Mastery Check #6 - Geologic History
The Earth is extremely old. Based on the evidence that we have, scientists estimate that the Earth formed 4.6 billion years ago. A summary of the history of the Earth is the geologic timescale pictured below on the left. This is not a scale model because you can see that the first three time periods take up about 4 billion years and all of the rest of the timer periods take up only 570 million years. The timescale on the right shows a model that is to scale.
Evidences of Earth's past history
How do we know this? Obviously, there were not people around 4 billion years ago to tell us about it. Scientists use clues that are found in rocks. There are many different techniques that are used, but we will focus on the following five tools:
Uniformitarianism
Since we were not there to see these processes millions of years ago, we have to assume that the same processes that we see happening on the Earth today happened in the same way in the past. For example, we see sediments being deposited by rivers into lakes and oceans today. We assume that rivers deposited sediments in a similar way millions of years ago. Other processes we assume are the same are volcanoes, earthquakes, animal and plant behavior, landslides, mountain ranges forming, plate movement and many more.
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Evidence from rock strata (rock layers)
- Evidence from rock strata (layers) - Different types of rocks form in different ways and in specific environments. Here are some examples:
- Sandstone - forms from beaches and deserts/sand dunes. A layer of rock made of sandstone was once a beach or sand dune.
- Limestone - forms in shallow oceans. A layer of limestone is evidence that there was once a shallow ocean there.
- Shale - forms in ocean water or at the bottom of lakes.
- The picture below shows the environments where each rock type can form.
Evidence from fossils
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Superposition
When sediments are deposited into lakes or oceans, they will sink all the way to the bottom. This means that layers that are older will be found below layers that are younger. When we look at layers of rocks, the ones at the bottom will be older and the ones at the top will be younger. For example, if a sandstone layer is found below a limestone layer, then you could conclude that there was a beach environment followed by a shallow ocean environment.
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