Look for: That the break looks like it is pretty far down

4.B.1 Construct an explanation using qualitative evidence from class investigations to explain what is happening to the bedrock below the observable surface when an earthquake causes a shift or break in the land.

Students arguing using a chain of logic to make the connection between the crack at the surface and what must be happening under the surface. Some arguments you may hear them debate with their partner:

• That the break looks like it is pretty far down.
• If the land was broken pretty far down below the surface, the bedrock was also broken apart here.
• The land at Ridgecrest not only broke apart, but it shifted up half a meter.
• The section of land that shifted up was about 10 miles long.
• For such a large section of land to shift up so much, it must mean the break is all the way through the bedrock to where the rock begins to move or melt.

As students are constructing their explanation with their small group, and if they are struggling to come to an agreement, encourage them to think back through the images they saw of the crack near Ridgecrest, how much the land shifted here, and what they have figured out about what it is like under the surface. In addition, using an analogy to help them visualize a break of this size in something solid might help. For example, you might ask the students to think of a large solid food like a pizza, or a pan of brownies or cake. We can use a knife to create an opening in the surface of the brownie, like we saw at the crack in the land near Ridgecrest, but if we want to remove a piece (like shifting it vertically), we need to cut all the way through the brownie. Encourage them to think about this in relation to the land at Ridgecrest shifting half a meter upward.

Reconvene the class. Explain that we will use the pink foam to represent the bedrock. Ask students how this is different from the bedrock we see on the surface of the Earth. Probe students to consider how this is spatially and structurally similar and different.

Suggested prompt

Sample student response

We are using this piece of pink foam to represent the bedrock that makes up this section of Earth where Ridgecrest is. How is this similar to the bedrock?

It is solid.

It is kind of bendy.

It is kind of thick.

It can probably still crack like what happened at Ridgecrest.

How is this different than the bedrock that we find on Earth’s surface?

It’s definitely smaller- Ridgecrest is a much larger area.

It’s a different color.

It may be softer or more bendy.

The bedrock is made of rock- I wonder if that makes a difference?

So it is much smaller than Ridgecrest. How might using something smaller like this to model that area help us in our investigation?

• Sometimes we can’t make changes on something as big as pieces of land. But we can use this to model what might happen at a different scale.
• We can see it without having to go to Ridgecrest or get heavy machinery or cause an earthquake, and still figure out what might be happening using this smaller item.

If we want to use this to help us make sense of what happens to the bedrock when there is an earthquake like near Ridgecrest, do you think this will provide us some evidence that we can use to help us figure this out?

• Yes… it is way smaller and it doesn’t have sediment, like sand on it, but we can use it to see how it breaks.
• It can still probably crack and move like the ground at Ridgecrest.
• It’s smaller, but we can’t move the ground at Ridgecrest on our own. We can move this and see what happens.

Ask students to share what they discussed with their partner(s). Students should argue that the bedrock either broke or was already broken before the earthquake occurred.✱

Use the foam board to demonstrate this by slowly pulling up on one corner of it and bending it up on a diagonal. Say, OK, I am starting to bend the foam board (bedrock ) slightly… which in reality is something that rocks can do up to a point before they break. Typically it’s hard to see such bending unless the forces on them are very great or the size of the rock being bent is very large.

Snap the board in half and then say, OK, so that is one thing that could have happened from the earthquake, the solid bedrock broke. But what if the bedrock was already broken along this line, can someone show me how these two pieces of bedrock would have shifted from the earthquake to result in the motion and change in elevation?

• Students should conclude that a break all the way through the bedrock would have to happen during the earthquake, or would have had to exist before the earthquake happened in order to explain what they observed at the surface of the land: shifting half a meter upward and moving northwest and southeast.
• Say, Let’s add to our model to represent what we just determined.

Add the representation of a crack below the surface all the way through the…