Hi. It's Mr. Andersen and today
I'm going to be talking about the law superposition.
Law of superposition essentially says the
older rock is normally going to be found on
the bottom. And so right here we're looking
at a section from the Grand Canyon. And you
can see that all the rock above this line
looks very similar. It's actually called Navajo
sandstone. But if I go down to the next layer
of rock, it's going to look the same. And
the next layer of rock is going to look the
same. And the next layer of rock is going
to look similar. So horizontally rock looks
the same, but the law of superposition says
the farther down you go, the older that rock
is going to be. Another thing that you should
understand is that rock is going to be laid
down horizontally. In other words rock will
be laid down flat because gravity is always
pulling down. Now an example of where you
might see this in your daily life might be
in your locker. And so I grabbed a sample
from a locker right here. So if I grab some
of the stuff out, it looks just like that.
We got a book and a notebook and some papers.
The stuff that's laid down most recently,
like this biology book is going to be on the
top. Or like, this looks like a geologic key.
We might then have a notebook that was used
fairly recently. But the farther I dig down,
I find crossword puzzles and doodling and
way at the bottom I might find like a blank,
a sheet of black paper. Some headphones. Those
are going to be found at the bottom. And so
that means that those were laid down more
distantly or in the past. And so the law of
superposition is fairly simple. But the problems
you can solve with it are fairly complex.
And so this would be an example of a geology
problem that you have to solve. And so we
can see all these different rock layers. So
the idea is to figure out well, which one
of these occurred most distantly and most
recently. So could you put it in order from
the thing that happened in the distant past
and the thing that just happened a little
bit ago? And so you might say some of these
things might be easy like probably this rock
here E was laid down a long time ago. And
this rock layer, F was laid down fairly recently.
But how do you do that? Well you have to use
the law of superposition and the law of horizontally
laid down rock and you can do pretty good
at it. When I was thinking about how I could
kind of explain this to you, the thing that
made most sense for me was to use a drawing
program. And so imagine I were to ask you
this problem right here. I were to ask you,
could you figure out all these rock layers,
which one was laid down the oldest or which
is the oldest? And which is the youngest?
And so some of this stuff would make sense.
Like this is probably older here. And younger
up here. But it's always apparent until you
actually see it take place. So let's take
a look at what happened here. So this is me
drawing it. I kind of sped up the drawing.
Now let me pause it after a little bit. Okay.
So let me pause it right here. So what happens?
Well we had this first layer of rock laid
down. And then a second and a third and a
fourth layer of rock was laid down. So we
know that the oldest rock is going to be the
rock at the bottom. But let's play what happens
next. Oh wow. Okay. So what happened there,
well we had a fault. In other words those
rock layers, we'll say that look like this,
were broken and lifted up. And when you're
looking at problems like this you try to match
up the sides and so you know that the rock
was laid down before the fault occurred. And
so we could say that this occurred before
the actual break because you couldn't have
a break in this until it was actually laid
down. Let's keep going again. So what happened
next? Well that whole rock layer was tilted.
That's kind of hard to figure out how that
might work. Oh, what's this? That's me using
an eraser. Okay. So what happened next? Well
it looks like we had erosion taking place.
And so there was rain and wind and it washed
a lot of that rock away. Can we tell that
that occurred after the tilting? I think that
would be kind of a hard one. So let's keep
going. What happens next then? Oh. It's a
lava. So we had a lava flow and then that
lava eventually was eroded. And so this is
horizontal now at this point. So we know that
this occurred before or after we had all of
this erosion and we had the tilting. Looks
like some more rock layers are going to be
added as well. So we're adding more rock layers.
What happens next? Looks like we have a little
bit more erosion taking place. And is that
it? No. We have this. What is that? Okay.
That is an intrusion. In other words that's
where molten rock is actually moving up. We
call this a dyke and then that's causing this
bulge or like an intrusion, igneous intrusion
or rock that's actually intruding into other
rock. And then it's hardening inside it. And
so when did that occur? Well we can look on
here and we know that it actually, since it's
cutting across all of these and cutting across
over here, it actually had to form after these
rock layers formed. And so if you know a little
bit about the law of superposition and the
law of horizontally laid down rock or horizontaility
I think is what it's called, you should be
able to put together some of the geology or
how things laid down. Now you normally don't
see it like this unless you're actually taking
a test. You usually see it something like
this. And so what do you know using law of
superposition? Let's take a look at this?
This is a road cut. Usually you can't see
geology because there's plants and stuff growing
over it. But if you cut out a road or you
cut out a trail or something like that you
can actually see the rock. And so here we
have several layers of rock that were at one
time laid down horizontally. And so we would
say the oldest rock was down here and the
youngest rock is up here. The whole thing
was then tilted. And then it was eroded. And
so we have a unconformity here which is going
to be an area where it was eroded. And then
we have new levels, layers of rock that are
added on top of that. Since these are horizontal,
we'd say this is the newest and this would
be the oldest. So I hope that's helpful.
