How to Interpret Events from Natural Phenomena

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  • 0:01 Layers Within the Earth
  • 1:23 Relative Dating
  • 4:06 Disruptions Within the Layers
  • 8:47 Lesson Summary
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Lesson Transcript
Instructor: Paul Brege

Paul has been teaching middle school science for the last 10 years, and has his bachelors degree in Elementary Education.

This lesson will explain what scientists can determine from looking at Earth's rock layers. It will cover terms used to describe parts within the layers of rock and how layers can change over time. Relative dating will also be discussed.

Layers Within the Earth

Have you ever had one of those big candy treats that have layer upon layer of sugary coating? They go by such names as gobstoppers and jawbreakers, and can be found in candy stores around the world. They've been featured in cartoons on TV, and even on the big screen in movies about chocolate factories. The largest jawbreaker in the world was made in Canada and has a circumference of 37.25 inches - that's bigger than a basketball!

But do you know how these hard candies are made? When a small ball of candy is rolled in liquid sugar and left to dry, a thin layer of candy sticks to the ball and makes the ball grow a bit larger. After repeated turns in different colored sugar baths, the treat grows bigger. If you were to take that jawbreaker and break it right down the middle you would see something like this.

Inside of a jawbreaker
layer of jawbreaker

Those layers of candy are just like the layers of rock beneath your feet right now. You are actually standing on something that is quite similar to a gigantic jawbreaker. Each layer is like a record of time passing, and the deeper you go, the farther back in the history of both the jawbreaker and the rock layers beneath you. This lesson will explain what scientists can determine from looking at those layers using a process called relative dating, and how we determine which fossils from those layers are older or younger. This lesson will also cover the special names given to parts within those layers, and how the layers can change over time.

Relative Dating

Within the world of geology, the term relative dating describes the method scientists use for determining the order of events without using actual ages. It's a way to compare things like rocks and rock layers, called strata, by using what is immediately around. It's a concept that most of us have undoubtedly used before. Think of it this way, when you throw things into the trash can the first items in would be at the bottom. Then, as time passed on, the last thing into the can would always be near top. Relative dating is just like that - it's all about order and sequence. When a rock layer is found above another layer of rock, the rock on top is usually younger than the rock below. The general rule of thumb is that the older layers are found at the bottom and newer layers are found at the top.

Of course, this is all relative to the other layers. If the oldest layer was from this morning and you were throwing away lots of trash, you might have many layers over a short time. That's why this is called 'relative' dating. Using this form of dating only gives a sequence of what happened before or after some other event, not an actual date the event happened or a true age.

This idea can be used not only with layers, but with items within the strata as well. If someone were to find a fossil and then after digging deeper were to find another fossil, the deeper fossil should be from longer ago, thus making it older. Think of this idea as similar to a fancy gelatin mold, the type where there are layers upon layers of different colors. If you started with a red flavor and maybe put slices of strawberry in that layer, once it hardens, those berries will be part of that strata; they would be trapped in there. If you were to then add a blue layer afterwards and put blueberries in that layer, those blueberries would stay within the blue layer; they wouldn't mix into the layer below. Adding a third orange layer with, say, carrots, would make our gelatin mold not only complete but gross and now full of vegetables.

Once the gelatin is all set up, if you were to cut across those strata, you would find that the top layer contained fossils of carrots, the next layer down would have fossilized blueberries and the oldest layer would have the fossilized remains of strawberries. Remember this is relative dating - nothing here is really that old; the layers form a sequence, even though here they were all created the same day. Had our gelatin mold been rock strata with fossils, the strawberry fossils would be much older than anything in that orange layer. Paleontologists use this type of reasoning to determine when in Earth's history organisms found buried in rock layers may have been alive. Things further down are usually much older than those found closer to the surface.

Disruptions Within the Layers

Rocks don't always form in layers. Sometimes they melt upward through existing rock. An intrusion is when molten rock invades preexisting layers. Does that word intrusion remind you of the word intruder? They have the same root word. An intruder breaks or forces into an area. Intrusive rocks do the same thing: they force their way into rock layers that were already present.

Frequently, molten rock occurs where volcanoes are forming. Liquid rock, called magma, forms under the Earth's crust and because it is less dense, starts to rise up through the layers of rock which have already been formed. Any layer in the way is simply melted through and added to the magma. This all happens at an incredibly slow rate. It could take tens of thousands of years before the magma reaches the surface to form a volcano, and sometimes it never erupts at all. If the magma simply cools beneath the surface and becomes solid rock, this would be a form of intrusion called a batholith.

A batholith
Bath layers

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