Mark Thomas - Geology: November 2014

Colton Crater from the flank of SP Mountain.

While I was climbing SP Mountain several years ago I noticed Colton Crater a few miles to the south. It was a rather short mountain but it had a monstrous crater with a gaping maw like it was chewing at the sky. It reminded me of the sand worms rising out the desert in Frank Herbert's famous science fiction novel "Dune". It intrigued me so after I climbed down from SP Crater I drove over to it.

On the northwest side of the volcano I found a gate through a barbed wire fence. A four wheel drive two track road took me up the steep side to a low saddle on the rim of the volcano. As I crested the rim and the crater came into view I was astonished. The crater bowl was massive and strangely empty. I've seen a lot of cinder cones and they all have been - you know - full of cinders, but this one looked like it had been swept clean.

4WD track up the NW side of the volcano.

At the time I didn't know it was called Colton Crater. Among the grasslands and cedar trees north of the San Francisco Peaks are dozens of cinder cones clustered together to the west of Highway 89. There are so many volcanoes around Flagstaff that the United States Geological Survey (USGS) couldn't come up with names for them all so they numbered them. On older USGS topographical maps, Colton Crater was volcano V160 and its associated lava flow 160. Only a few of the cinder cones in that area have names, such as SP Mountain, Antelope Hill, Red Hill, plus a few others. At some point V160 became Colton Crater, I assume in honor of the late Dr. Harold S. Colton, a zoologist and founder of the Museum of Northern Arizona in Flagstaff.

Colton Crater isn't colorful or symmetrical and much of its geology is hidden so it receives far less attention than the famous SP Mountain nearby. CC is mentioned in several hiking guide books but it has no trail. Very little has been written about its geology and even Wikipedia says nothing about it at all. But after several visits to the volcano, hikes around its rim, exploring its crater and some internet data mining I've concluded that Colton Crater is the most dynamic, amazing and psychotic cinder cone out there.

Somewhere between 200,000 and 800,000 years ago magma split the earth and Colton Crater was born. At first it was a typical cinder cone similar to many others surrounding it. For a few years it chugged away and piled up about 800 feet of cinders and splatter in successive layers that are visible in the crater walls on the southern interior slope.

Cinder and spatter layers in the southern interior slope.

As Colton Crater grew a branch split off of the main vent and formed two parasitic cones on the south side of the main cinder cone. Today one of them is slightly taller than its parent volcano. One or more lava flows were forced underneath the north and northeast cinder walls. The lighter more buoyant cinders were elevated and floated away on top of the lava flow in a process called rafting. Two low spots on the north and northeast crater rim were created that way.

Rafting is common at many cinder cones but at Colton Crater a lava flow carried away the north side of the cone that undermined a layer of welded spatter. At least 200 feet of it broke away and rotated 90° from its horizontal plane until it stood vertical. The monolith that remains today is often mistaken for a dyke, a vertical conduit that feeds magma to a volcano. The first time I saw it from the opposite side of the crater it fooled me until I examined it up close and realized it was a layer of welded lava spatter.

The lava and rafted cinders flowed four miles to the north. SP Mountain, a much younger volcano, is sitting on top Colton Craters lava flow. The flow is not well defined and difficult to see because it has been covered by a thick layer of soil, grass and cedar trees.

View to the north from the peak of Colton Crater.

Colton Crater had more in store including its most cataclysmic event yet to come.

The entire area surrounding Flagstaff is covered by a veneer of lava flows. Hidden beneath the lava is a 350' thick layer of Kaibab Limestone. This same layer is visible at the Grand Canyon as the top layer of sedimentary rock. Limestone is calcium carbonate rock which is slowly dissolved by acidic rainfall over a long period of time which forms caverns and sinkholes.

There are several such features not far from Colton Crater, including a cavern known as the Blowhole and the sinkholes The Citadel and Arrowhead. Before the roofs of the sinkholes collapsed both were hidden under thick layers of lava.

Arrowhead Sinkhole

The hypothesis at Colton Crater is a significant quantity of water channeled through a passage in the limestone and suddenly poured into the magma chamber under the volcano. The water flashed to steam expanding 1,700 times causing a phreatomagmatic (steam/magma) eruption. A gigantic explosion blasted the guts of the volcano into the sky. A sizable earthquake radiated away from the site as a pyroclastic mushroom cloud full of molten magma, pulverized limestone, shale and cinders surged upward into the stratosphere. The blast excavated the limestone and shale, any lava above it and the core of the original cinder cone leaving a crater hundreds of feet deeper than the volcano was tall. As the magma and rock showered back to earth, it was still hot enough to form a layer of welded tuff on the rim of what was left of the cinder cone.

