblages of different ages become juxtaposed and deformed.

feldspar

Feldspar — a collective name for a group of aluminosilicate minerals that make up about 60% of the Earth’s crust. They occur in igneous, metamorphic, and sedimentary rocks. Feldspars are divided into three main groups: plagioclase feldspars (albite, oligoclase, labradorite, bytownite, anorthite), potassium feldspars (orthoclase, microcline, sanidine), and potassium–barium feldspars (celsian). Impurities often color them. Geologists use potassium feldspars in radiometric dating (K–Ar, Ar–Ar), which allows them to determine the ages of rocks and reconstruct geological history. Examples include dating of mineralization in Cambrian Lamotte Sandstone (USA) and the age of lunar anorthosite “Genesis Rock” (~4 billion years).

feldspathoid

Feldspathoid — a framework aluminosilicate mineral of alkali metals (sodium, potassium, and more rarely calcium) chemically similar to feldspars but with lower silica content. Feldspathoids replace feldspars in alkaline igneous rocks where silica is insufficient for feldspar formation. Different feldspathoids exhibit slightly different crystal structures. Feldspathoids, chiefly nepheline and leucite, are characteristic of alkaline volcanic rocks that may interlay with sedimentary sequences. They help date paleoassemblages: nepheline phonolites of the Crescent Formation with rich molluscan faunas (Eocene, USA); leucite-bearing rocks of Cenozoic provinces that correlate volcanic activity with environmental changes. Feldspathoids include nepheline, leucite, sodalite, lazurite, hauyne, cancrinite, analcime, and nosean.

felsic rock

Felsic rocks — igneous rocks enriched in silica (more than 63–65% SiO₂), quartz, and feldspars, with a predominance of light elements (Si, O, Al, Na, K). They are typically light in color—white, gray, or pink. Felsic magmas have relatively low melting temperatures and high viscosity. They include granite, dacite, rhyolite, and their metamorphic equivalents (granite gneisses). Felsic tuffs and ash deposits often interbed with sedimentary rocks and help determine the age of strata and fossil assemblages. Examples: Eocene tuffs of the Green River Formation with fish and plant fossils; Pliocene ash beds in the Glenns Ferry Formation with mammalian fauna; Oligocene deposits of the John Day Fossil Beds with plants and mammals.

ferruginous quartzite

Ferruginous quartzites — metamorphic equivalents of Precambrian banded iron formations (BIF), typically dark red or gray rocks in which quartz layers alternate with hematite, magnetite, or iron silicates. They form through the metamorphism of ancient iron-rich sediments and volcanosedimentary sequences. A characteristic variety is itabirite, a finely layered metamorphic rock. Major deposits occur in the Quadrilátero Ferrífero (Brazil) and the Krivoy Rog Basin (Ukraine). Ferruginous quartzites record the Great Oxidation Event from roughly ~2.4–2.1 billion years ago. They also document the role of microbial life in iron precipitation. These rocks may contain some of the earliest traces of life, such as Grypania spiralis (~2.1 Ga, Negaunee Iron Formation, Michigan).

fissure volcano

Fissure volcano — a volcano in which eruptions occur through an elongated fracture in the Earth’s crust associated with major faults. Lava erupts along the entire fissure or at discrete segments, forming linear flows and chains of volcanic cones. Fissure eruptions produce extensive lava plateaus (flood basalts), driving global climate change and mass extinctions. Lavas and tuffs from these eruptions serve as markers for dating and correlating fossil assemblages. These rocks record brief, distinctive geological events identifiable by their chemical and stratigraphic characteristics. Examples: Laki (Iceland), Tolbachik (Kamchatka), Tarawera (New Zealand).

flint

Flint is a cryptocrystalline variety of quartz (SiO₂), a specific type of chert, occurring as nodules and layers within chalk and limestone. Typically, dark grey or black, it has extreme hardness and sharp edges when fractured. Ancient people used flint to create stone tools. Flint often contains or replaces fossils of marine organisms (radiolarians, sponges, fusulinids), providing exceptional preservation; nodules frequently form around organic remains, preserving Late Cretaceous marine biocenoses. Examples: Cretaceous flints of England and Europe with sea urchins Micraster, ammonites, and sponges.

