Geology Vocabulary: Earth Science Terms and Definitions

Geology is the study of Earth—its materials, structure, processes, and history. From the crystals in a granite countertop to the forces that raise mountain ranges and trigger earthquakes, geology vocabulary helps us understand the planet we call home. Whether you are a student preparing for an earth science exam, a hiker curious about the rocks beneath your feet, or someone fascinated by volcanoes and fossils, this guide provides clear definitions of essential geological terms organized by theme.

Earth's Structure

Understanding Earth's internal layers is foundational to geology.

Crust

The outermost solid layer of Earth, ranging from about 5 km thick beneath the oceans (oceanic crust) to 30–70 km beneath continents (continental crust). The crust is composed primarily of silicate rocks.

Mantle

The thick layer between the crust and the core, extending to about 2,900 km depth. The upper mantle is partially molten, allowing tectonic plates to move. The mantle makes up about 84% of Earth's volume.

Core

The innermost layer of Earth, divided into the liquid outer core (composed mainly of iron and nickel) and the solid inner core. The core generates Earth's magnetic field through convection in the outer core.

Lithosphere

The rigid outer layer of Earth, comprising the crust and the uppermost part of the mantle. The lithosphere is broken into tectonic plates.

Asthenosphere

The partially molten, ductile layer of the upper mantle beneath the lithosphere, on which tectonic plates slowly move.

Mohorovičić Discontinuity (Moho)

The boundary between Earth's crust and mantle, marked by a change in seismic wave velocity. It was discovered by Croatian seismologist Andrija Mohorovičić in 1909.

Minerals and Rocks

Minerals

Mineral

A naturally occurring, inorganic, solid substance with a definite chemical composition and crystalline structure. Quartz, feldspar, mica, and calcite are common minerals.

Crystal

A solid in which atoms are arranged in a highly ordered, repeating three-dimensional pattern. Crystals can be microscopic or spectacularly large.

Mohs Hardness Scale

A scale from 1 (talc, softest) to 10 (diamond, hardest) used to measure the relative hardness of minerals based on their ability to scratch one another.

Luster

The way a mineral's surface reflects light—described as metallic, vitreous (glassy), pearly, silky, or dull.

Cleavage

The tendency of a mineral to break along flat, predictable planes determined by its crystal structure, as opposed to fracture (irregular breaking).

Streak

The color of a mineral's powder when scratched across an unglazed porcelain plate. Streak is often more diagnostic than the mineral's external color.

Rock Types

Igneous Rock

Rock formed from the cooling and solidification of magma or lava. Intrusive (plutonic) igneous rocks like granite cool slowly underground; extrusive (volcanic) rocks like basalt cool rapidly at the surface.

Sedimentary Rock

Rock formed from the accumulation, compaction, and cementation of sediments—fragments of pre-existing rocks, minerals, or organic material. Sandstone, limestone, and shale are common examples.

Metamorphic Rock

Rock transformed from pre-existing rock by heat, pressure, or chemical processes without melting. Marble (from limestone) and slate (from shale) are metamorphic rocks.

Rock Cycle

The continuous process by which rocks are created, broken down, and reformed. Igneous rock weathers into sediment, which becomes sedimentary rock, which can be metamorphosed, and any rock type can melt to form new igneous rock.

Magma / Lava

Magma is molten rock beneath Earth's surface; lava is magma that has erupted onto the surface. The distinction is purely one of location.

Plate Tectonics

Plate tectonics is the unifying theory of geology, explaining how Earth's surface is divided into moving plates that interact to create mountains, earthquakes, volcanoes, and ocean basins.

Tectonic Plate

A massive, irregularly shaped slab of lithosphere that floats on the asthenosphere. Earth has about 15 major plates and several minor ones.

Divergent Boundary

A plate boundary where two plates move apart, allowing magma to rise and create new crust. The Mid-Atlantic Ridge is a classic divergent boundary.

