 Greetings and welcome to the Introduction to Astronomy! In this lecture we are going to continue our talk about the Earth and look at the surface and atmosphere of Earth. So we've previously looked at the interior, now let's look at the outer regions that we can actually see directly. So let's look at our Earth's surface here, and here is an image that we've looked at previously of our Earth, showing the various oceans and some of the continental land masses that are present. Now when we look at the rocks, the Earth's surface is made up primarily of rocky material, other than that which is water, but in honesty the water is a very small portion of Earth's surface. Yes, it covers three quarters of the surface of the Earth, but it doesn't go down to a very high depth. So it's only a few miles deep at most, and remember that it's thousands of miles down to Earth's core. So when we look at the rocks we will see three different types of rocks, we will see volcanic rocks called igneous rocks, rocks formed from sediment in water, sedimentary rocks, and rocks of these types that are called metamorphic that have been changed by very high temperatures or pressures, making different types of rocks. Now what do we see when we actually look at the surface in a little more detail, we see that the surface is broken up into plates. So the surface is constantly changing. And where we see things like earthquakes, volcanoes, and mountains are at the plate boundaries. So if we look at the region here, various volcanic and earthquake regions such as much of the west coast of the United States, the areas around Japan here are right on the edge of plates and other regions such as the Himalayan mountains, which are right up in here. So we have different regions that are generally around boundaries of plates, that's where we found earthquakes, volcanoes, and mountains. However this is not always the case, here's Hawaii in the middle of the Pacific plate. Not anywhere near the edge of a plate, but it is still volcanic and that is a weak spot in the crust that forms the Hawaiian volcanoes. But typically if you live toward the middle of a plate, you would not expect to have many strong earthquakes or volcanic activity. Now we can also look at what these plates are, these plates are moving. So all of these plates are in constant motion relative to one another. And in the early 1900s, 1915 Alfred Wegener noticed that there were some similarities between the continental boundaries. Generally one that's often seen is between South America and Africa, where it looks like they might fit together like pieces of a puzzle. And we also noted that there were similar fossils there. So how could some kind of creature be able to get across from Africa to South America or vice versa based on the incredible distances there, showing that maybe long ago these were connected. And we can look at how this might have changed over time. If we look at a number of different things here, here in the bottom is our present day, and if we went back 65 million years to the edge of the age of the dinosaurs, then we see that things were quite different than they are today. We can see that, for example, India is separate from everything else and was an island there. We can see that there was no Central America, North and South America are separated, and that North America and the Eurasian plate were actually touching. Now if we go back even further to 150 million years ago to the Jurassic era, again we still see India out here, Australia is down connected with Antarctica, and we see here what we had discussed, that the South American plate and the African plate look like they joined together very well here. And again, no Central America, and if we go back again we can go back to 200 million years and 225 million years to when all the continents were joined together in Pangea. So this is a constant changing. They have been slowly spreading apart for years, and for millions of years, in fact all of the time of Earth they have been changing, and that process will continue. So what we see as boundaries right now will not be the same 50 million years from now. As it goes we are spreading across the Atlantic Ocean, continues to get larger and larger, and the Pacific Ocean will get smaller as the plates slowly move across. Now the motion is only centimeters per year, so it's not easily noticeable, but over millions of years it makes a massive difference in where the actual boundaries are going to be. Now we can look a little bit about how these are changing, and here this map shows with the arrows the directions of different parts of the plate. So some are separating apart, some are coming closer together, and others are moving parallel to each other. So one place that is where they're parallel to each other is around the west coast of the US, and that is the San Andreas Fault region where two plates go side by side, and that gives us a very strong earthquake region. Now other plates are moving very strongly in one direction, so you can see where plates are coming together in various regions, but it shows you kind of the overall motion that things tend to move toward the Pacific Ocean, filling that in, and then separating across in the Atlantic Ocean. So that North America is moving towards this direction, and Eurasia is moving this direction, so kind of crunching everything out in between. So we will see that these would continue to change. Look for example at Australia, Australia is very strongly moving north, so what we could do if you could come back in 100 million years, Australia could be an equatorial region or maybe even be north of the equator, so it is moving very strongly to the north and will move up that direction. So again these change and the boundaries what we see today will not be permanent. Now what are the impacts? What do we see some of these? Well we get things called, we call rift zones, which are areas here, which was an example would be the mid ocean in the mid-Atlantic area, and the magma coming up is spreading