What is Plate Tectonics?
 

   The Earth's surface is covered by rigid, moving land masses called plates. Some plates, such as the North American Plate and the Eurasian Plate, are moving together. The study of the formation and movement of these plates is called Plate Tectonics.
  The Earth's crust and mantle were thought to represent two different materials. now we have learned that they are both similar in rock composition and physical properties. Both make up a layer known as the lithosphere. The lithosphere is composed of igneous rock basalt. Continental crust is composed more of igneous rock granite. Granite is less dense than basalt and therefore continents only occur  in the denser part of the lithosphere.
  The lithospheric plates are resting on a layer called the asthenosphere. The asthenosphere rock is partially melted, so the layer is able to flow. The flow makes up large slow moving convection currents. Within these currents, material will expand and rise while heating, but will contract and sink while cooling. When the currents are rising, the material comes to the Earth's surface and pushes the lithospheric plates apart where it is cooler, denser currents in the asthenosphere seem to sink and pull plates together.
 

Evidence of the existence of Plate Tectonics
 

  This idea of earth's solid surface moving is not new. We have many sources of evidence for the existence of plate tectonics.
    1) In the 17th century, when first reliable world maps were made, people noticed similarities in West coast Africa and East coast South America.
    2) Alfred Wegener, a German scientist who proposed the theory in 1912 also noted that the fossil remains of Mesosaurus, a small reptile that lived 270 million years ago, are found only in Brazil and South Africa, nowhere else in the world.  He also noted some distinctive rocks are found on both continents.
    3) In the 1960's, his theory was the subject of scorn and ridicule, but formations of earthquakes, volcanoes, etc. seemed to support Wegner's theory, and eventually evolved into the study of plate tectonics. Scientists have long observed that earthquakes do not occur randomly across the globe. but rather in limited belts, and most of the Earth's great volcanoes are also found there.  The belt's location became clear with the study of plate tectonics (the theory that the two had been one became continental drift).  These belts became known as plate boundaries.  The boundaries are in places where one plate is moving near another causing activity.  As if the stress becomes too much along the area, fractures form and earthquakes occur.  The boundaries are also areas of heat flow where molten rocks comes up and forms volcanoes.  The largest active belt is known as the Ring Of Fire, encircling the Pacific Ocean and 90% of all earthquakes occur there.
  4) Some Igneous rocks contain minerals react to magnetic reversals (the switching of the poles due to the Earth's rotation).  These minerals create a record of the direction of the magnetic poles at the time the rocks were formed.  When this was studied it was discovered that the Earth's crust has shifted or drifted since the rocks were formed and that the poles have reversed.  Magnetic polarity reversals also show bands of rocks.  Using these observations, scientists have found what happens when lithospheric plates move apart.  These areas are called spreading centers.  Lava builds up from deep in the Earth and new rocks keep being formed.  At the same time, older rocks keep moving away from the boundary area in both directions.  As the plates move apart, they carry their continents with them.
  5) Heat flow measures how much heat leaves the rocks of the lithosphere.  The heat flow is abnormally high in spreading centers, seeing as the convection currents move heat to the surface of the sea floor.  Because heated material expands, the spreading centers have a higher elevation than all the rest of the sea floor.
 

Types of Boundaries
 
 

Diverging Boundaries

  Diverging boundaries are where two lithospheric plates are moving apart.  The boundaries have mid ocean ridges.  Mid ocean ridges have deep valleys along the entire length.  These valleys are the boundaries between the lithospheric plates.  The rift valleys are broken into segments.  Fracture zones separate these segments and movements along these zones have been a source of earthquakes that occur along the ridges.  An example of a ridge is the Mid Atlantic ridge, or the East Pacific rise.

Sliding Boundaries

  At some boundaries, the lithospheric plates slide past each other. In California, for example, the North American Plate and Pacific Plate are sliding past each other along the San Andrea's Fault. What is a fault? A fault is a break in the Earth's crust from movement. The average movement rate is 5 centimeters a year. Places like these are the most likely spots for future earthquakes.

Converging Boundaries: Collision

    When two lithospheric plates move toward each other, it is called a converging boundary. Of both plates are carrying continents they may end up becoming one large continent. the collision can push the plate upwards and create a mountain range. The Himalayan Mountains is an example of this kind of boundary. Earthquakes occur along boundaries such as these also.

Converging Boundaries: Subduction

  Subduction occurs when two plates converge and one slides underneath the other. A deep sea trench is an example of this. The deep-sea trench that forms is accompanied by the formation of chains of volcanic islands. For example, the Pacific Plate is subducting the Philippine Plate. When an ocean and continental plate converge,, the ocean plate, (which is denser) will subduct. The deep-sea ocean trench is paralleled by a mountain chain / range on the continental plate above. The earthquakes that occur at these boundaries are deeper, while other boundaries only involve the lithospheric plate, which creates shallow earthquakes.
 

The Growth of Continents and Plate Tectonics
 

   A craton is an ancient core. They are usually the most significantly changed rocks. The shapes of the continents are different than when they first stand out. The North American craton is exposed in Eastern Canada at the surface. The craton shows the approximate shape from 2.5 billion years ago.
  Development of the continents comes from various sources.  One is deep-sea sediment.  When an oceanic plate subducts underneath a continental plate, some sea floor sediments are scraped off and become part of the continent on the other side of the subduction zone.  Another source is volcanic rock.  Volcanic rock chains on the subduction zones contribute sediment.  A third source is the sediments deposited by rivers flowing across the continent.  These sediments build on the continental margins.  They are not part of the active plate boundary.
  Thin-skinned thrusting is when there is a pushing of fairly thin, horizontal pieces of rock from continental margins which cover great distances along level surfaces.  When North America and Africa split over 650 million years ago, it created an early version of the Atlantic Ocean.  The closing of this ocean 150 million years later brought the two together again, also with parts of Europe.  It pushed thin pieces of ocean floor and volcanic islands onto the continental margin.  The Appalachians formed at this time.
  When a large piece of the lithospheric plate is moved over a great distance, it is called a terrane.  It then attaches to the end of a continent.  Each terrane is bounded by all sides by major faults.  The fossils and rocks found in them do not match those of neighboring terranes.  Also, the magnetic polarity does not match that of its neighbors.  Cache Creek of British Columbia is an example of one.

Links to other pages on this site: