accretion The addition of material to a plate or area of land.
asthenosphere The weaker layer of Earth’s upper mantle lying directly below the lithosphere and composed of plastic and partially molten rock.
back-arc basin A basin that forms behind an island arc as a result of two converging oceanic plates and the subduction of older and denser oceanic crust.
bomb Volcanic fragments larger than 64 mm (2.5 in) in diameter.
constructive margin Also called divergent margin.
continental rift A separation in Earth’s continental crust caused by two diverging lithospheric plates pushed apart by magma.
convergent margin The boundary created between two tectonic plates moving towards each other.
craton Stable area of continental crust, typically composed of crystalline basement rock.
diastrophism Natural reshaping of Earth’s crust leading to the formation of geological features, such as continents, mountains, or ocean basins.
divergent margin Boundary created between two tectonic plates moving apart where new crust is formed.
fumarole Vent in Earth’s surface that emits steam or volcanic gas.
Gondwana Also called Gondwanaland. Name given by geologist Edward Suess to an ancient supercontinent comprised of modern-day South America, Africa, Arabia, Madagascar, India, Australia, and Antarctica.
island arc Chain of islands formed by volcanic activity near converging oceanic plates.
isostasy State of equilibrium on the lithosphere, wherein forces supporting Earth’s crust balance those that push down on it. Isostasy affects continental and oceanic elevations.
kimberlite Igneous rock created in the asthenosphere that forms vertical pipelike structures in the surrounding rock as it ascends through Earth’s crust.
lithosphere The rigid outer layer of Earth’s surface that includes the crust and is about 100 km (60 miles) thick. It is composed of large and small plates that move and can cause seismic or volcanic activity.
mofette Also called Moffette. Fumarole containing a large quantity of carbon dioxide gas.
Mohorovicic Discontinuity Also called Moho. Border between Earth’s continental or oceanic crust and mantle underneath.
obduction The thrusting of oceanic crust up and onto continental crust during the process of subduction.
oceanic trench Narrow and long depression on the ocean floor, representing the deepest part of the ocean.
orogeny Formation of mountains by way of natural processes.
paleomagnetism The study of Earth’s magnetic field through observation of magnetic rocks and minerals.
Pangea Supercontinent formed from the joining of all of earth’s major landmasses between the early Permian and Jurassic periods, prior to the separation of continents.
polar wandering The movement of magnetic poles over Earth’s surface.
pyroclastic flow A combination of hot gases and glowing rock particles that slide down slopes as a result of volcanic activity.
rheology The study of how matter flows and deforms.
seafloor spreading The divergence of oceanic crust from midocean ridges.
seismic tomography An imaging technique that uses information from earthquakes to form three-dimensional representations of Earth’s interior.
solfatara A fumarole with a large quantity of sulfur.
stratigraphy The study of rock layers, especially in relation to sedimentary and igneous rocks.
subduction The process in which one denser tectonic plate sinks below another plate and into Earth’s mantle as the two plates converge.
supercontinent cycle The concept that a supercontinent fragments into pieces that can then collide and form new supercontinents.
terrane Group or formation of crustal or related rocks.
tillite Glacially deposited rock useful for determining the timing and extent of glacial episodes.
transform fault Boundary between two plates moving past each other laterally without forming or destroying crust.
Wilson Cycle The concept noting that ocean basins opened and closed.