The ongoing shenanigans in our planet’s crust are responsible for earthquakes, volcanic eruptions, the creation of mountains, and various other literally Earth-changing events. But even the most in-depth television or online coverage of such events can’t really illustrate the fractures in the earth’s crust that make these things happen.
You could build a giant, drill-nosed tunneling vehicle, a la the bad guys in the late ‘80s Ninja Turtles cartoon and take an underground journey to view a faultline in person. Or, you could use cheese. Both are good, but we’re not mechanical engineers over here, so we’re only going to show you the cheese demonstration.
Cut the Cheese…for Science
For this quick and easy Demo Science science demo, all you’ll need is some pre-sliced cheese and a cheese cutting device of some time. For the former, we suggest something simple like cheddar, since smelly little kids like the ones you’re likely performing this experiment with wouldn’t know good cheese if you stuck it up their noses, and there’s no point in wasting more exotic stuff. (That’s not a knock on cheddar, though—cheddar is the longtime MVP of the cheese game.) For the latter, we recommend a little-known gizmo called a “knife”, but do what you like.
Gather your students (or whoever) around and bust out your first slice o’ cheese. If you’re using the kind of cheese that comes in individually-wrapped slices, remove the plastic film before continuing. Then, with your cheese slicing device, make a small cut in the middle of the slice, parallel to two sides and perpendicular to the others. (Cut straight, not diagonally, basically.)
Grasp the edges of the cheese parallel to the cut, and pull them gently in a direction perpendicular to the cut. Tell your students to observe the shape of the fracture (cut) as it grows, and keep pulling slowly until the cheese in torn in twain. If anyone (probably Kevin) wants to eat the jankety, ripped up, manhandled deskcheese, let ‘em. #wastenotwantnot
Make two cuts in a fresh slice o’ cheese, approximately one inch apart, in the same direction, parallel/perpendicular to the squared edges of the slice, and diagonally offset from each other. (But don’t cut diagonally—again, straight cheese cuts.)
Then, repeat the previous cheese ripping process. Observations will be different this time, as the tips of the two separate fractures grow past each other. Barring some sort of weird cheese anomaly, the fractures will begin to curve toward each other and eventually consolidate into a single, jagged fracture.
Continue the process as many times as you want and/or as many times as your students will tolerate before going all little-kid-ADD on you. Handing out pieces of cheese as you go will help keep them interested as long as possible.
The Cheese. Stands. Alone.
Your cheese ripping adventures are more or less analogous to tension fractures in Earth’s crust. Pulling on the edges of the cheese slice simulates the tensional tectonic forces that tear at our planet’s mantle far below the surface.
Wherever there is an imperfection or weak spot in the Earth’s crust (or a cut in a slice of cheese), this tension cannot pass through it and instead becomes concentrated around the tips of the break, increasing as the fracture grows. Increased tension makes it easier for the fracture to expand. When the twin cuts in the second round of the demo curve toward each other and combine, it is because tension cannot be transferred in a straight line across the space betwixt the two.
Real-life, deep-in-the-earth tension fractures lead to earthquakes, and similar action can be seen in larger cracks in glaciers and, on a smaller scale, in damaged asphalt roads.