Thu November 17, 2011
S.P. Crater--Today on Land Lines, Michael Collier and Rose Houk take us to S.P. Crater near Flagstaff. Early cowboys gave this perfectly shaped cinder cone its initials--whose shape reminded them of a ……chamber pot.
Climbing the steep slopes of S.P. Crater, you take one step forward and two steps back in the loose black cinders. This beautifully symmetric cone, about thirty miles north of Flagstaff, reminded local cowboys a century ago of the shape of a chamber pot, thus the initials S.P. As the old wranglers used to say, Volcanoes happen.
At the top, you’re blessed with a breathtaking view of the San Francisco volcanic field--pinyon and juniper grasslands, a broken volcanic terrain that the cowboys summed up as malpais. Six hundred lava flows, cinder cones, and the San Francisco Peaks themselves make up this field. It sprawls across 1,800 square miles from Williams to Flagstaff, and north to Cameron.
What accounts for all of this volcanic activity? The Earth has been hot since the early days of its formation. Ever since, the Earth has been slowly cooling off. To do that, it has to let off steam, so magma--liquid rock--wells up from deep inside.
The S.P. explosion was a doozy. A fiery-red fountain shot high into the sky. Peanut- and papaya-sized cinders rained back down to build S.P.’s classic cone--thirty-two-degree slopes, 800 feet tall. As the volcano coughed up the last of its magma, a gaping 400-foot crater was gouged into the cone. The rim was fortified with welded spatter, and littered with fantastic volcanic bombs--spindle-shaped rocks that had spun like molten footballs as they flew through the air.
Most cinder cones around here are basaltic, but S.P. is a little different. Its lava tends toward andesite--more silica and less iron and magnesium. S.P.’s magma was a little stickier than basalt, making it harder for gas bubbles to escape. Compressed gas trapped near the top of the magma chamber also contributed to the fireworks. Like popping open a shaken can of soda, frothy lava burst out of the volcano’s single vent. After most of the gas escaped and the cinder fountains were exhausted, more languid lava broke out the bottom of the cone. This pattern cinder cone formation followed by a lava flow is very common among volcanic cones in northern Arizona and around the world. S.P’s glowing river of lava flowed north four miles across open country. It froze in place, forming a prickly rock--more treacherous to walk across than the cones softer cinders.
The pyrotechnics of the San Francisco volcanic field have migrated from west to east over the last six million years. The youngest, nearby Sunset Crater, blew up only a thousand years ago. S.P. Crater is also an upstart, just four or five thousand years old. Will it erupt again? Probably not. But volcanoes do happen around here. Don’t be too surprised if the fireworks start again someday.