Where Are Fossils Mostly Discovered?

The world's oldest rocks are in Greenland, between 3.9 and 3.8 billion years old. (left) The age of rocks are determined by the researches made on the decay of radioactive minerals. (right)

Fossils are widely dispersed just about everywhere on Earth. Almost no fossils are encountered in some types of rock, but large numbers are found in others. Geologists have divided rock types into three main groups:

1. Igneous
2. Sedimentary
3. Metamorphic

The igneous category includes granite or basalt-type rocks formed by the cooling of magma present in the depths of the Earth, or else emitted by volcanoes in the form of molten lava. Sedimentary rocks form when sand, silt, mud, and other small particles or substances carried in water are deposited on top of one another. Metamorphic rocks are igneous or sedimentary ones that have undergone structural changes due to high temperature and pressure deep in the Earth.

Few fossils are generally encountered in igneous seams. The rare examples discovered are fossils that have resulted when a plant or animal gets trapped inside molten lava. Very few fossils can survive the high temperatures and pressures that transform sedimentary strata into metamorphic rock. Almost all fossils are found in sedimentary seams or deposits.

Nearly all sedimentary rocks are formed by substances carried by wind or water or else from the erosion of still other rocks. Some forms, such as coal, are made of plant or animal remains. Clastic is the name given to sedimentary rock formed by minute particles or grains. Sandstone and schist are examples of such rocks. If there has been dissolution in the substances transported, then due either to chemical solution or vaporization, "organic" sedimentary beds form. Examples of such rocks are limestone and dolomite. In general, sedimentary rock seams are a mixture of clastic and organic seams. Fossils are usually seen in shales, schists, sandstone and limestone formed from calcium carbonate.