Glaciers and Whatnot
A blog about glaciers and whatnot...
Monday, March 18, 2024
CO₂ Fell So Antarctica Could Rise
Saturday, November 11, 2023
Ice Sheet Memory Foam Mattress
Ice sheets are beautifully complex systems, but it's easy to think of ice sheets and the environment as a one-way relationship: the environment changes, then the ice changes -- environment to ice. But ice sheets have an influence on the environment just as strongly as the environment has an influence on them. This post will talk about an interaction that I have been obsessed with (and nerding out over) recently: glacial isostatic adjustment (GIA, for short).
Imagine the most comfortable bed you can possibly sleep on. The type of mattress that you just sink in to and is the hardest goodbye on a Monday morning. Glacial isostatic adjustment is like that, but a several-kilometer thick ice sheet is you, and your mattress is the Earth's mantle.
When you lie down on a mattress, you exert your weight on that mattress -- compressing the inside of your beloved foam rectangular prism -- causing the surface on which you rest to sink. Ice sheets on continents are doing the same thing, just on a larger scale. Ice sheets are big. And, as you would expect from that detail, they are also very heavy. So heavy, in fact, that they weigh down the upper mantle beneath them. This causes the ground to subside.
"Thickness of the Ice Sheets" - xkcd comic |
The Earth's mantle is deformable, and we can think of the mantle as having a viscosity. Viscosity is the measure of how resistant a substance is to deformation. A substance that is very resistant to deformation is described as having a high viscosity, while low viscosity fluids are less resistant to deformation.
When falling onto your mattress after a long day, think about how far you sink into it, and how quickly. If it's fast, that mattress isn't effectively resisting your force. For the load of an ice sheet, this would mean the mantle below has a low viscosity. The mantle quickly adjusts to the stress applied by the ice sheet. Alternatively, if you slowly sink into your mattress, that's like an ice sheet above a high viscosity mantle. The adjustment is slow.
Your alarm blares, and now you have to get up and get ready for the day ahead of you. You muster up the courage to sit up, then get off your bed completely ... just slowly. Depending on the type of mattress you have, it will rebound as you get up at varying speeds. Memory foam mattresses rebound slowly. Very slowly. Other mattresses might rebound quickly, maybe a second or two after getting up. For an ice sheet (or lack thereof), this is called post-glacial rebound.
Some Sources:
ICE5G-D:
- Peltier, W. R., R. Drummond, and K. Roy (2012), Comment on “Ocean mass from GRACE GRACE and glacial isostatic adjustment” by D. P. Chambers et al., J. Geophys. Res., 117, B11403, doi:10.1029/2011JB008967.
- Peltier, W.R., 2004. Global Glacial Isostasy and the Surface of the Ice-Age Earth: The ICE-5G(VM2) model and GRACE, Ann. Rev. Earth Planet. Sci., 32, 111-149.
- A, G., J. Wahr, and S. Zhong (2013) "Computations of the viscoelastic response of a 3-D compressible Earth to surface loading: an application to Glacial Isostatic Adjustment in Antarctica and Canada", Geophys. J. Int., 192, 557–572, doi: 10.1093/gji/ggs030.
ICE6G-D:
- Peltier, W.R., Argus, D.F. and Drummond, R. (2018) Comment on "An Assessment of the ICE-6G_C (VM5a) Glacial Isostatic Adjustment Model" by Purcell et al. J. Geophys. Res. Solid Earth, 123, 2019-2018, doi:10.1002/2016JB013844.
Caron-2018:
- Caron, L., Ivins, E. R., Larour, E., Adhikari, S., Nilsson, J., & Blewitt, G. (2018). GIA model statistics for GRACE hydrology, cryosphere, and ocean science. Geophysical Research Letters, 45, 2203–2212. https://doi.org/10.1002/2017GL076644
Saturday, October 28, 2023
West Antarctica Is Vulnerable
Surface mass balance of Antarctica and Greenland from 1989-2009. Notice that Antarctica mostly has a positive surface mass balance. Figure from Van den Broeke et al., 2011. |
Observing Antarctica through surface mass balance is like watching people enter and leave through the front door of a hotel. We might not be capturing what is actually happening to the ice. Because of that back door, it is entirely possible for a glacier or an ice sheet to have a positive surface mass balance and still undergo thinning and retreat.
