What are the ways in which different rocks were formed?
Sedimentary rocks forms from sediments. Sediments are loose materials such as rock fragments, mineral grains and bits of plant and animal remains that have been transported and deposited. Layer upon layer of sediments build up as more and more sediments are deposited in a place. Each upper layer presses down on the layer below it. The pressure causes the layers to stick together or become compacted and form solid rock. This happens when the sediments are small. Try to press a lump of clay between your palms. Do the particles of clay stick together? This is similar to what happens when a layer of small sediments presses on the layer below it. Sand and pebbles are sediments that are large. Large sediments cannot be compacted. They are cemented together naturally from minerals dissolved in water. The cemented sediments form sedimentary rock. Most sedimentary rocks form as layers. The older layers are at the bottom because they were deposited first.
Some sedimentary rocks form from rock fragments. Here are some of them:
Conglomerate is a rounded mass of pebbles with much quartzite and feldspar. The sediments are held together by sand, mud and clay. This rock may be red, white or light brown. It has a coarse texture and is hard to scratch.
Sandstone consists of fine sand cemented together. It contains quartz grains. It contains quartz grains. It may be red, gray or brown. It is coarse-grained and filled with tiny holes or air spaces.
Shale is formed from fine mud, clay and silt. It has a smooth surface and can be split into layers. Shale may be red, brown, gray or black. Some have alternating dark and light layers.
Some sedimentary rocks form from chemical sediments. These chemical sediments come from dissolved minerals in seas and lakes. When water evaporates, the deposits of minerals form rocks. Examples of sedimentary rocks formed from chemical sediments are limestone, rock salt and rock gypsum.
Some sedimentary rocks are formed from remains of plants and animals. Many algae and animals that lived in the sea are lime-forming. Examples of lime-forming animals are corals, snails, mussels, oysters and may others. To make their shells, these animals get calcite dissolved in water. When they die, their shells pile up on the seafloor. After a long time, the shells become cemented and form into limestone.
Coal is another sedimentary rock. It is formed from mosses, ferns and tree parts that were buried in swampy places millions of years ago. Carbon from the plants becomes part of the sediments. The sediments change as they are compacted. After thousands of years, the sediments form coal.