Sedimentary rocks

Most of the rock exposed at Earth's surface is sedimentary, built grain by grain from pieces of older rock.

A sandy beach, a muddy riverbank and a lake floor covered in shells are all sediment: loose material on its way to becoming rock. The USGS notes that most rocks exposed at the surface are sedimentary, formed from particles of older rock broken apart by water or wind. Making one takes a chain of steps, and each step leaves its mark on the finished rock.

Breaking rock down

Weathering is the wearing down or breaking of rock in place. The BGS sorts it into three kinds. In physical weathering, water seeps into cracks and freezes. Ice is about 9% less dense than water (USGS), so it takes up roughly a tenth more room and wedges the crack wider. When it thaws, more water gets in, and the cycle repeats. In chemical weathering, rainwater that has dissolved carbon dioxide from the air is slightly acidic and slowly reacts with minerals in the rock. In biological weathering, roots grow in cracks and burrowing animals pry rock apart.

Erosion carries the pieces away. Gravity, wind, rain, rivers, the sea and glaciers all move rock fragments, and the pieces can travel a long way from the rock they came from. Deposition is the laying down of that sediment once the wind, water or ice carrying it slows down. Grains sort themselves as they settle. In the mid-1600s the Danish scientist Nicholas Steno noticed that the largest or heaviest particles settle first and the smallest last, and that slight changes in size or composition produce layers, called beds. Layering is the most obvious feature of sedimentary rocks.

From sediment to stone

As new layers pile up, older ones are buried. The weight above squeezes the grains closer together and pushes out water: this is compaction. Then minerals carried by water moving through the gaps grow around the grains and stick them together, the way cement binds sand in mortar. The Geological Society lists calcite, silica, iron oxides and clay minerals as common cements. Together, compaction and cementation turn loose sediment into solid rock.

Naming sedimentary rocks

Clastic sedimentary rocks are made of pieces, or clasts, of older rock, and they are named by grain size. From smallest to largest the grains are clay, silt and sand; grains larger than 2 mm are pebbles. Shale is made mostly of clay, siltstone of silt, sandstone of sand, and conglomerate of pebbles in a matrix of sand or mud. Other sedimentary rocks are made by living things. When large numbers of organisms die, their shells or skeletons can pile up into chert or limestone, and limestone can also form when minerals precipitate directly out of the water.

Sedimentary rocks keep a record that other rocks lose. The USGS says fossils are found almost exclusively in sedimentary rocks. Igneous rocks are too hot when they form, and the heat and pressure that make metamorphic rocks generally destroy fossils. Plants and animals that die in or near water are buried quickly by sediment, which is why the fossil record leans heavily towards sea life.