Antarctic ice mass change tracked through GRACE, a NASA mission that observes gravitational differences across the Earth's surface and time. This shows that the West Antarctic ice sheet is losing mass, despite a positive mass balance. From NASA |
And when more ice crosses the grounding line, we get sea level rise. Going forward, the grounding lines in Antarctica will be very important. But scientists are currently working on finding out what the future will hold for the ice sheet. And sometimes to answer questions about the future, we have to look to the past. But that's a different post...
- Antarctic Ice Sheet surface mass balance (antarcticglaciers.org)
- An introduction to Glacier Mass Balance (antarcticglaciers.org)
- Antarctic Ice Loss 2002-2020 : GRACE Tellus (nasa.gov)
- Van den Broeke, Michiel R., et al. “Ice Sheets and Sea Level: Thinking Outside the Box.” Surveys in Geophysics, vol. 32, no. 4-5, 22 June 2011, pp. 495–505, https://doi.org/10.1007/s10712-011-9137-z.
Saturday, August 5, 2023
Surface Strain Rates and Rock Glaciers
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 ideal stake setup pattern. From Nye (1959). |
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. |
Strain Rate (\(10^{-4}\) yr\(^{-1}\)) |
|
---|---|
Maximum | 14.7 |
Minimum | 0.1 |
Mean | 2.8\(\pm\)0.3 |
- 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.
- 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.
Saturday, July 29, 2023
Glacial Geology of the Mohawk Lakes (CO)
Mohawk Lakes via the Spruce Creek trail topo map. |
There is glacial geology all over the Rocky Mountains and if you hike a lot up there, you have likely seen it. Sometimes it's hard to see, like the old (very old) glacial moraines under the Independence Chair of Breckenridge (but I think that is another blog post). Other times, the glacial geology of the Rockies are staring you right in the face ready to be looked at and admired. These places are in the broad U-shaped valleys and cirques nestled under the high peaks of the area.
I completed a hike recently that was full of this stare-you-in-the-face glacial geology. So in-your-face about it, in fact, that a glacier is still there -- though much smaller than it once was. This in-your-face glacial geology (pyramid-shaped peaks, U-shaped valleys, cirques, moraines, lakes, etc.) were left behind by glaciers that once dominated this mountain range during the last ice age.
This hike was along the Spruce Creek trail going up to the Mohawk Lakes just outside of Breckenridge. After leaving the forest on the first couple miles of the hike, you enter a beautiful landscape sculpted by a long-retreated glacier. A steep climb parallel to a waterfall is where you get the first hints of ancient glaciation: exposed bedrock. Rather than jagged and rough, the bedrock that the trail meanders through are smooth and polished by the glacier that once filled the valley you are ascending. But it's not all smooth! The bedrock is decorated with lines like a freshly groomed ski run.
Striations on the exposed bedrock next to the trail. Sorry I forgot to put something on the ground as a size reference, but hopefully the plants in the shot will give some clarity of scale. |
These lines are called striations, which results from glaciers scraping jagged rocks and debris along the glacier-bed interface. The lines are typically parallel to the direction of ancient ice flow. And if you like geology beyond glaciers, you are looking at biotite gneiss from the early Protozoic.
The Mohawk Lakes are surrounded by this bedrock and make for one heck of a scenic area. But my favorite part is further up the trail. After passing Mohawk Lake (the one above the Lower) you climb just a little more to a tiny lake at the tree line. Here, you can see up the cirque all the way to Pacific Peak and the debris-covered glacier below it -- now much smaller than it was when it covered the bedrock you're walking through. You can see the multiple lakes ahead of you, which consist of kettles (possibly) and tarns.
With the bedrock now behind you there are wildflowers and out-of-place boulders scattered all the way up the valley. You are walking over the till of the Pinedale glaciation from the late Pleistocene sprinkled with erratic boulders. Till is simply glacial sediment deposits and features rocks and grains of all sizes in a poorly sorted mess. It seems uniform from the ground, but below those wildflowers are unsorted sediment deposits. The sediments here were deposited during the Pinedale glaciation. Assuming that the sediment deposits were from the glacier's retreat, likely between 21 thousand (glacial maximum) and 10 thousand years ago (end of the Pinedale glaciation).
