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Course: Middle school Earth and space science - NGSS > Unit 4
Lesson 2: Fossils and rock layersFossils and rock layers
Geologists use a geologic timescale to map Earth's 4.6-billion-year history. They study sedimentary rock layers, or strata, and fossils to understand past events. They use the Law of Superposition and the Law of Crosscutting Relationships to determine the relative ages of rock layers. This process, called relative dating, helps them trace the evolution of life on Earth. Created by Khan Academy.
Want to join the conversation?
- Is Earth a rock and a perfect sphere?(4 votes)
- If we're being pedantic, Earth isn't a sphere. It is a shape called an oblate spheroid. That means that rather than being perfectly round, it is wider around the equator than it is pole to pole. In that way, Earth is sort of shaped like a Skittle. But unlike Skittles, Earth wouldn't be good to eat because it is mostly rock.(16 votes)
- What is the known largest rock on the Earth, other than the Earth herself?(7 votes)
- Uluru in Australia often claims the title of the largest known singular rock, or monolith.
Also, the Earth is partially made up of rocks, but it isn’t just one big rock.(4 votes)
- How are fish found in fossils if when the die the float to the top?(5 votes)
- The belly becomes a guts-filled balloon and so they float up when they die(1 vote)
- I wish I can ride on a dinosaur 🦕(4 votes)
- Well, if you wish you can ride a dinosaur, you're going to have to find out how to build a time machine first, and that can take years. You're probably going to have to travel really far back in time until you reach the time of the dinosaurs. As soon as you get there, if you get there, you'll also have to dodge t-rex and velociraptor, but that alone will be tricky. Velociraptor is the fastest dinosaur on the planet earth, and unless you can run really fast, there is a 0.0000000000001% chance you'll escape the velociraptor. So, my recommendation is don't do it, unless you have a death wish. Good luck. How fast can you run?(4 votes)
- I would go to fight in WW2(4 votes)
- can diamonds melt?(5 votes)
- so yes it can(1 vote)
- I think that the earth is the oldest planet in the solar system besides the sun but not the big 6 bil.years(3 votes)
- All of the planets were created at the same time.(3 votes)
- what is the oldest rock you know of?(4 votes)
- There have been only 6,000 years since God created the world, vorb!(0 votes)
- 4:26is so vired and the name she gave it in4:31(3 votes)
- can fossils be more than just bones?(2 votes)
- yes, there is many types of fossils including bones, like an imprint of a shell on a rock.(2 votes)
Video transcript
- [Instructor] 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
geologic timescale and it breaks up all of Earth's history into units called eons and eras. And you may 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 breathe 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 did geologists
know when one rock layer is older than another? Well, when sedimentary rocks form, they're 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 scientists 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 ages 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 the other layers? Geologists would call this
section an igneous intrusion since it was formed for
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
crosscutting relationships. This rule says that when
two geologic features cross or intersect, the feature that cuts
through the other is younger, and 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 slice
through all the layers, as well as the igneous rock, the law of crosscutting relationships tells us that the faults must be the very youngest
geologic feature here. Other pieces of evidence, like fossils, could 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 timescale. 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 timescale 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 geologists 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 the first single cellular life formed about 3.8 billion years ago and the first multicellular life formed about 600 million years ago, and 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, ginger snap! Hyah!