- [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!