Geology of Kilbourne Hole

Kilbourne Hole by Lee Stockman (Vice President for Programs)

The February Field Trip visited one of New Mexico’s more unusual geological features. Most geologic formations developed over eons. Kilbourne Hole was not one of these, but a spectacular event that was sudden. It has been described as “One of the worlds most perfect example of a maar.” A maar is a volcanic event which occurs when magma comes into contact with water below the surface. About 28,000 years ago when the weight of the overburden could not hold in the pressure generated by the super heated water a violent explosion sent the overburden flying.

The hole left in the ground had hardly any crater walls above the surrounding land. Those who were on the field trip may have noticed the hard light colored material deposited in layers on the inside of the crater where we climbed down to collect. These layers are deposits of the ash that was expelled at the time of the explosion. An unusual characteristic of the magma that produced the explosion is that it contained peridot crystal pockets which came up with the magma from the Mantle originating some 7 miles deep in the earth. Today when we walk along the walls of Kilbourne Hole we find the peridot in the cooled pockets with a coating of the magma on the outside.

The garnet granules in with the Peridot have been dated by the uranium lead method at 1.375 billion years so it had formed long before the explosion that brought it to the surface. What the Crater looked like just after the explosion, we will never know but it must have been deep. Other maars in New Mexico had craters almost a mile deep. But during the ensuing years alluvial deposits have buried the bottom of the crater and today we see a playa and even evidence of human occupation in the form of a foundation for a building.

February Field Trip

Kilbourne Hole by Kyle Meredith Field Trip Coordinator

The day started out a little cooler than I expected, and never really got as warm as I hoped, but it was still a great start for the twentyfive or so people who showed up for our trip to Kilbourne Hole. I don’t know if anyone showed up in Tyrone due to the misinformation printed in the Daily Press, but let me emphasize here that the reliable information is always in the Beacon. If you’re not receiving your copy, please contact our editor, Elaine!


We had no difficulty reaching our first stop where we looked for yard rocks, some of which contained crystalline quartz with an amethystine quality. However, as I led us forward to Kilbourne Hole for lunch, I had forgotten which turn to make (my GPS unit had just kicked the bucket), and the map was worse than useless. Fortunately, we had other competent leaders who got us on the right road, and with one additional turnaround we made it to the overlook where the wind almost blew us over the edge.
After a quick lunch, we drove around to the other side of the Hole where we hiked a short ways to a superb collecting area. My disparaging assessment of what we could find was proven wrong—there was way more interesting stuff than just sandy peridot crystals, and Lee even found a larger crystal than I’ve ever seen come out of there.

As I predicted, though, the wind was fierce, and only got worse as we drove into it on our way home. By the time we got to Deming, we were driving through wind and sand, then wind and rain, and finally wind and snow. If you’ve never been to Kilbourne Hole, it’s worth the drive, but go with someone who knows how to get there. And expect wind.