flood basalt

Flood basalt — extensive lava plateaus composed predominantly of tholeiitic basalts with dolerites, gabbros, and rare picrites, formed by fissure eruptions associated with mantle plume activity and building what geologists call large igneous provinces. River incision and weathering of these sequences produce a characteristic stepped topography. Flood basalts correlate with several Phanerozoic mass extinctions, including the Siberian Traps (~252 Ma, Permian–Triassic boundary) and the Deccan Traps (~66 Ma, Cretaceous–Paleogene boundary). Other examples include the Ethiopian–Arabian and Paraná–Etendeka provinces. Synonyms: plateau basalt, traps, trap rock.

floodplain

Floodplain — a part of the flat floor of a river valley that is periodically inundated during seasonal floods and high-water events. In lowland rivers, floodplains may reach widths of 30–50 km. They form through lateral migration of the channel and consist of alluvium deposited during flooding. Floodplain deposits preserve remains of terrestrial organisms well, including pollen and mammal bones, and allow reconstruction of ancient landscapes and climates. Examples: Late Pleistocene floodplains of Siberia with mammoth fauna; Pliocene deposits of the Irvington Formation with rich mammalian assemblages.

fluvioglacial deposit

Fluvioglacial deposits — cross-bedded deposits of sand, gravel, and boulders formed by meltwater from melting glaciers, as well as by reworked material. Proglacial deposits accumulate in front of an ice margin and build outwash plains (sandur) and glaciofluvial terraces. Subglacial deposits form within ice tunnels and meltwater channels beneath the glacier. Pleistocene glaciofluvial deposits often contain bones of large mammals and pollen, allowing reconstruction of late-glacial landscapes and climate. Examples: mammoth remains in glaciofluvial gravels of the Alps and Vancouver Island; mammoth fauna from sandur deposits in Siberia and North America. Synonym: glaciofluvial deposit.

flysch

Flysch — thick sequences of marine sedimentary rocks that form in deep-water basins during the early stages of orogeny from turbidity currents on continental slopes. They consist of rhythmically alternating layers of sandstone, mudstone, siltstone, marl, and limestone, reaching thicknesses of thousands of meters. Flysch deposits form in deep-marine environments. Flysch commonly preserves abundant trace fossils (ichnofossils) and rare body fossils that reflect conditions in ancient ocean basins. Examples: graphoglyptids and echinoderm traces in the Cretaceous–Paleogene flysch of Zumaia (Spain); fish and crabs in the Carpathian flysch.

foothill

Foothills — transitional low-relief landscapes between mountains and plains, typically hilly or gently mountainous, with elevations of about 200–400 m. Foothills accumulate thick continental deposits rich in remains of terrestrial vertebrates, which help reconstruct stages of orogeny and paleoclimate. Examples: the Miocene foothills of the Sierra Nevada with mastodons, horses, and camels; the Siwalik foothills of the Himalayas with a rich Miocene–Pliocene mammalian fauna; the foothills of the Rocky Mountains with dinosaur trackways and Cretaceous faunas.

formation

Formation — in geology, a body of rock units grouped together based on similar lithological characteristics and age. In stratigraphy, it represents the fundamental unit of lithostratigraphy, defined by lithologic properties and traceable over large areas. Different tectonic settings produce characteristic formations (flysch, reef, deluvial, etc.). Their thickness may reach thousands of meters, and their areal extent may cover hundreds to thousands of square kilometers. Gaps in sedimentation may also occur. Formations serve as key units for correlation and paleoenvironmental reconstruction and often contain characteristic fossil assemblages. Examples: the Morrison Formation (dinosaurs) and the Green River Formation (fish and plants).

fumarole

Fumarole — an opening or fissure in the Earth’s crust through which hot volcanic gases escape, including water vapor, CO₂, SO₂, H₂S, and others. As the gases cool, water vapor condenses, forming hydrothermal manifestations. Fumaroles are common in areas of active volcanism and geothermal activity. Varieties include solfataras (dominated by sulfur gases) and mofettes (dominated by carbon dioxide). Hydrothermal mineralization in fumarolic systems often leads to exceptional preservation of organic remains. Examples: seeds and shoots of horsetails from the Clarno Nut Beds (John Day Formation, USA); fossilized microbial mats in ancient hydrothermal systems.

