Glaciers vs. Rock Glaciers

Are they ice? Rocks? Are they even glaciers? Those are the most common questions people ask about my research. Often, people ask about the differences between glaciers and rock glaciers – that’s exactly the right question.

Skillet Glacier on Mount Moran in the Grand Teton Mountain Range. Photo by Paepin D. Goff in 2016.

Teton Glacier between Grand Teton (left) and Mount Owen (right) in the Grand Teton Mountain Range. Photo by Paepin D. Goff in 2016.

If you aren’t familiar with rock glaciers, you may be familiar with glaciers. Glaciers are masses of perennial (long-lasting) ice that flow under their own weight. Year after year, snow piles up and compresses to create a mass of dense ice. They exist mostly in polar environments, but can be found at nearly every latitude – even equatorial regions! Glaciers that exist in high mountain environments are called “alpine glaciers,” whereas the continental glaciers are called “ice sheets.” Basically, a glacier = perennial ice + flow.

Possible rock glacier landform (or debris covered glacierette) in the Wind River Mountain Range. Photo by Simeon Caskey, 2015.

A rock glacier is a bit different. Firstly, whereas a glacier is ice, a rock glacier is a mixture of rock and ice. In some cases, rocks will overwhelm an existing glacier and, over time, become incorporated into the overall mass, influencing flow. In other cases, precipitation (rain, snow, hail, sleet, fog) and/or meltwater will enter the empty spaces between rocks in an established body of rocks. As temperature drops, that water freezes and unites the rocks into a single mass. We call this interstitial ice. If enough ice builds in open cavities, the mass can gain enough weight to begin flowing. Finally, a third kind of rock glacier forms when roughly equal inputs of rock and ice combine to create a rock-ice landform. With enough material, the landform can begin to flow.

Rock glacier taxonomy, or the study of rock glacier type classifications, is a really interesting area within the field. Scientists in this area are concerned with identifying how specific rock glaciers came to be by studying how fast they move, their sizes and shapes, their materials, and other characteristics that can hint at whether they are mostly rock, mostly ice, or roughly equal portions of each.

Part of what makes rock glaciers difficult to study is that they’re in remote locations. Whereas glaciers are in high mountain environments, rock glaciers are mostly in permafrost zones right below glaciers. In fact, it’s pretty common to find rock glaciers just down valley from an existing glacier or glacierette, which contributes to the ice input.

Aerial view of a rock glacier. Photo by Paepin D. Goff, 2016.

So, if these landforms are in remote areas, why study them? First, rock glaciers are indicators of what’s going on with Earth’s climate. Similar to glaciers, rock glaciers are a proxy (or representation) of increases in average temperature over time. Rock glaciers can also provide information about the health of their nearby environments. But, just as importantly, rock glaciers have direct connections to people in that they act as water towers. Meltwater from rock glaciers flows down valley and charges streams and rivers. Those streams and rivers are water sources, and in some cases, those water sources supply entire populations. (That is certainly the case for the Wind River Reservation in my study area.)

Rock glacier draining into a terminal pond in the Grand Teton Mountain Range. Photo from Google Earth, 2013.

As a cryopshere scientist, my work aims at identifying and characterizing rock glacier landforms for further study. My index of rock glaciers in the Greater Yellowstone Ecosystem helps to identify which landforms contribute to local water supplies. In cases where rock glaciers are melting faster than they’re growing, it’s important to keep an eye on them so we are prepared for large flows of meltwater that might present hazards for surrounding populations or for rock glaciers that stop flowing altogether, thus reducing the available water supply.

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