Hiking above the till. The trail did not pass through a location with a good view of what till looks like, but trust me bro ... it's down there. |
Then there are the glacial erratics. Boulders that were left behind by the glacier on top of the till. In the picture above, you can see some small erratics. I took a picture of a much larger one closer to the bedrock area.
A larger erratic boulder. |
Erratic boulders can provide valuable information about the previous extents of glaciers. Using the chemistry of the rock, scientists can know when an erratic boulder left the shade of the ice and into the outside world -- left behind as the glacier retreats upward.
Passing the lakes, you eventually arrive at a moraine -- a pile of glacial sediment deposits and erratics. Just beyond the moraine, however, is the star of the show: the debris-covered glacier.
Looking at the geologic map of the area, the debris-covered glacier is defined as "active rock glacier deposits" from the late Holocene (very recent). This was true, at least, in 2005 (Keller et al. 2005). So, why is there still a glacier here when most of the glaciers of Colorado have long disappeared?
Bird's eye view of the Spruce Creek rock glacier. |
Debris-covered glaciers, also called rock glaciers, are glaciers that are covered in debris. This debris is typically provided by landslides and rockfalls from the surrounding mountains. Note that there is glacier ice beneath all this debris. How thick this debris layer above the surface of the glacier ice is tends to increase in the lower elevations of a glacier's reach -- in the ablation area where ice melts off a glacier. This detail about where the debris layer is thickest is important for the mass balance (the balance between accumulation; ice gain, and ablation; ice loss) of the glacier.
A thick layer of debris is like a layer of armor for the glacier. When the climate warms, whether through natural or anthropogenic processes, the added warmth to the surrounding environment takes time to penetrate through the layer of debris and into the ice. In other words, the debris layer insulates the glacier. The response of rock glaciers to climate changes is therefore much slower than a glacier without a debris layer protecting it.
Now, 13 thousand feet in elevation in Colorado is still pretty darn warm. So, active rock glaciers are not abundant in the Ten Mile Range of the Rockies. This particular rock glacier is a gem. Or maybe was a gem. The last study done on this glacier was in 2005, and it was found that the glacier was moving very slowly back then (Leonard et al. 2005). It is possible that the glacier could no longer be flowing. But I think that it's pretty cool that this glacier was flowing in my lifetime.
Sources and Whatnot:
- “Cirque - an Overview | ScienceDirect Topics.” Www.sciencedirect.com, 2022, www.sciencedirect.com/topics/earth-and-planetary-sciences/cirque.
- “Glacial till and Glacial Flour (U.S. National Park Service).” Nps.gov, 2018, www.nps.gov/articles/glacialtillandglacialflour.htm.
- Leonard, Eric M., et al. “Kinematics of Spruce Creek Rock Glacier, Colorado, USA.” Journal of Glaciology, vol. 51, no. 173, 2005, pp. 259–268., doi:10.3189/172756505781829403.
- “Tarns (U.S. National Park Service).” Nps.gov, 2018, www.nps.gov/articles/tarns.htm.
- Wallace, C., Keller, J., McCalpin, J., Bartos, P., Route, E., Jones, N., Gutierrez, F., Williams, C., and Morgan, M. L., 2005, Geologic Map of the Breckenridge Quadrangle, Summit and Park Counties, Colorado: Colorado Geological Survey, Open-File Report OF-02-07, scale 1:24,000
CO₂ Fell So Antarctica Could Rise
A misconception that has been circulating around the internet since (I assume) long before I made my Twitter account, is that through Earth&...
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A misconception that has been circulating around the internet since (I assume) long before I made my Twitter account, is that through Earth&...
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Ice sheets are beautifully complex systems, but it's easy to think of ice sheets and the environment as a one-way relationship: the en...
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The West Antarctic ice sheet is the ice sheet in Antarctica that is the most vulnerable to climate change. But the ice sheet itself melting ...