PLATE TECTONICS: UNDERSTANDING PLATE TECTONICS

Objectives: To Know: 
1. How the earth is stratified
2. How the spreading, convergent, and transform plate boundaries relate to the distribution of continents, mountains,and earthquakes.

Did you Know?

Misconceptions:
1. The location of earthquakes is random.
2. Continents don't move.
3. Earth is molten, except for its crust.
4. Most of the world’s most spectacular scenery was created by cataclysmic events.
5. An earthquake measuring 6.5 on Richter scale is 1 time more powerful than a 5.5 earthquake.

Introduction--Why does volcanism, orogenies (mountain building events), and earthquakes occur where they do? Why are there fossils on the top of 20,000 ft.mountains? Why does Antarctica have coal deposits? Doesn't Africa and South America look like pieces in a puzzle? We may even be able to answer why Los Angeles and San Francisco will one day be suburbs of one another..

To understand plate tectonics we first need to understand the properties of the lithosphere, which includes the earth's crust and upper mantle.

Objective 1: How the earth is stratified

The Lithosphere's Stratification

The decay of radio active minerals is the power source for plate tectonics.

Objective 2: How spreading, convergent, and transform plate boundaries relate to the distribution of continents, mountains, and earthquakes.

The earth's crust is broken into eight major fragments or plates that wander the planet at varying speeds (1-15 cm/yr) and directions. Plate tectonics refer to creation, movement, and destruction of these plates. Plate tectonics helps explain mountain building (orogeny), volcanism, and earthquakes. Let's explore movement along three major classes of plate boundaries.

1) A spreading boundary occurs where two plates move away from one another like two conveyor belts moving in opposite directions. From this spreading zone, new crust is created along cracks called rift zones where magma wells up to the surface. An oceanic example of a spreading boundary occurs along the Atlantic Ocean's Mid-Atlantic Ridge (Fig. 1), where the North American and Eurasian Plates are created and moving away from each other. Consequently, the Atlantic Ocean is getting wider. An example of continental breakup at the surface occurred when Saudi Arabia split away from the African Plate forming the Red Sea.

2) A converging boundary occurs where two plates collide. There are three types of converging boundary.
A) In oceanic-continental convergence (Fig. 2), oceanic crust collides with less dense continental crust, oceanic crust sinks and is consumed beneath the continental crust through a process known as subduction. Subduction occurs where the Nazca Plate dives beneath the South American Plate. Volcanism and mountain building (orogenies) result. Offshore, subduction often results in the formation of deep oceanic trenches 8-10 km deep. Subduction also produces volcanism. As oceanic crust sinks it is subjected to tremendous pressure. Eventually portions of the plate melt to form magma. Under pressure magma then finds its way through fissures in the earth's crust to form volcanos and lava flows at the surface. Converging boundaries ringing the Pacific Ocean are consuming its oceanic crust; as a result, the Pacific Ocean is getting smaller.

B) A second type of converging boundary is where two oceanic plates collide (oceanic-oceanic convergence). As in oceanic-continental collisions, one plate is subducted under the other creating a trench. The Marianas Trench (almost 11,000 m) is an example of this plate interaction. Subduction also results in submarine volcanoes which rise above the surface to form to form island arcs, the curved pattern of the islands mirroring the curved pattern of the offshore subduction zone.

C) A third type of converging boundary is called continental-continental convergence (Fig. 3). Here, neither parcel of continental crust is subducted because continental rocks are relatively light and resist downward movement. The Himalayan Mountains (over 8,000 m) and Tibetan Plateau (around 4,600 m) result where the Indian-Australian Plate smashed into the Eurasian plate 50 million years ago. The term given for continent-continent collisions is suturing. Suturing causes the crust to buckle forming folded mountain belts.

3) Sometimes plates horizontally slide past one another with crust being neither created nor consumed. This type of boundary is termed a transform plate boundary. Most occur on the ocean floor. Pictured on the left (Fig. 4) is an example a transform plate boundary--the San Andreas Fault--in western California. Land on the western side of the plate boundary, which includes Los Angeles, is located on the Pacific Plate and is moving northwest. Located on the eastern side of the plate boundary is most of San Francisco, on the North American Plate, which is moving southeast. Being on opposite sides of the plate boundary, these cities are moving toward each other at a rate of 5 cm/yr. An eventual question to be resolved: Is Los Angeles a suburb or Los Angeles or is Los Angeles a suburb of San Francisco?

Where movement along any of the plate boundary becomes locked, stress builds, and earthquakes can occur. Earthquakes occur as plates suddenly lurch past one another; the seismic waves generated by the earthquakes can result in building collapse through violent ground shaking. Volcanism is commonly associated with spreading and converging boundaries
North American Plate - North America, western North Atlantic and Greenland
South American Plate - South America and western South Atlantic
Antarctic Plate - Antarctica and the "Southern Ocean"
Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
African Plate - Africa, eastern South Atlantic and western Indian Ocean
Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
Nazca Plate - eastern Pacific Ocean adjacent to South America Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)

About two dozen minor plates also exist. Some of these more famous plates are the Arabian, Caribbean, Cocos, Philippine, and Juan de Fuca plates.

Web Resources to Supplement this Lecture