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.