THE EARTH CRUST
In geology the Earth’s crust is the outer most layer of the planet.
Billions of years ago, the Earth started out as a hot, gooey ball of rock. Iron and nickel sank to the center of the Earth and became the core. The surface of the Earth slowly cooled off and hardened. This made the surface rocks became the crust.
Earth's crust occupies less than 1% of Earth's volume. The temperature of the earth’s crust increases with the distance, typically in the range from about 500 °C (900 °F) to 1,000 °C (1,800 °F) at the boundary with the underlying of the earth’s mantle.
The Earth's crust is divided into 15 major tectonic plates; the North American, Caribbean, South American, Scotia, Antarctic, Eurasian, Arabian, African, Indian, Philippine, Australian, Pacific, Juan de Fuca, Cocos, and Nazca plates.
Tectonic plates move along the surface of the mantle, which causes earthquakes, mountain formation, continental drift, volcanoes, and other geologic activity on the Earth’s crust.
The crust is divided into two types: oceanic crust and continental crust.
Continental crust
Continental crust is found under land masses, it is made up of less dense rocks such as granite. It’s about 10 and 75 kilometres (6 to 47 miles) thick.
Continental crust is much older than oceanic crust. Most of the oldest rocks in the world are found in the Nuvvuagittuq greenstone belt in Quebec, Canada. Continental crust formation has rocks that are about 4 billion years old. Compared to continental crust, oceanic crust is still being formed in places called mid-ocean ridges. Mid-ocean ridges are formed by magma from the mantle the erupts through cracks in the ocean floor, creating crust as it cools.
Oceanic crust
Oceanic crust, found under the ocean floor it is made of up dense rocks such as basalt. It is about 7 kilometres (4 miles) thick.
Oceanic crust is heavier than continental crust. Heavy oceanic crust is constantly sinking, very slowly, underneath lighter continental crust. This process is called subduction.
Volcanoes that are formed at a subduction zone are called volcanic arc. Volcanic arc exists where the oceanic crust of the Australian plate sub-ducts under the continental crust of the Eurasian plate. Indonesian Island Arc, which includes the islands of Sumatra and Java in Indonesia, have some of the most powerful volcanoes in the world.
Eventually, oceanic crust sinks low enough into the Earth's mantle. Once this has happened, the crust melts, and then rises up again as magma in the mid-ocean ridges. In this way the Earth forms a brand new oceanic crust once every 200 million years crust once every 200 million years or so.