ELAJ+1-5

toc

Back To Home Page

=The Theory of Plate Tectonics=

Vocabulary
driven by convection currents in the mantle.
 * Plates**: The lithosphere is broken into separate sections called plates.
 * Scientific**: Theory: A well-tested concept that explains wide range of observations.
 * Plate Tectonics**: The geological theory that states that pieces of Earth's lithosphere are in constant, slow motion,
 * Faults**: Breaks in Earth's crust where rocks have slipped past each other-form along these boundaries.
 * Transform boundary**: A place where two plates slip past each other, moving in opposite directions.
 * Divergent boundary**: The place where two plates move apart, or diverge.
 * Rift valley**: A deep valley that forms along the divergent boundary.
 * convergent boundary**: The plates where two plates come together, or converge.

A Theory of Plate Motion

 * How can Earth's plates move?
 * Answer: The plates of the lithosphere float on top of the astenoshpere. Convection currents rise in the asthenosphere and spread out beneath the lithosphere.
 * No plate can budge without affecting the order plates surrounding it.

Plate boundaries

 * The edges of different pieces of the lithosphere-Earth's rigid shell-meet at lines called plate boundaries.
 * There are 3 kinda of plate boundaries: transform boundaries, divergent boundaries, and convergent boundaries.
 * For each type of boundary, there is a different type of plate movement.

Transform Boundaries

 * Along transform boundaries crust is neither created nor destroyed.
 * Earthquakes occur frequently along these boundaries.

Divergent Boundaries

 * Most divergent boundaries occur at the mid-ocean ridge.
 * Divergent boundaries also occur on land.
 * When a divergent boundary forms on land, two of Earth's plates slide apart.
 * A deep vally called a rift valley forms along the divergent
 * Example: the Great Rift Valley in east Africa marks a deep crack in the African continent that runs for about 3,000 kilometers.
 * The rift may someday split the eastern part of Africa away from the rest on the continent.
 * As a rift valley widens, its floor drops, and eventually the floor may drop enough for the sea to fill the widening gap.

Convergent Boundaries

 * When two plates converge, the result is called a collision
 * Collisions may bring together oceanic crust and oceanic crust, oceanic crust and continental crust, or continental crust and continental crust.
 * When two plates collide, the density of the plates determines which one comes out on top.
 * Oceanic crust which is made mostly out of basalt is more dense than continental crust, which is made mostly out of granite,
 * Oceanic crust becomes cooler and denser as it spreads away from the mid-ocean ridge
 * Sometimes a plate carrying the oceanic crust collides with a plate carrying continental crust. The less dense continental crust can't sink under the more dense ocean crust
 * Instead, the oceanic plate begins to sin and plunges beneath the continental plate.
 * Both continental plates are mostly low-density the mantle.
 * Instead, the plates crash head-on.
 * The collision squeezes the crust into mighty mountain ranges

The Continents' Slow Dance

 * The plates amazingly slow rates: from about one to ten centimeters per year!
 * The North American and the Eurasian plates are floating apart at a rate of 1.5 centimeters per year
 * This doesn't seem much, but these plates have been moving for tens of millions of years.
 * About 260 million years ago, the continents were joined together forming a super continent called pangaea.
 * Then about 225 million years ago, Pangaea began to break apart.