G

gabbro

Gabbro — a coarse-grained mafic intrusive rock, the plutonic equivalent of basalt. It consists of calcic plagioclase (labradorite, bytownite) and clinopyroxene (augite), with minor amounts of olivine, hornblende, and other mafic minerals. The color ranges from dark gray to black. Gabbro forms by slow cooling of mafic magma at depth in plutons and layered intrusions; it also occurs in sills and dikes. It makes up the lower portion of the oceanic crust and represents a key component of ophiolite complexes. Their age and obduction help reconstruct the history of seafloor spreading and ocean closure. Examples: the ophiolites of Oman, Troodos (Cyprus), and the Appalachians.

geochronological scale

Geochronological scale — a scale of geological time showing the sequence of stages in the evolution of the Earth’s crust and the development of Earth’s biosphere. Scientists construct it based on data from paleontology and historical geology. See also: Geologic Time Scale.

geosyncline

Geosyncline — a historical concept describing large-scale subsidence of the Earth’s crust in continental platforms or oceanic sedimentary basins extending for hundreds of kilometers and characterized by gentle slopes. These basins accumulated thick (up to 3–5 km) sequences of sedimentary and volcanic rocks. In plan view, they show rounded or oval outlines. They represent extensive regional basins rather than individual folds or structural features and often develop above older aulacogens. The emergence of plate tectonic theory led to the reinterpretation of geosynclines as sedimentary basins formed above subsiding lithosphere. Examples: the Tethys Geosyncline, the Appalachian Geosyncline, the Vindhyan Basin.

glacier

Glacier — a mass of ice formed by the compaction and recrystallization of snow above the snow line, slowly flowing downslope under its own weight. In the geological record, glacial striations on bedrock, erratics, and tillites indicate the presence of glaciers. These features document ancient glaciations, including the Pleistocene (2.6 Ma–11.7 ka), when ice sheets covered large parts of Europe and North America, and the Cryogenian (720–635 Ma, Snowball Earth). Synonym: gletcher.

glauconite

Glauconite — a green hydrous potassium–iron aluminosilicate of the mica group that forms in marine shelf environments. It serves as an indicator of marine conditions and slow sedimentation, occurring in sands, sandstones, clays, marls, and limestones, often together with smectite. Glauconite has low resistance to weathering. Geologists use it for isotopic dating (K–Ar and Rb–Sr) of marine deposits, which makes it a valuable tool for stratigraphy and paleobasin reconstruction. It often occurs in association with rich marine faunas. Examples: the Main Glauconite Bed (Eocene of Texas, with mollusks, brachiopods, sharks, and fish); glauconitic sands of New Jersey containing mosasaur bones.

globular texture

Globular texture — in mineralogy, a rock texture characterized by rounded or radiating aggregates of crystals (spherulites) formed during rapid crystallization from a glassy or strongly supercooled melt. It occurs in paleovolcanic sequences as an indicator of specific cooling conditions. It is especially characteristic of Archean basalts (varioles) and rhyolites. Examples: spherulites in rhyolites of the Long Valley Caldera (USA, up to 20 cm in diameter); varioles in Archean basalts (~2.7–3.5 billion years old). Synonym: spherulitic texture.

gneiss

Gneiss — a common coarse-grained metamorphic rock, typically light in color, characterized by a distinct banded texture produced by the alternation of light-colored minerals (quartz, feldspars) and dark-colored minerals (biotite, hornblende, pyroxene, garnet). Gneisses constitute a major part of the continental crust, outcropping in crystalline shields (Baltic, Ukrainian, Canadian, and Aldan) and forming the basement of ancient platforms. They preserve some of the oldest Archean and late Hadean crustal materials. Examples: the Acasta Gneiss (Canada, 4.03–3.96 Ga—the oldest dated rocks of the Earth’s crust); the Itsaq Gneiss Complex (Greenland, ~3.8 Ga); Morton Gneiss (Minnesota, ~3.5 Ga).

gorge

Gorge — a narrow, deep valley with steep, often overhanging sides that a river carves into hard rocks; unlike a canyon, the river does not fully occupy its floor. Depth ranges from several tens of meters to over 1 km. In addition to erosional gorges, tectonic and karst gorges also occur. Gorges expose ancient rock strata, preserving a record of millions of years of geological history. Examples: Olduvai Gorge (Tanzania)—deposits up to 2 million years old containing hominin remains and stone tools; Letchworth Gorge (USA)—Devonian marine deposits (~380 Ma).