Convergent Boundary

A plate boundary where two plates collide. Oceanic crust may subduct (dive beneath) another plate, creating deep trenches and volcanic arcs; continental collisions build mountain ranges like the Himalayas.

Transform Boundary

A plate boundary where two plates slide horizontally past each other. The San Andreas Fault in California is a famous transform boundary.

Subduction

The process by which one tectonic plate slides beneath another into the mantle, typically oceanic crust beneath continental crust or another oceanic plate.

Rift

A fracture or zone of fractures in Earth's crust where the lithosphere is being pulled apart, often the initial stage of plate separation. The East African Rift is a modern example.

Continental Drift

The hypothesis (proposed by Alfred Wegener in 1912) that continents move across Earth's surface over geological time. Plate tectonics provided the mechanism for continental drift.

Pangaea

The supercontinent that existed approximately 300–200 million years ago, comprising all of Earth's landmasses before they broke apart into today's continents.

Seafloor Spreading

The process by which new oceanic crust forms at mid-ocean ridges as magma rises, pushes older crust apart, and solidifies.

Volcanoes and Igneous Processes

Volcano

An opening in Earth's crust through which magma, gases, and ash erupt onto the surface. Volcanoes occur primarily at plate boundaries and hotspots.

Eruption

The release of magma, gases, and pyroclastic material from a volcano. Eruptions range from gentle lava flows (effusive) to violent explosions (explosive).

Shield Volcano

A broad, gently sloping volcano built by successive flows of low-viscosity basaltic lava. Mauna Loa in Hawaii is the world's largest shield volcano.

Stratovolcano (Composite Volcano)

A steep, conical volcano built from alternating layers of lava, ash, and pyroclastic material. Mount Fuji and Mount Vesuvius are stratovolcanoes.

Caldera

A large, basin-shaped depression formed when a volcano's magma chamber empties during an eruption and the summit collapses inward. Yellowstone sits atop a massive caldera.

Hotspot

An area of anomalously high volcanic activity not located at a plate boundary, caused by a plume of hot mantle material rising from deep within Earth. The Hawaiian Islands formed over a hotspot.

Pyroclastic Flow

A fast-moving, deadly current of hot gas, ash, and rock fragments that races down a volcano's slopes during an explosive eruption.

Pumice

A very lightweight, porous volcanic rock formed when gas-rich lava cools rapidly. Pumice is the only rock that can float on water.

Earthquakes and Seismology

Earthquake

A sudden release of energy in Earth's crust, generating seismic waves that cause the ground to shake. Most earthquakes occur at plate boundaries.

Seismic Waves

Waves of energy that travel through Earth's interior and along its surface following an earthquake. P-waves (primary) are compressional and faster; S-waves (secondary) are shear waves and slower; surface waves cause the most damage.

Epicenter

The point on Earth's surface directly above the focus (or hypocenter)—the underground point where an earthquake originates.

Richter Scale / Moment Magnitude Scale

Scales used to measure earthquake magnitude. The moment magnitude scale (Mw) has largely replaced the Richter scale for scientific use and is logarithmic—each whole number increase represents roughly 32 times more energy released.

Fault

A fracture in Earth's crust along which movement has occurred. Normal faults (extensional), reverse/thrust faults (compressional), and strike-slip faults (lateral) correspond to different types of tectonic stress.

Tsunami

A series of ocean waves generated by a sudden displacement of the seafloor—often due to an underwater earthquake—capable of causing catastrophic coastal flooding.

Aftershock

A smaller earthquake that follows the main shock, resulting from readjustments along the fault. Aftershock sequences can continue for days, weeks, or even years.

Seismograph

An instrument that detects and records seismic waves, producing a visual record called a seismogram.

Weathering and Erosion

Weathering

The breakdown of rocks and minerals at or near Earth's surface. Mechanical (physical) weathering breaks rock into smaller pieces without changing its composition (frost wedging, root growth). Chemical weathering alters the chemical structure (oxidation, dissolution by acid rain).

