Coliseum Maar, Hopi Buttes, Arizona

Hopi Buttes is a volcanic field located in northeastern Arizona. The volcanic field consists of about 300 maars and diatremes in an area if 965 square miles (2,500 square km). This photos show a distant view of a few of the Hopi Buttes.

Aerial view of Coliseum maar. The maar is about 2,600 feet (800 m) in diameter. The inward dip of the beds and circular shape of the maar resemble the Roman Coliseum.

The maars are volcanic cones that form from phreatic explosions near the surface. The diatremes are subsurface pipes that fed the maars and were filled by volcanic material at the time of the eruption. They are now exposed because of lowering of the land surface by erosion. Most of the maars and diatremes at Hopi Buttes formed between 8.5 and 6 million years ago. The youngest volcanoes are 4.2 million years ago.

Two models have been proposed for the origin of the maars and diatremes. One model suggest that they formed from the rapid unmixing of gas from the magma as it ascended through the crust. The other model proposes that the maars and diatremes result from phreatic explosions caused by magma coming in contact with water saturated rock under a lake.

The tuff beds and lake deposits that make the maar dip about 35 degrees towards the center of the Coliseum. The dips may reflect the original deposits of the material or sagging of the center of the maar due to magma withdraw.