graben

Graben — an elongated block of the Earth’s crust that subsides relative to the surrounding terrain along a system of parallel tectonic faults and that fault scarps bound on both sides. Grabens may reach hundreds of kilometers in length and tens to hundreds of kilometers in width. They form in rift zones and record episodes of crustal extension from the Archean to the Cenozoic; the sedimentary and volcanic sequences accumulated within them preserve a fossil record that allows reconstruction of ancient rifting events and climate changes. Examples: the Upper Rhine Graben (Europe, Eocene–Oligocene); grabens of the Basin and Range Province (USA, Miocene–Pliocene); ancient Proterozoic grabens in the Belt Supergroup (USA).

grainstone

Grainstone — a granular limestone, a carbonate sedimentary rock in which grains (bioclasts, ooids, skeletal fragments) are in contact, while silt or micritic particles are nearly absent (<1%). It forms in high-energy environments—such as beaches, shoals, shallow shelf settings, and reef complexes—where currents winnow out fine material. Abundant bioclasts (crinoids, brachiopods, bryozoans, foraminifera) reflect the high-energy conditions of shallow Phanerozoic seas. Examples: Paleozoic crinoidal grainstones and Mesozoic oolitic–bioclastic grainstones.

granite

Granite — the most widespread coarse-grained intrusive felsic rock, forming a significant part of the continental crust since the Archean. It consists of quartz (20–40%), potassium feldspar, plagioclase, and micas (biotite, muscovite); the color is typically light gray or pinkish. Granite is less dense than basalt and forms through slow crystallization of magma in the deep parts of the crust, occurring in batholiths and laccoliths. Examples: the Yosemite and Sierra Nevada batholiths; Archean granites of the Canadian Shield.

graphite

Graphite — a natural crystalline form of carbon with a layered structure. It is black to steel-gray in color, with a metallic luster and high lubricating properties. Graphite forms during metamorphism and metasomatism; it often has a biogenic origin as a product of deep metamorphism of ancient organic matter and serves as a key indicator of early life on Earth. Examples: ~3.95 Ga graphite in rocks of Labrador; graphite in ferruginous quartzites of Greenland (>3.7 Ga). Synonym: plumbago (obsolete).

gravel

Gravel — an unconsolidated mixture of rounded rock fragments 2–63 mm in size, formed by weathering and erosion. It is common in fluvial, coastal, and morainic facies; when lithified, it forms gravelites and conglomerates. Gravel serves as an indicator of high-energy depositional environments (rivers, beaches, alluvial fans). The geological record preserves it since the Archean: quartz conglomerates of Jack Hills (~3 Ga) document early erosional processes and fluvial systems.

graywacke

Graywacke — a dense, dark sandstone composed of fine grains of quartz, feldspar, and rock fragments, with a significant proportion of clay-rich matrix. It forms in high-energy environments such as gravity flows and turbidity current systems in deep-marine basins. Graywacke is a classic component of flysch sequences and an indicator of ancient active continental margins; it may occasionally preserve fossil fragments. It is widely distributed in the Paleozoic and Mesozoic. Examples: Ordovician–Silurian sequences of Europe; the Franciscan Complex of California with Jurassic–Cretaceous ammonites. Synonyms: greywacke; grauwacke; greiwacke; apogrit (obsolete).

greenhouse effect

Greenhouse effect — the warming of the Earth’s surface and lower atmosphere: atmospheric gases transmit incoming shortwave solar radiation but absorb and re-emit part of the outgoing longwave infrared radiation from the Earth’s surface. The main greenhouse gases include water vapor, CO₂, methane, ozone, and nitrous oxide. Without them, the average surface temperature would be about –18 °C, and the planet would exist in a state of global glaciation. Intensification of the greenhouse effect in the geological past has caused abrupt warming events and mass extinctions.

greenstone belt

Greenstone belt — thick sequences of metamorphosed volcano-sedimentary rocks within Archean shields, named for the green color of metamorphic minerals (chlorite, epidote, and actinolite). These belts may reach up to 20 km in thickness, extend up to 1,000 km in length, and attain widths of 100–200 km. They date to about 2.5–3.5 billion years ago and typically overlie a basement of gray gneisses. Greenstone belts represent the most widespread complexes of the middle and late Archean. They preserve some of the earliest evidence of life, including stromatolites and microfossils, and host important deposits of gold and other metals. Examples: the Barberton Greenstone Belt (South Africa, ~3.5 Ga) and the Abitibi Greenstone Belt (Canada).

