Geology Vocabulary: Earth Science Terms and Definitions

Earth keeps records in stone. A cliff face, a beach pebble, a cave wall, or a line of mountains can all tell part of the planet’s story if you know the words geologists use. This guide explains core geology vocabulary in plain English, from Earth’s inner layers and rock types to fossils, groundwater, volcanoes, earthquakes, and plate motion. Use it for earth science class, field trips, exam review, or simply to make better sense of the ground under your feet.

What Earth Is Made Of Inside

The basic layers of the planet give geologists the framework for understanding many surface processes.

Crust

Earth’s outer solid shell. It is about 5 km thick under the oceans, where it is called oceanic crust, and roughly 30–70 km thick beneath continents, where it is called continental crust. Most crustal rocks are silicate rocks.

Mantle

The broad layer below the crust and above the core, reaching down to about 2,900 km. Parts of the upper mantle are partly molten, which helps tectonic plates shift. The mantle accounts for about 84% of Earth’s volume.

Core

Earth’s central layer. It includes a liquid outer core, made mostly of iron and nickel, and a solid inner core. Movement in the outer core produces Earth’s magnetic field.

Lithosphere

The stiff outer part of Earth, made up of the crust plus the uppermost mantle. This layer is divided into tectonic plates.

Asthenosphere

A ductile, partly molten zone of the upper mantle below the lithosphere. Tectonic plates move slowly over it.

Mohorovičić Discontinuity (Moho)

The boundary separating Earth’s crust from the mantle. Seismic waves change speed at this boundary. Croatian seismologist Andrija Mohorovičić identified it in 1909.

Rock and Mineral Terms

Words for Minerals

Mineral

A natural, inorganic solid with a definite chemical makeup and a crystalline structure. Common minerals include calcite, mica, feldspar, and quartz.

Crystal

A solid whose atoms are arranged in an orderly, repeating three-dimensional pattern. Some crystals are too small to see clearly; others grow to impressive sizes.

Mohs Hardness Scale

A 1-to-10 scale for comparing mineral hardness. Talc is 1, the softest reference mineral, and diamond is 10, the hardest. The scale is based on which minerals can scratch others.

Luster

The appearance of light reflecting from a mineral’s surface. Common descriptions include dull, silky, pearly, vitreous or glassy, and metallic.

Cleavage

A mineral’s tendency to split along flat, regular planes controlled by its crystal structure. This differs from fracture, which means a more irregular break.

Streak

The color of a mineral in powdered form, usually tested by rubbing it on an unglazed porcelain plate. Streak can be more useful for identification than the color seen on the mineral’s surface.

Main Categories of Rock

Igneous Rock

Rock that forms when magma or lava cools and solidifies. Intrusive, or plutonic, igneous rocks such as granite cool slowly below ground; extrusive, or volcanic, rocks such as basalt cool quickly at Earth’s surface.

Sedimentary Rock

Rock made when sediments are accumulated, compacted, and cemented together. These sediments may be pieces of older rock, minerals, or organic material. Shale, sandstone, and limestone are familiar examples.

Metamorphic Rock

Rock changed from an earlier rock by heat, pressure, or chemical action without fully melting. Slate forms from shale, and marble forms from limestone.

Rock Cycle

The ongoing set of processes that forms, breaks down, and remakes rocks. Igneous rocks can weather into sediment; sediment can become sedimentary rock; sedimentary or igneous rock can become metamorphic rock; and any rock type can melt and later form new igneous rock.

Magma / Lava

Magma is molten rock while it remains below Earth’s surface. Lava is the same material after it erupts at the surface. The difference is location.

How Plate Tectonics Works

Plate tectonics is the central theory that ties much of geology together. It explains Earth’s surface as a set of moving plates whose interactions form ocean basins, build mountains, generate earthquakes, and feed many volcanoes.

Tectonic Plate

A huge, uneven slab of lithosphere that rides on the asthenosphere. Earth has about 15 major tectonic plates, along with several smaller ones.

Divergent Boundary

A boundary where two plates move away from one another. Rising magma fills the gap and creates new crust. The Mid-Atlantic Ridge is a well-known example.

