From measurements of radioactive decay products, it is possible to measure the age pattern of the Earth’s continents. They are found to have an ancient nucleus, at least 2.5 billion years old, onto which successively younger extensions have been welded, a process still continuing today. The growth has not been regular and in particular there were world-wide interruptions marked by intense deformation and igneous activity around 2.7, 1.8 and 1.0 billion years ago. Although a surprisingly large proportion of the continental crust was already in existence 2.5 billion years ago, most of it is younger-in marked contrast to the surfaces of the other planets. The Earth must have been subjected to the same intense bombardment four billion years ago that was responsible for the large craters still seen on the other planets but tectonic activity since then has destroyed these early surface features.

The surface of the Earth is also being changed by weathering and erosion. Chemical reactions and frost shattering can weather rocks to produce debris which is worn away and transported elsewhere by erosion. Together, weathering and erosion result in the sculpturing and eventual lowering of the landscape. The main agents of erosion are rivers, glaciers, waves, currents and wind. Loose material can also fall, slide or creep downhill just under the force of gravity. Erosion is rapid in steep areas with high rainfall and in semi-arid regions only poorly protected by patchy vegetation, but slow in deserts and cold lowlands. It has been estimated that the average rate of erosion of the land is 8.6cm per 1000 years. Since this is rapid enough to wear Mount Everest down to sea level in only 100 million years we can see that erosion alone would have long ago destroyed craters formed four billion years ago. Today we see the net effect of tectonic mountain building activity and the levelling due to erosion.

The rock materials removed and carried away by weathering and erosion are generally deposited at the bottom of a river or ocean to form sediments. If the pressure is sufficiently high the sediments are compressed into a more compact and hardened state and are then known as SEDIMENTARY ROCKS. Such rocks cover more than two-thirds of the Earth’s surface. IGNEOUS ROCKS are produced by the solidification of magma. The third main type of rocks are the METAMORPHIC ROCKS, which are produced from other rocks by the action of heat (but not sufficient to melt the rocks) and pressure.

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