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Sedimentary Rocks: Definition, Types & Examples

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  • 0:02 Sedimentary Rocks
  • 1:48 Clastic Rocks
  • 3:56 Chemical & Organic Rocks
  • 5:38 Lesson Summary
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Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

How are sedimentary rocks different from other rocks? How and where do they form, and from what? You will learn everything you ever wanted to know about these rocks in this lesson.

Sedimentary Rocks

Sedimentary rocks are rocks composed of sediment. Isn't that something of a circular definition, you might ask? Not really. They're one of the three types of rocks (the other two being igneous and metamorphic), and are the rocks formed by processes acting on the earth's surface.

Sediment refers to particles, or grains, of weathered rock or mineral debris. This stuff comes in many shapes and sizes, and a wide range of compositions. But the thing all sediment types have in common is that they were deposited by one or more of Earth's surface geological processes, such as wind, rivers, waves, and glaciers.

The difference between a loose pile of sediment and a sedimentary rock is whether or not the sediment grains are stuck together. Lithification is the fancy term for the sedimentary rock-forming process. And the two main ways for sediment to become lithified are by compaction, where the layers' weight squeezes them together into rock, and cementation, where minerals form around the layers and bind them together.

A distinguishing property of all sedimentary rocks, such as the limestone in the upper half of the picture and the shale in the lower half, is that they form in beds, or layers, which are the result of how the sediment was deposited over time. Unless they are tilted by mountain building or some other movement, the layers are nearly horizontal.

There are a lot of different ways to categorize sedimentary rocks. To some extent, the way you prefer to do it depends on whether you're a 'lumper' or a 'splitter' when it comes to classifying. For the sake of brevity, and simplicity, we'll 'lump,' in which case there are two categories of rocks to consider.

Clastic Rocks

Clastic rocks are composed of fragments of rock and mineral debris produced by weathering of other rocks on the earth's surface. During weathering, large masses are physically broken up into smaller pieces, and those grains, or clasts, are given names based on their sizes. Many of the terms are likely familiar to you.

Clay and silt are very tiny grains which, when mixed with water, make mud. Rocks made from mud-sized sediments are called claystone, siltstone, or shale, a mixture of clay and silt. The clay and silt grains are held together mostly by electrical attractions on the surfaces of the grains, the result of them being squeezed tightly by compaction alone.

You probably know what sand grains look like. A sandstone is any rock composed mostly of sand, regardless of what the sand grains themselves are made of. Sandstone is usually formed from sand deposited on beaches and in sand dunes.

For sediment that is sand-size (or larger), compaction alone isn't sufficient to bind the grains and mineral cement is needed to hold the grains together. The three most common cementing minerals are calcite, quartz and hematite, which are deposited by groundwater in the spaces between grains.

Gravel is the name for clasts bigger than sand, up to about 2 inches across. When it comes to rock names, conglomerate refers to smooth and rounded gravel pieces and breccia to sharp-edged and angular pieces.

Larger pieces of sediment are called cobbles and boulders, the latter category reserved for clasts bigger than a bowling ball. Sediment larger than gravel-size is only rarely found in sedimentary rocks, due to the fact that big pieces are too heavy to be easily transported and deposited by wind and water. However, glaciers (moving ice) can and do deposit larger sediments; the term tillite refers to a rock made of glacial sediment which is typically mixed with sand and mud.

Chemical and Organic Rocks

The other major category of sedimentary rocks includes those formed from sediment that is produced by chemical reactions, either physical or biological. There's actually some overlap, as we'll see, which explains why the two types are often discussed together.

At one end of the spectrum is coal, a rock composed of dead plant material that was compressed, then altered over time by anaerobic bacteria, bacteria living in low-oxygen conditions. Coal comes in different types, depending on the extent of compaction and alteration. Obviously, we define the term 'sediment' pretty broadly to classify coal!

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