Earth

PLANET EARTH

To an astronaut high in space looks like a small mainly blue ball with white areas of cloud and green or brown land. Here on the ground, our Earth is a slightly flattened sphere measuring about 7,930 miles [12,800 km] across and 24,800 miles around. Almost three quarters of its surface is actually covered with water. some of the most active regions for making new areas of oceanic plates are in the middle of the oceans.

 Every hill, every valley, every stream, every cliff, and every pool has now been mapped on the Earth’s surface. So have the mountains and trenches below the waves, on the sea bed using sound waves in echo sounding or sonar. Maps such as this one distort the sizes of the land masses make those near the poles seem the larger.   The only way truly represents an Earth map is to put it onto the same shape as the Earth itself-a ball or globe.

INSIDE THE EARTH

The Earth seems vast and solid but inside, it is mostly molten or semi molten and is always on the move. The whole Earth is some 7,930 miles in diameter, but the hard outer layer, the crust, is only about 15-22 miles under the major land masses or continents, and3-6 miles, beneath the oceans. Below this is the thickest layer, the Mantle, which is 1,800 miles deep. Within the two part core .The outer core ,1,360 miles thick, is composed  of almost liquid iron rich rocks .The solid inner core ,1,550 miles across, is also mainly of iron and nickel.

The bulk of the Earth volume is the mantle .This is fairly firm in its upper region, but becomes semi melted or plastic deeper down. The mantle is made of rocky minerals, rich in silicon, magnesium, and iron .The fairly rigid upper 35-36 miles of mantle, plus the crust above, forms the lithosphere. This is cool and strong and is divided into huge lithospheric plates.

RESTLESS EARTH

On a world map , the east coastlines of South America bears  an uncanny likeness to the shape of the west coast of Africa .The edges  of the continental shelves of these two continents   several hundred feet under the sea’s  surface , and the resemblance is even more striking. The two would fit together like pieces of a giant jigsaw puzzle. In fact they once did .The idea of the continental drift, once dismissed out of hand, is now well established in science. The idea of continental drifts has been around for centuries.