Visiting Kilbourne Hole (Geology, Rockhounding, and Geocaching)

I have to admit that I was ashamed of how long it took me to finally drive out and visit the famous Kilbourne Hole.  I had heard that the dirt roads were a bit tricky to navigate so I brought along my roommate as a hiking partner and to read the driving directions (the good ol' buddy system).

Kilbourne Hole is a maar - a volcanic crater believed to be formed by a violent eruption as a magmatic body boils the groundwater at initial contact, producing a gaseous eruption which displaces country rock and brings up the steam and magmatic material.  At Kilbourne Hole, surge deposits were emplaced by a steam and debris (ash and rock) mixture and make up the well defined non-volcanic crater walls.

 What makes Kilbourne Hole special is that the basaltic magma which contacted with the groundwater to produce an eruption also contained an abundance of mantle xenoliths which are preserved inside basaltic bombs.  These bombs often shelter the xenoliths from the majority of weathering (which is very important to preserve unstable minerals from the mantle, which would oxidize relatively quickly).

What complicates finding these bomb xenoliths and also makes Kilbourne Hole more interesting is that the maar erupted through a the Afton flow which was a basaltic flow originating from cones to the northeast of Kilbourne Hole.  Since the Afton flow has been dated however, we know Kilbourne hole is younger than the ~ 70 - 81 ka flow.

Anyway, now that the basic geologic background of the area has been covered I'll talk about my trip.  It had rained maybe three days before I went to Kilbourne Hole so the roads were still somewhat washed out: perfect to test out my new '04 Jeep Grand Cherokee!  It handled everything we threw at it and after a bit of wandering to figure out where we were and almost hitting to large birds of prey we stirred up, we made it to the crater rim.  There was an abundance of shells for every type of firearm you could expect a rural area to harbor along the rim edge, but even the litter couldn't spoil the view.

I took a few pictures from the rim to fulfill the requirements for this geocache and then headed to the western side of the rim to find another.  After a good half hour of searching, we found the second cache and had worked up a ridiculous sweat.  It was August 8th when we went and temperatures were in the mid 90s (°F).  We rock-hounded around on the East side of the rim for a half hour or so, giving me a chance to use my newly purchased rock hammer to break open a few small bombs.  Eventually my roommate and I ran out of water so we headed back to the jeep.

I had brought an milk gallon and a 2-liter soda bottle of more water so we tore off our shirts and cooled off with that just in time for some confused border patrol agents to drive by in their Wrangler.  I didn't care I was just happy to cool off and then drive home with a wicked sunburn, a spaghetti sauce jar full of xenoliths, and a happy day out at Kilbourne Hole!

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Photos are all mine but much of the geologic information comes from this excellent NMBGMR page by Shari Kelley at New Mexico Tech, which compiles a solid arrangement of sources on the Kilbourne Hole area.