Sedimentary rocks are the second great rock class. Whereas igneous rocks are born hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata; hence they are also called stratified rocks. Depending on what they're made of, sedimentary rocks fall into one of three types.
How to Tell Sedimentary Rocks
The main thing about sedimentary rocks is that they were once sediment — mud and sand and gravel and clay — and were not greatly changed as they turned into rock. The following traits are all related to that.
- They're generally arranged in layers of sandy or clayey material (strata) like those you'll see in excavations or a hole dug in a sand dune.
- They're usually the color of sediment, that is, light brown to light gray.
- They may preserve signs of life and surface activity, like fossils, tracks, ripple marks and so on.
Clastic Sedimentary Rocks
The most common set of sedimentary rocks consists of the granular materials that occur in sediment. Sediment mostly consists of surface minerals — quartz and clays — that are made by the physical breakdown and chemical alteration of rocks. These are carried away by water or the wind and laid down in a different place. Sediment may also include pieces of stones and shells and other objects, not just grains of pure minerals. Geologists use the word clasts to denote particles of all these kinds, and rocks made of clasts are called clastic rocks.
Look around you at where the world's clastic sediment goes: sand and mud are carried down rivers to the sea, mostly. Sand is made of quartz, and mud is made of clay minerals. As these sediments are steadily buried over geologic time, they get packed together under pressure and low heat, not much more than 100 C. In these conditions the sediment is cemented into rock: sand becomes sandstone and clay become shale. If gravel or pebbles are part of the sediment, the rock that forms is conglomerate. If the rock is broken and recemented together, it is called breccia.
It's worth noting that some rocks commonly lumped in the igneous category are actually sedimentary. Tuff is consolidated ash that has fallen from the air in volcanic eruptions, making it just as sedimentary as a marine claystone. There is some movement in the profession to recognize this truth.
Organic Sedimentary Rocks
Another type of sediment actually arises in the sea as microscopic organisms — plankton — build shells out of dissolved calcium carbonate or silica. Dead plankton steadily shower their dust-sized shells onto the seafloor, where they accumulate in thick layers. That material turns to two more rock types, limestone (carbonate) and chert (silica). These are called organic sedimentary rocks, although they're not made of organic material as a chemist would define it.
Another type of sediment forms where dead plant material builds up into thick layers. With a small degree of compaction, this becomes peat; after much longer and deeper burial, it becomes coal. Coal and peat are organic in both the geological and the chemical sense.
Although peat is forming in parts of the world today, the great beds of coal that we mine formed during past ages in enormous swamps. There are no coal swamps around today because conditions do not favor them. The sea needs to be much higher. Most of the time, geologically speaking, the sea is hundreds of meters higher than today, and most of the continents are shallow seas. That's why we have sandstone, limestone, shale and coal over most of the central United States and elsewhere on the world's continents. (Sedimentary rocks also become exposed when the land rises. This is common around the edges of the Earth's lithospheric plates.
Chemical Sedimentary Rocks
These same ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate), starting with calcite, then gypsum, then halite. The resulting rocks are certain limestones, gypsum rock, and rock salt respectively. These rocks, called the evaporite sequence, are also part of the sedimentary clan.
In some cases, chert can also form by precipitation. This usually happens below the sediment surface, where different fluids can circulate and interact chemically.
Diagenesis: Underground Changes
All kinds of sedimentary rocks are subject to further changes during their stay underground. Fluids may penetrate them and change their chemistry; low temperatures and moderate pressures may change some of the minerals into other minerals. These processes, which are gentle and do not deform the rocks, are called diagenesis as opposed to metamorphism (although there is no well-defined boundary between the two).
The most important types of diagenesis involve the formation of dolomite mineralization in limestones, the formation of petroleum and of higher grades of coal, and the formation of many types of ore bodies. The industrially important zeolite minerals also form by diagenetic processes.
Sedimentary Rocks Are Stories
You can see that each type of sedimentary rock has a story behind it. The beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils or sedimentary structures such as marks left by water currents, mud cracks or more subtle features seen under the microscope or in the lab.
From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desert sand, organic rocks in peat bogs or lake beds, and evaporites in playas. These are called continental or terrigenous (land-formed) sedimentary rocks.
Sedimentary rocks are rich in geologic history of a special kind. While igneous and metamorphic rocks also have stories, they involve the deep Earth and require intensive work to decipher. But in sedimentary rocks, you can recognize, in very direct ways, what the world was like in the geologic past.
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