Jon Morris: Theory of Plate Tectonics (Week 4)

Continuing with our tradition of attempting to find interesting geological features in our home state, my family and I visited the San Francisco volcanic field, which is located near Flagstaff, Arizona.  Of course, we were only able to check out a small portion of it, because the San Francisco volcanic field covers 1,800 square miles.  In particular we decided to visit Sunset Crater, primarily because it is easily accessible as opposed to some of the other areas in the San Francisco volcanic field. 

While at the Sunset Crater National Monument, we learned that the Sunset Crater is actually the youngest in the string of volcanoes that are within the San Francisco volcanic field.  Interestingly, original estimates determined that the eruption of the Sunset Crater volcano began somewhere between A.D. 1064 and 1065.  They used tree ring dates, or dendrochronology, as the method of dating the eruption, but more recent evidence suggests that the eruption actually began around A.D. 1085.  Although we did not find definitive information about why the San Francisco volcanic field, and the Sunset Crater exists where they are, the common belief is that the area was formed from a geological hotspot, which are volcanic regions that are thought to be fed by underlying mantle that is deviating in heat, or inconsistent as to heat levels with the surrounding mantle.  Many believe that the position of hot spots on the surface of the Earth are independent of tectonic plate boundaries, however we do know that as the North American plate moves over the spot, new volcanoes appear.  The Sunset Crater is considered extinct, but we were astounded to find out that in 2015 there was an eruption scare after a website with images of steam rising from the Sunset Crater had some people believing the volcano was about to erupt again.  The cause of the steam was later determined to be from a forest fire, thank goodness!  If it had actually erupted again, it would not be the first time it potentially displaced people since it is known to have forced the Sinagua people to temporarily abandon their settlements when it erupted around A.D. 1085. 

The good news is that as far as we have been told, the Sunset Crater is extinct and not likely to erupt again, which is really good news because its initial eruption was responsible for the Bonito and Kana-a lava flows that extended about 2.5 kilometers northwest and 9.6 kilometers northeast.  Additional vents extended along a 10-kilometer fissure to the southeast and also produced a 6.4-kilometer lava flow to the east.  The Sunset Crater eruption produced a blanket of ash and lapilli (various shaped droplets of molten or semi-molten lava ejected from an eruption) covering an area of more than 8120 square miles.

These days the Sunset Crater has re-vegetated and is quite beautiful.  While we can feel fairly safe that the Sunset Crater is no longer going to erupt, it was scary to learn that the San Francisco volcanic field itself is still considered active and that a future eruption in the area is a statistical certainty.

Here are a couple of pictures from out trip, I highly recommend making the journey if you are able.  The region as a whole is quite remarkable, and there are many places to observe volcanoes in the area.

                               ("Sunset Crater Volcano National Monument (U.S. National Park Service)", 2018)

                                  (Cassidy, 2016)

Cassidy, J. (2016). Sunset Crater Volcano National Monument. Retrieved from https://www.azcentral.com/story/travel/arizona/2016/08/22/sunset-crater-volcano-national-monument/88806972/

Sunset Crater Volcano National Monument (U.S. National Park Service). (2018). Retrieved from https://www.nps.gov/sucr/index.htm