Callan Bentley over at Mountain Beltway just posted about Aden Crater, and I though i would share a few photos from Kilbourne Hole, a nearby maar volcanic crater. It’s only about 15 miles from my childhood home, and I went there quite a bit in high school, both for geology and just to get out into the desert (mainly to drink beer).
Geologically speaking, it is the crater that resulted from a magmato-phreatic explosion, or when, as Wikipedia says:
rising magma super-heats water-saturated earth, far enough below the surface that a high pressure can be contained. At some point, the pressure is too much, and a steam explosion occurs, throwing the earth out in a catastrophic event. Country rocks are fragmented and expelled in the atmosphere (together with fragments of the magma), eventually creating a deep crater, the bottom of which sits below the pre-eruptive ground surface.
For some nice images of other maars in action, go here. The main reason I like these features so much is that the ash cloud that the eruption produces (called a base surge) is a sediment gravity flow (just like a snow avalanche and a turbidity current). So, these are just really hot turbidity currents on land!
Looking on Google Earth at Kilbourne Hole and nearby Hunt Hole, you see one peculiar thing – there is a prominent ridge on the eastern side of the crater, but not the western side. This is thought to be due to westerly winds (i.e., winds out of the west blowing eastward) during the eruption that pushed most of the ash to the east. The wind in west Texas and southeastern New Mexico usually blows out of the west, and since this eruption only occurred ~25,000 years ago, I think that explanation makes good sense.
In the above photo, you can see the ridges on the eastern side of the crater, and I made a simple cross section showing the formation of the ridges and dunes from the explosion. Type in these coordinates in Google Earth to get you there – take a look at it yourself! 31°58’19.35″N, 106°57’45.23″W.
Looking north, you can see the right hand side (eastern) ridge is much higher than the western ridge. This is due to the wind blowing the exploded ash and bombs eastward. The crater is large – 2 x 3 km and 12 m deep – looks impressive from the ground:
One of the biggest attractions at Kilbourne Hole are the xenoliths full of beautiful green olivine – Callan has some nice photos here. However, we came to look at the base surge deposits on the rim of the crater, which formed when ash was falling after the explosion and piling up into big dunes and ridges on the eastern side of the crater. They are impressive, and quite thick:
In the above photo you can see many dune forms, which are all made of accretionary lapilli, which are basically sand-size ash clumps. If you want to learn more about these kinds of deposits, take a look at this thesis. A detailed photo of the lapilli is below.
Note that these deposits are normally graded (biggest grains on the bottom), as is common in many places where sediment gravity flows deposit sediment. Almost all turbidites are normally graded – see here and here for more info. And if you ever drive through the area, do yourself a favor and take a trip out to Kilbourne Hole (only takes 1 hour from downtown El Paso) – you’ll be glad you did.