Sedimentary rocks are made from layers, or strata, of mineral particles found in other rocks that have been weathered and from newly formed organic matter. Most inorganic minerals in sedimentary rocks are from igneous rocks.
When rock minerals are weathered, their chemical composition is changed, weakening the solid rock. The rock breaks up into particles of many sizes. When these particles are transported in a fluid—air, water, or glacial ice—we call them sediment.
Streams and rivers carry sediment to lower land levels, where it builds up. Most sediments accumulate on shallow seafloors bordering continents, but they also collect in inland valleys, lakes, and marshes. Wind and glacial ice are two other agents of transportation that can move sediment.
Over long spans of time, the sediments become compacted and harden to form sedimentary rock, with distinctive visible characteristics.
There are three major classes of sediment: clastic sediment, chemically precipitated sediment, and organic sediment.
Clastic sediment is made up of inorganic rock and mineral fragments, called clasts. These can come from igneous, sedimentary, or metamorphic rocks, and so they can include a very wide range of minerals. Quartz and feldspar usually dominate clastic sediment.
The size of the clastic sediment particles determines how easily and how far they are transported by water currents. Fine particles are easily suspended in fluids, while coarse particles tend to settle to the bottom. In this way, particles of different sizes are sorted in the fluid.
When layers of clastic sediment build up, the lower strata are pushed down by the weight of the sediments above them. This pressure compacts the sediments, squeezing out excess water.
Dissolved minerals recrystallize in the spaces between mineral particles in a process called cementation. Sandstone, a rock made of sand, and shale, a rock made of clay particles, are typical examples.
Chemically precipitated sediment is made of solid inorganic mineral compounds that precipitate from water solutions or are formed by organisms living in water. One of the most common sedimentary rocks formed by chemical precipitation is limestone.
The third class of sediment is organic sediment. This is made up of the tissues of plants and animals. Peat is an example of an organic sediment. This soft, fibrous, brown or black substance accumulates in bogs and marshes where the water stops the plant or animal remains from decaying.
Peat is a compound of hydrogen, carbon, and oxygen. Hydrocarbon compounds such as this are the most important type of organic sediment—one that we increasingly depend on for fuel. They formed from plant remains that built up over millions of years and were compacted under thick layers of inorganic clastic sediment. Hydrocarbons can be solid (peat and coal), liquid (petroleum), or gas (natural gas).
Coal is the only hydrocarbon that is a rock. We often find natural gas and petroleum in open interconnected pores in a thick sedimentary rock layer, such as in a porous sandstone. These fossil fuels, as they are known, took millions of years to build up. But our industrial society is consuming them rapidly.
They are nonrenewable resources—once they are gone, there will be no more. Even if we wait another thousand years for more fossil fuels to be created, the amount we gain will scarcely be measurable in comparison to the stores produced in the geologic past.