Huerfano Butte

Huerfano Butte: Volcanic Edifice or Hypabyssal Plug

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 that the magmas associated with Huerfano Butte vented to the surface. Notice the darker colored rock on the south side and the lighter colored rock on the north side and the notch at the top. These are the first clues that this feature was not as homogenous as once believed. Metamorphism of the surrounding Pierre Shale to argillite extends radially about 80 m from the butte. The medium to fine grained size of the alkali-grabbro and lack of adjacent ejecta deposits do not support the hypothesis that Huerfano Butte represents a volcanic edifice. It is more likely that Huerfano Butte is a hypabyssal plug.

Huerfano Butte is a biotite olivine alkali-gabbro cut by two east-west trending dikes, intrusions of monzonite and alkali-lamprophyre. The felsic dike, a biotite monzonite intrusion, is 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. The smaller mafic dike is only visible from the east side and appears to be a heavily weathered alkali-lamprophyre. This conclusion is based on a number of observations of similarly weathered alkali-lamprophyre dikes found in the Spanish Peaks region. 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.

Data for both the biotite gabbro and monzonite yield late-Oligocene ages, which are synchronous with the alkaline intrusive rocks of the Spanish Peaks, found 50 km to the southwest. The concordant age spectra for both the dike and the surrounding plug are identical; implying that during the intrusion of one of the dikes the age of the plug was reset. Consequently, the age of the alkali-gabbro plug is probably greater than 25.2 Ma. The lack of cross-cutting relationships between the alkali-lamprophyre and the monzonite dikes make it difficult to ascertain their order of intrusion. But, based on compositional similarities, it is likely that the alkali-lamprophyre and alkali-gabbro intruded around the same time. In addition to the temporal similarities, major element analysis suggests that the alkali-gabbro is geochemically associated with the earliest alkaline intrusive rocks of the Spanish Peaks region.

The information required to log this earthcache can be found at the listed coordinates. To log your visit:

1. Send me an email with answers to the following questions:

a. The information at the listed coordinates reveals that Huerfano Butte is what type of rock outcrop?

b. The summit of this rock outcrop is how many feet higher than the elevation at the listed coordinates?

2. Log your find and upload an image of your group with this rock outcrop.

Technical Information: Intrusive Landforms

Igneous rocks that have cooled underground are termed "intrusive", that is, the magma did not form in situ , but has intruded the surrounding rocks. Intrusive rocks are therefore always younger than the rocks surrounding them. Intrusive rocks can only be seen after erosion has removed the overlying rocks to expose them. As intrusive rocks form underground, they cool slowly, forming rocks of medium to coarse grain size. Rocks formed in large intrusive structures are termed "plutonic", those that form in small intrusions are called "hypabyssal".

Major (Plutonic) Intrusions

Batholiths

The largest igneous intrusions are batholiths. They are defined as being over 100 km² in extent, but may be over 250 km wide and over 1000 km long. Batholiths are some 20 to 30 km thick, which is a sizeable proportion of the continental crust, but compared to their lateral extent, they are somewhat tabular bodies. They are typically composite, being made up of a number of distinct, but associated intrusions. Walls of batholiths are generally near vertical.

A major problem associated with batholiths is just how they are emplaced - what happens to all the rock that they have intruded? The traditional explanation has been by emplacement by stoping . When a magma intrudes, it breaks off fragments of the overlying rock. Being denser than the magma, these rock fragments may sink through the magma. The fragments may sink all the way to the floor of the magma chamber, or they may become assimilated with the magma, thereby changing the magma composition and volume slightly, or they may remain trapped within the magma as a xenolith (xeno = "foreign", lith = "rock"). The major problem with this emplacement method is that these highly viscous magmas can bring up from depth high density blocks. If quite dense rocks can be carried up with the magma, it is hard to see how relatively low density blocks can sink through the same viscous magma. While stoping undoubtedly occurs, and is a factor in the emplacement of batholiths, it is not the sole answer. Similarly, the diapir model, or large rising blob of magma, is probably overly simplistic and overrated as an emplacement mechanism. This form of emplacement is likely to be limited to the deepest crustal regions. Many, if not most plutonic intrusions occur in extensional regimes, and the "room problem" may be partially solved by this extension of the crust. In the upper crust, plutons are likely to be fed by dykes to ballooning plutons that push the country rock aside and upwards.

