Section 11-3 What is the theory of plate tectonics? Describe the lithosphere and the asthenosphere. Review the difference between divergent and convergent plate boundaries. Explain the three types of convergent plate boundaries and define subduction zone. What is a transform fault boundary? Define convection current. What are the causes and effects of plate tectonics?

Plate tectonics is the theory that Earth’s crust and upper mantle are broken into sections called plates, that slowly move around on the mantle. The lithosphere is the rigid, outermost layer of Earth composed of the crust and upper mantle. It is about 100 kilometers thick. The asthenosphere is the plastic-like layer below the lithosphere in Earth’s mantle. The less dense lithosphere floats around on the denser asthenosphere.

Plate Boundaries Divergent boundaries occur when two plates are diverging, or moving away from each other. Rifts and valleys can form. Convergent boundaries occur when two plates are converging, or moving toward each other. There are three types of convergent boundaries.

The first type of convergent boundary occurs when an ocean-floor plate collides with a continental plate and the denser ocean plate sinks under the less dense continental plate. This is known as a subduction zone, an area where an ocean plate descends into the upper mantle. Volcanoes occur at subduction zones.

The second type of convergent plate occurs when two ocean plates collide. This also causes a subduction zone.

The third type of convergent plate occurs when two continental plates collide. No subduction zone usually occurs here. The collision forms mountain ranges or causes earthquakes.

A transform fault boundary occurs between two plates that are sliding past one another horizontally in opposite directions, or in the same direction but at different speeds. The San Andreas fault is a transform fault boundary. Earthquakes occur at this fault boundary.

Convection currents cause plate tectonic movement. A convection current is the cycle of heating, rising, cooling, and sinking of matter.

The formation of different landforms is the effect of plate tectonics. Diverging boundaries result in fault-block mountains, rift valleys, and mid-ocean ridges. Converging boundaries result in folded mountains, island arcs, volcanoes, and earthquakes. Transform boundaries result in earthquakes from strike-slip faults.

Rift Valley – a lowland created by the action of a rift