Tuesday, April 13, 2021

To the Rim of SP Crater

Of the more than 600 scoria (or cinder) cones within the San Francisco Volcanic Field, SP Crater stands out as the steepest of them all! One reason for this  is that its final eruptive phase delivered molten lava that welded into dense agglutinate that caps its rim. This hard carapace protects the top and steepens its sides. SP Crater is likely the second youngest crater in this volcanic field (Sunset Crater is the youngest) and has recently been dated at about 55,000 years old. On April 7, I visited and climbed this unique cone, located about 35 miles north of Flagstaff.

A Google Earth image of SP Crater. The image clearly shows the crater and its spectacular set of lava flows that erupted from its base and flowed downhill to the north (top). Note the variations in color on top of the lava flows - the older (and broader) of the flows are weathered and support grasses that appear as green-gray patches. These are seen mostly around the flow margins where the quenching of the lava produced lava levies. There are two lava lobes on the west (left) margin that spilled over these natural levies into a low-lying valley. Younger flows on top of the older flows are not weathered enough to allow for grasses and they appear jet black on top of the older flows. There are least eight other scoria cones in this image that are much older and much more weathered that SP Crater. Some of these have elongate rims suggesting structural control of the vents. The light color in the upper left is the Kaibab Formation bedrock.

SP Crater from an earlier trip in January, 2011 shows the steep sides of the scoria cone.

The hike I undertook with friends began on the west side SP Crater (left) and generally switchbacked up to its western rim. Then we hiked around to the south side of the rim. 

This is where the young rubble from SP Crater rests on the older material of an older cone.

Brad and Dawn on the indistinct social trail that climbs the west slope.

Volcanic bombs are seen on the slopes of the volcano. One would not want to be hit by one of these dense blobs of 1000°C  lava.

When hot spatter lands on the ground, it tends to squash and flatten out into distinct layers.
After achieving the top of the cone, a fantastic view to the south is obtained. In the far distance is the San Francisco Mountain stratovolcano. In the middle distance with a deep shadow in Colton Crater. It is a next on our tour of craters in Northern Arizona.

Close-up view of the agglutinate rim of the volcano. This hard carapace protects the unleaded scoria that lies beneath it. Through time the slope beneath this likley becomes oversteepened.

The agglutinate likely once completely covered the surface of the inner crater. It is now being undercut as scoria slips downward, causing the agglutinate cap to collapse. A possible sequence of events at SP Crater would be construction of a cone, possible phreatomagmatic eruption to create an inner crater, then fountaining of lava to produce agglutinate, and finally a lava leak in the north side of the cone to produce a lava flow.

More volcanic bombs lying on the rim.

Beautiful clear skies in an enchanted landscape.

Shadows begin to creep in from the west across the volcanic landscape.

On the way down and toward the lava flows. 


  1. sounds like the knees are OK!

  2. Geoff Webster8:18 AM

    Very special place that Northern Arizona.
    Thanks for sharing.

  3. Incredible! Sounds like quite the adventure.


If your comment will not post, email me with the problem.