guyot

Guyot — an isolated submarine mountain with a flat summit and steep slopes; it represents an extinct volcanic island truncated by wave abrasion and later submerged below sea level due to tectonic subsidence of the plate. Summit depths exceed 200 m, the platform diameter may surpass 10 km, and the total height from base to top may reach 4–5 km. Guyots are widespread in the Pacific Ocean and serve as markers for reconstructing plate movements. They preserve evidence of ancient tropical reefs of Cretaceous age, and fossilized corals, rudists, and limestones often cap their summits. Examples: guyots of the western Pacific Ocean, including Resolution Guyot. Synonym: tablemount.

H

hamada

Hamada — a desert landscape of high rocky plateaus (often composed of basalt), where wind deflation has removed loose material, exposing gravel, pebbles, boulders, and bedrock, forming a stony desert with little to no soil or vegetation cover. It develops under arid conditions over tens of thousands of years. Hamadas often expose ancient deposits and preserve traces of wetter Pleistocene and Holocene climates, as seen on the Tinrhert Plateau in the Sahara (Algeria), where Cretaceous deposits and ancient fluvial features occur. Examples: the Sahara Desert, the Negev Desert (Israel). Synonyms: hammada, nejd.

Hawaiian-type eruption

Hawaiian-type eruption — a relatively gentle effusion of hot, low-viscosity basaltic lava with low gas content. It involves weak explosions, and lava erupts as fire fountains, producing only small amounts of pyroclastics (ash and tephra). Owing to its high fluidity, lava flows may travel long distances, forming broad, gently sloping shield volcanoes. Eruptions may occur through a central crater as well as through networks of radial fissures or flank vents. These eruptions are typically long lasting and often preceded by earthquakes. Ancient shield volcanoes of the Hawaiian–Emperor chain represent examples of this activity in the geological past. Typical examples include Kīlauea and Mauna Loa (Hawaii) and Piton de la Fournaise (Réunion), as well as individual eruptive phases of Etna (Italy, 2013).

hematite

Hematite — iron oxide (Fe₂O₃), one of the principal iron ores. It occurs in effusive rocks, hydrothermal veins, and metasomatic formations. Its color ranges from black to cherry-red with a metallic luster. Precambrian banded iron formations (BIFs) are of particular importance, where hematite together with magnetite reflects the transition to an oxygenated atmosphere in the early Proterozoic. The red sands of Mars consist of basaltic particles coated with thin layers of maghemite and hematite. Examples: Hamersley formations (Australia), Transvaal formations (South Africa). Synonyms: red iron ore, iron glance.

highland

Highlands — extensive areas situated well above sea level, characterized by mountainous relief or elevated plateaus. They usually represent tectonically uplifted crustal blocks. Within them occur ancient crystalline and metamorphic complexes, as well as sedimentary and volcanogenic sequences that provide material for paleontological and facies reconstructions. The Scottish Highlands reflect the Caledonian continental collision and the closure of the Iapetus Ocean (~480–425 Ma). Examples: Cambrian Mountains (Wales), Central Highlands of Afghanistan, Central Highlands of Tasmania.

hornblende

Hornblende — a dark-colored rock-forming mineral of the amphibole group, representing a complex aluminosilicate of calcium, magnesium, and iron, with admixtures of sodium, potassium, fluorine, and hydroxyl groups. It has a vitreous luster, and its color is typically black, dark green, or brownish. Hornblende is widespread in magmatic (both intrusive and effusive) and metamorphic rocks. Its presence helps reconstruct conditions of regional metamorphism and tectonic history. Examples: Barrovian zones in the Scottish Highlands, amphibolites within many orogenic belts.

horst

Horst — in structural geology, an elongated crustal block uplifted relative to the surrounding terrain along parallel normal faults under tectonic extension, forming elevated ridges or plateaus. Horsts often expose ancient basement rocks. Examples: Sierra Nevada (USA)—a horst of the Basin and Range province uplifted in the Cenozoic; Vosges (France); and Schwarzwald (Germany), which bound the Rhine Graben.

