High School Earth Science—Semester A

Sample Lesson - Introduction

Lesson 7.04: Plate Tectonics

We've always had a love-hate relationship with puzzles.

Puzzles are both mentally stimulating and totally mindless, which means that after six hours spent sitting on the couch leaning over the coffee table, we can feel like we've really accomplished something. It's super satisfying to put that last piece in its spot and see all 1,000 little shapes come together to make a big picture.

At the same time, it's pretty frustrating to spend six hours sitting on the couch leaning over the coffee table and have nothing real to show for it but an achy back. It's exhausting to spend all night working on a puzzle and still not be finished. And it's absolutely infuriating to get to the end and realize that we were missing a piece all along.

Gah! If only we'd known before we put the other 999 pieces in place.
(Source)

In this lesson, we're going to look at the biggest puzzle on the planet. This puzzle only has a few dozen pieces, but since one piece covers more than 40 million square miles, it isn't a puzzle we can piece together on our coffee table.

Together, these puzzle pieces, or plates, cover Earth's surface, and we'll look at what they are, why they exist, and how they shape our planet. This is important stuff because the movement and position of the plates determine our planet's geography, climate, and biology. In short, they determine whether this is a place we can live. It's a good thing this puzzle isn't missing any pieces.

Sample Lesson - Reading

Reading 7.7.04: Earth's Puzzle Pieces

In the last few lessons, we've discussed how the earth is made up of layers. The top 60 miles or so—the lithosphere—is compact rock that includes both the crust we walk on and the upper, solid portion of the mantle. While it may look pretty solid to us, the lithosphere is not one connected sheet of rock, like the peel of an orange or the skin of an onion.

Instead, imagine taking a hard-boiled egg and rolling it gently under your hand until the shell cracks. The shell still covers the egg, but now it's cracked and broken into small sections. The lithosphere is likewise broken up into numerous tectonic plates (sometimes called lithospheric plates), which sit on top of the semi-solid asthenosphere. There are seven major plates, ten minor plates, and numerous smaller plates that fill in the gaps.

(Source)

Keep in mind that although there are also seven continents, these landmasses don't perfectly match up with the major plates. As we can see on the map above, Europe and Asia mostly share one giant plate, but parts of those continents, like India, are on entirely different plates. The two don't always neatly match, but what's happening below the surface is the main factor in what, geographically, is happening above.

Between a Rock and a Hard Place

While the plates are important, the real interesting stuff occurs at the places where they meet, because that's where the movement happens. The edges of tectonic plates are called plate boundaries, and there are three main types: divergent boundaries, convergent boundaries, and transform faults.

At divergent boundaries, two plates move away from one another at fissures in the earth. Powered by convection currents in the mantle, the crust is stretched and cracked. This allows magma to flow upward, causing the plates to move apart from one another as new plate material is created to fill in the gaps.

Divergent boundaries almost always occur where two similar plates meet. When two oceanic plates move apart, new ocean floor is created. The Mid-Atlantic Ridge is a great example of this, while the East African Rift is a good example of a divergent boundary between two continental plates.

At convergent boundaries, two plates move toward one another, but what happens when they collide depends on what kinds of crust the plates are. There are three different kinds of convergent plate boundaries.

Take a quick visit to the U.S. Geological Survey's website to learn about the differences between the three types of convergent boundaries. Make sure to take note of what geologic features exist as a result of the collision and why they occur. Also pay attention to the explanation of subduction zones and how different types of crust subduct based on how dense they are.

Transform faults are different from the other two because they don't involve the creation or destruction of new crust. Instead, they occur where the edges of two plates slide sideways past one another—although "slide" isn't really the right word when we're talking about huge pieces of rock 60 miles deep.

Instead, as the plates move past each other, they grind and scrape together like two giant pieces of sandpaper, building up huge amounts of energy that get released in the form of huge earthquakes. After an earthquake at a transform boundary, like the San Andreas fault in California, we might wake up to find our rosebushes are now in our neighbor's' yard.

Recap

Feeling like plate tectonics theory is a bit of a puzzle itself? Let's quickly look through the pieces:

• Tectonic plates are pieces of Earth's solid lithosphere that move on top of the semi-solid asthenosphere. They move as a result of convection currents in the mantle.
• At divergent boundaries, plates move away from one another and new plate material is created in the space left behind.
• At convergent boundaries, plates collide into one another. What happens next depends on whether the plates are oceanic or continental crust.
  • If a continental plate converges with an oceanic plate, then the denser oceanic plate will subduct, or sink, below the continental plate. This leads to the formation of magma, plus a convenient channel for it to find its way to the surface.
  • If two oceanic plates converge, then the older, denser plate will subduct.
  • Continental plates won't subduct. They'll just smash into each other to form mountain ranges.
• At transform faults, plates grind sideways past each other.
• These different boundaries result in different geologic features like volcanoes and earthquakes. Since we're afraid of both of these things, we can plan our next vacation by avoiding them.

