High School Earth Science: Rocks - Sedimentary Rocks — Flashcards

Sedimentary Rock Formation - Accumulated sediments harden into sedimentary rock by lithification. Two important steps are needed for sediments to lithify: compaction and cementation.    
Compaction occurs when sediments are squeezed together by the weight of all the sediments on top of them.    
Cementation occurs when spaces between sediments fill in with mineral crystals. The minerals crystallize out of water that seeps through the sediments.    

Sedimentary rocks can form from sediments with a wide range of sizes. The type of sedimentary rock that forms depends on the size of the sediments.    
- If sediments are large and rounded, they form rock called conglomerate.    
- If sediments are large and angular, they form rock called breccia.    
- Smaller, sand-sized sediments form sandstone.    
- Still smaller, silt-sized sediments form siltstone.    
- The smallest, clay-sized sediments form shale.    

When sediments settle out of calm water, they form horizontal layers. One layer is deposited first; then another layer is deposited on top of the first layer. More and more layers are deposited on the layers below them. Therefore, any layer of sedimentary rock is always younger than the layer below it, as long as the layers have not been disturbed.    
Biochemical sedimentary rocks form at the bottom of an ocean or salt lake. Living creatures remove ions—such as calcium, magnesium, and potassium—from the salt water to make shells and other tissues. When the organisms die, they sink to the ocean floor. Their shells become biochemical sediments, which may then become compacted and cemented into sedimentary rocks.