hyaloclastite

Hyaloclastite — a volcaniclastic breccia composed of glassy fragments formed when lava shatters as magma rapidly cools upon contact with water or ice. Rapid cooling causes volcanic glass to crack and hydrate. Hyaloclastites are characteristic of submarine and subglacial basaltic eruptions. Examples: Pleistocene sequences of Iceland, hyaloclastites in the core of the Hawaiian Scientific Drilling Project (Mauna Kea). Synonym: aquagene tuff.

hydrothermal spring

Hydrothermal spring — a source of geothermally heated groundwater enriched with minerals and gases, emerging at the Earth’s surface or the ocean floor. It forms when circulating waters in fractures heat up through contact with hot rocks or magmatic chambers. Hydrothermal springs are widespread in volcanic regions (geysers, fumaroles) and along mid-ocean ridges, where black smoker systems develop with chemosynthetic ecosystems. They represent one of the earliest habitats for life on Earth; sediments of the Nuvvuagittuq greenstone belt (Canada) reveal possible microfossils dated to 3.77–4.28 Ga.

I

iceland spar

Iceland spar — a transparent, coarse-crystalline variety of calcite, calcium carbonate (CaCO₃). It shows double refraction, splitting light into two separate rays. Usually colorless, it may occasionally show bluish, pink, or yellowish tints. Iceland spar forms mainly in limestones and dolomitic sedimentary rocks, hydrothermal veins, or evaporite deposits. The largest deposit lies in hydrothermal veins within basalts of eastern Iceland (Helgustaðir), mined from the 17th to the 20th century. The optical properties of Iceland spar played a crucial role in early microscopy and crystallography, aiding the study of fossil structures and thin sections of rocks.

Icelandic-type eruption

Icelandic-type eruption — a quiet effusive eruption where highly fluid basaltic lava pours from fissures with little pyroclastic material. Lava spreads over great distances, forming lava fields, shield volcanoes, and basalt plateaus. These eruptions occur along rift zones and divergent plate boundaries; gases escape freely through fractures, so strong explosions are rare. They resemble Hawaiian eruptions but differ by their fissure style of outpouring. The basaltic plateaus they create sometimes preserve interlava sediments; in West Iceland, Miocene layers contain fossil soils with plant and insect remains, recording ecosystems between successive flows. Examples: Laki fissure, Krafla eruptions (Iceland).

igneous rock

Igneous rocks — rocks that form when magma or lava cools and solidifies. Crystallization at or near the Earth’s surface produces extrusive (volcanic) rocks, while crystallization at depth produces intrusive (plutonic) rocks. Classification depends on SiO₂ content: acidic, intermediate, basic, and ultrabasic. Igneous rocks make up ~90% of the Earth’s crust and record the major stages of continental formation since the Archean. The oldest known igneous rocks are TTG complexes (tonalite–trondhjemite–granodiorite) dated to ~4.0 Ga. Interbeds between igneous flows sometimes contain fossil soils and sedimentary horizons, preserving traces of ecosystems that developed during volcanic pauses. Example: Acasta Gneiss, Canada.

ignimbrite

Ignimbrite — a solidified deposit of a pyroclastic flow, composed of a heterogeneous mixture of ash particles, pumice fragments, and volcanic glass crystals; at high temperatures, particles fuse to form welded tuffs. It originates from ash flows and glowing clouds during large explosive eruptions of acidic or intermediate composition. Colors range from light gray to dark brown and nearly black. Ignimbrites are products of supereruptions, forming sheets hundreds of meters thick and marking episodes of intense acidic volcanism, especially in the Oligocene–Miocene. Examples: ignimbrite flare-up in the Basin and Range (USA, 40–25 Ma); Taupo Volcanic Zone (New Zealand). Synonyms: ash‑flow tuff, welded tuff, flood tuff.

ilmenite

Ilmenite — the principal ore of titanium, iron–titanium oxide (FeTiO₃). A widespread accessory and rock‑forming mineral of the corundum group in igneous rocks of basic and ultrabasic composition: gabbros, troctolites, norites, anorthosites, as well as basalts and dolerites; it also occurs in metamorphic rocks and placer deposits. Ilmenite is a dark‑black, weakly magnetic mineral with metallic luster. Its presence in basalts and cumulates of layered intrusions reflects processes of early magmatic differentiation. Lunar soils, ~3.5–3.8 Ga in age, are notably rich in ilmenite. Synonyms: titanic iron ore, haplotypite, mohsite.

