Gravity can also change over time. As an example let's say that magma gradually accumulates in a chamber beneath the surface. This means that more and more mass is being added to the chamber, which should result in a gravity increase over time. Similarly, if mass is lost — maybe because magma drains from the subsurface — the gravity will decrease over time. Based on this principle, gravity can be used to monitor volcanic activity. In fact, gravity measurements have been done on more than 50 volcanoes around the world, including Yellowstone.

Gravity is measured using a specialized instrument called a gravimeter, or gravity meter — an uninspired name, perhaps, but it is at least descriptive. A gravimeter is essentially a very sensitive spring with a weight at the end of it. If the gravitational pull is strong, the spring becomes longer. Less gravitational pull means a shorter spring.

Past gravity studies of Yellowstone caldera have not yielded strong indications of gravity change, despite abundant ground deformation and earthquake activity that suggest the presence of magma beneath the surface. This might be in part because the magma is relatively deep — about 5 kilometers (3.1 miles) beneath the surface. The deeper the magma, the smaller the effect on gravity.