Last time, we discussed several aspects of the Earth's interior: its internal structure (names of the different layers), its composition, and its magnetic field. We began a detailed discussion of the Earth's crust, which is the part we are most familiar with, and talked about the kinds of rock found in the crust.
Plate tectonics is the idea that the Earth's crustal plates (there are roughly 12) slowly move on top of the mantle, resulting in "drifting" of continents. The interactions of the continental plates causes mountain formation, fault zones, earthquakes, rift zones, and volcanoes.
We have a scale model of the Earth and Moon, showing the Earth's crust (surface of the Earth with the oceans removed) and the Moon's surface. As they are passed around the room, think carefully about the differences you see between the two bodies.
The mechanism for plate tectonics is a process known as convection, which involves the rising of warm material from deeper in the mantle, the cooling as it reaches the surface, and the subsequent sinking of material. This is the same process you witness when you have a pot of water boiling on a stove. The crustal plates are floating on the mantle. Thus, they move around slightly as these "bubbles" of material rise and sink beneath them.
The Earth's atmosphere contains several different layers, just like the interior of the Earth. At the surface of the Earth, we are at the bottom of the atmosphere. The pressure at the Earth's surface is 1 bar; you can think of this as a measure of all of the atmosphere above us weighing down on us due to the force of gravity. If we travel to higher elevations, the atmospheric pressure will be smaller because there will be less atmosphere above us. If we go scuba diving in the ocean, the pressure will be larger because we will have the additional weight of the ocean water above us.
The lowest layer of the atmosphere is called the troposphere. This is the layer we live in, and where airplanes fly. Warm air rises, cools, and descends (CONVECTION), causing clouds and weather. The temperature DECREASES as you go up to higher elevations. (this is why Cloudcroft is always cooler than Las Cruces). The next layer is the stratosphere (cold, temperature INCREASES with height due to heating by absorbed sunlight).
Near the top of the stratosphere is a layer of ozone (O3). This molecule absorbs much of the Sun's harmful UV raadiation, and thus it protects us from long-term damage. The reduction of ozone in our atmosphere (due to pollutants created by human beings) is indeed a concern. Ozone can also be formed near the Earth's surface, but it is not good to have around - it forms other bad toxins when combined with automobile exhaust, it damages crops, it is bad for humans to breathe. You should understand the difference between "good ozone" (the stuff in the stratosphere) vs. "bad ozone" (the stuff near the surface of the Earth).
The atmospheric layers are shown here, along with a good description of the various layers here.
You should also be aware of the difference between weather and climate. Weather is the day-to-day circulation patterns that we see in our atmosphere. The atmosphere tries to distribute heat from warm areas to cooler areas. This causes winds, and the water cycle on Earth introduces clouds, rain, hurricanes, etc. Climate is long-term variations in the Earth's atmosphere (e.g. warming or cooling trends over decades or centuries). Climate changes are difficult to measure over short time scales, but can have more profound effects on life on Earth.
Isabel image from NASA, global warming image from EPA.