Sea-Floor Spreading and Plate Tectonics

This set of vocabulary flashcards covers the fundamental concepts, boundaries, and geological processes of plate tectonics and sea-floor spreading as described in the lecture notes.

Alfred Wegener

The German meteorologist who in 1912 argued that if continents could move vertically, horizontal motion or continental drift was also possible.

Sea-floor spreading

A process first proposed in 1960 whereby new sea floor is created as adjacent crust is moved apart to make room.

Plate tectonics

A unifying model of crustal development that joined the theories of continental drift and sea-floor spreading, supported by geophysical evidence.

Lithosphere

The coherent rigid outer shell of the earth (crust and upper mantle) approximately $100-150\,km$ thick.

Divergent boundaries

Also called spreading centers, these are sites where plates separate and magma rises to form new ocean crust, such as the Mid-Atlantic Ridge.

Convergent boundaries

Also known as subduction zones, these are sites where plates move together and old crust is destroyed as one plate plunges under another.

Transform faults

Boundaries where plates slip horizontally past one another with no crustal creation or destruction, often expressed as fracture zones.

Paleomagnetism

The force exerted by the earth's magnetic field preserved in rocks, used as critical evidence for sea-floor spreading.

Polarity epoch

A major reversal of the earth's magnetic field occurring approximately every half million years.

Events

Shorter magnetic reversals recorded within longer epochs, lasting from tens to hundreds of thousand years.

Positive anomaly

A stronger than average magnetic field found where rocks have the same magnetic polarity as the present-day field.

Negative anomaly

A weaker than average magnetic field measured where rocks preserve reverse polarity.

Hot spots

Fixed plumes of rising lava originating in the mantle that form chains of progressively older volcanoes as a lithospheric plate moves over them.

Exotic terrane

Also called a tectonostratigraphic terrane, it is a fault-bounded block of rock with a geological history different from adjacent rocks.

Miniplates

Also called microplates, these are blocks of continental rock floating on oceanic crust and transported with the oceanic plate.

Magma

Molten rock material within the earth; it is referred to as lava once it reaches the surface.

Wrangellia

An exotic terrane that was an island during Triassic time with a paleomagnetic signature indicating it formed $16^{\circ}$ from the equator.

Spreading center

A mid-ocean rise or ridge, expressed topographically, where molten basalt rises to create new sea floor.

Subduction zone

An elongate region where one crustal plate descends below another, characterized by trenches and deep-focus earthquakes.

San Andreas Fault

The transform fault boundary in California between the Pacific Plate and the North American Plate.