Subsequent week, I’ll head off into the wilds of Arizona and Utah to go to some nice geologic locales. Together with geoscience college students from Denison College, I’ll go to classics just like the Grand Canyon and Zion—together with a spot the place just one,000 years in the past, a number of the most up-to-date volcanism within the decrease 48 states occurred. It’s a place the place over 600 volcanic vents have been lively over the previous 6 million years. It’s a place the place lava flows dammed a river and despatched molten rock up and down a canyon for over 10 miles. It’s a place the place Native Individuals doubtless watched an eruption and made artwork with the lava that was pouring out over the land.
You possibly can see all these sights on the San Francisco volcanic discipline, close to Flagstaff, Arizona. There you’ll discover terrain coated with basalt lava flows and cinders, a composite volcano that can also be the tallest peak within the state of Arizona, and a few unusual rhyolite domes that produced folded rocks and steep flows.
Now, these of you with an concept of what causes volcanic exercise are presumably questioning: Why Arizona? It isn’t close to a subduction zone just like the Cascade Vary or Aleutians. It looks as if a spot that shouldn’t see a lot volcanism in any respect … besides that Arizona is on the sting of the Basin & Vary province, the place North America is stretching. A number of the a lot older volcanism in Arizona comes from this spreading, and the San Francisco volcanic discipline may very well be one other piece of that puzzle.
However it isn’t fairly that straightforward. In the event you have a look at the composition of the basalt erupted on the San Francisco volcanic discipline, it appears far more like lava you would possibly anticipate from Hawaii. This might imply that these volcanoes are fed by mantle plumes beneath the realm—although there isn’t a whole lot of proof past the lava composition.
We discover a whole lot of proof that the basalt that’s the supply of the SFVF melted the crust beneath the volcanic discipline. The lavas are fill of xenoliths, or “overseas rocks”. These samples of fabric from the crust fluctuate from olivine-and-pyroxene-rich chunks that may come from the underside of the crust all the way in which to chunks of limestone that come from close to the floor.
So, what are you able to see once you go to the San Francisco volcanic discipline? Effectively, right here’s a pattern of a number of the locations I’ll get to see for the primary time subsequent week.
Merriam Crater and Grand Falls
Merriam Crater and another close by volcanic vents erupted ~20,000 years in the past (based mostly on publicity courting of the lava flows and paleomagnetism) and despatched lava flows over 10 kilometers in direction of the Little Colorado River canyon. When it reached the canyon, the flows spilled down into the river and blocked it … after which proceeded to movement up and down the canyon over 15 kilometers. Not solely that, however the lava crammed the canyon at Grand Falls and continued to movement one other kilometer past the river. The channel of the Little Colorado River then rerouted round these lava flows (see above). I’m certain it could have been spetacular to see lava pouring into the 65-meter (~215 foot) deep canyon after which proceed flowing down the canyon. An ephemeral lake doubtless shaped after the eruption (though the Little Colorado is a really intermittent river) till the brand new channel was carved.
Within the northern a part of the San Francisco volcanic discipline lies SP Crater and its surrounding sea of volcanic vents. This space of volcanism has been steadily producing eruptions for over one million years—on common, one volcanic eruption each 15,000 years. A research by Conway and others in 1998 put an opportunity of an eruption within the space of SP Crater at ~13 % over the subsequent 1,000 years. SP Crater itself (above) is presumably 70,000 years previous (though the date just isn’t nice and it appears youthful than that age) and the youngest, known as V4626, was solely about 10-16,000 years in the past. (And marvel the place “SP Crater” acquired its identify? Apparently a rancher within the late 1800’s thought it regarded like a, effectively, shit pot.)
Sundown Crater is the youngest recognized eruption within the San Francisco volcanic discipline. It erupted solely about 1,000 years in the past, when the ancestors to the Pueblo lived within the space. When the eruption occurred, a 10-kilometer fissure opened, making a “curtain of fireside” constructed from a number of lava fountains that additionally produced lava flows. Rapidly the fissure eruptions coalesced right into a single vent that grew to become Sundown Crater. That vent continued to erupt, developing a 300-meter-tall cinder cone made principally of unfastened volcanic particles known as “scoria”.
Cinder cones are liable to breaches that allow lava flows escape, and that’s what occurred at Sundown Crater. Two lengthy lava flows come from the bottom of the cinder cone: Bonito and Kana’a. Within the former, chunks of the cinder cone had been carried tons of of meters away from the cone in the course of the eruption.
With individuals dwelling within the space, you would possibly anticipate that the eruption had some affect on their lives. There may be archaeological proof that folks moved 30 kilometers to the north as a result of a lot of the realm close to Sundown Crater was coated in coarse volcanic scoria. Nevertheless, as you bought additional away, solely volcanic ash fell, which truly helped the expansion of crops. There are additionally “corn rocks” which have impressions of maize cobs and it’s speculated that the Native Individuals close to the eruption might have deliberately put these cobs in/close to the lava to make these!
We do know that the San Francisco volcanic discipline remains to be an lively volcanic space, with the very best chance of a brand new eruption coming from someplace within the jap a part of the volcano discipline. Any new eruption is prone to take the type of one thing like Sundown Crater—so most likely spectacular, however not notably harmful to these dwelling close to the sphere.
There may be much more to see throughout the San Francisco volcanic discipline. If you wish to take a volcanic trip, take a look at a discipline information by Sarah Hanson that captures a whole lot of geology and historical past of those volcanic wonders. I’m excited to see these for the primary time subsequent week on the journey—make sure to watch on Twitter for footage as we discover the lively volcanoes of Arizona.
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