The surface

The Earth's crust is a thin layer containing rock material.

The surface of the crust consists of mountain ranges, valleys, flat plains, deserts, and vegetated areas of varying density. From space, it can be seen that oceans of water cover much of the surface. The surface has been shaped over millions of years by tectonic forces and volcanic action and erosion by wind, water and glaciers.

The four terrestrial planets - Mercury, Venus, Earth and Mars - contain evidence of volcanic action having occurred in their past. In the case of Earth, some volcanoes are still active and these often erupt, releasing molten rock material into the air or surrounding areas. There are about 500 volcanoes on Earth, most of which are found along plate boundaries.

A key feature of the Earth is the presence of liquid water on its surface. This water is found in the oceans, lakes, and rivers. Water also occurs in the polar ice-caps, and as vapour in the air. Water allows living things to survive on Earth.

Figure 6.7 Volcanic eruption on Earth. (Photo: J. Wilkinson)
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