Sedimentary Rocks Laying the rock down!
How do sedimentary rocks form? • Igneous rocks are the most common rocks on Earth, but because the majority of them exist below the surface we do not see a lot of them. • 75% of the rocks at the surface are sedimentary rocks. • Sediments are loose materials such as rock fragments.
What are sediments? • Sediments are any loose material that get trapped with rock fragments and through the process of cementation get included in sedimentary rocks. • Examples include but are not limited to: plant and animal remains, rock fragments, and mineral bits.
Why layering? • Sedimentary rocks often form as layers with the older layers on the bottom. • Sometimes forces within the Earth overturn layers of rock causing the oldest to no longer be on the bottom.
Classifying sedimentary rock. • Sedimentary rocks can be made of just about any material found in nature. Sediments come from weathered and eroded igneous, metamorphic, and sedimentary rocks. Sediments also come from the remains of plants and animals. Sedimentary rocks are usually classified as detrital, chemical or organic.
Detrital sedimentary rocks • Detrital sedimentary rocks are made from the broken fragments of other rocks. When rock is exposed to the air, water, and ice, pieces are broken off which is called weathering. The movement of weathered rock pieces – erosion – moves sediments to a new location where they can be deposited.
Detrital sedimentary rocks • Layers of sediment build up with pressure from the upper layers pushing down on the lower layers. If the sediments are small they stick together in a process called compaction. If they are large another process called cementation is needed. Cementation occurs when water moves through the large rock pieces binding them together.
Detrital sedimentary rocks • Detrital rocks are named and classified depending on the size of the sediments (see table 2). • Sediments range from clay to silt to sand to gravel. • They form shale, siltstone, sandstone, and conglomerate or breccia.
Chemical Sedimentary Rock • Chemical sedimentary rocks form when dissolved minerals come out of solution. • Examples are gypsum or limestone.
Organic sedimentary rocks • Rocks made from the remains of once living things are called organic sedimentary rocks. • Two of the most common are chalk and limestone. • Made from the mineral calcite. • CaCO3 • Mostly made from the shells of once-living ocean organisms.