Friday, 18 July 2014 09:05



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 our fingernails grow) in relationship to each other. The plates are moving 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

These plates lie atop a layer of partly molten rock called the asthenosphere. The plates can carry both continents and oceans, or exclusively one or the other. The Pacific Plate, for example, is entirely oceanic.

Continental plates are composed mainly of granite, while oceanic plates are mostly basalt, which is considerably heavier. Essentially, the continents are lighter and more buoyant; hence, they float higher on the earth's mantle than the ocean's crust does.

Convergent boundary: these are the places where plates crash or crunch together are called convergent boundaries also known as collisional boundaries. When plates converge, one slips under the other and is said to be subducted. At depths from 185 to 435 miles beneath the earth's surface, the subducted parts of the plate melt and become part of the molten mantle. As new plate material is being formed continuously, and the excess is melted into magma, the earth's rocky crust is constantly recycled.

If both converging plates have oceanic edges, either one might slip beneath the other. But when a plate carrying a continent converges on an oceanic plate, the more buoyant continental plate always slides over the heavier basaltic oceanic plate.

When two continental plates converge the two interact to create mountain ranges.

Divergent boundary: Places where plates are coming apart are called divergent boundaries also called as spreading centres. When the  Earth's brittle surface layer (the lithosphere) is 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 are formed where the magma reaches the surface.

Where a divergent boundary crosses the land, the rift valleys get formed which 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. 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, eg. Divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per year.

Transform boundary: Places where plates slide past each other are called transform boundaries. The plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other.

The most famous transform boundary in the world is the San Andreas Fault. Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas Fault hit San Francisco in 1906. Many buildings were finished by the quake, and maximum of the city was destroyed by the fires that followed.