MTEL Earth and Space Science (74) key terms

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

A place where two tectonic plates move apart, allowing hot mantle rock to rise and solidify. It forms mid-ocean ridges under oceans and rift valleys on continents, with shallow earthquakes and basaltic volcanism.

A place where two plates move toward each other. Oceanic crust sinks beneath a less dense plate, or two continents collide, producing deep earthquakes, volcanic arcs, trenches, and folded mountain ranges.

A boundary where two plates slide horizontally past each other without crust being created or destroyed. Stress builds until it releases as shallow earthquakes along faults such as the San Andreas Fault.

A convergent setting where one lithospheric plate descends beneath another into the mantle. It generates deep ocean trenches, powerful earthquakes, and chains of volcanoes fed by melted fluids rising from the sinking slab.

The process at mid-ocean ridges where new oceanic crust forms from upwelling magma and moves away from the ridge on both sides, recorded by symmetric magnetic stripes and increasing seafloor age with distance.

The record of Earth's past magnetic field preserved in magnetic minerals of rocks. Alternating normal and reversed stripes on the seafloor provided key evidence that new crust forms and spreads at ocean ridges.

A flip in the orientation of Earth's magnetic field, so that magnetic north and south swap. These flips occur irregularly over hundreds of thousands of years and are recorded as striped patterns in cooling lavas and seafloor rocks.

A curved chain of volcanic islands formed above a subducting oceanic plate, parallel to and landward of an ocean trench. Examples include the Aleutian and Mariana islands, built from andesitic magma.

A long, narrow, very deep depression in the ocean floor marking where a lithospheric plate bends downward into a subduction zone. Trenches host the deepest ocean waters and strong earthquakes.

A fracture in rock along which movement has occurred. Faults are classified as normal, reverse, or strike-slip depending on the direction of slip, and they are the sources of most earthquakes.

The point inside Earth where an earthquake rupture begins and seismic energy is released. Its depth below the surface distinguishes shallow, intermediate, and deep earthquakes and helps identify tectonic settings.

The point on Earth's surface directly above an earthquake's focus. It is located by comparing arrival times of seismic waves at several stations and is usually where shaking damage is greatest.

An elastic vibration traveling through Earth from an earthquake or explosion. P waves compress rock, S waves shear it, and surface waves travel along the surface, each with characteristic speeds and effects.

A primary, compressional seismic wave that pushes and pulls rock in the direction of travel. It is the fastest seismic wave, passes through solids, liquids, and gases, and arrives first at a station.

A secondary, shear seismic wave that shakes rock perpendicular to its direction of travel. It travels slower than a P wave and cannot pass through liquids, which revealed Earth's liquid outer core.

The boundary, commonly called the Moho, between Earth's crust and the mantle beneath it. Seismic waves speed up sharply there because the mantle rock is denser and more rigid than crustal rock.

The rigid outer layer of Earth made of the crust plus the uppermost mantle. It is broken into tectonic plates that move over the hotter, weaker asthenosphere below.

A hot, partially molten, weak layer of the upper mantle beneath the lithosphere. It flows slowly and allows the rigid tectonic plates above it to slide and shift over geologic time.

The thick layer of silicate rock between Earth's crust and core, making up most of Earth's volume. Slow convection in the mantle drives plate motion and feeds magma to volcanoes.

Earth's thin, outermost rocky layer, consisting of denser oceanic crust rich in basalt and thicker, lighter continental crust rich in granite. It is the layer people live on and mine for resources.

The ongoing set of processes that transform rock among igneous, sedimentary, and metamorphic types through melting, cooling, weathering, erosion, deposition, burial, and heat and pressure.

Rock formed when molten material cools and solidifies. Slow cooling underground makes coarse-grained intrusive rock such as granite; fast cooling at the surface makes fine-grained extrusive rock such as basalt.

Rock made from weathered fragments, dissolved minerals, or organic remains that are deposited, compacted, and cemented. Layers called strata and embedded fossils make these rocks key records of Earth history.

Rock changed in the solid state by heat, pressure, or chemically active fluids, often developing foliation or new minerals. Examples include slate from shale and marble from limestone.

A layered or banded texture in metamorphic rock produced when platy minerals align perpendicular to directed pressure. It is a visible sign of the stress the rock experienced during metamorphism.

A dark, fine-grained, mafic igneous rock rich in iron and magnesium. It forms most of the ocean floor and erupts in fluid lava flows at mid-ocean ridges and shield volcanoes.

A coarse-grained, felsic intrusive igneous rock dominated by quartz, feldspar, and mica. It forms from slowly cooled magma deep underground and is typical of continental crust and mountain cores.

A broad, gently sloping volcano built by many fluid basaltic lava flows. Low viscosity lets lava travel far, producing the largest volcanoes on Earth, such as those in Hawaii.

A steep, cone-shaped volcano built of alternating layers of viscous lava and ash. It forms above subduction zones and can erupt explosively, as Mount St. Helens and Mount Fuji did.

The breakdown of rock in place by physical, chemical, or biological processes. It produces sediment and soil without transporting material away, unlike erosion.

The removal and transport of weathered rock and soil by moving water, wind, ice, or gravity. It shapes landforms such as valleys and deltas and deposits sediment elsewhere.

The dropping of eroded sediment when the transporting agent loses energy, as when a river slows at its mouth or a glacier melts. Deposited material may later become sedimentary rock.

The principle that in undisturbed sedimentary rock layers, each layer is older than the one above it and younger than the one below. It is fundamental for relative dating of rocks and fossils.

Determining the order of past events without assigning numeric ages, using principles such as superposition, cross-cutting relationships, and fossil succession. It establishes which rocks or events came first.

Estimating the absolute age of a rock or mineral from the ratio of a radioactive parent isotope to its decay product, using the isotope's known half-life. It provides numeric ages for Earth materials.

The time required for half of the atoms of a radioactive isotope in a sample to decay. Each isotope has a fixed half-life, so measuring remaining parent atoms gives a clock for dating rocks.

Preserved remains or traces of ancient organisms, usually found in sedimentary rock. Fossils document evolution, allow correlation of rock layers, and indicate past environments and climates.