A Geologic Overview

Rachel  Kronyak

Just as Curiosity is slated to investigate the surface of Mars at Gale Crater, so too have we explored the San Francisco Volcanic Field as a Martian geologic analog.  This extension of young volcanoes straddles the boundary of the Colorado Plateau in northern Arizona and includes a variety of cinder cones, strato volcanoes, and shield volcanoes. Covering 1,800 square miles of land and around 6 million years of geologic time, the volcanic field is also home to a variety of organisms, as its biomes are quite diverse.  The source of the volcanic activity is believed to come from the complete subduction of the ancient Farallon plate underneath North America. As mantle material extrudes to the surface, the volcanoes we now observe are born.

Our first stop at this geologic wonderland was SP Crater. This 820-ft cinder cone has one of the most distinct lava flows in the area. Its last eruption, around 71,000 years ago, resulted in a basaltic andesite flow that can be observed for 4 miles around the cone. Olivine phenocrysts are easily observable in hand sample, as are secondary minerals such as opal. Hiking up the crater took about 60 minutes, pretty much on all fours, and was quite exhausting due to the cinders and 33° angle of repose. Volcanic bombs and flow structures were observed at the rim of the crater. The hike- rather, the RUN- down the cinder cone was an entirely different story, seeing as it only took 6 minutes (yes, I timed us). Excuse my non-scientific words, but it was incredible, hilarious, and unforgettable. A certain rock hammer was lost in the madness, but if given the chance, I wouldn’t do anything different.

Next up was Colton Crater, around 500 ft deep and a mile wide. This one falls into the category of maar- a crater that started off as a cinder cone but has since been reduced to a flattened rim due to an explosive eruption as magma came into contact with the water table. A younger, 10-m high infant cinder cone is present on the floor of the maar. The composition of Colton differs greatly from that of SP- no cinders are present except those on the cinder cone at the crater’s floor. Large outcrops and a dike intrusion can be observed inside the walls of the crater, and many of the rocks are pelagic tuff, granite, and some basement and ultramafic material. This hike was much more manageable and laid-back - no grueling climbing or running down was required.

Our final stop of the San Francisco Volcanic field was Sunset Crater. Its last eruption was in the year 1064, making it the region’s youngest cinder cone. Due to its status as a national monument, we were confined to walking around a paved path through its lava field. Much of the area displayed a reddish-yellow tint as a result of the oxidation of gases following volcanic activity. The aa lava is mainly composed to blocky, fine-grained basalt with phenocrysts of olivine and white, glassy feldspars. We also observed several instances of zenolith in the basalt- regions where magma erupted through wallrock and trapped chunks of Coconino sandstone in it. One such sandstone clast was measured to be around 150mm in diameter. Another interesting aspect of Sunset Crater is the lichen that observed to be thriving on the green/black basalt, but not the red basalt. Why? It is believed that the green basalt is younger and provides a fresher surface on which the lichen can grow. While it proved to be a much more relaxing hike than previous sites, climbing on the jagged basalt was probably not the smartest idea- my cut up shins did not thank me.

While we visited many other incredible places such as Meteor Crater, the Grand Canyon and Painted Desert, the San Francisco Volcanic Field was by far my favorite aspect of our geologic adventure.