impact

Impact — the collision of a meteorite with the surface of a planetary body, producing an impact crater. These rare, high‑energy events trigger shock metamorphism (shatter cones, planar deformation structures in quartz) and produce impactites—melt rocks and breccias. Large ancient structures are termed astroblemes. Some hypotheses link major impact events to mass extinctions. Synonym: impact event.

inland sea

Inland sea — a large continental water body, enclosed by land or connected to the ocean by a river or strait. Its salinity is usually intermediate between freshwater lakes and marine water. In geological terminology, inland seas refer to epicontinental basins formed during marine transgressions. An example is the “Pará Sea” of the Middle Miocene, which developed after the uplift of the Andes blocked the Amazon’s outflow. The Western Interior Seaway stretched from the Gulf of Mexico to Canada during the Cretaceous. This famous seaway hosted marine reptiles, turtles, and ammonites. Modern inland seas: Marmara Sea, Baltic Sea, Caspian Sea.

intermediate rock

Intermediate rock — an igneous rock with silica (SiO₂) content between mafic and felsic compositions, usually ~52–63%. It typically contains roughly equal proportions of light minerals (feldspars) and dark minerals (amphiboles, biotite, and pyroxenes). Intermediate rocks dominate island arcs and active continental margins, reflecting partial melting of subducting oceanic crust. They are widespread in Phanerozoic orogenic belts. Example: Andean arc, ranging from the Mesozoic to the present. Examples of rock types: andesite, dacite, diorite.

intrusion

Intrusion — in geology, this term has two meanings: 1) the process of molten magma penetrating surrounding rock masses without reaching the surface; 2) a magmatic body formed when magma solidifies within the Earth’s crust. Intrusions produce intrusive (plutonic) rocks; typical forms include batholiths, laccoliths, sills, dikes, and stocks. Contact metamorphism along their margins often alters the host sedimentary strata, and zones around intrusions may preserve fossiliferous sediments. Large intrusions mark stages of orogeny and continental crustal growth since the Archean. Examples: Sierra Nevada Batholith (USA); Bushveld Complex (South Africa, ~2.06 Ga). Synonyms: injection, irruption.

island arc

Island arc — a chain of volcanic islands that develops along a subduction zone, where one oceanic plate descends beneath another. Island arcs feature high seismicity, intense volcanism, and separate forearc basins from deep-sea trenches. They play a key role in continental crust growth through accretion; remnants of ancient arcs remain preserved in accretionary orogens. Sedimentary basins adjacent to island arcs often contain rich fossil assemblages, documenting ecosystems shaped by arc volcanism and subduction processes. Example: Chaitenia, accreted to Patagonia in the Devonian. Modern examples: Antillean, Kuril, Aleutian, Mariana, and other island arcs. Synonym: volcanic arc.

J

jaspilite

Jaspilite — a finely banded variety of banded iron formation (BIF) containing typically 15–25% Fe, dominated by quartz and iron oxides. Siliceous layers consist of red or brown‑pink jasper/chert, while iron‑rich layers are composed of silvery‑gray hematite or magnetite. Siderite interbeds may also occur within these structures. It formed mainly in the Precambrian by chemical precipitation in anoxic marine waters and later underwent low‑grade metamorphism, commonly recrystallizing silica to quartz. Some Archean samples preserve possible microfossils—tubular and filamentous structures—linking chemical sedimentation to early ecosystems. Examples: Nuvvuagittuq Supracrustal Belt (Canada, ~3.8–3.7 Ga); Negaunee Formation with macrofossil Grypania spiralis (Michigan, USA). Synonyms: jaspillite, jaspilyte, itabirite, jasper taconite.

joint

Joint — in structural geology, a planar fracture of natural origin in rock, without significant displacement. It forms due to tension, cooling, or pressure release; it commonly occurs in systems (joint sets). Joints may be open or filled with magma or precipitated minerals. Joints sometimes develop near fossils, which act as zones of weakness. Examples: columnar joints in basalts (Giant’s Causeway, ~60 million years); fractures in limestones that control karst caves containing paleontological finds. Synonyms: fracture (without displacement), joint set, joint system.

