3) Plate Tectonics (Chapter 2)

The theory of plate tectonics provides a framework that can explain

1. Continental drift

2. Earthquakes

3. Volcanoes

Continental drift: Alfred Wegner proposed that North America, South America, Africa and Europe were once part of a single continent (called Pangea). Over time, these continents drifted apart and the Atlantic Ocean opened the present day land masses. Wegner's ideas were based on; 1) the near perfect fit of these four continents if the Atlantic Ocean were closed, 2) Evidence for a common glaciation in the southern continents (South America, Africa, and Antarctica), 3) Similarities in rocks and fossils on continents separated by the Atlantic ocean.

CONTINENTAL DRIFT ANIMATION

Continental drift was not widely accepted since no explanation for drifting continents existed, until plate tectonics was proposed.

Lines of evidence leading to plate tectonics;

1. Mapping of the ocean floor revealed the presence of mid-ocean ridges and trenches. Mid-ocean ridges constitute rift zones that form interconnected sub-sea mountain ranges that span the globe. Trenches are deep troughs associated with active earthquakes zones and chains of volcanoes.

2. A record of magnetic reversals on either side of mid-ocean ridges. Magnetic reversals occur when the Earth's magnetic field essentially flips so the magnetic north becomes magnetic south and visa versa. The ocean floor consists of basalt, an igneous rocks formed by the cooling of lava. When the rock cools, magnetic minerals record the orientation of the Earth's magnetic field.

3. The ocean floor becomes progressively older on either side of the mid-ocean ridges.

These observations indicated that the ocean floor is continuously being formed at mid-ocean ridges through a process of sea floor spreading. (See figure 2-6 in your text)

If new ocean crust is being formed at mid-ocean ridges, it must be being consumed somewhere else. Oceanic trenches are located where sea floor is sliding into the mantle in a process called subduction. Subduction of oceanic crust provides an explanation for earthquake and volcanoes associated with trenches.

The Earth's surface is divided into large regions bounded by mid-ocean ridges, subduction zones, and faults. These regions are called plates. Plate boundaries are known as;

1. Divergent plate boundaries: Boundaries where plates move apart as new crust is formed (Mid-ocean ridges).

2. Convergent plate boundaries: Boundaries where two plates approach each other, and one plate is subducted beneath another. These are the subduction zone associated with trenches.

3. Transform plate boundaries: Boundaries where two plates slide horizontally past each other on either side of a transform fault. The San Andreas fault in California is a well known example.

PLATE BOUNDARY ANNIMATION

The Earth's surface is divided into at least 15 plates. As new crust plate forms at divergent plate boundaries, and is consumed at convergent plate boundaries, these plate jostle about and move relative to one another.

Most (but by no means all) earthquakes and volcanoes are associated with plate boundaries.

Layered structure of the Earth:

Layered Earth divided by composition

• Crust: Continental Crust: Granitic rocks 2.7 g/cm³

• Oceanic Crust Basalt: 3.0 g/cm³

• Mantle: Upper Mantle: Ultramafic 3.2 g/cm³

• Lower Mantle: Ultramafic 4.5 g/cm³

• Core: Iron

Layered Earth divided by strength properties.

• Lithosphere: Crust and part of upper mantle -  Rigid

• Asthenosphere: Rest of upper mantle  - Plastic

• Lower mantle - Rigid

Important points:

• Asthenosphere is plastic. It flows when under stress similar to a liquid

• Lithosphere is less dense than asthenosphere, and thus floats on asthenosphere.

• Similar to ice floating on water.

THUS: Plates can be thought of as rigid pieces of lithosphere floating on asthenosphere. Heat is transferred through the lithosphere by convection currents. Where convection causes upwelling, a divergent plate boundary forms in the lithosphere. Subduction appears to occur where convection currents flow back into the mantle. (SEE figure 2.8, 2.15, and 2.13) This convection may drive plate tectonics.