Inside the Earth and its plates

Look at a map of India. The Himalaya in the north, the plains of the Ganga, the Deccan plateau, the Thar desert. "Have they always looked like this?" No. The Earth's surface is always changing, very slowly. Forces from inside the Earth push it up and move it around; forces on the surface wear it down and reshape it.

A landform is a natural feature on the Earth's surface: a mountain, a valley, a plateau, a plain, a desert or a coast. Landforms are made by weathering, erosion, deposition and the movement of the Earth's crust.

To see what moves the surface, first look inside. Think of a boiled egg cut in half: a thin shell, a thick white, a yolk in the middle. The Earth has three main layers in the same pattern (Fig. 2.1):

From the surface to the centre is about 6,375 km.

Two more names you must know:

• The lithosphere is the crust plus the upper part of the mantle. It is rigid (stiff), about 100 km thick.
• Below it is the asthenosphere: hot and semi-molten (partly melted). It is soft enough that the lithosphere can slide over it.

Now the big idea. The book says the theory of plate tectonics was given by W.J. Morgan. It says the Earth's outer layer is not one single piece. The lithosphere is broken into large and small pieces called tectonic plates, like a cracked eggshell.

Tectonic plates are huge slabs of solid rock. They move very slowly, usually a few centimetres a year, about as fast as your fingernails grow. There are three types:

• continental plates, which carry continents;
• oceanic plates, which carry the ocean floor;
• mixed plates, which carry both.

The major plates (Fig. 2.3): the Pacific, Eurasian, African, North American, South American, Indo-Australian and Antarctic plates. India sits on the Indo-Australian plate.

"But what makes a slab of rock move?" Heat. Think of a pot of dal on the stove: the hot dal at the bottom rises, the cooler dal at the top sinks, round and round. Inside the Earth, heat from the core makes hot material in the mantle rise, while cooler material sinks. These convection currents push and pull the plates in different directions.

Watch out: the plates do not float on liquid. The asthenosphere is semi-molten, not a sea of lava, and the movement is a few centimetres a year, not something you can feel.