We had a very rare earthquake here in Maryland this week, and a pretty big one for us at 5.8 on the Richter scale. It turns out that there are over 100,000 earthquakes a year world wide that can be felt by people and almost 500,000 total that scientists can detect! Naturally, Beckett had a lot of questions, so we set up an experiment to help us better understand what was going on.
The easiest way to begin understanding what is happening during an earthquake is understanding the underlying forces that cause them. Beckett was familiar with plate tectonics from all the dinosaur books we've read over the years and the many maps and projections we've seen of Pangaea breaking up into the contintents we have today. Plate theory says that one of the main causes of seismic activity is the movement of giant plates of land floating on the mantle and bumping into one another or sliding against one another. So we decided to look into plate tectonics a little deeper.
We started by setting up a table with a large piece of wood and a large bucket of sand. We would use the wood as a base for the mantle we were going to make and the sand for the 'crust' or outermost layer of the earth. In plate theory, continental drift is the slow movement of the continents as they float on the mantle below them. The plates follow many of the same contours as the continents, and when these plates rub against one another or collide they release energy that we feel on the surface as tremors. The main thing to understand about these plates is that they have structure -- we can easily stick a shovel into the ground and move around dirt, but the plates and continents are giant structures that move and behave as units. Our wood and sand would let us see some of that structure. We wouldn't exactly see an earthquake, but we could see how different substances act differently next to each other.
For our first experiment we gently sprinkled mostly dry sand on the board. On the right we added rocks to see if there was any difference. There was:
The right side with the rocks not only moved first, it also seemed to move a bit faster. The rocks had much less surface area, less surface tension, and less friction -- all of which contributed to making that side move faster.
For our second experiment, we packed down the sand hard and firm. The board was much heavier and the strength and structure it had was clear as soon as Beckett started to lift the board:
For our next experiment, we added a common geological feature: a river. Using unpacked sand on the left and sand and stones on the right, Beckett poured water across out 'continent' and repeated the experiment. The water clearly changed the way the sand and stones moved. It seems that many of the earth's natural surface features could contribute to the unseen structures below and out of sight.
Next we sifted sand onto the board, divided it into two, sprayed the right side with plain water, and let it dry over night. I guessed that the surface tension of the water would provide a bit of extra stability and even after it dried, the sand on the right would have a shell and behave a bit differently than the unsprayed sand. It did:
Finally, we did one last experiment, using our now very dry and very well sifted sand. Beckett carefully sifted all of the sand onto the board with no rocks and no structure whatsoever. This time it was just plain clean sifted sand. We were surprised by what we saw:
The dry sand poured mostly as we expected -- it just flowed down the board like water. But about halfway through, there was a pause and large structure emerged -- it followed the contours of the wood grain in the board below it! We were using a very smooth and sanded piece of plywood, and yet the very tiny amount of wood grain on the surface was enough to catch the sand and give it a tiny bit of stability and structure before the whole thing slid off the board. Fascinating!
When we looked at the videos (sometimes in slow motion) we could easily see the 'plates' moving differently, sometimes faster, sometimes slower, and sometimes just moving in different ways.
There are lots of ways you can reproduce this experiment to see more: add different substances to the sand, or start with a different substance altogether. You could also do what earthquake safety engineers do, and build tiny wooden structures on the table and shake it to see what makes a building or substrate stronger.
The two fault zones near the epicenter of our earthquake are not very long -- less than 50 miles -- but they can still produce a lot of energy when bumping against another. If you have any earthquake stories, please leave a comment below -- we'd love to hear about any unusual or interesting things that you noticed before during or after the earthquake. Got to run though -- Hurricane Irene is on the way and Beckett and I are planning both safety and an indoor experiment to understand them!