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Chapter: 11th 12th std standard Geography earth space Higher secondary school College Notes

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Lithosphere and tectonic plates

Lithosphere and tectonic plates
Scientists believe the Earth began its life about 4.6 billion years ago. The continents probably began forming about 4.2 billion years ago as the Earth continued to cool. But it was not until the turn of the 20th century that scientists determined that our planet is made up of four main layers: the inner core, outer core, mantle, and crust.

LITHOSPHERE AND TECTONIC PLATES

 

Scientists believe the Earth began its life about 4.6 billion years ago. The continents probably began forming about 4.2 billion years ago as the Earth continued to cool. But it was not until the turn of the 20th century that scientists determined that our planet is made up of four main layers: the inner core, outer core, mantle, and crust. The core is composed mostly of iron and is so hot that the outer core is molten, with about 10% sulphur. The inner core is under such extreme pressure that it remains solid.

 

Most of the Earth's mass is in the mantle, which is composed of iron, magnesium, aluminium, silicon, and oxygen silicatecompounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner. The crust is much thinner than any of the other layers, and is composed of the least dense calcium and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle , so it can fracture in earthquakes.

 

The crust and the upper layer of the mantle together makeup a zone of rigid, brittle rock called the Lithosphere. The layer below the rigid lithosphere is a zone of about 50-100 km down, is especially soft and plastic, and is called the asthenosphere. The asthenosphere is the part of the mantle that flows and moves the plates of the Earth. A heavy load on the crust, like an ice cap, large glacial lake, or mountain range, can bend the lithosphere down into the asthenosphere, which can flow out of the way. The load will sink until it is supported by buoyancy.

 

The crust is composed of two basic rock types granite and basalt. The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt. Basaltic rocks of the ocean crust is much denser and heavier than the granitic rock of the continental crust. Because of this the continents ride on the denser oceanic plates.

The 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. These plates are all moving in different directions and at different speeds from 2 cm to 10 cm per year.

 

This theory of Plate tectonics explains 'how the earth works' and let us continue and learn more about the plates and their movements. 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.


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