Alluvial fan ↗
A fan-shaped deposit of sediment left where a fast-moving stream flows out of steep terrain onto a flatter plain, dropping its coarser material as it slows.
The language of geology
Big ideas.
Plain language.
Meet the words behind the rocks. Throughout the collection, tap an underlined term to see its meaning without leaving the page.
A
Anthracite ↗
The highest grade of coal — hard, shiny, and carbon-rich, having undergone the most heat and pressure of any coal type.
Atoll ↗
A ring-shaped coral reef surrounding a lagoon, formed when a volcanic island that the reef originally fringed has fully subsided beneath the ocean surface, leaving only the reef behind.
B
Biotite ↗
A dark, black-to-brown mica mineral, common as an accessory mineral in granite and other igneous and metamorphic rocks.
Breccia ↗
A clastic sedimentary rock made of angular (not rounded) rock fragments cemented together, indicating the fragments weren't transported far before burial. Compare to conglomerate.
C
Calcite ↗
A common, relatively soft mineral made of calcium carbonate; the primary component of limestone, chalk, and marble, and the reason all three react to acid.
Cementation ↗
The process by which dissolved minerals crystallize in the spaces between sediment grains, binding loose particles into solid rock.
Cinder cone ↗
The smallest type of volcano, built from a single eruptive episode ejecting small lava fragments that pile into a steep, cone-shaped hill around the vent.
Clast ↗
An individual fragment of broken rock or mineral that becomes part of a clastic sedimentary rock.
Clastic ↗
Sedimentary rock made from broken fragments (clasts) of other rocks, cemented together. Example: sandstone, made of compacted sand grains.
Cleavage (mineral) ↗
A mineral's tendency to break along flat, smooth, repeatable planes determined by its internal crystal structure — distinct from fracture, which is a rougher, less predictable break.
Coccolithophore ↗
A microscopic marine plankton with a shell made of calcite plates; the primary source material of chalk, accumulated in vast numbers on ancient sea floors.
Columnar jointing ↗
The formation of regular, often hexagonal rock columns as thick molten rock cools and contracts slowly and evenly. Example: Giant's Causeway (basalt), Devils Tower (an intrusive rock).
Compaction ↗
The process by which the weight of overlying material presses buried sediment together, squeezing out water and reducing space between particles.
Conchoidal fracture ↗
A smooth, curved breakage pattern (like a broken glass bottle) seen in materials with no crystal structure to break along. Example: obsidian.
Contact metamorphism ↗
Metamorphism caused by heat alone, from rock sitting near a body of magma, with little to no directional pressure involved — distinct from regional metamorphism's broad-scale heat-and-pressure.
Continental drift ↗
The historical observation that continents have moved relative to each other over geologic time — now understood to be a consequence of plate tectonics.
Continental ice sheet ↗
A glacier large enough to bury an entire landscape, flowing outward under its own weight rather than following existing valleys. Example: Antarctica, Greenland.
Convergent boundary ↗
A tectonic plate boundary where two plates collide, resulting in subduction (if one plate is oceanic) or folding/mountain-building (if both are continental).
Core (Earth's) ↗
The innermost layer of the Earth, made mostly of iron and nickel; divided into a solid inner core and a liquid outer core.
Cross-bedding ↗
Thin, angled internal layers within a rock, tilted in the direction wind (or sometimes water) was moving sediment when it was deposited — visible evidence of ancient dunes or currents preserved in stone. Example: Coconino Sandstone.
Crust (Earth's) ↗
The thin, outermost solid layer of the Earth, made of rock — either continental crust (thicker, lighter, forms landmasses) or oceanic crust (thinner, denser, mostly basalt).
Crystal ↗
A solid whose atoms are arranged in a repeating, orderly pattern. Rocks are usually made of many small crystals grown together; a "crystal" you can see and hold is just one that grew large and well-formed enough to show its shape.
