DSST Introduction to Geology key terms

The theory that Earth's outer shell is broken into rigid slabs that move over a softer layer, causing earthquakes, volcanoes, mountain building, and the drift of continents over geologic time.

The cool, rigid outer shell of Earth made of the crust plus the uppermost mantle. It is broken into tectonic plates that slide over the hotter, weaker rock beneath.

A hot, partially molten, slowly flowing layer of the upper mantle beneath the lithosphere. Its weak, plastic behavior allows rigid tectonic plates to move across it.

A boundary where two plates move apart, letting hot rock rise and form new crust. Examples include mid-ocean ridges and continental rift valleys such as East Africa's.

A boundary where two plates move toward each other. Ocean crust usually sinks beneath the other plate, while continental collision thickens crust and raises high mountains.

A boundary where two plates slide horizontally past one another. Crust is neither created nor destroyed, and shallow earthquakes are common along the fault line.

The process at mid-ocean ridges where magma rises, cools into new ocean crust, and spreads outward, making the ocean floor grow and recording magnetic reversals in striped patterns.

A place where one tectonic plate descends beneath another into the mantle. It produces deep ocean trenches, volcanic arcs, and powerful earthquakes along the dipping plate.

A convergent setting where two buoyant continental plates meet, so neither subducts easily. Crust crumples and thickens, forming high mountain belts such as the Himalayas.

A long, low trough formed where crust is pulled apart and drops between parallel faults. Continental rifting can eventually widen into a new ocean basin.

A fixed, unusually hot region in the mantle that melts rock and feeds volcanoes at the surface. Chains of volcanoes form as a plate moves over it.

A curved line of volcanoes that forms above a subducting plate, where rising magma reaches the surface. Arcs may sit on ocean crust or along a continental margin.

A mountain-building episode in which tectonic forces fold, fault, and thicken the crust. Orogenies typically accompany collisions or subduction and can last millions of years.

A fracture in rock along which the two sides have moved relative to each other. Movement may be sudden, causing an earthquake, or slow and continuous over long periods.

A fault where the hanging wall moves down relative to the footwall, produced by pulling or extension of the crust. It is common in rift valleys and basin-and-range settings.

Bending of layered rock under compressional stress without breaking. Upward arches are anticlines and downward troughs are synclines, both common in mountain belts.

The balance in which lighter crust floats on denser mantle rock, rising or sinking as weight is added or removed. Erosion and ice sheets can trigger slow vertical adjustments.

The record of Earth's past magnetic field preserved in rocks. Magnetic minerals align when lava cools, revealing ancient field directions and reversals that support plate motion reconstructions.

A naturally occurring, inorganic solid with a definite chemical composition and an ordered internal atomic structure. Quartz, feldspar, and calcite are common examples.

A ranking of minerals from 1 (talc, easily scratched) to 10 (diamond, scratches all others) based on scratch resistance. It helps identify unknown minerals in the field.

The tendency of a mineral to break along flat, smooth planes that follow weaknesses in its atomic structure. Mica splits into thin sheets; quartz instead breaks with curved surfaces.

Rock formed when molten material cools and solidifies. Slow cooling underground makes large crystals, while fast cooling at the surface makes fine-grained or glassy rock.

Rock made from deposited sediment or from chemical precipitation. Layers called strata preserve clues about past environments, and fossils occur mostly in these rocks.

Rock changed by heat, pressure, or chemically active fluids while remaining solid. New minerals and foliation develop, often in bands aligned perpendicular to directed pressure.

A layered or banded texture in metamorphic rock caused by parallel alignment of platy minerals or by separation of light and dark minerals under directed pressure.

The ongoing set of processes that transform one rock type into another through melting, cooling, weathering, erosion, deposition, burial, and metamorphism over geologic time.

The breakdown of rock in place by physical, chemical, or biological processes. Frost wedging and root growth are physical; dissolution and oxidation are chemical examples.

The removal and transport of weathered material by moving agents such as water, wind, ice, or gravity. It shapes valleys, coastlines, and depositional plains.

The laying down of sediment when a transporting agent loses energy and can no longer carry its load. Deposits build deltas, beaches, dunes, and alluvial fans.

The loose surface layer of weathered mineral grains, organic matter, water, and air that supports plant growth. Its character depends on climate, parent material, and time.

The upper surface of the zone where all pore spaces in rock or sediment are filled with water. It rises and falls with rainfall and pumping.

A body of rock or sediment that stores and transmits enough groundwater to supply wells or springs. Sand and gravel layers and fractured limestone make good aquifers.

The fraction of a material's volume that consists of open pore space. High porosity allows storage of water or oil but does not guarantee the material transmits fluids easily.

A large mass of ice that forms on land from compacted snow and moves slowly under its own weight. It erodes bedrock and deposits unsorted debris called till.

A broad bend in a river channel formed as water erodes the outer bank and deposits sediment on the inner bank. Meanders migrate and may be cut off to form oxbow lakes.

Placing rocks and events in chronological order without assigning numerical ages. It uses superposition, cross-cutting relationships, and fossil succession to establish sequences.

The principle that in undisturbed layered rocks, each layer is older than the one above it and younger than the one below. It is fundamental to reading geologic history.

A fossil of a species that lived over a short time span but was geographically widespread, making it useful for matching rock layers of the same age across regions.

The spontaneous change of an unstable isotope into another element at a steady rate. Measuring parent and daughter atoms lets scientists calculate numerical ages of rocks.

The time required for half of a radioactive parent isotope in a sample to decay into its daughter product. Each isotope has a fixed half-life used in radiometric dating.