Sunday Sensational Science

Sunset Crater


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.

These things happen when you live in the San Francisco Volcanic field.

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.

If they found a good vantage point on nearby mountains, they might have seen bits of the newly-birthed crater rafting away on lava flows. You can still see chunks of red oxidized agglutinate, pieces of the original cone, trapped in the Bonito flow. The explosions continued, filling Sunset Crater's gaping wounds with fresh scoria, and leaving the volcano with smooth, unblemished flanks.

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.

The visible interior of Sunset Crater is covered with a smooth coating of scoria, but we can get a look inside her if we head over to Red Mountain, many miles to the west. This is the eroded interior of a cinder cone. Steam and percolating water welded its layers of cinder and ash together with the same sorts of mineral oxides that cemented Sunset Crater, creating a volcanic material called tuff. We get this inside-look at the anatomy of a cinder cone because the entire western side of Red Mountain got itself rafted away by a lava flow, leaving an enormous ampitheater carved out of the cone. This time, there was wasn't any explosive action to replace the missing bits.

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.

Most of the flows around Sunset Crater are composed of a'a lava. There is a good reason why the Hawaiians call it a'a, which means, basically, "stony, rough lava." It's a stony, rough lava comprised of clinker, broken chunks of lava carried along the top by a dense, pasty core. As that hotter core oozes its way downslope, the clinker goes along for the ride, tumbling over the leading edge like a bunch of over-excited kids at a slow-motion water park. The tumbled chunks get buried as the flow ambles on. Thus, you get a sort of lava sandwich: clinker top and bottom, paste in the middle. Don't bite into a fresh flow, though: it's erupting at temperatures of 1000-1100 degrees C. That's 1800-2000 degrees F. That's bloody hot.

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 flow turned from pahoehoe to a'a as it lost its gas on the trip out from the mountain. Big cracks formed in its surface from the frictional drag of the liquid lava below the cooler crust, and as that weak crust collapsed when lava drained away from beneath it. It's the youngest and biggest of Sunset Crater's two flows. It covers almost two square miles, and ranges from 100 feet deep in its center to less than 6 feet along the margins. It filled in a basin surrounded by older volcanoes. The other major flow, Kana'a, flowed down an old stream bed for several miles, and never got more than 1000 feet wide. Sunset Crater's continued eruptions covered it in cinders, allowing a lot more vegetation to take root along its surface. Both flows, as well as the cinders, are alkai olivine basalt, which is composed of microscopic crystals of plagioclase, olivine and augite. The occasional big white chunk of stone embedded in the basalt is a xenolith, in this case formed when lava ripped a hunk of Kaibab limestone out of the underlying formations and took it along for the ride.

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.

If you've never used "lava" and "toothpaste" in the same sentence before, that's probably because you've never seen a squeeze-up. These form when gummy, partially-cooled lava squeezes its way through already-hardened cracks in the flow. It's pretty much the consistency of toothpaste. A close inspection will show you the marks left as it scrapes by the solid stone around it.



The Bonito flow also contains lava tubes, formed when molten rock drained from its solidified surroundings. Those tubes are cold - basalt's a terrible material for trapping heat. In colder, wetter Arizona days, the tubes used to contain ice full time. Now, they're usually dry, but frigid. Alas, Bonito's main tube collapsed some time ago, so you can't go exploring it anymore. This is good news for the claustrophobic set, not such good news for the spelunkers among us.



You can console yourself with a hornito. Yes, I know most of you associate the word "hornito" with tequila, for good reason - Hornitos is an excellent brand. In this case, though, hornito means a small spatter cone formed on the surface of a basalt flow. It's created when lava is forced up through the cooled surface. Hornitos are fed by the flow itself, rather than its own magma source as is the case with a regular spatter cone. They're steep-sided heaps of splattered lava that splashed down and over the developing cone, welding itself as it goes. The lava's still partially liquid when it falls, which is why it doesn't form distinct cinders, although the principle's roughly the same.


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.

Just past the hornito, you'll catch sight of something that looks like a mini-cinder cone. It's a cinder dune. This gives you some idea of just how much material Sunset Crater ejected.

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.