Monday, October 24, 2011



Colorado is blessed to have a wide variety of igneous rocks exposed in the central and western part of the state.  These are rocks that originated deep in the earth as liquid magma and then cooled and solidified either below the surface of the earth (intrusive) or on the surface of the earth (extrusive).  The latter are commonly known as volcanic rocks and since they cool very rapidly the individual mineral grains are tiny and magnification must be used to observe the minerals.  In fact, obsidian cools so rapidly that individual crystals are absent from the structure.  These extrusive rocks are “extruded” by explosive volcanoes, or as flows.

In contrast, intrusive igneous rocks “intrude” into preexisting rocks and cool very slowly.  Granite is a common intrusive igneous rock and one can easily see the individual grains of feldspar, quartz, and iron minerals in samples of the local Pikes Peak Granite.  Pegmatites develop near the margins of the magma chamber and cool very, very slowly.  Therefore, crystals can grow quite large; hence, the reported four foot crystals of smoky quartz near Devil’s Head (Pikes Peak pegmatites), and spodumene “logs” in South Dakota (Black Hills pegmatites) approaching 50 feet in length.
Geologists apply a wide variety of names to bodies of intrusive igneous rocks (plutons) including concordant (intrusive rocks are parallel to bedding planes) and discordant (intrusive rocks cut across preexisting rocks).  Further terms include (

  • batholith: large irregular discordant intrusions.
  • stock: smaller irregular discordant intrusions.
  • dike: a relatively narrow tabular discordant body, often with near-vertical attitude.
  • sill: a relatively thin tabular concordant body intruded along bedding planes, often near-horizontal when emplaced, but may be intruded into tilted beds or the entire package may be tilted by later deformation.
  • pipe or volcanic neck: circular or tube shaped nearly vertical body which may have been a feeder vent for a volcano.
  • laccolith: concordant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.
  • lopolith: concordant body with a relatively flat to sagging top and a shallow convex base (spoon-shaped), may have a feeder dike or pipe below.
  • phacolith: concordant lens-shaped pluton that typically occupies the crest of an anticline or the trough of a syncline.
To me, the most scenic plutons are the igneous dikes intruded into preexisting sedimentary rocks.  Dikes intruded into igneous or metamorphic rocks are usually of a similar hardness as the host rock and usually the weathering is similar.  However, dikes intruded into sedimentary rocks are almost always harder than the host rock and therefore tend to erode at a slower rate.  As a result, the dikes become topographic features and are readily noticed.  The most famous dikes in Colorado are those radial dikes associated with the Spanish Peaks in southern Colorado near La Veta.  In fact, these may be the most famous dike swarm in the United States.

However, I prefer my dikes to be infamous, unnamed, startling, surprising, and just something that will “knock your socks off” when driving a back- country road.   Such dikes are found north of Granby on lightly traveled Colorado Highway 125 near Willow Creek Pass.  

Colorado 125 intersects with US 40 slightly west of Granby (three miles) and then generally follows Willow Creek north from the intersection until it reaches the Pass in approximately 21.5 miles.  Along the way the road cuts through several exposures of the late Cretaceous Pierre Shale and the overlying basin fill rocks (arkosic sandstone, conglomerate, shale and some coal) assigned to the Coalmont Formation (Paleocene –Eocene) (Tweto, 1979).  Geologically, the Pass is interesting as it is near the intersection of North Park and South Park, bounded on the east by the Never Summer Range and on the west by the Rabbit Ears volcanics.  At 9621 feet it is one of the lowest places along the Continental Divide in Colorado.  After summiting the Pass, the road continues on for about 52 miles through Walden and finally to the Colorado-Wyoming border.  The scenery is spectacular for the entire trip.

There are several dikes exposed near Willow Creek Pass at approximately Mile Markers  18.2, 20.2, 20.7, 21.4, 21.8, 21.9  (Donnell, 1960 and see geologic maps of Tweto, 1979 and Toth and Soullier, 2000).  The rocks composing the dikes appear to be dacitic (high in plagioclase feldspar and iron and magnesium minerals) to rhyolitic (high silica and low iron and magnesium); all are porphyritic in texture (Toth and Soullier, 2000).  Porphyritic rocks have large grained crystals (phenocrysts) set in a fine- grained matrix and probably indicate the cooling of the magma in two different stages.  Immediately to the east of the dikes is Radial Mountain at 11,240 feet.  Although I could not locate information as to the naming of this peak, it appears that the dikes radiate out from the summit (for an interesting view see Google Maps at and zoom in on Willow Creek Pass on both the Terrain and Satellite programs), an outcrop of rocks similar to the dikes (Tweto, 1979).  Toth and Soullier (2000) believe the dikes are probably Oligocene in age, perhaps 24-28.8 Ma.  They may be related to crustal extension associated with the Rio Grande Rift to the south. 

One of the most interesting aspects of the Willow Creek dikes is
that these quite hard igneous rocks are intruded into the easily
erodible Coalmont Formation.  Therefore, the dikes stand out in
high relief and cause the reaction:  “Whoa!  Stop the car.  What is 
the world is that”.  Colorado 125 is a beautiful back-country road 
with some “up-close” geology available for all to observe. So think 
about doing what Steve   Miller sang: Goin' to the country and 
leavin' right away.  No time to  talk I got to make a getaway.

Donnell, J. R. Geological Road Logs of Colorado: Denver, Rocky
Mountain Association of Geologists, 1960.
Toth, M.I., and S. J. Soullier. Geology in Bankey, V., Soullier, S. J., 
and Toth, M.  I., eds.  Mineral Resource Potential and Geology of 
the Routt National Forest  and Middle Park Ranger District of the 
Arapaho National Forest, Colorado:  U. S. Geological Survey 
Professional Paper 1610, 2000.

Tweto, Ogden. Geologic Map of Colorado. U.S. Geological Survey
State Map Series, scale 1:500,000.