ES 9.3 Theory of Plate Tectonics

Earth’s Moving Plates

9.3 Plate Tectonics

During the 1960s, scientists realized that sea-floor spreading explained part of Wegener's idea of continental drift.

A Canadian geologist named J. Tuzo Wilson and other scientists came up with a new theory that would lead to a revolution in geology: plate tectonics.

Earth's Moving Plates

9.3 Plate Tectonics

Wilson suggested that the lithosphere is broken into several huge pieces called plates.

◆ In the theory of plate tectonics, Earth's lithospheric plates move slowly relatively to each other, driven by convection currents in the mantle.

Earth's Moving Plates

9.3 Plate Tectonics

Wilson had answered the question that Wegener had failed to: "What causes the lithospheric plates to move?"

Convection currents within the Earth drive plate motion. Hot material deep in the mantle moves upward by convection. At the same time, cooler denser slabs of lithosphere sink into the mantle.

Earth's Moving Plates

9.3 Plate Tectonics

Lithospheric plates move, on average, about 5 centimeters per year - about as fast as your fingernails grow.

The results of plate motion include earthquakes, volcanoes, and mountain building.

Types of Plate Boundaries

9.3 Plate Tectonics

Divergent boundaries are found where two plates move apart.

New ocean floor is created at divergent boundaries.

Divergent Boundary

9.3 Plate Tectonics

Divergent Boundaries

9.3 Plate Tectonics

◆ Most divergent boundaries are spreading centers located along the crests of mid-ocean ridges. Some spreading centers, however, are located on the continents.

Divergent plate boundaries are constructive because this is where new lithosphere is produced.

Spreading Center

Types of Plate Boundaries

9.3 Plate Tectonics

Convergent boundaries form where two plates move together.

Lithosphere can be destroyed at convergent boundaries.

Types of Plate Boundaries

9.3 Plate Tectonics

◆ At convergent boundaries, plates collide and interact - producing features including trenches, volcanoes, and mountain ranges.

9.3 Plate Tectonics

Convergent Boundaries

9.3 Plate Tectonics

Oceanic-Continental
When these plates collide, the more dense oceanic lithosphere sinks beneath the less dense continental lithosphere.

The oceanic plate melts and the newly formed magma rises to the surface and forms a continental volcanic arc.

9.3 Plate Tectonics

Convergent Boundaries

9.3 Plate Tectonics

Oceanic-Oceanic
When two oceanic plates converge, one is pushed beneath the other. This causes volcanic activity.

The plate that is pushed into the asthenosphere melts, forming volcanoes on the ocean floor. This activity eventually forms a chain of volcanic islands called a volcanic island arc.

9.3 Plate Tectonics

Convergent Boundaries

9.3 Plate Tectonics

Continental-Continental
When two continental plates collide, mountains are formed because neither plate is subducted. The plates simply collide and fold together.

This type of plate collision formed the Himalayas.

Collision of India and Asia

Types of Plate Boundaries

9.3 Plate Tectonics

Transform fault boundaries occur where two plates grind past each other.

Along transform boundaries, lithosphere is neither created nor destroyed.

Transform Fault Boundary

9.3 Plate Tectonics

Transform Fault Boundaries

9.3 Plate Tectonics

◆ At a transform fault boundary, plates grind past each other without destroying the lithosphere.

Although most transform faults are located in oceans, a few cut through the continents. One example is the San Andreas fault in California.

Transform Fault Boundaries

9.3 Plate Tectonics

Earthquakes are commonly triggered by movements along transform fault boundaries.

Three Types of Plate Boundaries

Review Key Concepts

9.2 Plate Tectonics

1. In your own words, explain the theory of plate tectonics.

2. List the three types of plate boundaries.

3. Why is a divergent boundary considered a constructive plate margin?
   
   

Review Key Terms

9.2 Plate Tectonics

• plate

• plate tectonics

• divergent boundary

• convergent boundary

• transform fault boundary

• continental volcanic arc

• volcanic island arc
  
  

Multiple Choice

Complex mountain systems such as the Himalayas are a result of

• oceanic-oceanic
  convergence

• oceanic-continental
  convergence

• continental volcanic
  arcs

• continental-continental convergence

Multiple Choice

What is the type of plate boundary where two plates move together, causing one of the slabs of lithosphere to descent into the mantle beneath an overriding plate?

• oceanic-continental
  convergent

• divergent

• transform fault

• continental-continental
  convergent