K

karst

Karst — in geomorphology, a set of processes and landforms resulting from the dissolution of soluble rocks by natural waters, leading to the formation of cavities and collapses. Soluble rocks include carbonates (limestone, dolomite, chalk), sulfates (gypsum, anhydrite), and evaporites (rock salt). Typical karst landforms are sinkholes, dolines, shafts, and caves. Karst cavities often act as natural traps, preserving rich faunal assemblages of the Pleistocene Epoch (mammoths, cave bears, dire wolves) as well as hominin remains. Examples: Mammoth Cave (USA) with marine fossils; Sterkfontein (South Africa) with australopithecine remains. Synonym: karst topography.

Katmai-type eruption

Katmai-type eruption — a powerful explosive eruption in which enormous volumes of pumice and ash, saturated with gases, generate incandescent pyroclastic flows (glowing clouds) that move at high velocity. The ash column may rise to 20 km, while the pyroclastic deposits can reach hundreds of cubic kilometers in volume. Acidic magmatic melt emerges from numerous fissures. Extensive fumarolic activity accompanies the eruption, and caldera collapse marks its culmination. A thick tephra layer blankets and levels the landscape over wide areas. Examples: Novarupta–Katmai (Alaska, 1912), which caused mass extinction of local fauna and flora; Mount Martin (Aleutian Range). Synonym: eruption of pyroclastic flows.

kerogen

Kerogen — an insoluble organic substance: a complex mixture of detrital and finely dispersed organic matter in sedimentary rocks, most commonly in shales. Plant, plankton, and marine remains transform into kerogen under increasing temperature and pressure. Further heating converts this material into bitumen and ultimately into oil and gas. Researchers sometimes call it proto-oil. Kerogen contains microfossils and preserves molecular and isotopic traces of life. Examples: Type I kerogen derived from the alga Gloeocapsomorpha prisca (Ordovician); the Green River Formation with remains of Botryococcus; Cambrian–Proterozoic black shales containing microfossils. Synonym: insoluble organic matter, kerabitumen (obsolete).

khondalite

Khondalite — a high-grade metamorphic (meta-sedimentary) rock, formed mainly during the Archean and Paleoproterozoic. It originates from the metamorphism of clay-rich sandstones and argillites. KKhondalite commonly occurs in ancient Precambrian belts alongside charnockites and kinzigites. These rocks indicate continental collision and the assembly of the Columbia supercontinent. Direct fossils are extremely rare due to the intense metamorphism. Examples: Khondalite Belt of the North China Craton (Ordos–Yinshan collision, ~1.95 Ga); Eastern Ghats Belt (India). Synonyms: Bezwada gneiss, Kailasa gneiss.

kimberlite

Kimberlite — a breccia-like or tuffaceous ultramafic igneous rock of mantle origin, enriched in magnesium and rare earth elements. It consists of a fine-grained matrix containing fragments of olivine, serpentine, pyrope, phlogopite, and calcite. Kimberlite fills vertical explosive pipes (diatremes) and occurs in dikes and sills; it is the principal source of diamonds. Its color is typically gray-green or bluish. Diatremes may preserve organic remains and xenoliths containing fossils, providing rare information on the paleoclimate of ancient cratons. Examples: Eocene wood of Metasequoia from the Panda pipe (~53 Ma); Cretaceous spores, pollen, and turtle bones from the Lac de Gras kimberlites. Synonym: kimberlitic breccia.

komatiite

Komatiite — an ultramafic volcanic rock containing 18–32% MgO, derived from high-temperature magma (1600–1800 °C); the most refractory volcanic rock on Earth. Its color is dark gray with a greenish tint. Komatiites occur mainly at the base of Archean greenstone belts; the oldest examples date to ~3.5–3.6 Ga. Associated sedimentary interbeds preserve some of the earliest traces of life, including stromatolites and microfossils. Examples: Barberton Greenstone Belt (South Africa, ~3.5–3.2 Ga) with early microbial mats; Pilbara Craton (Australia) with stromatolites ~3.48 Ga. Synonyms: spinifex-textured peridotitic komatiite.

The Geological Dictionary Terminology E–K presented here reflects the extraordinary diversity of Earth’s geological and biological history, from Archean greenstone belts to Pleistocene glaciations. Each definition links terminology to the fossil record and to specific formations and events across deep time.

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