 Greetings and welcome to the Introduction to Astronomy. In this lecture, we're going to continue, we're going to look at the Earth, and we're going to look primarily at the surface and the atmosphere of the Earth. So some details here on different parts of the Earth, not just the interior, but parts that we can actually see and study a little bit better. So we're going to start off by looking at the surface of the Earth here. And the surface of the Earth, we find a number of different types of rocks. And there are three primary different classifications of rocks that we see. And those are igneous rocks, which are volcanic. So from molten lava and then solidified. We find sedimentary rocks, which are formed from water, water dissolving various rocks and then recombining them into other types of rocks. And we can find metamorphic rocks, which are changed. They can be an igneous or sedimentary rock that are then changed by high temperatures or pressures. And where do we get those high temperatures or pressures? Well, deep down inside the Earth. So their rocks can actually be deformed. So rocks really are not solid, but can be slowly deformed under pressure over long periods of time. Now let's look at what the Earth's surface looks like here. And what we see is that the Earth's surface is divided into a number of different plates. And it is constantly changing. We are under a constant state of change on the Earth's surface. And you see a number of the plates here, including the North American plate, the Eurasian plate. Down here you can have the South American plate and the Antarctic plate. And the plate for Africa here and etc. India as a separate plate over here all by itself, the Australian plate. And these are all in constant motion relative to each other. There are currents down in the mantle of the Earth that plastic-y part below the crust of the Earth. And as they move, they cause the plates to move as well. So material collides into each other. Some places, plates are separating apart. Other places, things are coming together. So for example, where we have plate boundaries, that is generally where we get a lot of activity. And one of the primarily well-known ones is what we call the Ring of Fire of the Pacific. So going up through Alaska here, down the coast of North America, we get earthquakes. In the California region, we have volcanoes up in Oregon and Washington state. But as you go around, there's a big area here and going around through Japan and back up around. So you get a big ring around this plate where you get a lot of activity. Now it's not always that because we do have volcanoes right in the middle of a plate. Hawaii being one example. And there is Hawaii right in the middle of the Pacific plate, not anywhere near a plate boundary. And that would be what we call a hotspot volcano present because of a weak spot in the crust of the Pacific plate right there. But as these plates continually move, features will change. When plates collide together, we can form mountains. We can build up mountain ranges and volcanoes. And that happens, for example, with the Indian plate here, which is moving in this direction and colliding into the Eurasian plate. As those two continental plates collide, they're actually are forming the Himalayan Mountains. So some of the youngest and tallest mountains that we have in the earth. Other plates can be separating. And right down the middle here, we have the Mid-Atlantic Ridge, which is actually separating these various plates. As they move across, so new plates are being created, new crust is being created at that central portion. And over a hundred years ago, Alfred Wegener noted that there was a similarity between some of the continental boundaries. Primarily that the South America and Africa kind of look like they fit together like a jigsaw puzzle. That you could imagine putting the coast of South America right in the part of Africa here. And that, if you could imagine that they were slowly moving, that they might have been once put together. Now, just the similarity of the boundaries might have been coincidence, but finding similar fossils on them was also very important. That fossils in edge of South America and on the coast of Africa that were quite similar dating back to many millions of tens or hundreds of millions of years ago was very important for putting forth the idea of continental drift and plate tectonics. Now, let's look at how some of these motions have worked over the years. So 225 million years ago, all of the continents were connected into a great continent called Pangaea. So you can see here, you might be able to make out some of the things like North America, South America. There's Eurasia, there's Africa, Antarctica, Australia, and India. So India kind of its own separate plate here. And over time, 25 million years later, things had started to separate. So you have the North America and Eurasia were still connected and South America and Africa were connected. But India, you can see, starting to separate out here and move northward and Antarctica, again, moving and separating as well. And another 50 million years later, back in the Jurassic period, then North America kind of separating all by itself. At this point, India would have been an island continent. So it is still moving northward and over the next 100 million years continued to move northward, getting further and further north. And eventually, here it is colliding into the Eurasian plate. And that is where we are getting a great collision of those two continental plates and getting mountains created. Now those have happened elsewhere in previous times. Most big mountain ranges that we see can be associated with the motions of plates in the past. So things like the Rocky Mountains in North America or the Appalachian Mountains on the eastern part of North America were both created by previous collisions as these plates have moved around. So while they are slowly spreading apart and moving around relative to each other, these motions will continue. What we see today for the Earth is not what the continents would look like in 50 or 100 or 200 million years. These motions will continue and continents will separate and continents will come back together again. So it is a constant change and the boundaries that we see today are not permanent. What we see today, if we could come back in 100 million years, would be quite different continental boundaries. So what are some of the impacts that we get from these moving plates? Well, we get what we call rift zones, which is where the plates are separating. The Mid-Atlantic Ridge is one of these where the plates are separating in the middle of the Atlantic Ocean, pushing the United States, the North American plate, and the European Eurasian plate apart, getting further and further apart, just a few centimeters per year, but it is something measurable. And the East African Rift Valley, if you look down the eastern coast of Africa, there are a number of long lakes where Africa is stretching itself apart and splitting itself apart into another piece so that eventually the eastern portion of Africa could separate out from the rest of Africa. We also get subduction zones, which occur where ocean plate is pushed under a continental plate. So that happens at various plates