Gravity methods are an important geophysical tool for studying calderas because they measure small variations in the Earth’s gravitational field caused by differences in subsurface density, recorded in milliGals (mGal). Calderas form through magma chamber collapse and are filled with volcanic deposits, which create strong density contrasts with surrounding rocks. These contrasts produce gravity anomalies that can be used to interpret subsurface structure.
In the Valles Caldera, low-density volcanic deposits contrast with denser basement and intrusive rocks. The main unit is the Bandelier Tuff, a welded rhyolitic ash-flow tuff made of ash, pumice, and crystal fragments from large explosive eruptions. Because it is much less dense than the underlying basement, gravity data can help map caldera boundaries, depth, and collapse structure.
After collapse, rhyolitic magma formed domes like the Valles Rhyolite, along with obsidian and other volcanic deposits that also have relatively low density. Hydrothermal circulation within the caldera can further alter rocks and change their density, helping identify zones of fluid flow and geothermal activity.
Gravity surveys help image density differences between volcanic fill, intrusive bodies, basement rock, and altered zones. When combined with other geophysical methods, they improve understanding of magma storage, caldera formation, and subsurface fluid systems in the Valles Caldera.
-Kevin