Sedimentary rock forms in layers over time.
Sedimentary rocks are formed from small mineral particles called sediment. Natural forces, such as gravity, water or wind, can exert enough pressure on sediment to shape it into solid rock. Unlike igneous rocks, which are formed from cooled lava, sedimentary rocks do not float in water. This is because they are solid all the way through.
Sedimentary vs. Igneous Rocks
Sedimentary rocks form in layers, with each layer older than the one above it. In addition to mineral matter, they often contain plant and animal matter that was mixed in with the sediment. They are too dense to float in water. Igneous rocks such as obsidian and pumice, on the other hand, do float. They are formed from cooled lava, which often cools so quickly that air bubbles are trapped inside the rocks. These bubbles give igneous rocks the property of flotation.
The Composition of Sedimentary Rocks
When natural forces, such as precipitation and the melt cycle of snow and ice, act on a mountain formation, pieces of the mountain erode and crumble off. These sediments are carried down the mountain by streams and rivers and eventually settle at the bottom of the lake or ocean where they end up. Water pressure, as well as the pressure of the top layers of sediment on the bottom layers, form them into sedimentary rocks.
Types of Sedimentary Rocks
Sedimentary rocks are divided by geologists into six categories. Conglomerate rock consists of round rocks, like pebbles and boulders, cemented into a matrix. Breccia is the same formation, but with jagged rocks. Sandstone is made of cemented sand grains, sometimes of different colors. Shale is hardened clay that breaks into flat sections. Limestone is made of calcium carbonate and usually full of microscopic shells. Finally, gypsum is a soft sedimentary mineral containing salt from evaporated seawater.
Fossils in Sedimentary Rocks
Because sedimentary rocks form by gradual pressure over time, the delicate animal and plant matter mixed up in the sediment is preserved intact. This allows sedimentary rock to be a rich source of fossil deposits. Shells, shark teeth, the skulls of deer and elk and the intact skeletons of small animals can all be commonly found in sedimentary rock, depending on the geographical area. These fossils have not only scientific but economic importance, in the case of fossil fuels like petroleum, oil, coal and natural gas.
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