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?
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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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.
Thermoclines are very obvious when you encounter them. A few years back, I distinctly remember SCUBA diving in a glacial lake with a colleague of mine. The lake was stratified and at twenty feet deep, we hit the lake's thermocline. The warm water we swam in instantly became cold and we went no deeper. You could literally swim along at 19 feet deep and be warm, then reach your hand down to 20 feet and feel the cold water below. This phenomenon is due to stratification and is present in most lakes and reservoirs throughout the summer months.
Other Examples of Stratification
Remember that stratification refers to layering. Therefore, while sedimentary rock and water bodies are classic examples of stratification, they are not alone. Soils can also be stratified into various horizons (which is really just another term for layers), ice cores can show stratification patterns, and even the atmosphere that surrounds Earth is stratified.
Stratification occurs throughout nature in places such as rock, water, and soil. Stratification is the term used to describe items being composed of layers. Sedimentary rock is one such item that is stratified. Sedimentary rock is composed of several layers of sediment that have built up over millions of years. These layers can reveal what type of environment the rock was being formed in and how that environment changed. Additionally, lakes and reservoirs are often stratified into warm upper layers and cold bottom layers. The transition between these layers is called the thermocline. Stratification in water is often due to solar radiation heating the upper portion of the water column while the bottom remains unheated and cold.
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Stratification: Definition, Theory & Examples
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