Erosion

The transport of weathered material by water, wind, ice, or gravity. Erosion shapes landscapes over time, carving valleys, coastlines, and canyons.

Deposition

The laying down of eroded material (sediment) in a new location, building features like deltas, beaches, and floodplains.

Glacier

A massive, slow-moving river of ice formed from compacted snow over many years. Glaciers sculpt landscapes through erosion and deposition.

Moraine

Accumulations of rock and sediment deposited by a glacier, marking its former extent.

Stalactite / Stalagmite

Mineral formations in caves. Stalactites hang from the ceiling (formed by dripping mineral-rich water); stalagmites rise from the floor.

Geological Time

Geological Time Scale

The chronological framework dividing Earth's 4.6-billion-year history into eons, eras, periods, epochs, and ages based on significant geological and biological events.

Eon

The largest division of geological time. Earth's history is divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic (the current eon).

Era

A subdivision of an eon. The Phanerozoic Eon includes three eras: Paleozoic, Mesozoic, and Cenozoic.

Period

A subdivision of an era. The Mesozoic Era, for example, includes the Triassic, Jurassic, and Cretaceous periods.

Stratigraphy

The study of rock layers (strata) and their sequence, used to interpret Earth's history. Older layers generally lie beneath younger ones (the Law of Superposition).

Radiometric Dating

A technique for determining the age of rocks and minerals by measuring the decay of radioactive isotopes. Carbon-14 dating is used for recent organic material; uranium-lead dating for ancient rocks.

Unconformity

A gap in the geological record where rock layers are missing due to erosion or a pause in deposition, representing unrecorded time.

Fossils and Paleontology

Fossil

The preserved remains, impression, or trace of a once-living organism, typically found in sedimentary rock. Fossils provide evidence of past life and evolutionary history.

Paleontology

The scientific study of ancient life through the examination of fossils. Paleontology bridges biology and geology.

Index Fossil

A fossil of an organism that lived during a specific, limited time period, used to date the rock layer in which it is found.

Petrification

A fossilization process in which organic material is gradually replaced by minerals, turning the organism into stone while preserving its structure.

Trace Fossil

Evidence of an organism's activity (footprints, burrows, coprolites) rather than the organism itself.

Mass Extinction

A widespread, rapid decrease in biodiversity. Earth has experienced five major mass extinctions; the most famous (66 million years ago) killed the non-avian dinosaurs.

Water and Geology

Aquifer

A body of permeable rock or sediment that stores and transmits groundwater. Aquifers supply drinking water to billions of people worldwide.

Water Table

The upper surface of the zone of saturation in the ground, below which all pore spaces are filled with water.

Groundwater

Water stored underground in soil pores and rock fractures, a vital resource for drinking water, agriculture, and industry.

Karst

A landscape formed by the dissolution of soluble rocks (limestone, dolomite), characterized by sinkholes, caves, and underground drainage systems.

Hydrology

The scientific study of water's distribution, movement, and properties on and below Earth's surface and in the atmosphere.

Tips for Learning Geology Vocabulary

• Go outside. Observe rocks, landforms, and geological features in your area. Every landscape tells a geological story.
• Study word roots. "Geology" comes from Greek gē (earth) + logos (study). "Seismology" from seismos (earthquake). Roots make terms predictable.
• Collect rocks and minerals. Handling specimens while learning their names creates multisensory memory connections.
• Visit geological sites. National parks, canyons, caves, and volcanic areas offer direct experience with geological vocabulary.
• Connect to science vocabulary broadly. Geology overlaps with chemistry, physics, and biology.
• Build your English vocabulary. Understanding Greek and Latin roots accelerates geology vocabulary acquisition.

Geology vocabulary is the language of Earth itself—a planet that has been reshaping its surface for 4.6 billion years. By learning these terms, you gain a deeper appreciation for the landscapes, resources, and forces that sustain life. Explore more at dictionary.wiki.