D
Deflation (wind erosion) ↗
The removal of loose, fine material by wind, leaving coarser rock and gravel behind.
Deposition ↗
The laying down of sediment carried by water, wind, or ice, once it slows or stops moving — the step before burial and compaction turn it into sedimentary rock.
Directional pressure ↗
Pressure applied more strongly from one direction than others, causing minerals to align and produce foliation.
Divergent boundary ↗
A tectonic plate boundary where two plates move apart, allowing magma to rise and create new crust. Example: mid-ocean ridges, continental rift valleys.
Dome mountain ↗
A mountain formed when rising magma pushes up overlying crust without erupting at the surface, creating a broad dome shape. Example: the Black Hills.
E
Earthquake ↗
A sudden release of energy in the Earth's crust, usually caused by movement along a fault, producing seismic waves felt as shaking at the surface.
Evaporite ↗
Sedimentary rock formed when a body of water evaporates, leaving dissolved minerals behind to crystallize. Example: rock salt.
Extrusive (volcanic) ↗
Igneous rock that cooled at the Earth's surface, usually fast, resulting in small or no visible crystals. Example: basalt, rhyolite.
F
Fault ↗
A fracture in rock along which the two sides have visibly moved relative to each other, often the site of earthquakes when that movement happens suddenly.
Fault-block mountain ↗
A mountain formed when tectonic stress fractures crust into blocks that shift vertically relative to each other along faults. Example: the Sierra Nevada.
Feldspar ↗
The most common mineral group in Earth's crust; typically light-colored (white, pink, gray) and a major ingredient in granite, diorite, and many other rocks.
Fissile ↗
Able to split easily into thin, flat sheets, characteristic of shale.
Flow banding ↗
Streaky, parallel lines within volcanic rock, formed by the movement/flow of viscous lava as it cools and solidifies. Example: rhyolite.
Fluorescence (mineral) ↗
The property some minerals have of glowing a distinct color when exposed to ultraviolet light, due to trace elements or structural quirks in the mineral, even though the same mineral looks ordinary under regular light. Example: sodalite in Yooperlites.
Fold ↗
A bend in layered rock caused by compressive tectonic forces, without breaking — a key feature of mountain-building.
Fold mountain ↗
A mountain formed when colliding continental plates crumple and fold the crust upward, since neither plate is dense enough to subduct. Example: the Himalayas.
Foliated ↗
Metamorphic rock with a banded or layered appearance, caused by minerals aligning under directional pressure. Example: gneiss, slate.
Foliation ↗
The banded or layered texture in metamorphic rock caused by minerals aligning under directional pressure.
Fossil ↗
The preserved remains or trace of an ancient organism, typically found in sedimentary rock, which forms under conditions gentle enough to bury and preserve organic material before it fully decomposes.
Fringing reef / barrier reef ↗
Two stages in a coral reef's relationship to a subsiding volcanic island: a fringing reef grows directly against the shoreline; as the island sinks and the reef grows upward to stay near the surface, it becomes a barrier reef, separated from the shrinking island by a lagoon.
G
Garnet ↗
A hard, often red (though color varies) mineral that commonly grows during metamorphism; found as distinct crystals embedded within schist and other metamorphic rocks.
Geyser ↗
A hot spring where constricted underground plumbing causes pressure to periodically build and release explosively, ejecting superheated water and steam through the surface opening in a semi-predictable eruption cycle. Example: Old Faithful, Yellowstone.
Glacial erratic ↗
A rock or boulder transported by a glacier far from its original source rock, then left behind when the ice retreated or melted — often the only clue to rock types not native to the area where they're found. Example: Yooperlites, found in Michigan but originally sourced from Ontario.
Glacial period / interglacial period ↗
Within an ice age, a glacial period is a colder stretch when ice sheets expand; an interglacial period is a warmer stretch (like the one we're in now) when they retreat.
Grain size ↗
How large the individual mineral crystals in a rock are; the single biggest visual clue to how a rock formed. Coarse-grained rocks cooled slowly (crystals had time to grow); fine-grained rocks cooled fast.