Sample Lesson - Activity

Activity 7.04a: Where in the World?

Remember maps? We're not talking Google Maps here. We mean good old-fashioned paper maps that you had to read and interpret yourself.

These maps could be a pain to read if you didn’t know what you were doing, though. These days, you can just ask Siri for directions, and she'll tell you exactly how to get there, how long the trip will take, and if there are any In-N-Outs along the way. That's pretty cool—at least until the battery runs out. Know who’s got your back then? Old-school maps.

So, it's still be helpful to understand maps. You don't want to be one of those drivers who follows their GPS right into a lake, do you?

While we can't guarantee you won't end up swimming for the shore, we can at least help you steer clear of some other natural features. In this lesson, you'll be mapping volcanoes, earthquakes, and a few other things.

There are two parts to this activity. First, we'll work with a paper map. Then, the real fun begins, because you'll build some 3D models for some of the features you'll map out in the first part.

Materials

• A way of printing out the world map linked below
• Colored pencils or markers
• Scratch paper
• Some clay or Play-Doh; if you want to make your own, here's Shmoop's quick and easy recipe for two-ingredient dough (you can pick up the ingredients at the dollar store)
  
  • Mix 1 cup of hair conditioner, body lotion, or shaving cream with 2 cups of corn starch.
  • Mix together until you have a moldable dough.
  • Optional: add food coloring to make it pretty.

Part I

Step One: Download and print out this handy-dandy world map.

Step Two: Hop on the internet. You'll need to look for the locations of major volcanoes around the world. Once you've found them, plot them on the map. You can choose whatever symbol you want, like little triangles or circles, but make sure you let us know what it is by drawing a legend.

Step Three: Now, find the locations of major earthquakes and do the same thing. Plot the earthquakes on your map using a symbol you choose, and identify that symbol on your map legend.

Step Four: Looking at your map, make a 3–4 sentence hypothesis about plate boundaries. Where would you expect to see transform faults? How about convergent boundaries? (Be sure to identify which type of convergent boundary you're talking about, like oceanic-oceanic.) Why? Write your hypothesis out on a separate sheet of paper, or better yet, type it up in a doc.

Step Five: Search online for the locations of the tectonic plates (or look at the map from this lesson's Reading). Draw them on your map, too. You don't need to do all the minor plates, but you should include at least the 10–12 biggest ones.

Step Six: Take a look at your map. What do you notice? Write a paragraph or two (5–10 sentences) below your hypothesis describing your observations. Be sure to explain why different features occur where they do. Also, be as specific as possible when talking about the different types of plate boundaries.

Part II

Now that you've got a good understanding of where plate boundaries are and what happens when different plates meet, let's build some models and play around with some clay or Play-Doh. You'll also be answering some questions along the way, so you might want a notebook or some scratch paper to write in.

Step One: Make six "plates" out of your dough. They don't have to be in the shape of the real tectonic plates, but that would win extra cool points. How big you make them will depend on how much material you have.

Step Two: Grab a couple of your plates and place them next to each other. Then, push them together until you have a mountain range. With some small pieces of paper, label the two plates (are they continental or oceanic?), the type of boundary formed, and the direction in which each plate is moving.

Where on your map from Part I would you expect to find mountains like the ones you've just created? What can we say about the densities of these two plates?

Step Three: Take a couple more plates and place them next to each other. Their edges should touch, but don't push them together. Rather, try to slide them past each other. Yeah, there will be some "damage" to the plates, but that's life, and we're modeling real stuff here. With some small pieces of paper, label the two plates (continental or oceanic), the type of boundary, and the direction in which each plate is moving.

Where on your map from Part I would you find a plate boundary like this? Was new crust created during this interaction? Why or why not?

Step Four: After placing the last two plates next to each other, push them together so that one plate subducts under the other. With some small pieces of paper, label the two plates (continental or oceanic), the type of boundary, and the direction in which each plate is moving.

Where on your map from Part I would you find a plate boundary like this? Was there a noticeable difference between the size of the plate that subducted and the other that floated on top?

Step Five: Snap some clear pictures of your models. Make sure that the labels are legible.

When you're all done cleaning up, collect all of your notes and answers to our questions. Here's a list of everything you'll be turning in for this activity:

• Your filled-in, labeled map (from Part I, Steps One through Five).
• The page with your 3–4 sentence hypothesis (from Part I, Step Four) and 5–10 sentence observations (from Part I, Step Five).
• The pictures of your 3D models (from Part II, Step Five).
• Your model observations (from Part II, Steps Two through Four).

Stick all of those in some documents and upload them using the button below. Then put a copy of everything in your car, just in case Siri tries to lead you into a transform fault or something.

Sample Lesson - Activity