Lesson

Background

Humans don't live in the oceans. Humans on the continents. If the entire Earth was covered by water, humans probably would not have evolved as they have. So what has provided us with continents on Earth?

1. Earth as we see it has ocean areas and land areas. But, what would a completely dry Earth look like if all the water was gone?The map image below shows elevations on Earth with all the water removed - note these elevations exist whether there are oceans or not.

1. a. Are elevations evenly distributed from high to low, or does it seem like there is a lot of high (red) and a lot of low (green), but not as much in the middle (yellow)?
2. b. Also, looking at the yellow around the red, does it seem like there is relatively small amount of land in the transition from red to green?

2. The two main elevations in image above exist regardless of water on the planet.
   1. However, why are we not usually aware of the lower land that is green in this map?
   2. What happens to water when it rains, especially on the red elevations, that makes this so?

3. 
   
   1. Knowing this fact about the Earth, what questions would you have?
   2. If you wanted to answer those questions, what other questions might you need to ask to get enough data to come up with an explanation?

4. As a basic concept, elements combine to make minerals as naturally occurring compounds, and minerals then aggregate together physically to make rocks. There are three major classes (groups) of rocks, do you remember what they are from previous science classes?
5. We can characterize the composition of much of Earth's crust by the names of the igneous rocks that match the crustal composition. In the map image, the red areas and nearby yellow areas generally are the composition of the igneous rock granite, while the green areas and enclosed (mid-ocean) yellow areas are generally the composition of the igneous rock basalt. You can find these rock names outlined in red in the chart below. The areas within the red outline shows mineral names and relative amounts of the mineral composition of these rocks.
   1. What minerals do you find in granite?
   2. What minerals in basalt?

6. The mineral compositions you found in the previous question affect the chemical properties of these rocks. One property you have studied, density, is important here, along with the understanding that lower density materials can float on higher density materials. For Earth, there is a layer under the crust called the asthenosphere that is solid but deformable (peanut butter is a similar deformable solid but the layer in the Earth is much more solid), so less-dense layers of the crust can float on this moveable solid layer. So then the densities of the granite and basalt crust become important because that will determine how these areas float on the asthenosphere.
   1. Examine the data in the table below. Note that the minerals, their densities and percent abundance are presented and use to calculate overall densities for granite and basalt.
   2. Which rock is more dense, granite or basalt?
   3. If both are floating on the deformable asthenosphere, which will float higher?

7. It's also true that granite crust is usually thicker than basalt crust. Putting all these facts together, draw your conclusion about why Earth has two main levels of elevation, and state your evidence and reasoning. Why is this chemistry important to life on Earth?
8. We have looked at density of the outer layers of rock on Earth. Expanding our view, examine the image below and look for a pattern.
   1. How do the densities of major Earth materials vary from the center of the planet to the atmosphere?
   2. Does this help you see why you have studied density in chemistry class?