Organic Sedimentary Rocks

This image of Australia's Great Barrier Reef shows organic sedimentary rocks in production! Click on image for full size (54K JPG) Courtesy of Abi Howe, American Geological Institute

The sediment in an organic sedimentary rock is made of fossils!

The mineral parts of animals, such as bones and shells, are much more likely to be preserved than the soft tissues, which typically decay. The bones, shells, and other skeletal parts can become cemented together over time to make sedimentary rock. Usually the bones are made of calcite, or a similar mineral called aragonite, and the resulting organic rock is called limestone. Sometimes the skeletal material is made of silica, such as with diatom algae and some other types of plankton, and the resulting organic rock is called chert.

Plants compressed together over millions of years make an organic sedimentary rock called coal. Coal doesn’t look like it is made of sediment. In fact, it is often difficult to see the plant fossils within it because they have become so compacted over time, some of the more volatile materials are no longer present, and all that is left is the organic carbon.

This image of Australia's Great Barrier Reef shows organic sedimentary rocks in production! Click on image for full size (54K JPG) Courtesy of Abi Howe, American Geological Institute

The sediment in an organic sedimentary rock is made of fossils!

The hard parts of animals, such as bones and shells, can become cemented together over time to make rock. Usually the bones and shells are made of calcite, or similar minerals, and the organic rock that is made from them is called limestone. Some types of microorganisms that live in the ocean or lakes have tiny skeletons made of silica. The organic rock made from their skeletons is called chert.

Plant remains squashed deep underground over millions of years make an organic sedimentary rock called coal. Coal doesn’t look like it is made of sediment. It is also often difficult to see the plant fossils within it because they have become so compacted over time, the less stable materials have left, and all that remains is carbon.

Look under the sea at Australia's Great Barrier Reef! This is one place where organic sedimentary rocks are being made. Click on image for full size (54K JPG) Courtesy of Abi Howe, American Geological Institute

Rocks made of fossils are called organic sedimentary rocks.

This picture shows one place where organic rocks are being made on the Earth today. This is a coral reef where the skeletons of coral, clams, fish, and algae are being bound together to make a rock called limestone.

Some organic rocks are made of plants! They form when plant that have died are squashed deep underground for millions of years. The rock that they make is called coal. Coal doesn’t look like it is made of sediment. It is also often difficult to see the plant fossils within it because they have become so packed together.