The Earth is composed of about 15 large plates and many more tiny plates.

The Earth has three compositional layers:

CRUST MANTLE CORE

The CRUST is the outer skin of the Earth and it is extremely thin compared to the other two layers.

Two types of crust on earth:

OCEANIC CRUST and CONTINENTAL CRUST.

Oceanic crust is under the oceans and is considerable denser than continental crust. (More Mg than the continental crust.)

Oceanic and continental crusts differ in thickness.

Oceanic Crust = 5 � 10 km thick

Continental Crust = 35 � 40 km thick.

Because the crust and the uppermost mantle are rigid, they collectively make up the LITHOSPHERE.

LITHOSPHERE = outer rocky layer of earth, including the uppermost mantle (mantle lithosphere).

The Lithosphere (crust and uppermost mantle) makes up the PLATES of Plate Tectonic Theory. (i.e., the base of the lithosphere is the base of the plates.)

Oceanic Lithosphere averages about 70 km thick.

Continental Lithosphere averages about 130 km thick.

The boundary between the CRUST and the MANTLE is known as the Moho.

Mohorovicic Discontinuity: fundamental change in rock properties from crust to mantle.

AESTHENOSPHERE = the soft, plastic, flowing portion of the mantle directly beneath the mantle lithosphere.

Aesthenos = "weak" in Greek

The aesthenosphere lies directly below the lithosphere and provides the lubrication for the tectonic plates to move.

(In other word, the Lithosphere rides the Aesthenosphere and the Lithosphere moves as the Aesthenosphere below it moves.)

OVERHEAD, Fig. 1-9

The Earth�s internal heat engine works by hot material moving to the surface and cold, dense lithosphere sinking down through the aesthenosphere into mantle. (Mantle Convection)

This internal heat engine drives TECTONIC FORCES.

Virtually all geologists accept the theory of Plate Tectonics.

NOTE: To a scientist, a theory is a hypothesis that has been rigorously tested by all scientific means available: THE SCIENTIFIC METHOD.

1. Raise a question or a bunch a questions:

Why are there marine fossils on top of Mt. Everest?

Why are there Mid-Ocean Ridges?

2. Gather Data

Let�s go to the source. The Oceans.

Where are these fossils being deposited? Ocean Basins.

What is the character of the Mid-Ocean Ridge? Parallel Ridges.

3. Hypotheses Proposed

Always better to have with multiple working hypotheses.

New oceanic crust is generated at Mid-Ocean ridges.

The continents drift (Alfred Wegener). Africa-S. America fit.

4. Prediction

If new crust is generated at the Mid-Ocean ridges, then it must be older as you move away from the Mid-Ocean Ridge.

5. Test the Predictions

Drill into the deep oceans and age-date the oceanic crust. As the theory predicted, the basalts and sediments were older as you moved away from the Mid-Ocean Ridge.

OVERHEAD, Box 1.3, Fig. 2

6. Hypothesis becomes a Theory

To most People, a "theory" is what scientists regard as a hypothesis. Like the airline crash offshore from New York in 1996, there were various theories. First, it was a spark in the gas tank. Second, it was bomb on the plane. Third, it was a surface-to-air missile.

To the scientist, these are hypotheses.

So, when people say, "Plate Tectonics, it is just a theory!" or "Evolution is just a theory", it implies that the scientific support is weak.

However, the reality is that SCIENTIFIC THEORIES are rigorously tested and overwhelmingly verified that they come as close as possible to being truth.

PLATE TECTONICS = movement and deformation of the earth�s crust.

Three basic concepts:

1. The outer layers of the earth (lithosphere) are divided into rigid plates.
2. The plates move with respect to one another, sliding on the plastic aesthenosphere.
3. Most deformation of the earth�s crust takes place near plate boundaries (where mountains, volcanoes, earthquakes form).

The most important concept is #3, Plate Boundaries.

1. Divergent boundaries exist where plates are moving apart.
2. Convergent boundaries exist where plates are plates move toward each other.
3. Transform boundaries exist where two plates are sliding past each other.

DIVERGENT BOUNDARIES

These mainly occur under the oceans.

In the Oceans, these divergent boundaries are marked by topographically high Mid-Ocean Ridges.

Best Examples = Mid-Atlantic Ridge and East Pacific Rise.

Seafloor spreading was one of the hypotheses that led to the Theory of Plate Tectonics.

Seafloor Spreading = the generation of new oceanic crust at Mid-oceanic ridges.

Paleomagnetism = the "fossil" or remnant magnetic field of a rock.

(palaios = ancient in Greek)

In order to understand paleomagnetism, we must know what the Earth�s magnetic field is.

Earth�s Magnetic Field = A region of magnetic force. Think of the Earth as having a huge magnet inside of it with the magnetic poles and the North and South Poles.

Magnets generate a field because they have a positive end and a negative end.

(DRAW ON BOARD)

(picture of magnet, with + and � ends)

(Go to Computer and show the magnetic field of Earth.)

Tell them that if you had little magnets floating in a liquid or suspended in air, they would align themselves according to the local magnetic field.

This is how a compass works � it is suspended in air.

Local Magnetic Field = the acute inclination of the magnetic force with the horizon.

(Show on Computer)

At the North and South Poles, the magnetic field is 90�, vertical.

At the equator the magnetic field is 0�, horizontal.

The Earth�s magnetic field reverses periodically but at irregular intervals once every 300,000 to 500,000 years.

It is still not clear why the magnetic field reverses, but there is evidence in the rock record both on land and in the oceans.

What changes are just the magnetic lines of force or magnetic North and South Poles, not the geographic Poles.

Reverse Polarity = when the magnetic lines of force leave the Earth near the geographic North Pole and reenter near the geographic South Pole.

So, at Mid-Ocean Ridges, basaltic lava erupts and cools to form new oceanic crust.

(OVERHEAD ON COMPUTER)

Basaltic lava contains small minerals called MAGNETITE. These Fe-oxide minerals will align themselves according to the local magnetic field.

This is because they are suspended in the molten rock, basaltic lava. When the lava cools, they become frozen in space � the space according to the local magnetic field at that time.

This is Paleomagnetism.

How do we measure Paleomagnetism?

Magnetometer = instrument that measures the amount and direction of residual magnetism in a rock sample.

In the 1950�s geologists towed sensitive magnetometers just above the seafloor and found that the ocean floor formed a pattern of symmetrical magnetic stripes or bands on both sides of a spreading ridge.

(OVERHEAD ON COMPUTER)

They determined that these magnetic stripes or magnetic anomalies were due to the reversals in Earth�s magnetic field at the times when the rocks were being formed at the Mid-Oceanic Ridges.

(OVERHEAD ON COMPUTER)

What results is a pattern of alternating magnetic stripes that can be correlated to the magnetic stripes on the other side of the spreading center.

Geologist can also age-date the rocks and correlate them based on magnetic reversals.

The magnetic stripes can vary is width depending on how long it took before the next magnetic reversal.

So, what develops is a very unsystematic pattern of stripes � fat one followed by a skinny one followed by another fat one followed by four skinny ones. This pattern of Fat � skinny �Fat � four skinny indicates a special time period � between 1 and 3.5 million years ago.

So, anywhere on earth where we see this pattern means that the ocean floor is between 1 and 3.5 million years old.

CONVERGENT MARGINS

Ocean-Ocean Convergence

Ocean-Continent Convergence

Continent-Continent Convergence

HOT SPOTS AND HAWAII

ATOLLS, BARRIER REEFS, FRINGING REEFS