Surely you’re aware that water, specifically the Colorado River, carved out the Grand Canyon (and don’t call you Shirley). There’s a good chance your students know this, too. So we all know that water can do a number on rock, given enough time. But, is there a quicker way to achieve this? Can water break down rock in less than several thousand years? You bet your sweet bippy it can!
1000s of Years? Try Overnight, Brah
Essentially, the erosion that created the Grand Canyon (and other, lesser canyons) is just big rocks being broken down into smaller rocks by water. To demonstrate this on a more reasonable timetable, you’ll need a few pieces of sandstone—which I’m sure you just keep lying around—some large zip lock plastic bags (with or without the ®), and water. Also, a bucket or tub to hold said water, and a freezer. (Since bringing a freezer into your classroom is likely unfeasible, it may be easier to use one in the school cafeteria kitchen.)
This is a three-day process, so make sure to let your students (or whoever) know that ahead of time. You know how impatient smelly little kids can be.
Day 1: put your sandstone slabs in the bucket, then fill the bucket with water. Let the sandstone soak overnight. That’s it. Pretty boring first day.
Day 2: take the sandstone pieces out of the water, letting the excess surface water drip off. Then, put them in your zipper bags and seal the bags tightly. Stick ‘em in the freezer overnight, and that’s it for day two. Also pretty boring.
Day 3: pull the sandstone out of the freezer and see what happened. Finally, something interesting! If everything went as it should, you’ll find that you have many more pieces of sandstone that you started with, and that they’re much smaller than your original slabs.
So, Some Sort of Science?
Exactly, dear reader, exactly. The sandstone absorbed some of the water through its porous surface—that porosity is why we suggest sandstone instead of, say, marble. While the stones were in the freezer, the water froze (whaddaya know?) and expanded, causing the rocks to break along tiny, nigh invisible, naturally-occurring joints and cracks.
You can re-try this accelerated form of erosion using different types of rock. Ask students to predict how much each type of stone will be affected by it, then experiment experiment experiment!