Welded Tuff.

The dark fragments are magma and lava, the white stones are Kaibab Limestone, the red ones are cinders and the Moenkopi Formation which would have been on top of the limestone.

The crater had indeed been swept clean and the two parasitic cones, lava flow and countryside was buried under a debris blanket of ash and rock. On the southern rim it is 100 feet thick and can be seen above the topmost spatter layer.

The crater left behind is 4,100 feet across and 1,100 feet deep from the highest point of its rim. The bottom of the crater bowl is 250 feet below the elevation of the surrounding plain. It is three times the width and depth of the pit left behind by the largest nuclear bomb ever detonated on earth and Meteor Crater could nest inside it.

In a last gasp from the decapitated volcano a new cinder cone started to build in the bottom but within a week or two it lost pressure and froze where it stands today.

An alluvial fan of loose soil washed down the slope and partially buried the small cinder cone.

I witnessed a steam explosion when I was 11 years old. For Christmas that year I received a kit for making toy soldiers out of lead; melt the lead in a cauldron on a hot plate, pour it into a mold and let it cool. I thought the cooling to be too slow so I filled a bucket with water to drop the mold into which I did. It sizzled and steamed and quickly cooled. Great! No problem. The problem came when I lifted the mold out of the water and inadvertently passed it over the cauldron still on the hot plate. A single drop of water dripped from the wet mold and fell directly into the liquid lead. There was a loud 'BANG' and hot lead exploded out of the cauldron landing in blobs and droplets on the table and my blue jeans. I literally jumped out of my pants and escaped with only minor burns.

They just don't sell toys like that anymore.

To get to Colton Crater drive north on Highway 89 towards Cameron. One mile past the turnoff to Wupatki National Monument is a dirt road to the left. It is just before Hanks Trading Post and Gas Station. This road takes you straight towards SP Mountain in a little over 5 miles. Head south of SP for 2 miles to the base of Colton Crater. There are several roads and none of them are marked but CC can hardly be missed.

Coordinates: 35° 32' 41" N x 111° 38' 12" W

On the NNW side of the crater a 4WD road passes through a gate in the barbed wire fence which ascends to a low point on the rim. You are on Babbitt Ranch property with their permission. Treat it with respect and close the gate to keep cattle where they belong.

Hiking from the location just described there is a track part way to the peak at 7,375 feet elevation. It is a steep 700 foot climb with rewarding views and a small Geocache hidden in a rock carine. To hike around the entire rim is a difficult 2.5 miles. On the NE side the footing is treacherous over large, loose cinders. I would not do it again. If you are really a geo-geek like me you can hike down to the bottom of the crater and check it out. It's less than a mile round trip and a drop of 400 feet.

References:

They Came to the Mountain by Platt Cline

United States Geological Survey http://ngmdb.usgs.gov/maps/mapview/

Lunar Exploration Initiative 2008 http://www.lpi.usra.edu/science/kring/lunar_exploration/ExpandingBPLF.pdf
NAU Professor Michael Ort - http://www.cox7.com/stem-journals/colton-crater
Google Earth

Sandstone concretions grow in sedimentary rocks. As acidic rain or groundwater seeps through the strata it dissolves minerals such as calcite or iron carbonate which is slowly redeposited around some foreign object such as a fossil bone or shell fragment. Concretions are simply sandstone grains cemented together by solidified minerals.

They may form over millions of years into any size or shape but usually as a sphere. They will continue to grow until the ground dries up or erosion exposes them where we may find them.

I found these five foot sandstone concretions in a
hidden location on the Hopi Reservation in Arizona.

Besides these rolling around several others are still buried.

One is shattered exposing layers like an onion.

Unfortunately the site was a party ground for the locals. There was glass and trash scattered about and shots had been fired at the one on the pedestal. Look close and you can see the pockmarks on it. Too bad!

These concretions in Arizona are on the high desert and 5 to 6 feet in diameter. I did a net search and found some other notable locations where they are found.