Groundmass ↗
The fine-grained background material surrounding larger, visible crystals in an igneous rock — common in rocks that cooled in two stages (slow, then fast).
Guyot ↗
A flat-topped, fully submerged seamount — the remains of a volcanic island that sank before coral reef growth could keep pace, its peak worn flat by wave erosion while it was still near the surface.
H
Hardness ↗
How resistant a mineral is to scratching, measured on the 1–10 Mohs scale (talc is 1, diamond is 10). A fingernail scratches minerals softer than about 2.5; a steel knife scratches up to about 5.5; quartz (7) scratches glass.
Hoodoo ↗
A tall, thin, irregularly-shaped rock spire, typically formed by frost-wedging widening cracks in layered rock of varying hardness. Example: Bryce Canyon.
Hornblende ↗
A common dark, greenish-black mineral found in igneous and metamorphic rocks like diorite and gneiss.
Hot spring ↗
A pool of water heated by nearby underground magma or hot rock, reaching the surface through relatively open plumbing as continuous flow rather than a geyser's periodic eruption.
Hotspot ↗
A stationary plume of unusually hot material rising from deep in the mantle, capable of melting through the crust above it regardless of plate boundary activity. Example: Hawaii.
I
Ice age ↗
An extended period of Earth's history cold enough for large ice sheets to persist, itself made up of alternating colder glacial and warmer interglacial periods.
Igneous ↗
Rock formed from cooled and solidified magma or lava. One of the three main rock categories.
Impact metamorphism ↗
Metamorphism caused by the extreme, near-instantaneous heat and pressure of a meteor impact — distinct from both regional and contact metamorphism in both cause and speed.
Inselberg ↗
An isolated hill or rock mass standing alone on an otherwise flat landscape, having resisted erosion that stripped away the softer rock that once surrounded it. Example: Uluru.
Intrusive (plutonic) ↗
Igneous rock that cooled slowly underground, resulting in large, visible crystals. Example: granite, gabbro.
Isostasy ↗
The equilibrium between the Earth's crust and the denser mantle beneath it, in which a heavier load on the crust (such as a massive volcanic island) causes that section of crust to sink somewhat, similar to a heavy object settling into a soft surface.
K
Karst ↗
A landscape formed by the dissolution of soluble rock, typically limestone, by slightly acidic groundwater — producing caves, sinkholes, and underground drainage.
Kimberlite pipe ↗
A narrow, carrot-shaped volcanic conduit that rapidly transports material from deep in the mantle to the surface — notable as the primary source of naturally occurring diamonds.
L
Lava ↗
Magma that has reached the Earth's surface, generally through a volcanic eruption.
Lignite ↗
A low grade, soft, brown coal — the least metamorphosed and lowest-carbon form of coal.
Luster ↗
How a mineral's surface reflects light — described as metallic, glassy, dull, pearly, etc. It's one of the first things geologists note about a specimen.
M
Magma ↗
Molten rock beneath the Earth's surface. Becomes lava if it reaches the surface, or cools underground into intrusive igneous rock if it doesn't.
Mantle ↗
The thick layer of Earth between the crust and the core, made of hot, slowly-flowing rock. Source of the magma that forms igneous rock, and of peridotite specifically.
Mantle convection ↗
The slow circulation of solid but hot, flowing mantle rock, driven by heat differences (hot material rising, cooler material sinking) — the underlying force that drives tectonic plate movement.
Matrix ↗
The finer-grained material surrounding larger grains, clasts, or crystals within a rock. Example: the fine material filling gaps between pebbles in conglomerate.
Melting ↗
The transition of solid rock into liquid magma, occurring when rock is heated past its melting point (or pressure is reduced enough) deep within the Earth.
Metamorphic ↗
Rock that started as igneous, sedimentary, or even another metamorphic rock, and was transformed by heat and/or pressure without fully melting. One of the three main rock categories.
