Section 2: The Theory of Plate Tectonics Preview Objectives How Continents Move Tectonic Plates Types of Plate Boundaries Causes of Plate Motion Plate Tectonics

Objectives Summarize the theory of plate tectonics. Identify and describe the three types of plate boundaries. List and describe three causes of plate movement.

How Continents Move plate tectonics the theory that explains how large pieces of the lithospehere, called plates, move and change shape. lithosphere the solid, outer layer of Earth that consists of the crust and the rigid upper part of the mantle. asthenosphere the solid, plastic layer of the mantle beneath the lithosphere; made of mantle rock that flows very slowly, which allows tectonic plates to move on top of it.

How Continents Move, continued The lithosphere forms the thin outer shell of Earth and is broken into several blocks or tectonic plates. The tectonic plates ride on the asthenoshpere in much the same way that blocks of wood float on water. Tectonic plates can include continental crust, oceanic crust, or both. Continents and oceans are carried along on the moving tectonic plates in the same way that passengers are carried by a bus.

Tectonic Plates Scientists have identified about 15 major tectonic plates. Scientists identify plate boundaries primarily by studying data from earthquakes. The locations of volcanoes can also help identify the locations of plate boundaries.

Tectonic Plates, continued The boundaries of tectonic plates do not always match the outlines of continents.

Tectonic Plates, continued Reading Check How do scientists identify locations of plate boundaries? Scientists use the locations of earthquakes, volcanoes, trenches, and mid-ocean ridges to outline tectonic plates.

Types of Plate Boundaries Tectonic plate boundaries may be in the middle of the ocean floor, around the edges of continents, or even within continents. The three types of plate boundaries are divergent boundaries, convergent boundaries, and transform boundaries. Each plate boundary is associated with a characteristic type of geologic activity.

Types of Plate Boundaries, continued

Types of Plate Boundaries, continued

Types of Plate Boundaries, continued

Causes of Plate Motion Many scientists think that the movement of tectonic plates is partly due to convection. Convection is the movement of heated material due to differences in density that are caused by differences in temperatures.

Causes of Plate Motion, continued The convection process can be modeled by boiling water in a pot on the stove. As the water at the bottom of the pot is heated, the water at the bottom expands and becomes less dense than the cooler water above it. The cooler, denser water sinks, and the warmer water rises to the surface to create a cycle called a convection cell.

Causes of Plate Motion, continued Mantle Convection Scientists think that tectonic plates are part of a convection system. Energy generated by Earth’s core and radioactivity within the mantle heat the mantle. This heated material rises through the cooler, denser material around it.

Causes of Plate Motion, continued Mantle Convection As the hot material rises, the cooler, denser material flows away from the hot material and sinks into the mantle to replace the rising material. As the mantle material moves, it drags the overlying tectonic plates along with it.

Ridge Push As the cooling rock sinks, the asthenosphere below it exerts force on the rest of the plate called ridge push. This force pushes the rest of the plate away from the mid-ocean ridge.

D. Slab Pull When the leading edge of a plate near a mid ocean ridge cools it becomes denser and eventually begins to sink back into the asthenosphere. As this leading edge is subducted (sinking), it pulls the rest of the plate behind it downward into the mantle exerting a force called slab pull.

Causes of Plate Motion, continued

Plate Tectonics

Types of Plate Boundaries A. General Info Tectonic plate boundaries may be in the middle of the ocean floor, around the edges of continents, or even within continents, and each plate boundary is associated with a characteristic type of geologic activity. ･ The plates move about 3-15 cm/year and where they meet one of three types of boundaries or margins are formed: Convergent boundaries, Divergent boundaries & Transform/Lateral boundaries.

Types of Plate Boundaries Convergent Margins- occur when plates move toward each other. At the plate boundaries, they may produce 2 types of zones: Type 1: Subduction Zone - one plate may be forced under the other into the mantle and old crust is destroyed as it is recycled into the mantle. As the subducted plate is forced back into the mantle, it often melts and finds its way back up to the surface through cracks in the crust, causing volcanoes to form. Type 2: Folding/Rift Zone- both plates may be forced upward causing folding at weak points in the plate somewhere behind the boundaries. As the crust crumples and is forced upwards it often forms mountains.

Types of Plate Boundaries

Types of Plate Boundaries Divergent Margins- when plates move away from each other & form a spreading zone where magma escapes from the mantle and forms new crust. Volcanoes are also often found at these boundaries where magma escapes the mantle. Transform/Lateral Margins- when plates simply move past each other, no crust is created or destroyed at these boundaries. These margins form fracture/fault zones where earthquakes often occur due to the pressure that builds up along the fault.

Types of Plate Boundaries

Types of Plate Boundaries