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  • 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.

Hi. It's Paul Andersen and this is disciplinary core idea ESS1C. It's on the

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ESS1C - 地球の歴史 (ESS1C - The History of the Earth)

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    Mark Lin に公開 2021 年 01 月 14 日
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