Plate Tectonics Tectonics

The Problem with Continental Drift

• While Wegener was able to find evidence of continental drift, there were 2 major problems with his ideas: – Wegener could not explain what could cause the

continents to drift– Geologists did not believe that the continents

could simply “plow through” the oceanic crust as they moved.

The Plate Tectonic Model

• The lithosphere is broken into rigid pieces called “plates”.

• Earth’s surface contains 7 major plates along with many other smaller plates.

• Each plate “floats” and moves around on the plastic-like layer of the mantle known as the asthenosphere.

The 7 major plates are the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic Plates.

What causes the plates to move?

• The transfer of heat in Earth’s interior causes convection to occur.

• Convection currents in the mantle provide the energy for plate motion.

A model for convection inside the Earth

3 Types of Plate Boundaries

• Divergent Boundary – two plates are moving apart.

• Convergent Boundary – two plates are moving together.

• Transform Boundary – two plates slide past each other.

Divergent Boundaries

New lithosphere is produced as molten rock from the mantle rises to the surface at divergent boundaries

Continental Rifting•Occurs when plates

diverge within continents forming rift valleys •Can break apart

land + form new bodies of water•The East African Rift Valley may eventually break up Africa

Mid-Ocean Ridges•Occur when plates move

apart on the seafloor and create rift valleys. •New seafloor is created

at ocean ridges by the process of seafloor spreading.

Seafloor Spreading• Theory put forth by Harry Hess in the 1960’s. • As plates separate on the seafloor, molten

rock from inside the Earth (magma) rises to the surface.

• The magma cools when it reaches the Earth’s surface and creates new oceanic crust.

• As a result, the youngest rocks on the seafloor are found at mid-ocean ridges. The further away from the ridge you get, the older the rocks will be.

Convergent Boundaries

Old lithosphere is “destroyed” as plates descend into the mantle at convergent boundaries.

Subduction Zones

• Subduction occurs when a less dense continental plate collides with a more dense oceanic plate

• The more dense oceanic plate “subducts” or goes down under the less dense continental plate.

Continental Volcanic Arcs• When subduction occurs, rock can melt forming magma. • Less dense magma rises to the surface and in some cases

can lead to volcanic activity. • Include Andes Mountains in South America and the Cascade Mountains in the western United States.

Formation of Mountains• Can happen from the collision of 2 continental plates.• The ongoing collision of India with Asia started about 45

million years ago and produced the Himalayan Mountain Range.

Transform Boundaries

Plates slide past each other without creating or destroying lithosphere at transform boundaries.

Transform Faults• The San Andreas Fault connects

two divergent boundaries, the Juan De Fuca ridge and the Gulf of California. • Plate movements along this

transform fault are responsible for most of California’s earthquakes.

Earth in 50 million years?