The Himalayan Mountains, formed by plate tectonics
Plate tectonic theory states that the Earth's surface is made up of a number of small portions called plates. The movement of these plates is responsible for major geological features of the Earth such as volcanic island arcs, deep sea trenches and mountain ranges. Earthquakes also result from the movement of these plates and collisions along their boundaries.
Earth's crust
The crust is the Earth's outermost layer. It ranges in thickness from 10 to 65 kilometers, with the outermost 35 kilometers being brittle enough to produce earthquakes. The boundary between the crust and the mantle below it is called the Mohorovicic discontinuity, or moho.
Mantle
The mantle comprises the main bulk of the Earth and varies in depth from about 40 to 3480 kilometers. It is the layer between the crust and the metallic core of the planet. The uppermost part of the mantle is rigid and, with the crust, forms the plates of plate tectonics.
Lithosphere
The lithosphere refers to the crust and the uppermost portion of the mantle and is the solid outer part of the Earth. It is about 100 kilometers in depth, with older lithosphere being somewhat thicker than that which is more newly formed.
Asthenosphere
Below the lithosphere is the layer known as the asthenosphere. It is partially molten and exists at depths of 100 to 700 kilometers. It is marked by low seismic wave velocities and high seismic wave attenuation. The lithosphere is pushed down into the asthenosphere during subduction.
Subduction
Subduction is the process of the lithosphere of one plate sliding down and below another plate when the plates converge. The areas between the two plates are called the subduction zones and are found at convergent plate boundaries. In order for subduction to occur, one plate must be heavier than the other. These boundaries are hotbeds of geologic change.
Plate Boundaries
There are thee kinds of plate boundaries: divergent, convergent and transform boundaries. All plate movement affects the other plates, though they move at different relative speeds and somewhat independently of each other. The energies released at these boundaries transforms Earth's surface in a variety of ways.
Divergent Boundaries
Divergent boundaries are where plates are pulling apart and new crust is being created. Oceans are born and grow wider as plates diverge. Rifts are formed when land masses are affected by divergent boundaries. Iceland is splitting along the divergent boundary known as the Mid-Atlantic Ridge and will eventually split into two separate land masses.
Convergent Boundaries
Molokini Island in Hawaii, formed by convergence of 2 oceanic plates
At convergent boundaries, crust is recycled as one plate is subducted beneath another plate. There are three types of convergent boundaries. Oceanic plates push into and are subducted beneath a continental plate, forming mountain ranges. These boundaries also produce strong earthquakes. Deep ocean trenches such as the Mariana Trench in the Pacific Ocean are caused by two oceanic plates converging. This convergence also creates volcanic island arcs. The convergence of two continental plates does not result in subduction as they are relatively light. When they meet, the crust buckles and is pushed upwards. A prime example of this type of convergence is the Himalayan Mountains and the Tibetan Plateau.
Transform Boundaries
Transform boundaries are where two plates are sliding horizontally past each other. They are more commonly known as faults. Most are found on the ocean floor, though some do occur on land. One of the most famous is the San Andreas fault in California. It is about 1300 kilometers in length and has been moving at a rate of about 5 centimeters per year for 10 million years. Pressure builds up along these boundaries and is released by earthquakes
Pangaea
In the history of plate tectonics, there was a supercontinent known as Pangaea about 225 million years ago. It was broken into smaller pieces due to the movement of the tectonic plates and drifted to form the continents we know today.
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