Most sedimentary rocks are formed by layers of sediment accumulating on top of each other over time. Chemical sedimentary rocks form as the result of minerals in water. As water evaporates it leaves behind these mineral deposits in a crystallized form. Over time these deposits harden to form rock. Chemical sedimentary rocks form three basic groups chert, carbonate rocks, and evaporate rocks.
Chert is often formed through biochemical processes. The rock itself is made of silicon dioxide which is left behind by dead marine life. Some marine creatures such as glass sponges, diatoms, and radiolarians have tiny tiny shells that are made of silicon. When these creatures die these shells will fall to the bottom of the ocean where they will accumulate over time hardening to form silicon. Influxes of sea water can bring and deposit these materials as the water retreats they are left to harden. In ancient time’s seas where higher so it’s possible that some chert deposits found inland are the result of ancient seas. Plate tectonics have a role in bringing deposits of chert to the surface along the sea bed up as they collide into each other. Chert also forms when tiny silicon dioxide particles become embedded in surrounding rock such as limestone.
Chert was once widely sought after for the manufacture of weapons. Early cultures like the Native Americans would use it to form arrow heads and hatchets due to its relative strength and the ability to easy manipulate it. It was also widely used through Europe for the formations and tools thousands of years ago. Early rifles required flint to fire which is made of chert. With modern techniques and technologies chert does not have the significance that it once did.
Carbonate rocks are made up primarily of the mineral carbonate and they form as the result of biological and chemical processes. Carbonate rocks that are formed through biological processes are made up the shells of animals that have absorbed carbonate from water. As they die the shells collect together and fuse with other material to create rocks. During the chemical process carbonate is carried through the water and deposits through precipitation. This can be seen in environments such as caves where minerals in the water are left behind creating some impressive works of beauty in the process.
Rocks made up carbonates have been long been used by people. Marble a type of carbonate rock has been used in architecture and art for thousands of years. The ancient Greeks especially valued marble and many ancient temples were built from it. Limestone is another carbonate rock that is valuable in building construction and is widely used in cement today. Trona is used in tooth paste, while pot ash is used in fertilizers.
Evaporate rocks are formed as water evaporates depositing minerals left behind in the process. This process frequently occurs in dry environments. A shallow body of water where there is not an outflow in conjunction with evaporation that occurs faster than water inflow is a perfect condition to see this. The Great Salt Lake is a perfect example of this occurring. The climate is hot and dry and water going in has nowhere to go. Overtime the water has developed a very high salinity. As the water evaporates salt deposits get left behind. When bodies of water that have a high salt content completely dry up they leave behind salt flats.
Evaporate rocks have a very important function in the world. Salt is an important mineral needed for the human body. It has also long been used to preserve and flavor foods. Other evaporate minerals are important as well. Nitrates are widely used in fertilizers and play a role in explosive manufacturing as well. Halite deposits have been used to store nuclear waste such as facilities in New Mexico. They are chosen for their stability. Salt caverns are also used to store oil due to the hardness and the ability of the rock to seal itself if damaged. It is in caverns like these the United States keeps its strategic oil reserves. Chemical sedimentary rocks play a valuable role in people’s daily lives from securing waste to providing building material.