where you have the ocean plates, which are denser, will actually sink down below the less dense continental plates. We can get fault zones, such as in California, in the United States, where plates are moving side to side. Instead of colliding with each other or pushing down below each other, they move side by side and stresses will build up and then be released in massive earthquakes. We can get mountain building zones where continental plates are colliding. So when you get a continental plate and an ocean plate colliding, the ocean plate is denser and will get pushed down below, but two continental plates of similar density will collide together, pushing up great mountain ranges. And volcanoes can occur. Often on a plate boundary, as in the Ring of Fire, which is part of volcanoes in Washington and Oregon and in Japan. And we can also get hotspot volcanoes like Hawaii, which are in the middle of a plate and just a weak spot in the plate where material is welling up from below. So now let's look a little bit at the atmosphere of the earth. And the atmosphere of the earth is divided into a number of different layers. The troposphere is where pretty much most things occur. For us, that is where airplanes are flying. That's where most of the clouds are present. And that is this very smallest layer here up to about 18 kilometers. Up above that, we have the stratosphere and the temperature begins to increase here. So here the temperature decreases as you get higher. So as you go up, the temperature is actually decreasing. When you get into stratosphere, the temperature actually increases as you move through the stratosphere. And that is where the ozone layer is that protects us from ultraviolet radiation. Up above that, we have the mesosphere up here going further up. And again, the temperature now decreases as you move through this layer. So temperature changes are not constant. Temperatures do not get continually colder as you get further out or continually hotter. And remember that that is because temperature is a measure of the motion of the particles. So in some cases where you get less and less particles, but they're being excited more and moving around more, the temperatures will actually increase. Now up in the mesosphere, the temperature again decreases and that's where meteors are formed. So meteors hitting through the atmosphere will then strike the mesosphere up at close to 80 kilometers or so. And that is where the atmosphere gets dense enough that they will start to burn and where we will see meteors or shooting stars. Now the atmosphere never completely goes away. It just constantly thins out. And in the thermosphere out here, this atmosphere is very, very thin. And that is actually reaching the edge of space now. This is where things like the space shuttle or the International Space Station would occur. And the temperatures are increasing as you go further up here. And it's also where you would see the aurora. So where the auroral activity would occur, that's where the particles from the sun are exciting particles in the upper Earth's atmosphere and causing them to glow. When you get this high, you can actually get things in relatively stable orbit. So things like the International Space Station would orbit in the thermosphere. Now what is the atmosphere made up of? Well, if we look at dry air, and that's because water vapor is very variable and can range from almost nothing in very dry air to a couple percent in very wet air or more. We're going to look at just dry air as a relative comparison. And in that case, we have 78% nitrogen, 21% oxygen, 1% argon, and everything else is just trace amounts. This is not the original atmosphere that the Earth had. The original atmosphere would have had things like methane, ammonia, water vapor, carbon dioxide. So you see that the atmosphere that we have today is nothing like the atmosphere that was originally formed on the Earth. This was from outgassing, from volcanic eruptions that melted rocks and released gases trapped in them, and possibly from cometary impacts, bringing some of the icy materials to the Earth. So what we want to look at, really, is that the atmosphere of the Earth that we see today is not the atmosphere that it always had, and that if we were to go back in time, billions of years, the atmosphere on Earth would be unbreathable. It would not have any oxygen. It would have had a very different composition than it has today. Now, finally, the last thing to look at the atmosphere is to talk a little bit about weather and climate. So let's take a look at those. And we want to define, first of all, what do we mean by weather? Weather is the short-term variations in atmospheric conditions. How do things change from day to day, week to week, et cetera? The climate, on the other hand, is long-term variations, and officially we measure this over a 30-year time span. The idea of 30 years is good enough that we smooth out any variations on shorter time scales, but we can see long-term trends. We can still see long-term trends that occur over longer time scales. And one example of those long-term trends is the ice ages. These are examples that can change that occur periodically, and we do get ice ages on the Earth every once in a while, where the temperatures will get significantly colder. Now, these are believed to be caused by what we call the Milankovitch effect. The Milankovitch effect is that these long-term changes, things that we're looking at on tens and hundreds of thousands of years' scales, that's not the temperatures that we see changing in decades or even centuries, but on very long-time scales, depend on two things. They depend on the changing shape of the Earth's orbit and the changing tilt of the Earth's axis. Remember that the Earth has an elliptical orbit, and that does change slightly. It can be more circular or a little less circular. The tilt of the Earth's axis, remember, 23.5 degrees, is not always exactly 23.5 degrees. It can be a little bit more or a little bit less, and that can have an impact on the seasons. And how all of these change over the long-term can actually give us these long-term examples when they combine together. Just right, we can get the ice ages, and then as they change again, we can warm and come out of an ice age. So, let's finish up here and look at our summary. And what we have is that the Earth's surface itself is in a constant state of change. The continents that we see today are not the continents we saw hundreds of millions of years ago, and are not the same shapes of the continents we will see hundreds of millions of years from now. These moving plates cause things like volcanoes, earthquakes, and rift valleys that occur on the surface of the Earth. Looking at the atmosphere, the atmosphere of the Earth that we see today has changed. It is not the same as it was originally. And then we looked at the climate of the Earth, the long-term changes, and that changes over long periods due to astronomical effects. Things like the tilt of the Earth, and like the shape of the Earth's orbit as those change over long times, they can actually have an impact on the long-term climate of the Earth. So, that concludes our lecture on the surface and the atmosphere of the Earth. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.