Convergent Boundary

A boundary where plates move toward each other. Oceanic crust may be forced beneath another plate, producing trenches and volcanic arcs; when continents collide, major mountain belts such as the Himalayas can form.

Transform Boundary

A boundary where two plates move sideways past each other. California’s San Andreas Fault is a famous transform boundary.

Subduction

The process in which one tectonic plate sinks beneath another into the mantle. This usually involves oceanic crust descending below continental crust or another oceanic plate.

Rift

A crack or belt of cracks where Earth’s lithosphere is being stretched apart. Rifting is often an early step in plate separation. The East African Rift is an active modern example.

Continental Drift

Alfred Wegener’s 1912 hypothesis that continents shift across Earth’s surface over geological time. Plate tectonics later supplied the mechanism that explained how this movement occurs.

Pangaea

A supercontinent that existed about 300–200 million years ago. It included Earth’s landmasses before they split apart into the continents known now.

Seafloor Spreading

The formation of new oceanic crust at mid-ocean ridges. Magma rises, hardens, and pushes older crust away from the ridge.

Volcanoes and Molten Rock Processes

Volcano

An opening in Earth’s crust where magma, ash, and gases reach the surface. Volcanoes are most common at plate boundaries and hotspots.

Eruption

The discharge of magma, volcanic gases, and pyroclastic material from a volcano. Some eruptions produce quiet lava flows, called effusive eruptions; others are explosive and violent.

Shield Volcano

A wide volcano with gentle slopes, built by repeated flows of runny basaltic lava. Hawaii’s Mauna Loa is the largest shield volcano in the world.

Stratovolcano (Composite Volcano)

A steep, cone-shaped volcano made of alternating layers of lava, ash, and pyroclastic material. Mount Vesuvius and Mount Fuji are stratovolcanoes.

Caldera

A large, bowl-like depression that forms when a volcano’s magma chamber drains during an eruption and the summit collapses inward. Yellowstone lies above a huge caldera.

Hotspot

A zone of unusually strong volcanic activity away from a plate boundary. It is caused by hot mantle material rising from deep inside Earth. The Hawaiian Islands formed above a hotspot.

Pyroclastic Flow

A fast, extremely dangerous flow of hot ash, gas, and broken rock that rushes down a volcano during an explosive eruption.

Pumice

A highly porous, very light volcanic rock produced when gas-rich lava cools quickly. Pumice is the only rock that can float on water.

Earthquake and Seismic Vocabulary

Earthquake

A sudden release of energy in Earth’s crust that sends seismic waves through the ground. Most earthquakes happen along plate boundaries.

Seismic Waves

Energy waves produced by earthquakes that move through Earth and across its surface. P-waves, or primary waves, are faster compressional waves; S-waves, or secondary waves, are slower shear waves; surface waves usually cause the greatest damage.

Epicenter

The spot on Earth’s surface directly above the focus, also called the hypocenter. The focus is the underground point where the earthquake begins.

Richter Scale / Moment Magnitude Scale

Magnitude scales used for earthquakes. The moment magnitude scale (Mw) has mostly replaced the Richter scale in scientific work. It is logarithmic, meaning each whole-number step represents about 32 times more released energy.

Fault

A break in Earth’s crust where movement has taken place. Normal faults form under extension, reverse/thrust faults under compression, and strike-slip faults under lateral stress.

Tsunami

A train of ocean waves caused by sudden seafloor displacement, often from an underwater earthquake. Tsunamis can produce devastating flooding along coasts.

Aftershock

A smaller earthquake that occurs after the main shock as the fault zone adjusts. Aftershocks may continue for days, weeks, or even years.

Seismograph

A device that senses and records seismic waves. The visual record it produces is called a seismogram.

How Rocks Break Down and Move

Weathering

The breaking down of rocks and minerals at or near Earth’s surface. Mechanical (physical) weathering reduces rock to smaller pieces without changing its composition, as in frost wedging or root growth. Chemical weathering changes the rock’s chemistry, as with oxidation or acid-rain dissolution.

Erosion

The removal and transport of weathered material by moving water, wind, ice, or gravity. Over long periods, erosion helps carve valleys, canyons, coastlines, and other landforms.

