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Plate Tectonics


Aftershock A smaller earthquake that occurs after a previous large earthquake in the same area.
Ash Dust-sized particles of rock produced by the explosive eruption of some volcanoes. This material may be carried in the air for long distances from the volcano which formed it.
Asthenosphere Part of the earth's mantle that lies below the lithosphere, at depths between about 100 and 350km. The rock here is relatively soft because of its high temperature and relatively low pressure. It is this layer upon which the tectonic plates move.
Batholith A very large mass of igneous intrusive rock (often granite) that forms from cooled magma deep in the earth's crust.
Composite volcano Large, steep-sided, symmetrical cone-shaped volcano formed from alternating layers of lava flows, volcanic ash, cinders, blocks and bombs.
Conservative plate margin A type of plate margin where two tectonic plates are moving past one another with no addition or destruction of plate material.
Constructive plate margin A type of plate margin where new crust is generated as the plates pull away from each other. These are found at mid-oceanic ridges.
Continental crust This is made of old, low density rocks such as granite. It is generally 35-70km thick and mostly over 1500 million years old.
Continental drift A hypothesis, proposed by Alfred Wegener, that today's continents are the result of the break-up of a single supercontinent. The fragments then drifted to their present positions.
Convection current Currents in the mantle that are the driving force in the movement of the tectonic plates. It is thought that they are initiated by hotspots deep in the mantle.
Core The innermost layers of the earth. The core comprises of two concentric spheres. The inner core is believed to be made of solid iron and the outer core, liquid iron.
Crater A circular depression in the ground caused by volcanic activity.
Crust The outermost layer of the earth.
Destructive plate margin A type of plate margin where crust is destroyed as two plates converge. These are usually associated with island arcs or young fold mountains.
Dormant A description of the state of a volcano between eruptions when it gives out very little gas and lava.
Dykes Steep, sheet-like intrusions, varying in thickness from a few millimetres to tens or metres across. They occupy vertical weaknesses in the rock. They often cut across rock bedding and form low ridges.
Epicentre The point on the earth's surface directly above the focus of an earthquake.
Eruption This is said to occur when a volcano gives off large quantities of lava and gas.
Extinct A description of a volcano that has not erupted for at least 25,000 years.
Extrusive activity Volcanic activity that results from magma reaching the surface.
Fault A fracture in the earth's crust that marks the point where two adjacent masses of rock are moving in different directions.
Focus The point below the surface where an earthquake occurs.
Geyser A type of hot spring that erupts periodically, throwing a column of hot water and steam into the air.
Hazard The potential threat to humans from a naturally occurring process or event.
Hot spring A point where heated groundwater emerges onto the earth's surface.
Hotspot An area deep within the mantle where the temperatures are high enough to initiate convection. They are associated with spreading ridges and isolated chains of volcanic islands found away from plate boundaries.
Igneous rock A rock formed from the cooling of magma.
Intrusive activity Igneous activity that results from the movement of magma within the crust.
Island arc A destructive plate boundary where oceanic crust is subducted beneath oceanic crust.
Lahar A mudflow composed of pyroclastic material and water that flows down from a volcano, usually along a river valley.
Lava Molten rock expelled by a volcano during an eruption.
Magma Molten rock that is found beneath the surface of the earth.
Mantle The layer of the earth between the core and the crust.
Moment magnitude scale A scale used to measure the size of earthquakes in terms of the energy released.
Oceanic crust The type of crust that underlies the ocean basins. It is generally between 5km and 10km thick, composed predominantly of basic igneous rock.
Ocean trench A deep depression in the sea floor created at a destructive plate boundary.
Palaeomagnetism A record of the history of the earth's magnetic field, preserved in magnetic minerals in volcanic rocks.
Plate tectonics The theory that states that the earth's crust is made up of several rigid plates moving relative to one another.
Plume A hot column of magma which rises up from deep within the earth.
Rift valley A long, deep valley found in the centre of a spreading ridge. It is formed between parallel faults where a block of the crust has sunk down.
Seafloor spreading The theory that the ocean floor is moving away from the mid-oceanic ridge and across the deep ocean basin, to disappear beneath continents and island arcs.
Seismic waves Shock waves caused by sudden movement along a fault.
Shield volcano A large, low-angled volcano composed of layers of low viscosity basaltic lava.
Sill An igneous intrusion between bedding planes of sedimentary rock layers.
Subduction The process whereby one crustal plate descends below another. This occurs at destructive plate margins.
Surface waves Seismic waves that travel along the surface of the earth. They include Rayleigh waves and Love waves.
Tectonic plates A series of rigid sections of the earth's crust. They float on the upper mantle and move relative to one another.
Tephra Any type of rock fragment that is forcibly ejected from a volcano during an eruption.
Tsunami Sea wave that can be generated by undersea earthquakes, volcanic eruptions and landslides into the sea.
Vent An opening, or rupture, in the earth's surface which allows hot magma, ash and gases to escape from below the surface.
Volcanic bombs Rocks that are more than 5mm in diameter that are thrown into the air by a volcanic eruption.
Volcanic explosive index (VEI) A scale used to measure the explosiveness of volcanoes.
Young fold mountains Mountains formed at a destructive plate margin or collision zone.
Created by: AdeleR