Chemically and/or Organically Formed Sedimentary Rocks The table above is from the Scheme for Sedimentary Rock Identification on page 7 in the reference tables.

Chemically and/or Organic Sedimentary Rocks Chemical sedimentary rocks form when minerals precipitate from water (Figure 8). This occurs in dry climates were evaporation rates are high Ex: rock salt Figure 8. Under the comments section, highlighted in blue, in your reference tables you can see that these types of sedimentary rocks form the precipitation of a solution, like sea water.

Chemical Sedimentary Rock Examples On the left is rock gypsum forming in Death Valley, California. Due to the arid climate, evaporation rates are high. Rock salt is forming for the same reasons around Salt Lake in Utah, which can be seen on the right.

Chemically and/or Organic Sedimentary Rocks Organic sedimentary rocks are formed from the shells of marine animals or from plant material (Figure 9). The most common organic sedimentary rock is limestone Almost all caves are formed from limestone Figure 9. Under the texture and comments section you can see that both coal and limestone can form from biological material.

Example of Organically Formed Sedimentary Rocks -Limestone There are many different types of limestone, all form from the deposition of calcium carbonate (calcite). On the left is chalk which forms when millions of tiny marine animals. In the middle is a limestone composed of shell fragments and is called coquina. When either of these rocks become buried miles below the surface of the Earth, heat and pressure turn the rock into the limestone you see on the right.

phytoplankton foraminifera coral bivalves like claims All of these different plants and animals make their shells or hard parts by extracting calcium carbonate (calcite) from seawater. When they die the are deposited on the sea floor and later turn into limestone.

Cliffs of Dover in England is limestone made from the remains of tiny marine organisms.

Coal Formation About 400 million years ago the only plants that existed on Earth were moss-like plants that didn’t grow above a few feet tall. It was around this time period that some plants evolved the ability grow into the large size that we see today. Bacteria and fungi weren’t able to decompose this material so the organic material built up into thick layers. This organic material was later buried and pushed deep into the earth were heat and pressure converted it into coal.