Granite in the Sandia Mountains

A road cut on the east side of the Sandia Mountains, in central New Mexico, exposes enough Sandia granite and the Sandia Formation to provide evidence that weathering along the Great Unconformity is taking place at various rates. The percent biotite and grain size in four different weathering ranks were used to determine granite composition at different states of weathering. The results yielded the interpretation that granite contains more biotite when it is fresh and the biotite weathers out with time. In addition, the middle ranks of granite weathering do not show significant variations in percent biotite from one another. The various states of weathering and their percent biotite values have led to the conclusion that biotite composition is a good indicator of how weathered granite is. In addition, tectonics in the area can explain the fractures and joints that give the granite blocks their shape. Tectonics and weathering of the Sandia Formation limestone also explain why some portions of the road cut are more weathered than others.

A road cut along New Mexico Highway 536 on the east side of the Sandia Mountains exposes Sandia granite and the Great Unconformity at the Doc Long Picnic Area. The Sandia granite underlies the Great Unconformity and the Sandia Formation, comprised of sandstones, shales, and limestones overlies it. Although granite is highly resistant to weathering, in this area, granite is found in various states of weathering, from granitic corestones to grus, as seen in Figure 1. It is interesting to note that the road cut has only been present 60-70 years and grus has either already been formed or grus that was already present 60-70 years ago now mantles the road cut. Also, corestones are permeated with fractures and/or joints that are both vertical and horizontal. These features appear to hold more information on weathering happening around and within the granite. The weathering, or lack thereof, of granite and, in particular, biotite in the granite may also provide more evidence as to how weathering is occurring in the area and at what relative rate.

Several different small sites approximately 30 x 30 cm in area were observed in some detail. Each site was assessed by its weathering rank, mean grain size, biotite percentage, and anything noteworthy, such as rock type and mineralogy. The weathering rank was determined auditory by striking the outcrop with a rock hammer and deciding whether the sound was a sharper “clink” or a dull “thud,” as seen in Figure 2. Sharp “clink” sounds were given a weathering rank of 1, indicating that it is a fresher corestone rock and dull “thud” sounds were given a weathering rank of 4, indicating more highly weathered or grus-like material. Sounds in between weathering ranks 1 and 4 were given ranks 2 and 3 accordingly. Mean grain size was determined by approximating an average grain size at each site using a millimeter ruler, provided in the lab handout. And, finally, biotite percentage was determined by comparing biotite in the rocks to illustrations of percent biotite in the lab handout.

After all observations in the field were made, the class’ group data was compiled into an Excel spreadsheet. The data was then manipulated into a table, Table 1, to illustrate the different values and histogram representations of mean grain size and percent biotite in each weathering rank, Figure 3, and the frequency of percent biotite class boundaries in weathering ranks, Figure 4. Figures 3 and 4 will be discussed later in the paper. In addition, a t-test was performed to determine whether or not there was a significant difference between mean values, this will also be discussed in detail later in the paper.

Sandia granite is mostly composed of feldspars (mostly potassium feldspar and plagioclase), quartz, and biotite in various percentages and grain sizes. Figure 3 illustrates a comparison between mean grain size and percent biotite for the different weathering ranks. The mean percent biotite decreases as the granite is weathered. In contrast, the mean grain size does not appear to have a trend. The error bars in the histogram represent the standard deviation of mean grain size and percent biotite for each rank. The error bars for the standard deviation of mean percent biotite are mostly above the mean percent biotite, except for in weathering rank 4. The error bars for the standard deviation of mean grain size are also mostly above the mean grain size, except for in weathering rank 2 where there is also a larger mean grain size than any other rank. The closeness of the error bars to the mean values indicates that the mean values are a fairly good approximation of the mean percent biotite and mean grain size in each weathering rank.

In addition to comparing mean grain size and percent biotite the values for percent biotite were categorized into class boundaries according to their frequency, as seen in Figure 4. The overall distribution can be very loosely interpreted as a bell curve; however, there is at least one major outlier. The frequency for percent biotite in the class boundary of 5 for weathering rank 1 greatly exceeds the expected value. This outlier could be a mistake in data collection or data compilation, or, most likely, it represents the high biotite composition in the fresher granite that makes up weathering rank 1. Weathering ranks 2 and 3 are shown to have continued medium percent biotite composition. Weathering rank 4, however, has greatly diminished values of percent biotite and frequency. This is to be expected though because the biotite is being weathered out of the granite and grus over time.

