Huerfano Butte is a conical-shaped circular plug located just east of mile marker 60 on I-25 north of Walsenburg, CO. When viewed from a distance Huerfano Butte appears to be a volcanic neck, but there is no evidence (unlike Goemmer Butte) that the magmas associated with Huerfano Butte vented to the surface. Figure 1 is looking east on I-25. Notice the darker colored rock on the south side and the lighter colored rock on the northside and the notch at the top. These are the first clues that this feature was not as homogeous as once believed.
Huerfano Butte is a plug of alkali basalt. The butte was cut by two subsequent intrusions of monzonite and alkali-lamprophyre. The monzonite intrusion is visible in figure 1 A & B as the light-colored rock cutting through the middle of the plug and extending through the notch and over the top. The notch is present because the monzonite easily weathers compared to the very hard alkali-basalt.
Well-developed jointing is evident in figure 2. Figure 3 shows the fine-grained nature of the alkali-basalt indicating its shallow intrusive nature. Figure 4 is a sample of the lighter-colored east-west trending monzonite (sp34ts) dike that intruded the alkali-basalt plug. The alkali-lamprophyre is only visible from the east side (Figure 5). The monzonite dike was dated at 25.2 ma. The alkali-basalt plug is undoubtedly older, perhaps somewhere around 26-27 ma. This age is consistent with other dated occurrences of alkali-basalts and lamprophyres in the area.
For a more technical overview click here to read the abstract of a paper presented at the 1995 Rocky Mountain Meeting of the AGU entitled "What's the Scoop on Huerfano Butte?"
Figure 2-Photograph of well-developed jointing on south side of plug. The outer portion of the plug is composed of fine-grained alkali-basalt with distinct columnar-type joints.
Figure 3 - Photograph of a piece of the alkali-basalt.
Figure 4 - Photograph of the monzonite dike that cuts through the plug.
Figure 5 - East-west trending alkali-lamprophyre dike cutting through Huerfano Butte.
Figure 6 - Closeup of the Lamprophyre dike. The white material is caliche.