Stocks

Stocks are similar to batholiths, but smaller, having an area of less than 100 km². They, too can be composite bodies. Some stocks are just the top of a larger batholith, that only has a relatively small part of it exposed at the surface.

A useful term used to describe a major intrusion whose extent and relationships are uncertain, is pluton.

Laccoliths

Laccoliths intrude between parallel layers of rock at relatively shallow depths. The low pressure allows the magma to dome up the overlying rock, so the intrusion becomes a lenticular, mushroom shaped body. Laccoliths are generally formed from acidic, viscous magmas that bulge upwards rather than spreading laterally. The thickness / diameter of laccoliths is greater than 1 / 10, otherwise it is termed a sill.

Lopoliths

Lopoliths are concordant (parallel to layering) intrusions that are saucer shaped. They are formed in a similar manner to laccoliths, but are produced from dense, mafic magma that depresses the overlying strata. Many lopoliths contain layered gabbroic rocks. Some are very large with thicknesses of many kilometres. The Bushveldt lopolith in southern Africa is several hundred kilometres across and contains the richest platinum deposits known.

Minor (Hypabyssal) Intrusions

Dykes

Dykes are discordant tabular sheets that cut across the layering of the rock it intrudes and are commonly steeply inclined. In regions of crustal extension, fractures may form which are filled by magma from a deep source, or intrusive magma may promote fracturing and extension of the crust. Dykes in outcrop range from a few metres in length to many kilometres, and range from a few centimetres wide to over 100 m, although the Great Dyke of Zimbabwe is a gabbroic mass nearly 500 km long and about 8 km wide. Because dykes intrude relatively cool rocks, they frequently display a chilled margin, with grain size becoming coarser towards the centre where the rate of cooling has been slower. Dykes may occur in swarms of parallel dykes, particularly where there has been crustal extension. Veins are very thin dykes.

Aplite dykes are common in granitic bodies. They are light coloured, equigranular and fine to medium grained. They are formed from the last residues of melt after most of the crystallisation of the granitoid was completed, and hence are rich in quartz and alkali feldspar and sometimes muscovite.

Pegmatite dykes also represent crystallisation from a residual melt fraction, but pegmatites are formed from a water-rich fluid, and are very coarse grained. Occasionally, pegmatites contain minerals such as tourmaline, garnet, apatite, beryl, topaz, spodumene, magnetite, sphene, and zircon, and numerous other rare minerals. Most, however, just contain quartz, alkali feldspar, micas and tourmaline. The occurrence of rare minerals is due to the progressive concentration of trace elements into the last fraction of melt, as these elements are not constituents of the common minerals that have crystallised during the solidification of the bulk of the magma.

Sills

Sills are similar to dykes, but are concordant , that is, they intrude parallel to the layering of the country rock. Thicknesses range from metres to hundreds of metres. Because they form by lifting and separating adjacent rock layers, sills only form within a few kilometres of the surface. The Palisades Sill, in New Jersey, U.S.A., is a dolerite sill which demonstrates magmatic differentiation by fractional crystallisation. It contains a base layer of olivine-rich dolerite formed by crystal settling of the early crystallising olivine, a central dolerite, which comprises the bulk of the sill and an upper layer of quartz dolerite, with thin lenses of quartz + alkali feldspar + pyroxene which represent late melt fraction after the more mafic minerals have crystallised. The chilled margins of the sill are basalt.

Volcanic Necks / Plugs

Plugs represent the cylindrical feeder pipe, or conduit, of a volcano. The magma that solidified in the conduit is harder and more resistant to erosion than the pyroclastic deposits and lavas that make up the flanks of the volcano. After the volcano becomes extinct, therefore, the plug often remains standing like a spire over the landscape.

Ring Dykes

Ring dykes are large, near vertical dykes with a circular outcrop pattern. Their thickness varies from hundreds of metres to several kilometres, and the diameter can be up to 30 km. Thicker dykes contain plutonic rocks, rather than hypabyssal. They are centred around a deeper intrusion. The central section may be a block that has sunken into the underlying magma, the ring dykes representing the fracture zone around the sunken block.

Cone Sheets

Cone sheets are minor intrusions which occur as a dyke swarm with a concentric distribution. They dip towards a focus, generally several kilometres deep, at angles between 20^o and 70^o, but typically at around 45^o.