ESS1C – The History of the Earth

Hi. It’s Paul Andersen and this
is disciplinary core idea ESS1C. It’s on the history of planet earth. And the history of
planet earth is literally written in the rocks. And so if we look at those rocks, these are
sedimentary rocks, they’re going to be laid down over time. So there’s erosion and deposition.
And one thing you should understand is that we can learn a sequence from how those rocks
are laid down. And so the oldest rocks are going to be laid down first. And then newer
rocks are going to be laid down on top of that. But we can have earthquakes and fractures
and volcanism. And all of those if we interpret them correctly are going to give us a little
bit of a history of our planet. What are some other clues we can get? Well the fossils found
within those rocks are going to tell us what life was like at that time. But they’re also
going to allow us to look at rocks in a certain area and compare those to rocks in a different
area. And also we can actually get measures of the rocks. We can look at the radioactive
decay of elements within those rocks and we get a pretty good idea of how old those rocks
are. And so there’s a huge amount of evidence we can get to the history of earth. One of
the cool things we find is that when you look on the continents, and so it’s kind of hard
to see this map of the earth. Here’s North America. Here’s South America right here.
One thing we found is that the continents are going to be really really old. And the
oceans are going to be relatively young. And so the oceans are going to be less than 100
million years. And some of them are going to be less that 10 million years old. But
if we look on the continents they’re going to be sometimes billions of years old. We’re
going to have some rocks in Montana that are almost 4 billion years old. That means that
those rocks go back to the formation of the earth itself. And so one thing you should
understand is that the earth is not static. It’s incredibly dynamic. It’s shifting all
the time. And so one of the best explanations for that is plate tectonics. And so all of
these continents and oceans are these giant plates that are constantly shifting on top
of the mantle. The surface of the earth, even though it’s cooled down and it’s not moving,
we’re going to have erosion on the surface. We’re going to have volcanism where we’re
building up new land as well. And so the earth is incredibly dynamic in nature. And so it’s
hard to look at the earth and figure out, you know, where is the evidence on the earth?
And so what happened in those early formations of the earth. And so one way to figure that
out is to actually look to space. And so we can look to asteroids. Or things that have
fallen to earth. And this is a meteorite right here. And that tells us more what the earth
was like in its earliest days. And we can also look out at other planets. We can look
elsewhere in the solar system and it tells us a little bit more, what happened on our
planet. And so if you look at the moon you see it’s just littered with craters. Well
our planet has been hit by just as many things and we should have just as many craters, but
that erosion is clearing it up. And so we get lots of evidence at at least the dynamics
of our earth and how it is changing. And we start to come up with a picture of the earth’s
history. And so we have to really deal in really really long time to understand this.
And so we think that the earth is around 4.6 billion years old. And this first period of
time it was mostly molten. It hadn’t cooled to the point where anything could be on its
surface. But we have these periods of time. And so we have you know massive bombardment
of our planet. We start to get an appreciable atmosphere. And humans show up, look at this,
way up here. So way, way later. And so the earth has been around for a long time. And
each of these divisions are really divisions of life. We look at the fossil evidence and
where life, where we had massive extinctions. And where we had new adaptive radiation of
new organisms. And then we kind of get a picture of the life. And so that’s getting into a
little bit more depth. So where do you teach this or how do you teach this? In the lower
elementary grades you want to give your students this information that events can occur in
cycles. Like the daily cycles of day and night and day and night. And then we can have other
events that just have a beginning and an end. So like a volcano. It starts, there’s an eruption
and then it ends. And also you want to have them understand the idea of time scale. That
we can have events that are really, really short. So what’s an earth event that’s short?
Like an earthquake is going to occur really really quickly. And then we can have long
events. And so the Grand Canyon was carved out by the Colorado River over millions of
years of time. And so that slow erosion takes a huge amount of time. Likewise all of this
rock being laid down took a long time as well. As you move into the upper elementary grades
you want to talk about the dynamics of the earth. And how it’s constantly changing. And
some of those events could be erosion. Some of those events could you know building up
of the earth like through volcanism. Tearing down of the the earth as well. And so since
it’s so dynamic, where do we look for evidence of what’s going one? Well we look again into
the earth itself. We want to dig down. We want to look at what the earth looked like
over time. We can use fossil evidence remember to figure out what happened at given periods
of time. We can even use tree rings to figure out what was happening at different time periods.
And then ice cores. Ice cores from Antarctica, from Greenland. We can dig down and what you
get are not only telling us how much ice was laid down during different periods of time,
but we can actually collect bubbles from this and it tells us what the atmosphere looked
like during different periods of time. As we move into middle school we want to talk
about this history of the earth. How long it is and how do we know some of these things?
We could use fossil evidence to figure out how old these things are. We call that relative
dating because we’re comparing fossils in one area to fossils in another area. But we
don’t have a specific time period. And we want to talk more about that or absolute dating
as students move into high school. And so radio active decay, in other words, as lead
decays down into, so lead 212 down into lead 208, we can measure the amounts of that. And
that’s going to tell us how long it’s been since these started to decay. A really good
one when we’re looking at rocks is uranium lead. As uranium is breaking down into lead,
it has a half life of billions of years. And so we can use that to measure the age of these
rocks, which are incredibly old. And what we find are these patterns. The idea that
the continents are really really old. The oceans are incredibly young. And that will
become more important as we get into plate tectonics. And where else can we look on our
planet for history of earth? Remember anything that falls to our planet, like this meteorite
is going to give us evidence about what it used to look like. And I hope that was helpful.

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