An pyroclastic flow is a fast-moving suspension of gas and volcanic particles ejected by an explosive volcanic eruption. The rocks formed from these flows are called tuffs and can range from loose, unconsolidated ash to densely welded rock, depending on the temperature and pressure during formation. Our home sits on an outcrop of a very densely welded tuff and the rock type is given the name "welded tuff". It was formed during the eruption of the Tucson Caldera 73.1 million years ago (the sample used for age dating came from a location very near our driveway). Welded tuff is incredibly dense and strong, trying to drill or dig through it is very difficult even with heavy machinery. Fill dirt was brought in to build the pad that our home's foundation (a concrete slab) was built on.

Our house sitting on an outcrop of welded tuff. Wasson Peak is in the background. The rock that makes up Wasson peak is mostly a volcanic megabreccia and contains clasts of limestone and other sedimentary rock, some the size of a building.

Welded tuff outcrop close-up near our driveway.