Mica ↗
A mineral group that splits into thin, flexible, shiny sheets; a common accessory mineral in granite and a defining ingredient in schist.
Mid-ocean ridge ↗
An underwater mountain chain formed at a divergent boundary, where new oceanic crust is continuously created as plates pull apart.
Mineral vs. Rock ↗
A mineral is a naturally occurring solid with a specific chemical composition and crystal structure (like quartz or feldspar). A rock is made of one or more minerals grouped together (like granite, which is quartz + feldspar + mica).
Moraine ↗
An unsorted pile of rock debris deposited directly by a glacier, typically marking the former edge or endpoint of the ice.
N
Non-foliated ↗
Metamorphic rock with no banded/layered texture, either from even pressure or minerals that don't align. Example: marble, quartzite.
O
Olivine ↗
A green mineral common in rocks sourced from the mantle; the main ingredient in peridotite, and the gem-quality version is called peridot.
Ophiolite ↗
A slice of ocean floor and upper mantle that has been thrust up and preserved on continental crust, usually by tectonic collision — one of the few places mantle rock like peridotite is directly accessible at the surface.
Orogeny ↗
The geologic process of mountain-building, typically driven by tectonic plate collision, involving folding, faulting, and uplift of the crust over long timescales.
P
Parent rock (protolith) ↗
The original rock that a metamorphic rock formed from before being altered by heat and/or pressure. Example: shale is the parent rock (protolith) of slate.
Plagioclase feldspar ↗
A common variety of feldspar, typically white to gray, and a major mineral component of diorite, gabbro, and many other igneous rocks.
Plate boundary ↗
The edge where two tectonic plates meet, categorized by how they interact: divergent (pulling apart, forming new crust), convergent (colliding, forming mountains or subduction zones), or transform (sliding past each other, as at fault lines like the San Andreas).
Plate tectonics ↗
The scientific theory describing Earth's crust as broken into large moving plates, whose interactions at their boundaries drive mountain-building, volcanism, earthquakes, and the long-term rearrangement of continents.
Plucking / abrasion ↗
The two mechanisms of glacial erosion: plucking rips loose chunks of bedrock frozen to the base of a moving glacier; abrasion is the grinding/scratching effect of rock debris embedded in glacial ice dragging against surrounding bedrock.
Precipitate (mineral) ↗
A mineral that forms by crystallizing directly out of a solution, rather than from melting or compaction. Example: halite precipitating as saltwater evaporates.
Pyroxene ↗
A dark, dense mineral group common in mafic igneous rocks like gabbro and peridotite.
Q
Quartz ↗
One of the most common minerals in Earth's crust; hard (7 on the Mohs scale), often clear or white, glassy luster. A major ingredient in granite and rhyolite, and the sole ingredient of pure quartz sand.
R
Recrystallization ↗
The process by which existing mineral crystals grow, rearrange, or reform into new mineral crystals under heat and/or pressure, without melting — the core mechanism of metamorphism.
Regional metamorphism ↗
Metamorphism caused by heat and pressure applied over a broad area, typically during mountain-building — the process behind this site's foliated rock sequence (slate, phyllite, schist, gneiss).
Rift (rift valley) ↗
A linear zone where continental crust is being pulled apart at a divergent boundary, often forming a valley as the crust stretches and drops. Example: the East African Rift Valley.
Ring of Fire ↗
A roughly horseshoe-shaped zone around the Pacific Ocean where most of the world's subduction zones are concentrated, producing the majority of Earth's volcanic eruptions and earthquakes.
Rock cycle ↗
The ongoing process by which rock transforms between igneous, sedimentary, and metamorphic forms over geologic time, through melting, weathering, compaction, heat, and pressure.
S
Seamount ↗
An underwater mountain of volcanic origin that never grew tall enough to break the ocean's surface, or that has since subsided back below it.
