Rick Bennett downloads data from a solar-powered GPS antenna in Petrified Forest National Park.

Arizona is an exceptional place to study the stability of the ground that we live on.

Processes deep within the Earth are stretching Arizona, increasing the state’s land area by about 3,000 square feet each year — about the size of a five-bedroom house.

To understand the behavior of the region’s rocks and the potential for damaging earthquakes, my students and I are using GPS technology to measure Arizona’s expansion.

Our work is helping us understand whether the ground will stretch tranquilly or rupture violently under ever-increasing tension. It may depend on how slowly the stretching takes place.

Rocks that compose Earth’s outer layers may behave much as expected —hard and strong, their shape might change only by fracturing and sliding along a fault, causing earthquakes. But rocks buried miles below the surface might also lengthen harmlessly under tension, like the elastic in worn-out socks.

To measure the rate of stretching, we are recording GPS data from more than 50 locations throughout the state. Our preliminary results suggest motions of only a fraction of an inch each year distributed across many tens of miles within Arizona. We’re still figuring out whether these motions will result in earthquakes.

Like the folds in a paper fan as it opens, our Sky Islands mountains are slowly spreading apart, riding west into the sunset, passengers on the flowing rock below.