Stratification: Definition, Theory & Examples

Lesson Transcript
Instructor: Ryan Hultzman
This lesson introduces the concept of stratification and includes examples of naturally stratified items. It also explains why items are stratified and what stratification can tell us about past environments. A summary and brief quiz are included.

Definition of Stratification

To begin this lesson, please consider the following: What do the rocks pictured below have in common with a lake during summertime?

Stratified Rock

At first glance it may appear these two items have little in common. However, both the rocks from our picture and the lake during summertime are each stratified. In other words, they both contain layers. Stratified rock is made of visible layers of sediment, while the lake contains a warm upper layer and a cold bottom layer. This layering is caused by different factors that we'll explore throughout this lesson.

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  • 0:00 Definition of Stratification
  • 0:34 Causes of Stratification
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Causes of Stratification

Stratified rock and water layers are caused by different factors. For the purposes of simplicity and clarification, let's start by exploring how layers of stratified rock are formed. We'll then move on to water.

If you refer back to our picture, you may recognize this as sedimentary rock. Sedimentary rock is rock that was formed by layers of sediment being laid down over the course of time. These sediment layers create the banding pattern visible in stratified rock. The sediments themselves also teach us about the environment in which the rock was formed.

For example, if we have a layer of shale overlain by a layer of limestone, then we know the environment was once mud-covered before flooding and growing into a shallow sea. This is because shale forms from old mud flats and limestone forms in shallow seas. It's like if the Everglades were ever swallowed up by the Atlantic due to rising sea levels. The same pattern of sedimentary rock could be expected to form and evidence of the event would reside in the stratified layers.

Water is stratified in a much different way. And one of the best places to find stratified layers of water is in large lakes or reservoirs. Imagine you're diving down to the bottom of such an area. The top layer of water is warm and comfortable to swim in. This is because the top layer of water is continually heated by solar radiation. However, the sun's rays can only warm so much water, so at some point the warm comfortable water will quickly turn cold. This transition is called a thermocline.

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