Sedimentary Rocks

Everyday we see them, rocks. They appear on our morning commute, or during the daily workout. You see them everywhere, but where did they come from? What kinds of rocks are there and how do you tell them apart? This is the first of a series of articles where I will cover the different rock types and discuss the processes by which many of the rocks you see everyday are formed. From my previous article on the Rock Cycle, we learned that there are three types of rocks, sedimentary, igneous, and metamorphic. We will begin our look at the rocks around us by exploring sedimentary rocks in depth.

Sedimentary rocks are by far the most common of all rock types. Sedimentary rocks cover most of the ocean floor and three quarters of the land surface. They are almost always deposited in horizontal beds that distinguish them from igneous and metamorphic rocks.

Sedimentary rocks are formed from loose sediment; generally this sediment has come from rocks that have eroded from other rocks. The rocks are formed in a process known as diagenesis. Diagenesis is the process by which unconsolidated sediments like sands, clays, silts, or organic materials are either chemically of physically turned into sedimentary rocks.

There are three different types of sedimentary rocks. These types depend on what the original sediment was that the rock formed from. The first of these are sedimentary rocks formed from grains of sediment known as clasts. Rocks formed from clasts are called clastic sedimentary rocks. Clastics are named based on the grain size of the clast material. If the clast size is larger than 2 millimeters, the rock is called a conglomerate. Rocks with clast sizes between .0625 and 2 millimeters are called sandstones. Siltstones form from clasts of .004 to .0625 millimeters in size. If the clast size is less than .004 millimeters the rock is called a claystone. Siltstones and claystones are also referred to as mudstone or shale. Clastics are the most common of all sedimentary rocks, with shale accounting for 81 percent of all sedimentary rocks and sandstone for 11 percent.

The second type of sedimentary rocks are those that form from chemical precipitation. Chemically precipitated rocks form from a simple process. Generally, these rocks begin in water that contains high concentrations of minerals. As the minerals in the water become highly concentrated, they begin to precipitate out of the water and are deposited on floor of the body of water. Limestone, probably the most common chemically deposited sedimentary rock, forms when calcium carbonate is precipitated from shallow seawater. Gypsum forms from the precipitation of calcium sulfate. Shallow water high in sodium chloride yields halite, or rock salt.

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