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Ryan Anderson • Mar 16, 2009

Planetary Surface Processes Field Trip: Day 3

SP Flow and Sunset Crater

This article originally appeared on Ryan Anderson's "The Martian Chronicles" blog and is reposted here with permission.

(This is day 3 of a week-long field trip to Arizona. Catch up with days 1 and 2!)

Today was all about volcanoes. We started early, driving north out of Flagstaff and skirting around the huge San Francisco peaks, which are the remnants of a huge stratovolcano (think Mt. Fuki or Mt. Rainier). The volcano formed between about 1 and 0.4 million years ago. It is currently 12,633 feet high, but in its pristine state it may have been several thousand feet higher. This is because it lost its peak, either by an explosive eruption or by erosion. You can imagine its profile by mentally extending the slopes in this photo up above the mountain to meet at a sharp summit.

San Francisco peaks
San Francisco peaks In the background are the San Francisco peaks, the remains of a huge stratovolcano that has lost its peak either due to an explosive eruption (like Mt. St. Helens) or erosion. Somewhat closer is an eroded cinder cone with a very broad crater. This photo was taken from the rim of SP Mountain, another cinder cone.Image: Ryan Anderson

Our main destination today was SP Mountain, a pristine cinder cone volcano with a spectacular lava flow extending four miles from its base. Cinder cones are volcanoes formed by loose rocks called cinders. As they spew forth from the vent, the cinders form a pile that gradually grows to become a cone.

The class had been given various satellite and aerial imagery of the SP mountain and flow, and made geologic maps of the area. Today we actually got to go out in the field and test their hypotheses. This activity was modeled after the same one that I did in Hawaii last summer, and it’s a really great way to realize just how complex geology can be and how difficult it is to interpret things with just satellite imagery. It’s humbling to realize how wrong we are in interpreting things even on earth, just think how much we must be getting wrong about Mars!

SP mountain
SP mountain A view of SP mountain, a ~1000 foot tall cinder cone. You can see that the rim is capped with solid rock, but most of the cone is loose chunks of volcanic rock, ranging in size from small pebbles to huge boulders. It's quite an exhausting climb!Image: Ryan Anderson
A view of SP flow from the rim of SP cinder cone
A view of SP flow from the rim of SP cinder cone (click for a larger image)Image: Ryan Anderson

The term cinders refers to small chunks that have flown through the air and mostly cooled before they landed, but not all the chunks are small. Some, like the one in the picture below are much larger, and are called “bombs”. These often are spindle or football-shaped, because they form when a blob of lava is flung up from inside the crater, stretching it like silly putty until it breaks.

"Bomb" rock
"Bomb" rock This large "bomb" is a lava rock that was launched from deep inside the SP crater, flew through the air and landed near the rim. It has an elongated shape because it stretched out as it was flung from the vent. Note my pocket knife for scale, and there are many bombs much bigger than this one!Image: Ryan Anderson

We explored all around the rim of the volcano, and walked through part of the lava flow, which is much more rugged up-close than it looks from above:

The lava flow up close
The lava flow up close The lava flow is very rugged up-close! (note the geologists for scale) The lava in northern Arizona tends to have more silica in it than the very fluid hawaiian basalts, and more silica means that the lava in Arizona is more viscous. It forms much thicker, blockier flows than those seen in Hawaii.Image: Ryan Anderson
Sunset crater volcano
Sunset crater volcano Sunset crater, the youngest volcano in Arizona, erupted about 1000 years ago.Image: Ryan Anderson

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