Chapter 2: Plate Tectonics

What is plate tectonics?Plate tectonics is the study of the origin and

arrangement of the broad physical features of the Earth.

Plate tectonics is the basic idea that the Earth is divided into a few large plates that move slowly and change in size.

Intense geological activity (earthquakes, volcanoes, mountain building) occurs where the plates meet. Where the plates meet are called plate boundaries. Plates can move toward each other, away from each other or slip past one another.

Major Plates of the Earth

Fifteen Major Plates of the Earth

African Plate Antarctic Plate Eurasian Plate Indo-Australian Plate North American Plate Pacific Plate South American Plate Arabian Plate Caribbean Plate Cocos Plate Indian Plate Juan de Fuca Plate Nazca Plate Philippine Sea Plate Scotia Plate

Why is plate tectonics a unifying theory?

Plate tectonics is a unifying theory because it helps to explain earthquakes, volcano formation and mountain belts.

Origin of the Theory:The theory of plate tectonics began in

the 1960’s by combining two older ideas:

1. Continental Drift2. Seafloor Spreading

Continental Drift Videos

Continental Drift Theory: A Little Nuts!

Continental Drift: Past, Present & Future

Continental Drift TheoryThis theory was developed by Alfred

Wegener in the early 1900’s. His evidence for this theory included the following:

1. Coastline fit (i.e. South America & Africa seem to fit together on a map)

2. Similar fossils & rocks can be found on different continents

3. Polar wandering- There is evidence that the North Magnetic Pole has changed position, partly because the continents moved.

Coastline Fit

Similar Animals & Plants

Similar Rocks

Magnetic Reversals

Seafloor Spreading The theory of seafloor spreading was developed

by Harry Hess in 1962 He believed that the seafloor was moving away

from ridges (creating a “spreading centre”) and toward trenches (called “subduction zones” because one of the plates sinks or subducts)

What is the evidence for seafloor spreading?1. Careful observation of the ocean floor2. Simple theory of convection: hot magma rises,

cold magma sinks)3. Heavier, denser and thinner oceanic crust sinks

under the continental crust to form trenches

Seafloor Spreading at the Ridges

Sinking in the Subduction Zones

What is subduction?Subduction occurs when the thinner, cooler, denser

and older oceanic crust sinks under the thicker, lighter and newer continental crust. Subduction causes the following:

1. Earthquakes (plates are colliding)2. Volcanoes (magma comes to the surface)3. Mountains (crust lifts upwards)4. Trenches (formed when the ocean sinks sharply

underneath the continent)

How old is the seafloor?The seafloor is approximately 200 million years old.

This is a useful fact when explaining the theory of seafloor spreading (in other words, oceanic crust is very new compared to the continent)

Subduction

What are plates?A plate is a large, mobile slab of rock

that is part of the Earth’s surface. A plate can be made entirely of seafloor or a combination of continental crust and oceanic crusts.

Plates on Modern Earth

What is the lithosphere?The lithosphere is the outer most part of

Earth that includes the rigid outer shell (or the crust) and the rocks of the upper mantle. The lithosphere ranges from 70-200km in thickness.Remember: The lithosphere is youngest by the ridges and oldest by the continents.

The plates move from 1 – 10 cm per year. This movement is detected using GPS.

Layers of the Earth