Chapter One: Plate Tectonics Section 5: The Theory of Plate Tectonics Pages 42-47

A Theory of Plate Motion Combine what you know about continental drift and sea-floor spreading into one single theory… the theory of plate tectonics.

Theory of Plate Tectonics Plate tectonics is the geological theory that states that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle. This theory explains the formation, movement, and subduction of Earth’s plates.

Bumper Cars No plate can move without affecting the other plates surrounding it. As the plates move, they collide, pull apart, or grind past each other, producing spectacular changes in Earth’s surface. These changes include volcanoes, mountain ranges, and deep-sea trenches.

Earth’s Lithospheric Plates (43)

Plate Boundaries The edges of different pieces of the lithosphere meet at lines called plate boundaries. Faults form along these boundaries. Faults are breaks in Earth’s crust where rocks have slipped past each other.

Transform Boundary Crust is neither created nor destroyed. Two plates slide past one another, moving in opposite directions. Earthquakes occur frequently along these boundaries. Ex. San Andreas Fault

Divergent Boundary Where two plates move apart or diverge. Most occur at the mid ocean ridge (sea-floor spreading). When they occur on land, a rift valley forms along the boundary.

Rift Valley A rift valley is a deep valley that forms when two plates move apart. In east Africa there is a deep crack about 3,000 km long called the Great Rift Valley. As a rift valley widens, its floor drops. Eventually, the floor may drop enough for the sea to fill the gap.

Convergent Boundary Place where two plates come together, or converge; causes a collision. Collisions may bring together the different types of crust: oceanic/continental (O-C) oceanic/oceanic (O-O) continental/continental (C-C)

Converging O-C When oceanic and continental plates collide, the oceanic plate is subducted. Oceanic is the denser plate so it sinks beneath the continental plate.

Converging O-O When two oceanic plates collide, one plate is subducted through a trench. The colder, older plate will subduct.

Converging C-C Where two continental plates collide, the crust buckles and mountain ranges form. NO subduction takes place; neither plate is dense enough to sink into the mantle.

Slow Moving Continents The plates move at slow rates, about 1-10 centimeters per year; as fast as your fingernails grow. The North American and Eurasian plates are floating apart at a rate of 2.5 cm per year.

Words Worth Knowing (ISN #44) Plate Plate tectonics Fault Transform boundary Divergent boundary Rift valley Convergent boundary