Midway Atoll Rock Challenge

Midway's Sand Island is a small point of land in the middle of the North Pacific Ocean.  It's only 2 square miles in size!  It's so small, and so far away from a continent, that there are only two kinds of rocks here naturally: pumice and limestone.

Pumice is a rock made in nature by volcanoes.   Albatross sometimes accidentally swallow pumice when they swallow their food, flying fish eggs.  The eggs are attached to the floating pumice, and so the pumice gets swallowed right along with the fish eggs.  Hopefully, most albatross get rid of the pumice by barfing it up in a bolus.

Limestone is a natural rock made in the ocean by corals, mollusks, and some other animals.  Corals are small jelly-like animals that can take minerals out of seawater to make themselves a limestone "house."   In fact, some corals make a whole colony of these houses.  A lot of coral colonies together is called a coral reef.

Midway also has cement, since that's used in lots of construction, for example: the buildings and the boat pier.  Cement is hard like a rock, but of course it's made by people, not nature.

Try the Rock Challenge!  There are ten "rocks" in the picture above.  Actually, there are 8 pieces of pumice, 1 piece of coral, and a piece of cement.  Can you correctly identify which are the pumice and which are the coral and cement?  If you'd like a better view of all 10 rocks, click on the picture to enlarge it.

How can we find out for sure?  There's a hint in the 2nd paragraph above.  When you think you know which rocks are which, check the next post for the answer!