Fossils and rock layers | The geosphere | Middle school Earth and space science | Khan Academy

Have you ever wanted to travel back in time? Would you go meet your younger self? Would you go and ride a dinosaur, or would you meticulously create a timeline of the earth's 4.6 billion year long history based on major geological events?

Even though geologists can't go back in time, they've been able to do just that. Their timeline is called the geologic time scale and it breaks up all of Earth's history into units called eons and eras.

You might be wondering, how can we map out what happened on Earth billions of years ago? The answer is rocks. Geologists look for clues in sedimentary rock layers, which we call strata. The fossil record, which are the remains of organisms that have been preserved in rock layers, can give us glimpses of creatures that breathed long ago. Also, the difference in mineral composition in different rock layers can tell geologists when a volcano erupted or an asteroid hit.

But how do geologists know when one rock layer is older than another? Well, when sedimentary rocks form, they are deposited in layers, one on top of the other. So, unless the layers are disturbed or turned over, the layers at the bottom are always older than the layers at the top. Geologists call this rule the law of superposition.

So, if a geologist finds a fossil of an ammonite in a layer of rock above a fossil of a dunkleosteus, the scientist would know that the dunkleosteus is older than the ammonite. This process is called relative dating because it helps scientists determine the relative ages of rock layers and fossils.

But what does relative age mean? Let's say that I told you my friend Ava is younger than her sister but older than her cat. Then I wouldn't be telling you Ava's exact age, but her relative age in relation to her sister and her cat.

Let's take a look at another example of relative ages, this time with rocks. Imagine that one day you found a big rock with many layers like this. Because the rock is a sedimentary rock and you remember the law of superposition, you know that the layers at the bottom are older and you know that the layers up here are younger.

But what about this section of rock right here that slices through all the other layers? Geologists would call this section an igneous intrusion since it was formed from molten rock that has cooled. This particular igneous intrusion isn't clearly on top of or below any of the sedimentary rock layers, so we can't use the law of superposition here.

Luckily, geologists have figured out another rule called the law of cross-cutting relationships. This rule says that when two geological features cross or intersect, the feature that cuts through the other is younger. This makes sense because you can't break something before it even exists. So, since our igneous intrusion cuts through the layers of the sedimentary rock, we know that it must be even younger than the rock layers.

And what about this fault or break over here? Well, since it slices through all the layers, as well as the igneous rock, the law of cross-cutting relationships tells us that the faults must be the very youngest geological feature here.

Other pieces of evidence, like fossils, can be used for relative dating. For example, suppose a geologist found a rock containing fossils of extinct organisms and another rock that contained fossils of modern organisms. The geologist could then deduce that the rock with the extinct organisms is older.

Certain kinds of fossils called index fossils are especially helpful when determining the relative age of a rock. Index fossils are fossils that are only found on a specific section of the geologic time scale. I always remember what an index fossil is like this: just like I use my index finger to point to give directions, index fossils can point to when on the geologic time scale a rock was formed.

So, if a geologist finds the same index fossil in rock layers in two different rocks, they know that those rock layers must have been formed around the same time period. But fossils aren't the only thing that can help geologists figure out a rock's relative age. For example, if a geologist found two rock layers that contained evidence of the same geologic event, like a layer of ash from a major volcanic eruption, the geologist could deduce that those rock layers were made at the same time.

Relative dating is how geologists figured out that stegosauruses were extinct long before T-rex's even existed. It also helped them figure out that the first single-celled life formed about 3.8 billion years ago and the first multi-cellular life formed about 600 million years ago. They figured out that modern humans didn't even exist until about 200,000 years ago, which is right at the very end of the geologic time scale.

In other words, it took a really, really long time for life to evolve into what it looks like today. We humans are very young in the grand scheme of the Earth's history.

Now, if you'll excuse me, I'm gonna go saddle and ride this brontosaurus. Let's go, Gingersnap!