Back To CourseEarth Science: Middle School
12 chapters | 101 lessons
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Paul has been teaching middle school science for the last 10 years, and has his bachelors degree in Elementary Education.
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.
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.
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.
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.
The picture here shows where a batholith would form if magma were to cool at location number one, it's that whole big area far below the surface. There are many other types of intrusions as well. For example, if a rock is formed within a horizontal crack or between layers in a rock, that intrusion would be called a sill (like a window sill) as shown in area number two, and if that intrusion was formed in a vertical crack it would be called a dike as seen in area number three. If the intrusion forms a dome shape, it is called a laccolith shown here as point four.
Faults can also play a role in disrupting the layers of rocks. A fault is a break in the rocks where the layers can slide past each other. When a fault occurs where there are nicely formed layers, the results can look something like this.
Faults makes the layers shift up or down, changing the way the layers now appear in this picture. The line on top was all connected before the faults moved the sections to their current areas. This is similar to what happens when a tree root grows underneath a section of sidewalk. The cracks in the sidewalk are like faults. They both break up the layers that were already laid down. When layers move like this, it can be confusing and difficult to understand which strata were previously connected. If only real rocks were as distinctly patterned as our pictures here!
A great way to summarize all this information up is to look at a diagram like this. This picture shows three strata, two of which are cut by an intrusion. Let's unpack things a bit. Look at the bottom layer labeled C and colored blue. We already talked about how this would be the first strata to be laid down because it's at the bottom. The layer above it, which looks like pink bricks, is younger because it is on top of the blue layer. The top layer shown here in a light tan color is even younger still.
There is also an intrusion on this diagram shown in gray and labeled D. You can see that the intrusion happened after section C and B because it has cut through both bottom layers. Since these layers are fully formed on both sides of the intrusion, we can be sure the intrusion was after these layers were made. Think of it like a burrowing rodent: you can't dig a tunnel up and out unless the dirt layers are already there. The intrusion can't happen unless the layers are already there, either.
If we reveal a bit more of this picture, more information comes into play. Can you see how the intrusion goes all the way across the layers now? See where it actually breaks the surface? Look at the line that cuts the picture. Do you remember what we call a break in the rocks where the layers can slide past each other? This is called a fault, and the line on this picture is an example of what it would look like if the left side was pushed up and some of the A strata was worn away. Although the top two layers are especially visible, look at the intrusion. See how it splits right at the fault? If someone were to ask you whether the fault happened first or the intrusion, what would you think? If you look closely, this picture shows that the intrusion actually goes through the A layer first, then after it's cooled, the fault happens and the intrusion is moved along with all the other layers. If the fault had happened first, the intrusion would not have been split like we see here.
Remember, this is all relative to the other layers. We don't know if these layers were from a million years ago, or if they were just created last month. All we know by looking at this diagram is the bottom C layer happened first, then the B layer and then the A layer. After that, the intrusion formed and lastly, a fault slid the layers past each other.
In this lesson, we looked at something called relative dating. Relative dating is a way that scientists determine the order of events without using actual ages. Remember our rule of thumb, older rocks are normally found on the bottom, while younger rocks are found at the top. Intrusions form when molten rock invades preexisting layers. Lastly, we looked at something called a fault, which is a break in the rocks where layers can slide past each other.
When this lesson is over, you should be able to:
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Back To CourseEarth Science: Middle School
12 chapters | 101 lessons