Rock Strata: Definition & Explanation

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  • 0:00 Definition of Rock Strata
  • 1:25 Seeing the Strata
  • 3:05 Importance of Sedimentary Rock
  • 4:00 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).

Rock strata are found almost everywhere, even on the tops of mountains. In this lesson, we'll learn what rock strata look like, how they form, and their importance to geologists. Then you can test your knowledge with a short quiz.

Definition of Rock Strata

Have you ever heard a weather forecaster talk about status clouds? Those are clouds that are spread out in layers, but are not all that thick vertically. Well, rock strata are pretty much the same thing - except they aren't clouds, they aren't in the sky, and they're made of sediment and not water droplets.

The term rock strata refers to stacked-up layers of sedimentary rock. Other kinds of rocks can have layering in them, but the word strata is reserved for sedimentary rocks - rocks composed of individual fragments of minerals or other rocks.

Geologists tend to use the term 'rock strata' in a generic sense when referring to many rock layers that appear over large areas. The singular form stratum, which is a Latin word that means 'spread out', can be used for a single layer, but individual rock layers (called beds) are more commonly referred to using a specific name.

The root word also lends itself to other geologic terms. The sub-discipline of geology that involves study of rock strata is called stratigraphy. Layering of rocks or sediment is also called stratification. A sequence of sedimentary layers stacked one atop the other is known as a stratigraphic section. And geologists sometimes refer to something formed in layers as a stratiform deposit. You get the idea.

Seeing the Strata

Strata are distinguished from one another on the basis of their physical composition. Fortunately, the layering of sedimentary rocks is made obvious because the compositional differences almost always produce layers with different thicknesses and colors.

Sedimentary rocks form as more-or-less horizontal layers because of the way in which sediment, sand, mud, rock debris, shell fragments and so on, is deposited. All sedimentary rocks form when moving water, wind or ice erodes, transports and deposits sediment. Wherever sediment is deposited - for example on a lakebed, the seafloor, in a river delta, or a sand dune - it spreads out in thin layers. Over long periods of time, those layers build up into much thicker ones. Compaction under the weight of overlying sediment squeezes the sediment together, and water seeping through leaves behind mineral cement that binds the sediment. What was once mud or sand is compressed and hardened into a stratum of sedimentary rock.

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