During the past thirty million years, the northern part of Pinal County has had two periods of volcanic activity. From 30 to 15 million years ago there were explosive volcanoes and frim 8 to 6 million years ago there were fluid volcanoes.
Explosive Volcanoes
From 30 to 15 million years ago there were explosive volcanoes in Pinal County centered mainly in the Superstition Mountains.
[Image - Mt St Helens, Washington, an explosive volcano.]
The eruptions were violent sending volcanic ash over thousands of square miles. The volcanic rock that formed from the ash is called tuff. These explosive volcanic rocks make up the Superstition Mountains and are found near Olberg, the hills by the Florence Hospital, North Butte, South Butte, and Box Canyon.
Fluid Volcanoes
From about 8 to 6 million years ago a series of fluid volcanic eruptions took place in Pinal County. The volcanoes and the volcanic rocks are similar to the ones forming in Hawaii today.
[Image - A fluid volcano in Hawaii.]
The eruptions were not violent and the magma flowed overran an area of hundreds of square miles. The rock that formed is basalt. Much of the basalt has eroded away. It is found on the top of Walker Butte, Poston Butte, the hills by the Florence Hospital and numerous other small hills scattered around the area. This fluid volcanic activity has moved north forming the White Mountains, the San Francisco Peaks and the last eruption less that a thousand years ago was at Sunset Crater.
[Image - Rough map of some places mentioned in the text, labeled in blue.]
North Butte, east of Florence, showing layers of Tuff>>
<<Walker Butte showing Basalt overlaying Granite
Volcano's of Pinal County Map Puzzle
