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From Castle Rock, we headed south on I-25 past Pueblo to Huerfano
Butte. Huerfano Butte is a small basaltic intrusion
which may be related to a dike structure, or it could be an
eroded volcanic plug.
Some of the basalt here is unusual in that it is the rare, large crystal form called "gabbro". As with the granite and rhyolite we've already seen, gabbro is identical in composition to basalt. The difference is cooling time; gabbro is an intrusive, coarse grained rock which solidified underground over a long period of time and it contains larger mineral crystals than basalt. Huerfano Butte (huerfano is the Spanish word for "orphan") was an important landmark in times past. |
| Huerfano Butte. Near I-25 exit 60, south of Colorado City, CO. |
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South of Huerfano Butte is Walsenburg and the highway 160
exit. About twelve miles west of Walsenburg on highway 160
towards La Veta pass is a large rhyolitic dome called Mt.
Maestas (also spelled Mestas).
Local construction workers report thick ice deposits on the lower slopes of the north face (right side) of this mountain. Tractor operators grading a private road found many feet of clear ice buried only a few feet under the soil in several places. |
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Looking west at Mt. Maestas near Walsenburg, CO.
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A pull out on westbound 160 allows a closer view of the south
face of Mt. Maestas and the large scree deposits around its
base.
The rhyolite is weathering and eroding into cobble sized clasts and several landslides are visible on the steep slopes of the dome. |
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Scree deposits at base of Mt. Maestas. Near La Veta Pass, CO.
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Looking south at the twin laccoliths of the Spanish Peaks from a
rest stop on highway 160.
These laccoliths solidified underground and were exposed by the erosion of overlying material. The West Spanish Peak (on the right) laccolith may have supplied magma to a stratovolcano which developed above it. The volcano has long since eroded away. A small neck, Goemmer Butte, is visible to the right and just below the West Peak and occurs along one of the extensive set of dikes which radiate from around the West Peak. |
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Spanish Peaks, Walsenburg, CO.
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Highway 12 branches off westbound 160 and heads south through the
small village of La Veta. The highway crosses several of the
large dikes associated with West Spanish Peak.
Some of these dikes can be traced for miles on topographic maps. This dike rises about a hundred feet above the highway. Note the smaller, parallel dike to the left of the larger one. Dikes may form as magma rises along faults or fractures in the local rock. The fractures and faults themselves may be related to volcanic activity. Large magmatic intrusions, like laccoliths, may warp and fracture the overlying rock. The heat from a large magma body may cause fractures and faults in the surrounding material due to the stress of thermal expansion. Underground steam explosions can also cause fracturing. |
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Spanish Peaks Dike, La Veta, CO.
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Looking at the dike almost end-on. This is the contact zone between the dike and the local sediments. The sediments were "contact metamorphized" in the baked-zone into a hard conglomerate. The contact zone runs vertically just to the left of center. |
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Spanish Peaks Dike, La Veta, CO.
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