The winter of 1064-65* wasn’t a particularly good one for the locals. There was the 6 mile fissure that opened and began pumping out lava. Then one end of the fissure started throwing scoria at them, collapsing the roofs of their pit houses under hot heaps of fresh cinders and ash. Then more lava flows filled in forested valleys. By the end of it, fields of corn lay buried, the landscape had undergone a fairly dramatic makeover, and the severely surprised Sinagua had discovered the art of basalt corn cobble making. They relocated to points less explosive nearby, where a beautiful new volcano formed a backdrop and gave a fertility boost to their fields.
Northern Arizona would be a flat, arid plateau if it wasn’t for the hot spot beneath it. For 6 million years, volcanoes have erupted here, steadily marching east. The field extends from Williams in the west to the banks of the Little Colorado River in the east, and from just below Flagstaff in the south nearly to Cameron in the north – an area of roughly 1,800 square miles. It contains the highest point in Arizona – 12,633′ Humphreys Peak – as well as the youngest volcano, Sunset Crater. If you’re looking for a particular type of volcano, chances are the San Francisco Volcanic Field has it, from lava domes to a stratovolcano to dozens of cinder cones of all shapes and sizes. With 600+ volcanoes to choose from, you can’t complain. And if you’re really lucky, you might get a chance to see a new volcano born, since the field’s still potentially active. Geologists think any future eruptions will be small enough to get a spectacular show without inconveniencing the locals too much.
Sunset Crater would have put on quite the show itself. The six mile long curtain of fire served as the opening act, and while it probably wasn’t quite as vigorous as many Hawaiian fissure eruptions, brilliant red lava shooting up from the ground is still an impressive sight. But that was merely the prelude. Activity along the fissure slowed quickly, becoming concentrated at the northern end, where the real show was starting. Explosions ejected fragments of lava high into the air; as those fragments cooled mid-air, the dissolved gasses within them exsolved and created dozens of vesticles, peppering the fragments with petrified bubbles. They rained down around the vent, piling into a cone. The heat, the smell, and the noise would have been overwhelming.
We have a good idea what the Sinagua saw. It would have been quite a bit like Paricutín’s birth:
Loud, isn’t it? One begins to understand why the Sinagua fed it corn: I imagine they were trying desperately to calm it down.
As the eruption waned, something wonderful happened. Fumaroles formed near the crater’s rim, venting hot gasses that oxidized the basalt scoria. The iron contained within basically rusted, painting the rim in gorgeous sunset colors. The fumaroles cemented the rim with silica, gypsum and iron oxide; as a finishing touch, they deposited sulfur compounds, opal, hematite, jarosite, and magnetite. Nature had created a masterpiece.
When all was said and done, the volcano topped out at 1000 feet in height, a mile in width, and contained a crater 400 feet deep, which itself hosts a 160 foot deep secondary crater. The local Sinagua might have considered it a decent consolation prize for getting volcanically evicted from their forested valley.
Lava can do some pretty outrageous things. And thanks to Northern Arizona’s cool, dry climate, we can get a nearly unweathered view of its antics. Rain and snowmelt just sink right in without disturbing the surface of the flows too much. If it wasn’t for the lichens and hardy bushes peppering the flows, you’d think they’d just erupted last week.
Back when I was a wee kiddie, our teachers showed us a video of an a’a flow filmed in Iceland. I’ll never forget the sound. As the clinker tumbles, it makes a cacophony like a monstrous china cabinet getting knocked over. This video from Hawaii demonstrates that nicely:
Is that, or is that not, simply awesome?
There are two ways a’a is formed from a basalt flow. One of them is when the basalt is high in gas bubbles and (relatively) low in temperature, thus high in viscosity. The other is when the strain rate of the flow is high – such as when it hits steep ground. Remember this, as it will factor in to the following discussion.
Sunset Crater’s lava flows weren’t limited to a’a. The Bonito flow began close to the western margin of the volcano as pahoehoe, a Hawaiian word meaning “smooth, unbroken lava.” It’s a far more liquid basalt that forms a beautiful, smooth surface, sculpted into undulating billows or ropy loops. It forms that way because of the way very fluid lava moves under a congealing surface crust. It’s hot stuff, 1100-1200 degrees C (2000-2100 degrees F), with a low gas bubble content.
Now, the interesting thing is this: pahoehoe can easily turn to a’a, depending on how the flow goes. If pahoehoe hits an uphill climb, it’ll cool down, slow down, and get all clinkered up. Same thing can happen as the flow cools further from its eruption site. Isn’t that neat?
You can get an idea of what something like that looks like from this video of pahoehoe and a’a flows merging:
Pahoehoe also means “good to walk” in Hawaiian. When I was a kid visiting the Crater, our field trip guide explained the name origins thusly: the Hawaiians, walking barefoot over flows, would try to tiptoe over the rough stuff, exclaiming: “Ah! Ah!” And then, when their feet hit the smooth, soothing surface, they’d sigh in relief: “Mmmm, pahoehoe!”
You’ll never forget the difference now, will you?
The Bonito lava flow is where you’ll see most of the interesting formations. For a crash course in lava, there’s nothing better than the trail that meanders through it. You can take an online field trip, but we’ll hit some of the high points here.
Watching one form is a fascinating experience:
Sunset Crater’s hornito used to be taller, but volcanoes aren’t all that good at welding, and people broke it down by sitting on it and taking away chunks. It’s still an impressive feature, though.
Sunset Crater is a geologists’ dream. There are few places in the continental United States where volcanism is so wonderfully demonstrated, without all the pesky plants in the way. And, just over the horizon, ancient oceans lie exposed, and prehistoric apartments look out over Painted Desert vistas.
But that’s a story for another Sunday Sensational Science. For now, I’ll just leave you with a portrait of Sunset Crater and her lava flows, and let you ponder the power of hot rock to create a work of art:
* Scientists are still bickering over the dates. The paleomagnetic guys think the dendrochronology guys are full of horse hockey, and the dendrochronology guys are busy arguing over whether the eruption or drought or beetles caused the slow tree growth that year. The dates used in this post were acquired from logs used in Wupatki’s roof, and they fit comfortably within the paleomagnetic dates, so we’re running with ’em.
All images except the map of the volcanic field are courtesy of yours truly and her traveling companion. Clicky for large, glorious versions.