Shale Rock: Formation, Uses & Facts Video

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  • 0:01 What Is Shale?
  • 1:02 Formation of Shale
  • 2:40 Uses for Shale
  • 4:00 Interesting Facts
  • 4:40 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about shale, a sedimentary rock formed from clay minerals. Discover how it forms and its uses, including its application as an energy resource and a source of many of the world's fossils.

What is Shale?

These days, when you hear the word shale it is almost always in reference to the growing oil shale industry. But there is more to it than simply an energy resource. Shale is a type of sedimentary rock that forms in muddy, clay mineral-filled environments. Like all sedimentary rock, it forms from the weathering by-products of other rocks, usually those containing clay minerals and quartz.

Unlike some other sedimentary rocks, like sandstone, shale is fine grained, meaning you cannot pick out individual mineral grains within the rock with the naked eye or even by touch. Because it forms in muddy, aquatic environments, shale often traps organic material in its rock matrix when it is forming, allowing for the formation of fossil fuel-like shale deposits. New technology has made mining these deposits more economically feasible in recent years.

Formation of Shale

Shale, like all sedimentary rocks, forms as a result of the compaction and cementation of materials that have weathered and eroded off of other, pre-existing rocks. These pre-existing rocks can be igneous, metamorphic or even other sedimentary rocks. For shale, it specifically forms from a muddy mixture of clay minerals (like kaolinite, montmorillonite or illite) and silicates (like quartz or chert). Some deposits also contain traces of mica minerals, which tend to give the rocks a greenish color, or iron minerals, which will give the rocks a reddish color. Browns and yellow browns are also possible but not as common. Most of the oil and natural gas-bearing shales tend to be dark gray to black in color, due to the large amounts of organic matter they contain.

Eventually the minerals in the mud settle out of the liquid mud solution and become compacted. It is this compacting that allows for rock formation, rather than remaining a permanent mud deposit. This settling out and compaction process can only happen in slow moving to still water bodies, such as lakes or deltas, where there is little water flow to prevent the settling out process from occurring. You can also sometimes see shale deposits along continental shelves, where again, the water currents would be slow moving. These slow moving waters also allow for many fossils to form within shale deposits, making shale one of the most common, if not the most common, fossil-bearing rock types.

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