Page 19 - Curriculum Visions Dynamic Book

basalt: an igneous rock with a low proportion of silica (usually below 55%). It has microscopically small crystals.
granite: an igneous rock with a high proportion of silica (usually over 65%). It has well-developed large crystals. The largest pink, grey or white crystals are feldspar.
half-life: the time it takes for the radiation coming from a sample of a radioactive element to decrease by half.
igneous rock: a rock that has solidified from molten rock, either volcanic lava
on the Earth’s surface or magma deep underground. In either case the rock develops a network of interlocking crystals.
 The round table was thought to have belonged to King Arthur of England and therefore date from the 6th century. Carbon dating revealed that it was, in fact, made in the 13th century.
Carbon-14
Carbon-14 is a radioactive isotope found in living things. It is produced high in the atmosphere where nitrogen-14 is bombarded by cosmic rays.
All forms of carbon dioxide are absorbed by plants and used to form tissues. As long as the plants, or animals that feed on them, are alive, there is
a constant amount of radioactivity
in the tissues, because old, decaying carbon-14 is replaced by further high activity carbon-14 from the air. As soon as the organism dies, the carbon is no longer absorbed and so the amount of radioactivity begins to decrease.
By measuring the radioactivity of carbon in a sample of organic material – timber, cloth, seeds, mummified bodies, etc. – its age can be accurately determined. Carbon-14 has a long half-life (about 5600 years). It can readily be used to date materials that are up to ten thousand years old – and a few even older samples have also been dated.
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