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Absolute plate motion: The motion of a plate with respect to hot spots, which appear to be at relatively fixed locations with respect to the Earth's mantle.
Azimuth: A description of direction at a point on the Earth's surface with respect to north, measured in degrees, minutes, and seconds.
B 
Bathymetric map: A map showing sea-floor depths.
C
Caldera: A large, circular depression in a volcanic area formed by eruption or collapse.
Cinder cone: A steep, cone-shaped hill formed around a volcanic vent by the eruption of rock fragments.
Continent-continent collision: A tectonic collision between two large land masses.
Convergent plate boundary: A zone where two tectonic plates are colliding as they move towards each other.
D
Divergent plate boundary: A zone where two tectonic plates are pulling apart as they move away from each other.
E
Earthquake: A sudden movement in the crust of the Earth caused by the rapid release of tectonic strain.
Epicenter: The point on the Earth's surface directly above the focus or hypocenter of an earthquake.
F
Fault: A fracture in the Earth's crust along which relative movement has taken place. Faults are classified into three types based on the sense of fault movement into normal, reverse, or strike-slip faults.
Frames of Reference: A system of locations used to describe the position or movement of an object relative to these locations.
G
Geoid: An equipotential surface, defined by a surface of equal gravitational force, that approximates where sea level would be in the absence of land.
Global plate motion model: A systematic description of tectonic plate movement for the entire planet.
Global Positioning System (GPS): A navigation system, based on a network of satellites, used to measure exact positions on the Earth.
H
Hot spot volcano: A volcano formed by a rising plume of magma that is not located at a plate boundary. A hot spot is considered to be relatively fixed with respect to the Earth's deep mantle.
I
Intraplate earthquake: An earthquake that occurs within a plate, as opposed to those occurring at a plate boundary.
Intraplate volcano: A volcano located within a plate, as opposed to one located at a plate boundary.
L
Latitude: A measurement of position in the north-south direction relative to the Earth's equator. The equator has a latitude of 0 degrees. The north pole has a latitude of 90 degrees north and the south pole has a latitude of 90 degrees south.
Lava: Magma or molten rock that has reached the Earth's surface.
Lithosphere: The outer, rigid layer of the Earth, located above the asthenosphere. It includes the crust and the uppermost mantle.
Longitude: A measurement of position in the east-west direction relative to the prime meridian, which runs through Greenwich, England.
M
Magma: Molten rock that cools to form igneous rocks.
Mantle: The layer of the Earth between the crust and the core, form a depth of about 40 to 2900 kilometers, composed of dense silicate rock.
Mid-ocean ridge: A submarine mountain chain located along the divergent plate boundaries in the ocean. New oceanic crust forms as magma cools and solidifies at the ridge.
Mohorovicic discontinuity: The boundary between the crust and mantle.
N
No-Net-Rotation: A reference frame describing the "average" velocity for the movement of the global system of tectonic plates.
Normal fault: A fault along which the hanging wall appears to move down relative to the footwall, caused by extensional stresses.
"NUVEL-1A" plate model: A commonly used model for predicting relative motion for any point on the Earth's surface, developed by Charles DeMets and colleagues at Northwestern University.
Physiography: The shape of the land surface.
Plate: A piece of the Earth's lithosphere which is internally rigid and moves independently over the asthenosphere.
Plate boundary: The edge between adjacent plates, classified by the relative motion taking place: convergent, divergent, or transform.
Plate tectonics: The unifying theory of geology, which hypothesizes that the Earth is broken into a mosaic of rigid lithospheric plates which move across the Earth's surface. The theory has helped to explain much in global-scale geology, including the formation of mountains and the distribution of earthquakes and volcanoes.
P-wave: The primary, or fastest traveling wave moving away from a seismic event, characterized by compressional vibration.
R
Reference frame: A system of locations used to describe the position or movement of an object relative to these locations.
'Relative' plate motion: The relative movement that takes place along an individual plate boundary. Controls the amount and type of earthquake activity, the presence and style of volcanism, and the geologic processes that take place along the plate boundary and in neighboring areas of the adjacent plates.
Reverse fault: A fault along which the hanging wall appears to move up relative to the footwall, caused by compressional stresses.
S
Sea-floor spreading: The concept that new oceanic crust is created at the mid-ocean ridge as a result of divergence between plates.
Satellite geodesy: High-precision measurements of the size and shape of the Earth, and surveying of precise locations on the Earth's surface, using satellite systems such as GPS.
Satellite image: A picture of the Earth's surface taken from a satellite.
Seismic activity: The distribution of earthquakes about the Earth's surface.
Seismology: The study of earthquakes, seismic waves, and their propagation through the Earth.
Shield volcano: A large, broad volcanic cone with gentle slopes.
Strike-slip fault: A fault along which movement takes place parallel to the trace, or strike, of the fault.
Subduction zone: A convergent plate boundary where a dense oceanic plate plunges into Earth's interior.
S-wave: The secondary wave, traveling slower than the P- wave, and consisting of a seismic vibration transverse to the direction of travel.
T
Topography: The pattern of elevation of the land surface.
Transform plate boundary: An edge between two plates with motion parallel to the boundary.
V
Vector addition and subtraction: A calculation based on combining measurements of magnitude and direction of individual vectors.
Vector: A physical quantity characterized by measurement of both magnitude and direction.
Volcanic ash: A volcanic sediment less than 4 mm in diameter, which is usually made of glass.
Volcanic crater: A well-defined circular depression formed by the explosive eruption of volcanic rock.
Volcano: A vent in the crust where magmas, gas, and ash erupt, and the structure formed by the eruption.