Texas Science Teacher Resource Hub

Some rocks look solid but have tons of tiny holes. Pick up a piece of pumice (the rough volcanic rock you can get at any drugstore) and you'll see and feel the holes. Sandstone has microscopic spaces between every grain of sand. Limestone is full of tiny gaps. The water and oil that humans pump out of the ground have been sitting in those tiny spaces for thousands or millions of years. The rock doesn't have to be obviously squishy. The holes just have to be there.

Almost no underground water sits in open caves or rivers. The Edwards Aquifer in central Texas — the water source for San Antonio — isn't a lake. It's water held in millions of tiny holes inside limestone rock. When you drink Texas tap water from that aquifer, that water just came out of a rock. Picture a sponge a hundred feet thick stretching for hundreds of square miles. That's an aquifer. The "river" is the rock.

The holes have to connect to each other. A rock can have lots of separate, sealed-off pockets and still be useless for storage because nothing can move in or move out. The two physical properties scientists check are porosity (how much empty space) AND permeability (whether the empty spaces connect). A great storage rock has both. A bad one might have lots of holes but they're closed off, so the resource can't get in to be stored or out to be used.

Granite is one of the worst storage rocks. Look at a piece up close — you'll see tightly interlocked crystals with almost no space between them. There's nowhere for water, oil, or natural gas to fit. That's why oil companies don't drill into granite layers and water wells aren't dug into granite bedrock. The rocks that store our natural resources are mostly sedimentary — sandstone, limestone, and certain shales — because they formed from grains piled together with spaces in between.