Ch.4 Plate Tectonics

Continental drift

A theory which originally proposed that the continents are rafted about. It has essentially been replaced by the plate tectonics theory.

Supercontinent

A large landmass that contains all, or nearly all, of the existing continents.

Pangea

The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.

Theory of Plate tectonics

A well-tested theory which proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.

Lithosphere

The rigid outer layer of Earth, including the crust and upper mantle.

Asthenosphere

A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers (60 miles) and in some regions extends as deep as 700 kilometers (430 miles). The rock within this zone is easily deformed.

Lithospheric plates or plates

A coherent unit of Earth’s rigid outer layer that includes the crust and upper unit. Also called simply a plate.

Divergent plate boundaries or Spreading centers

A region where the rigid plates are moving apart, typified by the mid-ocean ridges. Also called a spreading center.

Oceanic ridge system

A continuous elevated zone on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometers (300 to 3000 miles). The rifts at the crests of ridges represent divergent plate boundaries.

Rift valley

A long, narrow trough bounded by normal faults. It represents a region where divergence is taking place.

Seafloor spreading

The process of producing new seafloor between two diverging plates.

Continental rift

A theory which originally proposed that the continents are rafted about. It has essentially been replaced by the plate tectonics theory.

Convergent plate boundaries or Subduction zones

A boundary in which two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle. It can also involve the collision of two continental plates to create a mountain system. Also called a convergent plate boundary.

Deep-ocean trenches

An elongated depression in the seafloor produced by bending of oceanic crust during subduction. Also called simply a trench.

Partial melting

The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

Continental volcanic arcs

Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent.

Volcanic island arc or island arc

A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic slab beneath another is occurring. Also called simply an island arc.

Transform plate boundary

A boundary in which two plates slide past one another without creating or destroying lithosphere. Equivalent to a transform fault.

Fracture zone

Any break or rupture in rock along which no appreciable movement has taken place.

Mantle plume

A mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core–mantle boundary.

Hot spot

A concentration of heat in the mantle capable of producing magma, which in turn extrudes onto Earth’s surface. The intraplate volcanism that produced the Hawaiian islands is one example.

Hot-spot track

A chain of volcanic structures produced as a lithospheric plate moves over a mantle plume.

Curie point

The temperature above which a material loses its magnetization.

Paleomagnetism

The natural remnant magnetism in rock bodies; the permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized. Also called fossil magnetism.

Magnetic reversal

A change in Earth’s magnetic field from normal to reverse or vice versa.

Normal polarity

A magnetic field in which the north and south magnetic poles are at the same ends of Earth (relative to the rotational axis) as at present.

Reverse polarity

A magnetic field opposite to that which exists at present.

Magnetic time scale

A scale that shows the ages of magnetic reversals and is based on the polarity of lava flows of various ages.

Magnetometers

A sensitive instrument used to measure the intensity of Earth’s magnetic field at various points.

Transform fault

A major strike-slip fault that cuts through the lithosphere and accommodates motion between two plates. Equivalent to a transform plate boundary.