The BIG Idea The scientific theory of plate tectonics states that Earth’s lithosphere is broken up into rigid plates that move over Earth’s surface.

The Plate Tectonics Theory The theory of plate tectonics, proposed in the late 1960s, states that Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other. Plate tectonics suggests that Earth’s surface is divided into large plates of rigid rock and each plate moves over Earth’s hot and semiplastic mantle.

The Plate Tectonics Theory (cont.) Geologists use the word tectonic to describe the forces that shape Earth’s surface and the rock structures that form as a result. The cold and rigid outermost rock layer of the Earth is called the lithosphere. The lithosphere is made up of the crust and the solid, uppermost mantle.

Earth’s Tectonic Plates

The Plate Tectonics Theory (cont.) The lithosphere is thin below mid-ocean ridges and thick below continents. Earth’s tectonic plates are large pieces of the lithosphere that fit together like the pieces of a giant jigsaw puzzle. The layer of Earth below the lithosphere, called the asthenosphere, is so hot that it behaves like a plastic material and enables Earth’s plates to move.

Evidence for Plate Tectonics Scientists can measure how fast continents move using a network of satellites called the Global Positioning System. Because plates are rigid, tectonic activity occurs where plates meet.

Evidence for Plate Tectonics (cont.) Volcanoes form where plates separate along a mid-ocean ridge or continental rift or collide along a subduction zone. Mountains can form where two continents collide.

Notice the relationship between earthquake epicenters, volcanoes, and plate boundaries.

Plate Motion Earth’s plates move because the asthenosphere moves underneath the lithosphere. Convection is the circulation of material caused by differences in density. Hot mantle material rises upward and comes in contact with Earth’s crust.

Plate Motion (cont.) As the mantle cools, it becomes denser and then sinks, forming a convection current. Convection currents in the asthenosphere act like a conveyor belt moving the lithosphere above it. There are three forces that interact to cause plate motion: basal drag, ridge push, and slab pull.

Plate Motion (cont.) Basal drag refers to how convection currents in the asthenosphere circulate and drag the lithosphere like a conveyor belt. Rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge with a force called ridge push.

As a slab, or dense plate, sinks, it pulls on the rest of the plate with a force called slab pull.

A Theory in Progress Plate tectonics has become the unifying theory of geology, but several unanswered questions remain. Why is Earth the only planet in the solar system that has plate tectonic activity? Why do some earthquakes and volcanoes occur far away from plate boundaries?

What forces dominate plate motion? What will scientists study next? This is a 3-D image of seismic wave velocities from a new technique called anisotropy.

Plate Boundaries A divergent plate boundary forms where two plates separate. When the seafloor spreads at a mid-ocean ridge, lava erupts, cools, and forms new oceanic crust. Divergent plate boundaries can also exist in the middle of a continent, where they pull continents apart and form rift valleys.

Plate Boundaries (cont.) A transform plate boundary forms where two plates slide past each other. As they move past one another, the plates can get stuck and stop moving. Stress builds up where the plates are stuck until they eventually break and suddenly move apart, resulting in a rapid release of energy as earthquakes.

Plate Boundaries (cont.) Convergent plate boundaries form where two plates collide. The denser plate sinks below the more buoyant plate in a process called subduction. The area where a denser plate descends into Earth along a convergent plate boundary is called a subduction zone.

Plate Boundaries (cont.) When an oceanic plate and a continental plate collide, the denser oceanic plate subducts under the edge of the continent, creating a deep ocean trench and a line of volcanoes above the subducting plate on the edge of the continent. When two continental plates collide, neither plate is subducted, and mountains form from uplifted rock.

Lesson 3: The Theory of Plate Tectonics Types of plate boundaries, the location of earthquakes, volcanoes, and mountain ranges, and satellite measurement of plate motion support the theory of plate tectonics. Mantle convection, ridge push, and slab pull are the forces that cause plate motion. Radioactivity in the mantle and thermal energy from the core produce the energy for convection.