The maximum displacement of a vibrating material, or the "size" of a wave from crest to trough.
Waves whose line of propagation is through the body of a medium. These include P-waves (primary waves), which move extremely fast and arelongitudinal, and S-waves (secondary waves), which are move somewhat less fast and are transverse. Compare with surface waves.
A form of stress produced by the action of equal and opposite forces, the effect of which is to reduce the length of a material. Compression is a form of pressure.
The theory that the configuration of Earth's continents was once different than it is today, that some of the individual landmasses of today once were joined in other continental forms, and that these landmasses later separated and moved to their present locations.
A tectonic process whereby plates move toward each other. Usually associated with subduction, convergence typically occurs in the ocean, creating an oceanic trench. It is one of the three ways, along with divergence and transform motion, that plates interact.
The center of Earth, an area constituting about 16% of the planet's volume and 32% of its mass. Made primarily of iron and another, lighter element (possibly sulfur), it is divided between a solid inner core with a radius of about 760 mi. (1,220 km) and a liquid outer core about1,750 mi. (2,820 km) thick.
The uppermost division of the solid earth, representing less than 1% of its volume and varying in depth from 3-37 mi. (5-60 km). Below the crust is the mantle.
A tectonic process whereby plates move away from each other. Divergence results in the separation of plates and most often is associated either with seafloor spreading or the formation of rift valleys. It is one of the three ways, along with convergence and transform motion, that plates interact.
The response of solids to stress.
The point on Earth's surface directly above the hypocenter, or the focal point from which an earthquake originates.
An area of fracturing, as a result of stress, between rocks.
An area of rock that has been bent by stress.
A branch of the earth sciences that combines aspects of geology and physics. Geophysics addresses the planet's physical processes as well as its gravitational, magnetic, and electric properties and the means by which energy is transmitted through its interior.
Internal thermal energy that flows from one body of matter to another.
A region of high volcanic activity.
Where earthquakes areconcerned, intensity refers to the amount of damage to humans and buildings. Subjective and qualitative (as opposed to magnitude, which is objective and quantitative), intensity is measured by the Mercalliscale.
The energy that an object possesses by virtue of its motion.
The upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle.
A wave in which the movement of vibration is in the same direction as the wave itself. This is contrasted with a transverse wave.
See surface waves.
Where earthquakes areconcerned, magnitude refers to the amount of energy released by the quake as well as the amplitude of the seismic waves. Objective and quantitative (as opposed to intensity, which is subjective and qualitative), magnitude is measured by the Richter scale.
The thick, dense layer of rock, approximately 1,429 mi. (2,300 km) thick, between Earth's crust and its core. In reference to the other terrestrial planets, mantle simply means the area of dense rock between the crust and core.
See intensity.
Boundaries between plates.
The name both of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that shape them. As atheory, it explains the processes that have shaped Earth in terms of plates and their movement. Plate tectonics theory brings together aspects of continental drift, seafloor spreading, seismic and volcanic activity, and the structures of Earth's crust to provide a unifying model of Earth's evolution. It is one of the dominant concepts in the modern earth sciences.
Large, movable segments of the lithosphere.
The act or state of traveling from one place to another.
See body waves.
A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei); beta particles (either electrons or subatomic particles called positrons ); or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.
See surface waves.
See magnitude.
A packet of energy resulting from the disturbance that accompanies a strain on rocks in the lithosphere.
An instrument designed to record information regarding seismic waves.
The study of seismic waves as well as the movements and vibrations that produce them.
An instrument for detecting seismic waves.
A form of stress resulting from equal and opposite forces that do not act along the same line. If a thick, hard-bound book is lying flat and one pushes the front cover from the side so that the covers and pages no longer constitute parallel planes, this is an example of shear.
The ratio between the change in dimension experienced by an object that has been subjected to stress and the original dimensions of the object.
In general terms, any attempt to deform a solid. Types of stress includetension, compression, and shear.
A tectonic process thatresults when plates converge, and one plate forces the other down into Earth's mantle. As a result, the subducted plate eventually undergoes partial melting.
Seismic waves whose line of propagation is along the surface of a medium such as the solid earth. These waves tend to be slower and more destructive than body waves. Examples include Rayleigh waves (waves with both transverse and longitudinal characteristics) and Love waves (purely longitudinal). Compare with body waves.
See body waves.
The study of tectonism, including its causes and effects, most notably mountain building.
The deformation of the lithosphere.
A form of stress produced by a force that acts to stretch a material.
Heat energy, a form of kinetic energy produced by the motion of atomic or molecular particles in relation to one another. The greater the relative motion of these particles, the greater the thermal energy.
A tectonic process whereby plates slide past eachother. It is one of the three ways, along with convergence and divergence, that plates interact.
A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving. Compare with longitudinal wave.
A tidal wave produced by an earthquake or volcanic eruption. The term comes from the Japanese words for "harbor" and "wave."
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