Sedimentary rocks are made up of fragments of existing rocks which are called sediments. They are deposited in shallow waters, basins, and on ocean floors in layers which eventually become cemented or bonded together to form a solid rock. This process, called lithification, occurs in three ways; compaction, cementation and crystallization.
Sedimentary rocks are divided into two groups based on the type of sediments within them. Most of these rocks are formed through a process called lithification, which occurs when rock is formed by the compacting and cementing of sediments that have been carried and deposited in a certain area. When the particles within the sediment become rounded from weathering, it creates sedimentary rocks that are called clastic rocks. Another group of sedimentary rocks are created by chemical precipitation or by crystallization. These are called nonclastic rocks.
Compaction occurs when the weight of the materials above the sediments presses them together to form a rock. Rock particles along with the remains of organic material such as shells are deposited in the sea where they slowly accumulate and create layers. The process takes thousands of years, but eventually thick deposits form on land or on the ocean floor. The weight of the top layers compact the layers below, and over time they form sedimentary rocks such as sandstone and shale.
Cementation occurs when precipitation from the minerals around the sediments bond them together, making a solid rock. Most sedimentary rocks are bonded together by calcite and quartz, which act like cement to hold the pieces of sediment together. High pressures combine with high temperatures speed up this process by dissolving the minerals. If the sediment continues to be deposited in the same depositional basin, then new layers cover the old sediments. The dissolved minerals are forced into empty spaces between the grains, acting as a cement to bond them together and form sedimentary rock such as limestone.
When a rock is formed from a solution, it is called crystallization. Typically sedimentary rocks form in shallow areas of the water or in lakes in arid regions such as a desert where the rate of evaporation is higher than the precipitation. Water is lost through evaporation, leaving behind the dissolved minerals which form crystals. As the water continues to evaporate, more crystals are formed and they accumulate on the ocean or lake floor, becoming sedimentary rocks such as coal.