Volcanoes and Lava Flows
A thousand years ago, around the year 1085 CE, this landscape would have been unrecognizable, in the midst of a violent volcanic eruption that radically changed the lives of the people who lived here. The ground shook with earthquakes for days or weeks before the eruption began, prompting mass evacuations of the pithouses and fields that now lie buried by lava. Then the ground split open along a great fissure nearly 6 miles (11 km) long, and lava erupted out of it to heights of 850 feet (260 meters) or more.
The Lava Flows Today
The San Francisco Volcanic Field
San Francisco Mountain lost its top around 550,000 years ago. It's not clear whether it had a major eruption, similar to the 1980 eruption of Mount St Helens, or whether it collapsed in an event called a "gravitational collapse." There is good evidence for both theories, and holes in both theories as well.
The San Francisco Volcanic Field is massive, covering thousands of square miles. It is likely to be the result of a process called crustal delamination. This is a geologic process by which the underside of the earth's crust is separated from the upper crust, as a result of plate movement, metamorphosis of rocks, or instabilities in mantle currents below. This detachment allows hot magma to contact the cooler crust, melting it and forcing molten rocks upwards through the crust to the surface.
Volcanic activity over the past 3 million years has produced hundreds of cinder cones, dozens of lava domes and lava flows, and the stratovolcano of the Peaks themselves, which still stands nearly 5000 feet (8000 meters) over the surrounding landscape and is home to the highest peak in Arizona. Volcanism is a part of life in this part of the world, and the field is still geologically active. In the long term, there's no doubt more eruptions will occur here and more cinder cones will form, and we - or our descendants - will adapt and learn to live with them, just like our predecessors and ancestors did.
Last updated: October 15, 2021