Sedimentary rock covers most land yet makes up just 8 percent of the crust
Walk across the continents and nearly three quarters of the exposed ground is sedimentary rock. Dig down, though, and it turns out to be a thin skin: only about 8 percent of the crust's volume, draped over igneous and metamorphic rock. That skin still holds most of what we know about Earth's past.
These rocks begin as loose particles that settle and later get cemented. Some particles are mineral debris weathered from older rock or spat out as blobs of lava; wind, water, ice and landslides deliver them. Others are biological, the shells, bodies and droppings of aquatic creatures sinking as marine snow. Still others precipitate straight out of mineral-laden water. They build up in layers, or strata, often inside large sedimentary basins, and similar rocks have even been found on Mars.
Geologists sort them into four families by origin. Clastic rocks are cemented fragments, graded on the Udden-Wentworth scale: gravel above 2 millimetres makes conglomerate if rounded and breccia if angular, sand from 0.06 to 2 millimetres makes sandstone, and finer mud makes mudrock, with shale reserved for mudrock that splits into sheets. Biochemical rocks come from living things, such as limestone built from coral, mollusc and foraminifera skeletons, chert from diatoms and radiolaria, and coal from compressed plants. Chemical rocks precipitate inorganically, like rock salt and gypsum. A catch-all fourth group covers volcanic tuff and breccias shattered by meteorite impacts.
Sandstones have their own naming grammar under the widely used Dott scheme. A quartz sandstone has more than 90 percent quartz grains; below that, whichever of feldspar or rock fragments dominates sets the name. A clean sandstone with open pores is an arenite, while one with over 10 percent muddy filling is a wacke, so combining the terms yields six possible names. Outside specialist circles, older words like greywacke and arkose persist.
The practical stakes are high. Sedimentary layers supply coal, oil and gas, drinking water and ores, and knowing their properties guides the building of roads, tunnels and canals. Read in sequence, the strata are the main archive for reconstructing ancient geography, past climates and the history of life, the subject matter of sedimentology.
Source: Sedimentary rock