apart, so the two plates are moving apart and new crust is being formed here. You can also get this as said in the mid-Atlantic and also the East African rift valley. Now there are also subduction zones, shown in the second image here. Here you have a one crust and you have this one being pushed down below it, and of course as it melts that forms volcanic activity that we would then see where one is going under the other. We have fault zones where plates are moving side by side and we have mountain building zones where continental plates are colliding, pushing up mountains, so this would be where India crashes into the Eurasian plate and is pushing up the Himalayan mountains, very fresh new mountains that have formed. In volcanoes you can have two types, you can have those at a plate boundary, which are something like we see the subduction zone here where you can produce a volcano where melted material then erupts back up. Those are present in the west coast of the U.S. in Washington and Oregon and around Japan. You can also have a hot spot, a weaker spot in the crust where material is welling up in between in the middle of a plate, and that is an example of the Hawaiian islands. So depending on exactly what is happening with the plates you can get one of these, one or more of these events occurring. Now let's move on and look at Earth's atmosphere and here we see a sketch of the atmosphere. Pretty much the point we're familiar with is the troposphere. The troposphere is the bottom layer, that's where the airplanes and most of the clouds are. During this, in this region the temperature decreases so as you move higher up the temperature starts to decrease with altitude. So the temperature gets colder and colder. As you're moving up higher in the atmosphere the temperature is decreased. Now when you get to the stratosphere, which contains the ozone layer, the temperature then begins to increase again. And the mesosphere, which is where the meteors are, up here again you're getting temperature decreasing. And then finally once you get up to the ionosphere, you're getting out to the very edge of the atmosphere, and that is where we get the auroral activity and where the International Space Station is. The International Space Station is then not in the atmosphere. The outer reaches of the Earth's atmosphere are very tenuous, so there's not a lot of material there, but there is enough that satellites in low Earth orbit can be dragged back down to Earth by friction and will come back to Earth because of friction with the upper portions of Earth's atmosphere. Now what is the atmosphere made of? So let's take a look at that and we have that our atmosphere, our current atmosphere is about 78% nitrogen, 21% oxygen, 1% argon and traces of anything else. Now note, this is very specific for dry air, meaning no water. So we can look at it as a pie chart here, here is the nitrogen and oxygen, which make up the vast majority of the atmosphere, and this little tiny bit, this tiny fraction, most of the yellow there is argon, but the tiny fraction of others is things like carbon dioxide, hydrogen, neon, helium, methane and krypton, various other gases that make up very small proportions of Earth's atmosphere. Now water can vary from very little to a couple percent depending on where you happen to be located, so that's why we look at dry air. Now this is very different than what our original atmosphere would have been like. Our original atmosphere would have had things like methane, ammonia, water vapor and carbon dioxide. And if you note, those are not among the top three things that we see in the atmosphere today. So the atmosphere has changed. This original atmosphere was formed by volcanic eruptions expelling gases out into space and cometary impacts bringing some of those lighter material into Earth and that atmosphere has then changed over billions of years to the atmosphere that we see today. Now let's take a look and summarize a couple of things and bring a couple of terms that we'll look at here and talk about a little in future lectures, but that is weather and climate. Weather and climate are two different things. Weather is short-term variation. So what is the weather going to be like tomorrow? That is a very short-term variation. How does the weather change over that period of time? The climate is long-term variations. The official measurement is over 30-year time spans which smooths out any variations due to short-term effects, but you can see the long-term trends. So what are some long-term things? Well, ice ages are one of these. That's an example of a long-term change in our climate which occur every once in a while. So you'll have an ice age where the temperatures will decrease. Now often this is caused by what is called the Melankovitch effect. And these long-term changes depend on two things. They depend on the ellipticity of Earth's orbit, which we know the Earth's orbit is elliptical, but it actually varies slightly from almost perfectly circular to slightly more elliptical than it is now, and the changing tilt of Earth's axis. Earth's axis is tilted at 23.5 degrees, but it can vary a little bit either way of that. And depending on those two things can give us long-term changes which can make the overall temperature hotter or colder and causes things like the ice age. So we will look a little bit more detail in climate and future lectures. So let's go ahead and finish up with our summary. We looked instead, of course, that the Earth's surface is in a constant state of change. We have moving plates which give us things like volcanoes, earthquakes, rift valleys, and more. The atmosphere of Earth has also changed from its original composition. It is not the same as it originally was. And the climate of Earth does is known to change over long periods of time due to astronomical effects such as tilt of Earth's orbit and the ellipticity of Earth's orbit. So that concludes this lecture on Earth's surface and atmosphere. 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.