Deposition

The settling or placement of sediment in a new area. Deposition builds landforms such as floodplains, beaches, and deltas.

Glacier

A large, slow-flowing mass of ice made from snow compacted over many years. Glaciers reshape landscapes through both erosion and deposition.

Moraine

A pile or ridge of rock and sediment left by a glacier. Moraines can show where a glacier once reached.

Stalactite / Stalagmite

Cave mineral formations made by dripping mineral-rich water. Stalactites hang from cave ceilings, while stalagmites grow upward from cave floors.

The Vocabulary of Deep Time

Geological Time Scale

The timeline used to organize Earth’s 4.6-billion-year history. It divides time into eons, eras, periods, epochs, and ages, based on major geological and biological changes.

Eon

The largest unit of geological time. Earth’s history is divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic, which is the current eon.

Era

A division within an eon. The Phanerozoic Eon contains the Paleozoic, Mesozoic, and Cenozoic eras.

Period

A division within an era. For instance, the Mesozoic Era is divided into the Triassic, Jurassic, and Cretaceous periods.

Stratigraphy

The study of rock layers, or strata, and the order in which they formed. It helps scientists interpret Earth’s history. In general, older layers lie below younger layers, a principle called the Law of Superposition.

Radiometric Dating

A method for finding the age of rocks and minerals by measuring radioactive isotope decay. Carbon-14 dating is used for relatively recent organic material, while uranium-lead dating is used for very old rocks.

Unconformity

A missing interval in the rock record, caused by erosion or by a time when deposition stopped. An unconformity represents time that was not preserved in the layers.

Fossils and the Study of Past Life

Fossil

A preserved remain, mark, impression, or trace of an organism that once lived. Fossils are usually found in sedimentary rock and provide evidence for past life and evolution.

Paleontology

The scientific study of ancient life through fossils. Paleontology connects geology with biology.

Index Fossil

A fossil from an organism that lived during a short, specific span of time. Index fossils help date the rock layers where they occur.

Petrification

A fossilization process in which minerals slowly replace organic material. The organism turns to stone while much of its structure remains preserved.

Trace Fossil

Evidence of what an organism did rather than the body of the organism itself. Examples include burrows, footprints, and coprolites.

Mass Extinction

A rapid, widespread loss of biodiversity. Earth has had five major mass extinctions; the best-known event, 66 million years ago, wiped out the non-avian dinosaurs.

Water’s Role in Geology

Aquifer

A layer or body of permeable rock or sediment that can store and transmit groundwater. Aquifers provide drinking water for billions of people around the world.

Water Table

The upper boundary of the saturated zone underground. Below the water table, pore spaces in soil and rock are filled with water.

Groundwater

Water held below the surface in soil pores and rock fractures. It is an essential resource for drinking water, farming, and industry.

Karst

A landscape created when soluble rocks, such as limestone or dolomite, dissolve. Karst areas commonly include caves, sinkholes, and underground drainage systems.

Hydrology

The scientific study of water: where it is found, how it moves, and what properties it has on Earth’s surface, below ground, and in the atmosphere.

Practical Ways to Learn Geology Words

• Collect rocks and minerals. When you handle specimens while learning their names, the terms become easier to remember.
• Learn word roots. "Geology" comes from Greek gē meaning earth and logos meaning study. "Seismology" comes from seismos, meaning earthquake. Roots make many terms easier to predict.
• Visit geological places. Caves, canyons, national parks, coastlines, and volcanic areas turn vocabulary into something you can see.
• Look closely outdoors. Notice local rocks, slopes, streambeds, cliffs, and landforms. Every landscape has a geological history.
• Build your English vocabulary. Familiarity with Greek and Latin roots makes geology terms much easier to learn.
• Connect geology with broader science vocabulary. Earth science uses ideas from chemistry, physics, and biology, so related science terms often reinforce one another.

Geology terms give you a practical way to read Earth’s evidence: minerals, layers, faults, fossils, water, and volcanic rock all become more meaningful when the vocabulary is clear. Keep these words handy as you study, hike, travel, or look at the landscape around you. Explore more word guides at dictionary.wiki.