Violent Landscape

The twisted, brilliant land forms of Sunset Crater bear witness to convulsions of the Earth — which may yet renew themselves



Tom Brossart photo

Sunset Crater offers a vivid glimpse into the titanic forces that continue to shape the surface of the Earth.

Let us say that a gnat with a lifespan of a few minutes came across the vast terrain of a human being lying stretched in the grass.

Astonished and a little fearful, the gnat freezes motionless — anxious to know whether this mountain of a creature is dead or merely sleeping.

How long would the gnat crouch in that forest of hairs before concluding that this was the mere ruin of a human being — an ancient landscape fallen into spectacular ruin?

A minute? Two? A whole gnat’s lifespan?

So what do you think: Is the volcano whose violent eruption 1,000 years ago created the lurid red cinder cone of Sunset Crater extinct?

And need we fret about the cataclysm that blasted 4,000 feet off the top of the San Francisco Peaks some 400,000 years ago — an explosion that must have made Mount St. Helens’ look like a child’s sparkler?

In fact, the same forces that raised up the Rocky Mountains and dug out the Grand Canyon in the past six million years also created a whole chain of volcanoes marching across Northern Arizona — from Williams to Sunset Crater National Monument.

Do you suppose we’re all done?

Not likely — in fact, almost certainly not.

And that thought makes the 600 cinder cones and slumped volcanoes scattered across the high plateau around Flagstaff not merely beautiful and wild — but frightening as well.

A visit to that violent landscape — especially a drive and hike through Sunset Crater — offers a vivid glimpse into the titanic forces that continue to shape the surface of the Earth — and which will one day plunge this landscape back into the furnace.

Geologists still don’t know how to fully account for the shifts deep down in the Earth’s semi-molten mantle that have shaped North America. In general terms, the movements at the surface are believed driven by slow, inexorable convection currents in the molten core, transmitted outward through the semi-molten mantle to the brittle crust.

The current drive the movement of the continents — for instance breaking up a single supercontinent some 200 million years ago and scattering the pieces across the planet.

Sometimes, those deep currents also somehow create what’s called a hot spot — where molten rock miles beneath the surface wells up against the cool underside of a crustal plate drifting past on top. That blowtorch of upwelling rock periodically forces its way to the surface and can remain relatively stable for millions of years. Such a hot spot can create a whole chain of volcanoes as the crust drifts past overhead. The long chain of the Hawaiian Islands, which includes a chain of submerged seamounts, is perhaps the planet’s best-known example of the hot spot.

Some 6 million years ago, something shifted deep in the Earth and a vast chunk of North America began to rise, thrusting upward at about the speed your fingernails grow.

That uplift created the Rocky Mountains — and also the Grand Canyon, as several rivers that eventually combined to form the Colorado River chewed away at the layers of soft, sedimentary rock — mostly limestone and sandstone laid down in vanished deserts and disappearing seas. The southernmost edge of that uplift formed the Mogollon Rim — chain of nearly 1,000-foot-high cliffs stretching from Camp Verde nearly to New Mexico.

In Northern Arizona, what amounted to a small hot spot developed, spawning a 50-mile-long chain of volcanoes scattered across 1,800 square miles and starting with Bill Williams Mountain. Each peak in its turn spewed lava then subsided — only to have another mountain emerge from the ground further east.


Tom Brossart photo

A series of fiery eruptions built Sunset Crater 930 years ago.

Mt. Humphreys proved the largest of these peaks, a stratovolcano built up to a towering height of 16,000 feet by a succession of eruptions spread over perhaps half a million years. Most of the volcanoes in the region were made of magma low in silica and therefore not sticky enough to build great peaks. But Mt. Humphreys had sticky, silica-rich rock so it piled layer on layer — like Japan’s famous Mt. Fuji. Mt. Humphrey’s was likely the tallest peak in North America at one time.

But 400,000 years ago, disaster overtook the mountain and an explosion eerily like the much smaller outburst of Mt. St. Helens. Pressure built up inside the mountain from accumulating lava and steam, but instead of escaping out the summit and depositing another layer — the explosion blasted out of the bulging south side. The convulsions left the blasted out bowl of the Inner Basin, now filled with aspen and crushed rock layers that hold Flagstaff’s water supply.

The explosion blasted into the atmosphere the 4,000 feet of rock off the top of the mountain — nearly four times as much height as St. Helens lost.

Fortunately, no one was around to witness the unimaginable disaster. The evidence of our DNA and a scattering of bones and stone tools suggests that the first modern human beings spread out of Africa about 100,000 years ago, long after the mountain had fallen silent.

However, human beings had a ringside seat for the next episode in this violent geological history — the series of eruptions that built Sunset Crater 930 years ago.

Fortunately for the Native Americans who dry-farmed their crops and built their pithouses through the region, the eruptions that eventually built the cinder cone grew gradually enough that they had time to move — in some cases even salvaging the beams from their houses.

The Sunset Crater eruption started in about A.D. 1064 when a nine-mile-long fissure opened in the ground to unleash towering jets of molten rock known as a “curtain of fire.”

After an initial outburst, the eruptions shifted to the northwest end of the fissure and began to build the Sunset Crater cinder cone that still dominates the landscape.


Tom Brossart photo

The most recent eruptions at Sunset Crater stretch back 6 million years.

The already thin, runny lava bearing a load of gases escaped out the vent to the surface. Released from the great pressure of its confinement, the gases and molten rock blasted out into the cooling air. The lava condensed into little gobs, then fell back to earth to create a mound of cinders around the vent.

The eruptions continued off and on for perhaps 150 years, creating a vivid landscape that offers near textbook examples of volcanic land forms.

Termed a scoria cone for the light, air-filled volcanic rock, rusted red by the oxidation of their iron content, also produced a succession of major lava flows, which have given the landscape a wonderful complexity today. Other minerals in the rock tinted the cinders with all the colors of a sunset, including sulfur and gypsum that added hues of yellow, purple and green.

The seven-mile-long Kana-a flow and the massive Bonito lava flow covered two square miles with a river of black lava glowing at 2,200 degrees before it froze into a jagged landscape.

The molten rock flowing out of vents in the side of the cinder cone also formed a network of lava tubes, some more than a mile long. These tubes form when the outer edges of a lava flow cool, insulating the molten rock further from the surface. As long as the tubes run downhill, the insulated lava inside can remain molten and flow for miles.

The jagged, doomed landscape also features an array of other molten rock forms like squeeze ups, formed by the cooling of lava pools. Such pools first form a roof of rock — like a lava tube with nowhere for the pooled lava to escape. The layer closest to the surface hardens, cracks then settles on top of the still molten rock below. The molten rock is then “squeezed up” through the fractures in the rock on top of the pool. As the semi-molten rock squeezes through these cracks, it solidifies into these remarkable forms.

Sunset Crater eventually fell silent, the most recent outbreak in a chain of eruptions stretching back 6 million years.

The eruptions proved only an interruption in the Native American use of the area. In fact, archeologists believe that the population boomed in the decades following the eruptions. One theory holds that cinders spread over hundreds of square miles insulated the soil and so extended the growing season by a few weeks on average. Others argue that a period of increased rainfall that followed the eruptions led to a more generalized population boom.

Either way, these high mountain farmers built up remarkable settlements at Wupatki and Walnut Canyon and elsewhere after the eruptions, only to mysteriously abandon the region about 400 years later.

Geologists report not a peep in the 1,000 years since the violent birth of Sunset Crater.

So is it over? Are the volcanoes extinct? Is the monster asleep?

Just ask the gnat: It’s been nearly a minute.


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