Geyser
A geyser is a hot spring with plumbing tight enough to let heat, water, and pressure build until water and steam burst upward.
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Fumarole
A fumarole is a vent or crack where steam and volcanic gases escape from hot rock, magma-heated groundwater, or a hydrothermal system. Fumaroles can reveal volcanic heat, gas chemistry, sulfur deposition, hydrothermal alteration, unrest, and hazards even when no lava is erupting.
Caldera
A caldera is a large volcanic depression, usually formed when the ground above a magma reservoir collapses after magma is withdrawn during an eruption or intrusion. Calderas can host lakes, lava domes, resurgent uplifts, hydrothermal systems, renewed eruptions, and long-lived volcanic hazards.
Lahar
A lahar is a fast-moving volcanic mudflow or debris flow made of water, ash, rock fragments, sediment, and volcanic debris. Lahars can rush down river valleys during or after eruptions, travel far from a volcano, bury channels and floodplains, and threaten communities long after explosive activity has stopped.
Tephra
Tephra is volcanic material blasted into the air during an eruption and deposited from the atmosphere. It includes ash, lapilli, blocks, bombs, pumice, glass shards, crystals, and older rock fragments, and it can record eruption style, wind direction, hazards, and geologic time markers.
Pyroclastic flow
A pyroclastic flow is a fast-moving, ground-hugging mixture of hot volcanic gas, ash, pumice, and rock fragments. It is one of the most dangerous volcanic hazards because it can move rapidly down slopes and valleys, remain extremely hot, and destroy or bury almost everything in its path.
Lava
Lava is molten or partly molten rock that has erupted onto a planetary surface. On Earth, lava flows, fountains, domes, tubes, and volcanic fragments reveal magma composition, eruption style, cooling rate, gas release, terrain, and volcanic hazards.
Magma
Magma is molten or partly molten rock beneath Earth surface. It can contain liquid melt, crystals, dissolved gases, and bubbles, and it is the source material for igneous rocks, volcanic eruptions, lava flows, intrusions, and many mineral deposits.
Sedimentary rock
Sedimentary rock forms from particles, minerals, organic material, or chemical precipitates deposited at or near Earth surface. It often preserves layers, fossils, sedimentary structures, and environmental clues that help geologists reconstruct rivers, deserts, lakes, coastlines, oceans, and ancient climates.
Igneous rock
Igneous rock forms when molten rock cools and solidifies. It includes intrusive rocks that crystallize below the surface, extrusive volcanic rocks that cool at or near the surface, and many textures that record cooling rate, gas content, mineral composition, and eruption or intrusion history.
Rock cycle
The rock cycle describes how rocks form, break down, transform, melt, and reform through geologic time. It connects igneous, sedimentary, and metamorphic rocks through processes such as cooling, weathering, erosion, burial, lithification, metamorphism, uplift, and melting.
Metamorphism
Metamorphism is the solid-state transformation of existing rock as temperature, pressure, stress, and chemically active fluids change. It creates metamorphic minerals, textures, and rock types that record burial, mountain building, subduction, intrusion, and deformation.
Orogeny
Orogeny is the long geologic process of mountain building. It usually happens where tectonic plates converge, compressing, folding, faulting, thickening, heating, uplifting, and eroding crust into mountain belts over millions of years.
Subduction
Subduction is the plate-tectonic process in which one tectonic plate sinks beneath another and descends into the mantle. It recycles oceanic crust, builds volcanic arcs, forms deep-sea trenches, and creates some of the largest earthquakes and tsunamis on Earth.
Seafloor spreading
Seafloor spreading is the process that creates new oceanic crust at ocean ridges and moves older crust away from the ridge axis. It links volcanism, earthquakes, magnetic stripes, ocean-floor age patterns, and plate tectonics into one moving system.
Paleomagnetism
Paleomagnetism is the study of Earth's ancient magnetic field as recorded by rocks, sediments, and minerals. It helps scientists reconstruct magnetic reversals, date rock sequences, trace volcanic histories, and test how tectonic plates moved through geologic time.
Taphonomy
Taphonomy is the study of what happens to organisms, traces, and remains after death or activity and before they are interpreted as fossils. It connects decay, burial, transport, mineral change, erosion, discovery, and sampling bias into one record-forming story.
Fossilization
Fossilization is the set of physical, chemical, and biological processes that turn remains, impressions, or traces of organisms into fossils. It usually depends on rapid burial, mineral-rich water, low disturbance, and enough geologic time for preservation to outlast decay.
Trace fossils
Trace fossils are preserved evidence of what organisms did, rather than the preserved bodies of the organisms themselves. Footprints, burrows, trails, nests, bite marks, root traces, and coprolites can reveal movement, feeding, hiding, nesting, and interactions in ancient environments.
Index fossils
Index fossils, also called guide fossils, are fossils used to help identify and correlate rock layers of similar age. The best index fossils were widespread, abundant, easy to recognize, and lived for a relatively short interval of geologic time.
Fossil record
The fossil record is the evidence of past life preserved in rocks, sediments, amber, tar, ice, and other natural archives. It documents evolution, extinction, ancient environments, and the changing diversity of life, while also carrying gaps and biases shaped by preservation and discovery.
Mass extinctions
Mass extinctions are geologically rapid losses of a large share of Earth's biodiversity. They are recognized from the fossil record and usually reflect severe environmental disruption, such as climate shifts, ocean chemistry change, volcanism, asteroid impact, or combinations of stresses.
Geologic time scale
The geologic time scale is the framework scientists use to organize Earth's 4.6-billion-year history. It links rock layers, fossils, radiometric ages, climate events, mass extinctions, and tectonic changes into named intervals such as eons, eras, periods, epochs, and ages.
Stratigraphy
Stratigraphy is the study of rock layers and layered deposits. It helps geologists arrange events in relative order, correlate rocks from place to place, interpret ancient environments, and connect field evidence with geologic time.
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