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Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).
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.
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.
As it turns out, rock strata are present almost everywhere. That's because the processes that create them are acting on the earth's surface. Sedimentary strata can even be found atop mountain ranges, where rock layers have been lifted thousands of feet. While it is possible for layering to form in other rocks, it is the singular feature of sedimentary rocks. So if you pass through a highway excavation and see distinct layers of rock, you can be reasonably sure you are looking at sedimentary rock strata.
Strata are very important geologic features because geologists can use them to construct interpretations of the sequence of geologic events that happened far in the past.
For example, the kind of sediment in a particular stratum tells geologists something about how the sediment was deposited, and where. Sandstone forms from beach, floodplain or delta sand deposits, while limestone is made of the shell fragments of ocean-dwelling organisms that collected on a seafloor.
Because we know that sediment is originally deposited in flat-lying layers, if rock strata are tilted or offset by a fault, it means there has been tectonic activity in an area. Geologists also use sequences of rock strata to determine the ages of different beds relative to one another. Older beds are at the bottom of a stack of layers, assuming they have not been tipped over!
Rock strata refers to stacked-up layers of sedimentary rock. Other kinds of rock can have layering in them but the word 'strata' is reserved for sedimentary rocks, rocks composed of individual fragments of minerals or other rocks. 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 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.
Strata are very important geologic features because geologists can use them to construct interpretations of the sequence of geologic events that happened far in the past.
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