Sediment ↗
Loose particles of rock, mineral, or organic material, deposited by water, wind, or ice, that can eventually become sedimentary rock through burial and compaction.
Sedimentary ↗
Rock formed from compacted and cemented sediment, or minerals precipitated from water. One of the three main rock categories.
Shield volcano ↗
A broad, gently-sloped volcano built from low-viscosity basaltic lava that flows easily and spreads over a wide area. Example: Hawaii's volcanoes.
Shocked quartz ↗
Ordinary quartz crystals bearing microscopic fracture patterns that only form under the extreme, near-instantaneous pressure of a meteor impact — one of the most reliable pieces of evidence geologists use to confirm an impact site.
Sinkhole ↗
A depression or hole in the ground formed when the roof of an underground cavity — typically a cave dissolved out of limestone — collapses.
Sodalite ↗
A blue-to-gray mineral, sometimes fluorescent (glowing orange under UV light due to trace element content) — the mineral responsible for Yooperlites' glow.
Specific gravity ↗
A mineral's density relative to water, used as an identification test — a mineral with a specific gravity of 3 is three times as dense as an equal volume of water.
Stalactite / stalagmite ↗
Cave mineral formations built from calcite redepositing out of dripping, evaporating water: stalactites hang from a cave ceiling, stalagmites grow up from the floor, and the two sometimes meet to form a solid column.
Strata / bedding ↗
Distinct layers within sedimentary rock, each representing a separate deposit of material over time.
Stratovolcano (composite volcano) ↗
A tall, steep-sided, symmetrical volcano built from higher-viscosity andesitic/rhyolitic magma and explosive eruptions. Example: Mount Fuji, Mount St. Helens.
Streak ↗
The color of the powder a mineral leaves when scraped across an unglazed porcelain plate — often a more reliable identifier than the mineral's surface color, which can vary.
Subduction ↗
Where one tectonic plate is forced beneath another, often melting as it descends and feeding the magma that forms volcanoes like the Andes or Cascades.
Subsidence ↗
The gradual sinking of a section of the Earth's surface, whether from crustal cooling, the weight of an overlying mass, or other causes. In the context of volcanic islands, subsidence is what ultimately turns an island into an atoll or guyot.
T
Tectonic plate ↗
A large, rigid slab of Earth's crust and uppermost mantle that moves slowly over the more fluid layer beneath it, interacting with neighboring plates at its boundaries.
Transform boundary ↗
A tectonic plate boundary where two plates slide past each other horizontally, without creating or destroying crust, often producing earthquakes. Example: the San Andreas Fault.
Travertine ↗
A form of limestone that forms rapidly at the surface, typically at hot springs, as dissolved calcium carbonate precipitates out of cooling, evaporating water. Example: Pamukkale.
Tsunami ↗
A series of large ocean waves caused by the sudden displacement of a massive volume of water, most commonly from vertical seafloor movement during a subduction-zone earthquake. Barely noticeable in open ocean; dangerous only as it slows and grows taller approaching shallow coastal water.
U
Uplift ↗
The geologic raising of the Earth's surface, typically driven by tectonic forces (plate collision) or by the crust rebounding after erosion removes overlying weight.
V
Vesicle ↗
A small cavity in volcanic rock formed by a trapped gas bubble as the rock cooled. Example: pumice (many, tightly packed) and basalt (fewer, scattered).
Viscosity ↗
A fluid's resistance to flow — in magma, viscosity is determined largely by silica content, and it's the main factor determining whether a volcano erupts gently (low viscosity) or explosively (high viscosity).
Volcano ↗
An opening in the Earth's crust through which magma, gas, and ash erupt to the surface, typically forming a characteristic landform (cone, shield, etc.) from accumulated erupted material.
W
Weathering vs. Erosion ↗
Weathering is the breakdown of rock in place (from water, ice, temperature change, chemical reaction); erosion is the movement of that broken material elsewhere by water, wind, or ice. They usually happen together but are technically different steps.