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| The Earth's Crust | The surface of planet earth is constantly changing. Surface (exogenic) forces, weathering and erosion. Internal (endogenic) forces, moving magma. The solid crust of the earth is made up of continental crust and oceanic crust. Oceanic crusts,more dense, made of heavier rocks,eg. basalt. Oceanic crust, younger than the continental crust. Continental crusts, less dense, lighter rocks, eg. granite. Continental crust, older than the oceanic crust. |
| The Theory Of Plate Tectonics | Plate tectonics is the study of the processes which cause the movement of the earth's plates and the landforms that result. The theory of plate tectonics was proposed in 1912 by Wegener. He believed the earth was once one big landmass called Pangaea. This then began to split into two continents called Gondwanaland and Laurasia. These then split into the land we see today. This was based on the distribution around the world of identical fossils. The idea of continents moving around is known as continental drift. |
| Types of Plate Boundaries | Plate boundaries are the edges of plates. There are three types of plate boundaries: 1) Destructive (Convergent), 2) Constructive (Divergent) and 3) Conservative (Transform). |
| Destructive (Convergent) Plate Boundaries | Destructive (Convergent) Plate Boundaries At these types of boundaries plates move together and collide, destroying the crust. There are three types of destructive boundaries: 1) Collision between oceanic and continental plates, 2) Collision between two oceanic plates and 3) Collision between two continental plates. 1) Oceanic-Continental Plate Collision - Subduction Zones This is when the heavier oceanic crust collides and slides beneath the lighter continental plate. When it slides into the mantle it melts and its recycled. This is called subduction. The melted crust turns to magma and rises through the continental crust above and creates a volcano such as Mount Saint Helens in the United States. Earthquakes are common in these areas. Where these two plates collide a deep trench is created, these mark subduction zones. Sediments are carried on the sea bed of the sinking oceanic plate and are piled against the continent. These sediments create areas called terranes. 2) Oceanic-Oceanic Plate Collision - Trenches and Island Arcs When two oceanic plates collide one usually subducts beneath the other. This creates a deep trench. An example of this is the Mariana Trench, which is at the Pacific and the Philippine plate. Oceanic-oceanic collision zones can usually be found by volcano islands known as island arcs. The volcanoes form when the suducted plate melts and creates rising bubbles of magma called plumes and they break through the crust above. Some islands created this way are the Japanese and the Philippine Islands. 3) Continental-Continental Plate Collision - Fold Mountains When two continental plates collide neither of them tend to subduct. The plates tend to move upwards or sideways which can cause earthquakes such as the one in Pakistan in 2005. The collision can cause the plates to fold and create fold mountains such as the Himalayas and the Alps. |
| Constructive (Divergent) Plate Boundaries - Mid-Ocean Ridges | Constructive (Divergent) Plate Boundaries - Mid-Ocean Ridges At these boundaries plates move apart because of convectional currents. New crust is created as the crust splits creating rift valleys for magma to rise and fill the gap. This solidifies and crates new land, this is an endless cycle. This process is known as sea-floor spreading which was proposed by Harry Hess. An example of this is the Mid-Atlantic ridge. |
| Conservative (Transform) Plate Boundaries - Fault Lines | Conservative (Transform) Plate Boundaries - Fault Lines At these boundaries two plates slide past each other, they are neither creating nor destroying land at these boundaries. These boundaries are known for creating shallow boundaries. An example of this type of boundary is the San Andreas Fault in California. |
| Tectonic Activity In The Middle Of Plates - Hotspot | Tectonic Activity In The Middle Of Plates - Hotspot Sometimes volcanoes are found thousands of miles from the edges of plates. They occur in areas where high temperatures in the mantle produce a rising fountain (plume) of molten rock known as a hotspot |
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