Furthermore, although this is a small dataset and the distributions are not truly normal, a t-test was performed to determine whether or not a significant difference between mean values is valid. A t-test between the mean values of weathering ranks 1 and 4 yielded the conclusion that the difference in mean values of percent biotite is significant at the 95% confidence level because the P(T<=t) one tail value is less than 0.05; in fact, the actual value is 0.006, which is far below 0.05. Conversely, a t-test between weathering ranks 3 and 4 determined the difference in those means were not significant at the 95% confidence level because a value of 0.01 was calculated, more than 0.05. The difference in the t-test results in comparing weathering ranks 1 and 4 and ranks 3 and 4 were expected because the mean values of percent biotite of weathering rank 4 at the Doc Long site was much less than at weathering rank 1 and only slightly less than rank 3. Again, this occurrence is also represented visually in Figures 3 and 4.

Considering this road cut is relatively new, the surficial weathering that is taking place is also very recent. Since it is relatively recent, the feldspars, quartz, and biotite are still present in both the granitic corestones and grus; however, the biotite are more weathered out than the feldspar and the quartz is the least weathered. Biotite being more weathered is expected because it is a mica that is weaker in structure than feldspars and, especially, the much stronger quartz, both of which contain more silica (Klein, 2007). Another interesting observation in the Doc Long area road cut is the absence of a weathered rind around the granite blocks, as seen in Figure 5. A weathered rind surrounding an unweathered rock is expected in granite because that is the way weathering processes affect granite (Birkeland, 1999).

The granite may be undergoing chemical decomposition by interactions with rainfall and, perhaps, subsequent small amounts of infiltration; however, since grus is present, the granite appears to be going through significant disintegration caused by shrinking and expanding of the granite. The rock may be shrinking and expanding because of temperature variations during day and night warming and cooling, rock unloading caused by construction during the creation of the road cut, and, especially, biotite expansion. Ritter et. al. (2011) notes similar biotite expansion and, subsequent, disintegration in the Laramie Range of Colorado and in Texas.

In addition, the same rounded corners seen in Sandia granite blocks are also explained in Ritter et. al. (2011) as exfoliation. Exfoliation is taking place because biotite expansion is happening within the vertical fractures and joints that shape the granite blocks peel off the fresh granite over time. The fractures and joints in the blocks are most likely created by tectonic activity in the area, such as the Sandia uplift or the Laramide Orogeny. The expansion and contraction in the fractures and joints is allowing additional weathering to take place near the surface of the granite and creates many of the core stones seen in the Doc Long area. The tectonic activity and expansion and contraction in the fractures and joints may have also helped with the development of the corestone landscape of the lower Sandia Mountains.

As the overlain Great Unconformity and Sandia Formation above the Sandia granite become more exposed, the granite gives way to limestone. Limestone is a lot more susceptible to weathering, especially chemical weathering caused by rainfall; therefore, it is no surprise that most of the road cut in the Doc Long area is comprised of Sandia granite. However, causes of the varying stages of weathering occurring in the Sandia granite along the road cut cannot be so easily concluded. There are a variety of reasons, including that the more northern portion of the road cut may have been overlain by a thinner limestone deposit that became more weathered over time and, in turn, weathered the Sandia granite more too; or the more southern portion of the road cut may have been lifted more due to tectonic activity in the area. Further analysis would have to be done to come to a more conclusive answer. Nonetheless, since the road cut is relatively young and the Sandia Formation is about 300 Ma and the Sandia Granite is about 1440 Ma, it can be safe to say that the weathering below the Great Unconformity is due to the near-surface processes that occurred prior to the Pennsylvanian transgression.

Birkeland, P., 1999, Soils and Geomorphology, 3^rd ed. New York, Oxford University Press. p. 76.

Klein, C. and Dutrow, B., 2007, Mineral Science, 23^rd ed, New Jersey, John Wiley and Sons, Inc. p. 470 and 528.

Ritter, D.F, Kochel, C.R., and Miller, J.R., 2011, Process Geomorphology, 5th Ed, Illiniois, Waveland Press, Inc. p. 90-93.