TNPSC - Geography - 11th Standard Text Books

1. Scientists believe the Earth began its life about 4.6 billion years ago
2. Four main layers of Earch : The inner core, outer core, mantle, and crust 
3. The core is composed mostly of iron and is so hot that the outer core is molten, with about 10% sulphur.
4. Most of the Earth’s mass is in the mantle, which is composed of iron, magnesium, aluminium, silicon, and oxygen silicate compounds. 
5. 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. 
6. 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. 
7. The crust is composed of two basic rock types granite and basalt.
8. The oceanic crust consists of a volcanic lava rock called basalt. 
9. 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. 
10. Several minor plates also exist, including the Arabian, Nazca, and Philippines plates. These
11. plates are all moving in different directions and at different speeds from 2 cm to 10 cm per year