One of the most beautiful Colorado trails I've hiked was the Spruce Creek trail, passing the Mohawk Lakes, and ascending a glacier-carved cirque. I wrote about the glacial geology of this place in this blog post. At the end of this cirque is a debris-covered glacier (also called a rock glacier) that may or may not still be active.
To recap, a rock glacier is a glacier that has a surface covered in debris -- mainly from rockfalls and avalanches from nearby mountains. This means that beneath its rocky outer shell is a frozen core of ice nougat. But what makes that inner core of ice active as opposed to inactive? Glacier ice is considered "active" when the following things are true:
- First and foremost, the ice needs to stick around for multiple years. The ice needs to be perennial in a sense that you could revisit the pile of ice years later and it could possibly still be there. Of course, no glacier is truly permanent.
- The ice within the glacier needs to form from the compaction from snow into ice. Essentially, if the ice at some depth below the glacier's surface isn't being crushed by the weight of the ice above it -- enough to turn fluffy snow into hard ice -- it's not a glacier. So, that snow field that you see at the top of that mountain every August isn't a glacier. It's just a pile of snow.
- A glacier needs to show evidence of internal deformation currently taking place. Meaning that more than snow being compacted to ice and perennialism, the ice needs to be flowing and deforming. The glacier ice crystals need to be rearranging themselves under pressure.
The pressure within a glacier, also called stress (i.e.: a force per unit of area across a cross-sectional surface) is the result of the weight of the ice. Stress, \(\tau\), is typically measured in pascals, which is a unit of pressure. With enough stress acting on the ice, the ice within the glacier will deform. How much the ice deforms (stretching or compressing) is called strain. Strain, \(\epsilon\), is represented as the ratio of a glacier's dimensions after deformation to its dimensions before deformation. Strain is therefore dimensionless.
Glaciologists, however, are more interested in the strain rate \(\dot{\epsilon}\) which has a dimension of [time]\(^{-1}\). Fundamentally, stress and strain rate are related through a power law famously known as Glen's Law:
\[\dot{\epsilon}=A\tau^n\]
Where \(A\) is a quantity dependent on the temperature of the ice and \(n\) is some constant (typically equal to 3).
Sometimes in glaciology, it is important to test the strain rate at the surface of a glacier. For example, if you want to determine whether or not the Spruce Creek rock glacier is still active, finding the surface strain rate (if any) would be ideal. A glacier like the Spruce Creek rock glacier, however, (if moving) moves very slowly. Thankfully, way back in the 1950s, a field method that allowed scientists to observe very small surface strain rates was developed. And this method was adopted (and slightly modified) by scientists in 14-year-long study particularly on the Spruce Creek rock glacier, from 1985 to 1999.
The original method described by Nye (1959) is to simply place a number of stakes on the surface of a glacier. These stakes should be grouped into squares with 4 stakes at each corner and 1 stake in the center.
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| The ideal stake setup pattern. From Nye (1959). |
These squares should be distributed along the length of the glacier. Then, for some period of time, the length of the sides and diagonals of the squares are measured and recorded. The strain rate is found by using the ratio of the final length \(l_2\) and the initial length \(l_1\) across a time interval of \(\Delta t\).
\[\dot{\epsilon}=\frac{1}{\Delta t}\ln\left( \frac{l_2}{l_1} \right)\]
With 4 sides and 4 diagonals for each square, each square yields eight surface strain rates. Nye reduces this to four surface strain rates by averaging the strain rates for parallel sides. For example, the measured strain rates for the length of \(b_1\) and \(b_2\) would be averaged to one value.
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| The surface strain rates for 4 directions at every square from the Nye (1959) study. I boxed the strain rates for each direction in red. |
To sum it up simply, however, the distance between each stake is measured and used to calculate the surface strain rate.
This method was essentially used in the 1985-1999 study on the Spruce Creek rock glacier in Colorado. However, the rock part of the rock glacier didn't exactly allow for the use of stakes. Leonard and others (the folks who studied the rock glacier) used boulders instead of stakes, since there is no lack of boulders on the surface of the rock glacier.
| Strain Rate (\(10^{-4}\) yr\(^{-1}\)) |
|
|---|---|
| Maximum | 14.7 |
| Minimum | 0.1 |
| Mean | 2.8\(\pm\)0.3 |
An important observation from the data above is how small the strain rates are. The mean strain rate is 2.8\(\times 10^{-4}\) yr\(^{-1}\). That is 0.00028 [years]\(^{-1}\)! This means that over the course of the study period, the Spruce Creek rock glacier was deforming, just very slowly. In terms of the surface velocity of the glacier (unit of distance per unit of time calculated on three transects of the rock glacier's surface), the flow averaged from 4 to 7 centimeters per year (Leonard et al., 2005) from 1991 to 2000.
So, why the slow flow? Leonard and others list several possible explanations:
- The Spruce Creek rock glacier has a shallow slope. And because stress is determined by gravity, and therefore slope, a low slope would mean low flow rates. However, they bring up the possibility that this could not be the case, citing Frauenfelder et al. (2003), who argues that slope and surface velocity have little correlation for rock glaciers.
- Temperature could control the flow rate of rock glaciers. The modern climate (well, the climate from 1985-1999) of the mountains where the rock glacier resides might be the cause of the slow-moving flow.
- Most of the internal structure of the Spruce Creek rock glacier could be non-deformable (debris-rich), with the deformation mainly occurring where there is more ice. However, the internal structure of the Spruce Creek rock glacier is a mystery.
- The glacier could be stagnating as a result of ice loss due to climate change coming out of the Little Ice Age and human-caused global warming. According to Leonard et al. (2005), the glacier has been slowing down over the past century.
The data on rock glaciers, and the Spruce Creek rock glacier is lacking. So, it is hard to determine the cause. Could be one, could be more, or it could be none of the above.
I haven't found a study on the status of the Spruce Creek rock glacier today (or more recently). So, whether or not the rock glacier is active today is a mystery to me and to scientists.
References:
- Cuffey, K M, and W S B Paterson. The Physics of Glaciers. 4th ed., Amsterdam, Butterworth-Heinemann, Cop, 2010, pp. 54–55.
- Frauenfelder, R., W. Haeberli, and M. Hoelzle. "Rockglacier occurrence and related terrain parameters in a study area of the Eastern Swiss Alps." Proceedings 8th International Conference on Permafrost. Swets and Zeitlinger, Lisse. 2003.
- Leonard, Eric M., et al. “Kinematics of Spruce Creek Rock Glacier, Colorado, USA.” Journal of Glaciology, vol. 51, no. 173, 2005, pp. 259–268, https://doi.org/10.3189/172756505781829403.
- Nye, J F. “A Method of Determining the Strain-Rate Tensor at the Surface of a Glacier.” Journal of Glaciology, vol. 3, no. 25, 1 Jan. 1959, pp. 409–419, https://doi.org/10.1017/s0022143000017093.
