A region of extremely high pressure underlying the lithosphere, where rocks are deformed by enormous stresses. The asthenosphere liesat a depth of about 60-215 mi. (about100-350 km).
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 is associated most often either with seafloor spreading or with the formation of rift valleys. It is one of the three ways, along with convergence and transform motion, that plates interact.
One of two principal forms of tectonism, the other beingorogenesis. Derived from the Greek words epeiros ("mainland") and genesis ("origins"), epeirogenesis takes the form of either uplift or subsidence.
An area of fracturing between rocks resulting from stress.
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.
The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology.
The upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle.
An abbreviation used by earth scientists, meaning "million years." When an event is designated as, for instance, 160 Ma, it means that it happened 160 million years ago.
The thick, dense layer of rock, approximately 1,429 mi. (2,300 km) thick, between Earth's crust and its core.
Sub marine mountain ridges where new seafloor is created by seafloor spreading.
A deep depression in the ocean floor caused by the convergence of plates and the resulting subduction of one plate.
One of two principal forms of tectonism, the other being epeirogenesis. Derived from the Greek words oros ("mountain") and genesis ("origin"), orogenesis involves the formation of mountain ranges by means of folding, faulting, and volcanic activity. The processes of oro-genesis play a major role in plate tectonics.
An area of historical geology devoted to studying the direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks.
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 fashion them. As a theory, 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.
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.
The gathering of data without actual contact with the materials or objects being studied.
A split between two bodies (for example, two plates) that once were joined.
A long trough bounded by two or more faults.
The theory that seafloors crack open along thecrests of mid-ocean ridges and that new seafloor forms in those areas.
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 are no longer perfectly aligned, this is an example of shear.
In general terms, any attempt to deform a solid. Types of stress includetension, compression, and shear. More specifically, stress is the ratio of force to unit area F/A, where F is force and A area.
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.
A depression in Earth's crust.
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.
A general statement derived from a hypothesis that has withstood sufficient testing.
A tectonic process whereby plates slide past eachother. It is one of the three ways, along with convergence and divergence, that plates interact.