Saturday, 28 November 2015


The theory of plate tectonics has done for geology what Charles Darwin's theory of evolution did for biology. It provides geology with a comprehensive theory that explains "how the Earth works." The theory was formulated in the 1960s and 1970s as new information was obtained about the nature of the ocean floor, Earth's ancient magnetism, the distribution of volcanoes and earthquakes, the flow of heat from Earth's interior, and the worldwide distribution of plant and animal fossils.

The theory states that Earth's outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates also exist, including the Arabian, Nazca, and Philippines plates.
The plates are all moving in different directions and at different speeds (from 2 cm to 10 cm per year--about the speed at which your fingernails grow) in relationship to each other. The plates are moving around like cars in a demolition derby, which means they sometimes crash together, pull apart, or sideswipe each other. The place where the two plates meet is called a plate boundary. Boundaries have different names depending on how the two plates are moving in relationship to each other

crashing: Convergent Boundaries,
pulling apart: Divergent Boundaries,
or sideswiping: Transform Boundaries

With respect to plate boundaries is your home located in the middle of, or near the boundary of a plate? What does this mean for you tectonically?


In plate tectonics, a convergent boundary, also known as a destructive plate boundary (because of subduction), is an actively deforming region where two (or more) tectonic plates or fragments of the lithosphere move toward one another and collide. As a result of pressure, friction, and plate material melting in the mantle,earthquakes and volcanoes are common near convergent boundaries. When two plates move towards one another, they form either a subduction zone or acontinental collision. This depends on the nature of the plates involved. In a subduction zone, the subducting plate, which is normally a plate with oceanic crust, moves beneath the other plate, which can be made of either oceanic or continental crust.

The edge of the continental plate in the drawing has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth. A trench has formed at the bend. All that folding and bending makes rock in both plates break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth's hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions where it finally reaches the surface. An example of this type of collision is found on the west coast of South America where the oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the Pacific Ocean. The Himalayas in India are the result of two continental plates (the Indo-Australian and Eurasian plates) colliding head on.

Mountains, earthquakes, and volcanoes form where plates collide. Millions of people live in and visit the beautiful mountain ranges being built by plate collisions. For example, the Rockies in North America, the Alps in Europe, the Pontic Mountains in Turkey, the Zagros Mountains in Iran, and the Himalayas in central Asia were formed by plate collisions. Each year, thousands of people are killed by earthquakes and volcanic eruptions in those mountains. Occasionally, big eruptions or earthquakes kill large numbers of people. In 1883 an eruption of Krakatau volcano in Indonesia killed 37,000 people. In 1983 an eruption-caused mudslide on Nevada del Ruiz in Columbia killed 25,000 people. In 1976, an earthquake in Tangshan, China killed an astounding 750,000 people.

On the other hand, earthquakes and volcanoes occurring in areas where few people live harm no one. If we choose to live near convergent plate boundaries, we can build buildings that can resist earthquakes, and we can evacuate areas around volcanoes when they threaten to erupt. Yes, convergent boundaries are dangerous places to live, but with preparation and watchfulness, the danger can be lessened somewhat.


A divergent boundary is when the earths brittle surface layer (the lithosphere) is being pulled apart. It typically breaks along parallel faults that tilt slightly outward from each other.  As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere). The sinking of the block forms a central valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface. Places where plates are coming apart are called divergent boundaries. When a divergent boundary crosses land the rift valley which forms. Why does the magma have to fill up the cracks in the lithosphere? It fills the cracks in the lithosphere because it sinks into the lithosphere and forms a new crust. Is it a dangerous place to live on? Yes, because if you would live there it would be dangerous, caused by the magma that flows up and the plates split apart.

Where a divergent boundary crosses the land, the rift valleys which form are typically 30 to 50 kilometers wide. Examples include the East Africa rift in Kenya and Ethiopia, and the Rio Grande rift in New Mexico. Where a divergent boundary crosses the ocean floor, the rift valley is much narrower, only a kilometer or less across, and it runs along the top of a midoceanic ridge. Oceanic ridges rise a kilometer or so above the ocean floor and form a global network tens of thousands of miles long. Examples include the Mid-Atlantic ridge and the East Pacific Rise.

Plate separation is a slow process. For example, divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per year.


Transform boundaries are places where plates slide horizontally past each other moving in opposite directions. The result of two plates pushing against one another is massive amounts of energy built up. Occasionally this built up energy is released suddenly in the form of large earthquakes.

 Perhaps the most famous transform boundary in the world is the San Andreas fault.  The San Andreas fault zone is about 1,300 kilometers long and slices through two thirds of California at an average rate of about 5 centimeters per year

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