Scientists have determined that Earth's surface consists of a number of rigid, BUT MOVING, pieces called Plates.

• Some areas find plates moving apart
• Some areas find plates moving together
• Some areas find plates sliding passed each other
• The study of these plate movements is called Plate Tectonics.

Earth is divided into about 1 dozen (12) major plates

• S. American plate and African plate are moving apart
• Indian plate is colliding with Eurasian plate
• Pacific plate and N. American plate are sliding passed each other in California
• Where plates move together, a Trench is often formed by the overlapping of plates
• Where plates move apart, a Ridge is formed
• We'll talk much more later in the chapter…

So what are the Plates like?

• The crust and the mantle were thought to represent two different materials
• Now it is known that the crust and uppermost mantle are very similar in composition!
• Scientists have now called this region the Lithosphere
• The lithosphere is rigid but broken into plates
• The plates average about 100 km thick (80miles)
• Most of the rock in the lithosphere is igneous (volcanic) in origin

The lithospheric plates rest on a liquid layer

• This liquid layer is made up of super hot magma
• The liquid layer is called the Asthenosphere
• This layer is thought to cause plate movement because it allows the rigid lithosphere to float on it much like a raft floating on water.
• The rock of the asthenosphere is similar to the lithosphere - just melted!
• The rising and sinking of the heated, liquid magma causes movement in the overlying plates
• This is similar to watching noodles boiling in a pot on the stove…a rolling action takes place

Evidence of Plate Tectonics

• Ever notice how the continents seem to have parts of them that look as if they could fit together like pieces of a puzzle?
• This started a theory that these continents were in fact once connected as one
• This led to the theory of Continental Drift
• This was professionally proposed in 1912 by famed geologist Alfred Wegener
• Besides "shape", Wegener also found evidence in the Mesosaurus (a small reptile from 270 mil yr. Ago) existed in Brazil and South Africa - but nowhere else on earth!
• In other words, IF the two continents were connected - this animal lived where the split later occurred
• Also, the rock structure in the questionable area was the same!

Earthquakes, Volcanoes, and Plate Tectonics

• Earthquakes don't just occur randomly but in belts (certain areas) around the world
• The belts where these events occur is what is called the Plate Boundaries
• As stresses build up from the excessive heat in the asthenosphere, the pressure gets released in the weakest area - the plate boundary - the place where plates are joined or seamed.
• The largest active belt in the world is the Pacific Ocean…90% of all the world's earthquakes occur here

Magnetism

• Some igneous rock contain minerals that are magnetic
• They provide record of earth's magnetic direction at the time the rock formed
• We have found that earth has changed its polarity four major times in the last 4 million years
• Where plates are moving apart, the new layers that form often show a change in polarity
• These areas are called Spreading Centers
• Lava wells up in the gap between the plates
• At the same time, older rock layers are pushed away from the spreading center
• So as the lithospheric plates get moved apart, they carry the continents with them

Heat Flow

• Heat flow is the measure of the amount of heat leaving the rocks of the lithosphere
• Heat flow values are high in the area of the spreading center and decrease away from the center
• Because heated materials expand, spreading centers have higher elevations than the rest of the seafloor

Kinds of Plate Boundaries

• Diverging Boundaries (spreading centers) are places where two lithospheric plates are moving apart.
• Most diverging boundaries have mid-ocean ridges - most occur in oceanic areas
• These ridges have deep valleys along their entire length - called rift valleys
• The mid-Atlantic Ridge is an example of a diverging boundary - it separates N. America from the Eurasian Plate and the S. American plate from the African plate

Sliding Boundaries

• Sliding Boundaries occur where plates are trying to slide past each other
• In California, the N. American plate is sliding past the Pacific Plate at the San Andreas Fault
• A Fault is a break or crack in Earth's crust along which movement has occurred
• California's coastline is moving northwest with respect to the continental U.S.
• The SA Fault moves about 5 cm. per year!

Converging Boundaries

• Converging Boundaries form when two plates are coming together or converging
• If the two converging plates are carrying continents - then this may form one large continent
• This form of converging boundary is called a Collision Boundary - often creating a mountain range at the boundary!
• The Himalayan Mountains in India/China are a perfect example of a collision boundary
• The Mountains are growing about 5 cm higher each year. Mt. Everest is part of this range
• The Appalachian Mts. are thought to be formed this way as well when the U.S. collided with the African Plate
• Subduction Boundary is formed when one plate plunges down under another overriding plate.
• It is another form of a converging boundary
• An obvious feature of the subduction boundary is the formation of a deep-sea trench
• Subduction boundaries can form either from one ocean plate subducting under another ocean plate or by an ocean plate subducting under a continental plate
• It has been found that when two ocean plates collide (one subducts), a volcanic chain seems to occur on the overriding plate
• Average rate of plate subduction appears to be about 1 cm per year
• Note: When an ocean plate collides with a continental plate, the ocean plate subducts because it is heavier and denser.
• Note 2: This collisions may also give way to a volcanic mountain range
• The subduction boundary will also produce very powerful, deep earthquakes

Continental Growth

• The ancestor of most modern continents today were actually much smaller than they appear today
• Through plate movement and creation, margins have been added to the continents
• The ancient continental cores are called Cratons - the oldest rock of the continent
• The Canadian Shield is an example of the craton of N. America - it extends through central Canada and the upper midwest of the U.S. - that's us!
• The outer parts of the Shield lie under the ground in parts of the western U.S. (Plain states) and the Appalachian Mt. Region
• This craton is about 2.5 billion years old - the rest of N. America formed after this.
• Current continental growth is being accomplished from volcanic activity, sediment from rivers/streams, and mountain building from plates colliding/subducting

Additional Continental Growth

• Thin-skinned Thrusting can cause continental growth also - this pushes "sheets" of rock over the land causing a surface buildup of rock material
• A Terrane is a large block of lithospheric plate that has been moved a great distance and attached to the edge of a continent. They are identified by:

1. Bounded on all sides by major fault lines
2. Rocks do not match those of the continent
3. Magnetic polarity does not match continent
4. Fossils are not similar to those on the continent

• Cache Creek of Canada is a perfect example - it has been found to be from the Japan area!