Tag Archives: Rock glaciers

Understanding Rock Glaciers

Rock Glacier
A rock glacier in the Chugach, Alaska mountains. Public Domain

Rock Glacier – Overview 

A rock glacier is a body of rock, mud, ice, snow, and water that rests on top of mountains. These sentiments head down towards the valley below due to the force of gravity.

Typically, rock glaciers are covered with debris on the topmost layer, cemented with snow and ice underneath.

If you look at the surface of these glaciers, you will find very little. At a glance, it may not even seem like a glacier. Once the ice below the glacier’s surface begins to move, the rocks on the surface adjust to the movement. As a result, ridges or flow features may appear on the glacier’s surface. 

Rock glaciers are typically small in size. One of the largest may be only a few feet thick but and a few miles long. The rocks on the surface are of varying sizes depending upon the source of supply. 

These geological wonders are one of the most spectacular natural phenomena on Earth. Let’s delve deeper and find out more about how they are formed and what are some of the different types of rock glaciers. 

Changes in Rock Glaciers 

Over time, rock glaciers may grow or shrink. Both the ice mass and rock mass can change depending upon several factors. 

The mass under the surface may change due to weather conditions. It can change due to heavy rainfall, avalanches, spring discharges, and local runoffs. Moreover, the heat and sunlight can cause the underlying snow and ice to melt, which eventually reduces ice mass and causes a downward movement of the glacier. These components can also be lost due to evaporation. 

On the other hand, the rock mass may grow due to talus (large rock fragments) from the mountain or the valley walls. It may also grow as a result of landslides. Rock mass decreases as the glacier is carried down the slope due to ice mass movements. 

Formation of Rock Glaciers 

These glacial mass formations can develop for a variety of reasons. Some of them result from the melting of ice covered with debris due to a landslide. Such glaciers are a common feature of steep-sided valleys where mountainous rocks respond to Earth’s gravitational pull. 

Types of Rock Glaciers 

Depending upon the activity, ice mass, and shape, these entities can be classified into several categories. Let’s look at each of the different types.

Based on the current activity level, rock glaciers can be classified into active, inactive, and fossil rock glaciers. 

Active Rock Glaciers 

Active glaciers contain a significant amount of ice and due to their large ice mass, they move down the slope at speeds ranging between a few inches/year to several feet/year. They typically have well-defined and prominent ridges and a steep frontal slope due to deforming ice. 

Inactive Rock Glaciers 

As the name suggests, inactive glaciers are passive, which means there is very limited downward movement. Such glaciers may still have ice, but the underlying ice mass is not deforming. As a result, there is virtually no activity. Apart from the movement, an additional difference is in the frontal slope. Since there is no movement, the frontal slope is gentler than an active rock glacier’s slope. 

Fossil Fuel Glaciers 

The third type is based on fossil rock glaciers. There is no underlying ice mass as all of the ice has already molten. Therefore, the top surface is often covered with vegetation, and the frontal slope is significantly less steep than the other two types. 

How rock glaciers are formed is another way to classify them. Typically the two classifications are:


This type was active glaciers at some point. They were either detached from their main body or melted due to weathering and other forces. Over time, the deforming ice was covered with talus from the mountains. However, a distinguishing feature is an average temperature that remained around 32F  (0℃) with limited precipitation. So the glacier, which was initially reliant on rain and snow now feeds on talus. And the underlying ice plays a role in the downward movement. 

Periglacial Rock 

Periglacial rock glaciers are formed as a result of the freezing and thawing of the underlying material. The talus that feeds the glacier underwent periglacial processes. The intense freezing and melting of snow at the core of the rock glacier accompanied by talus accumulation of the top results in the forming of a periglacial rock glacier. 

Classification According to the Shape – Tongue-Shaped Rock Glaciers and Lobate Rock Glaciers


Tongue-shaped rock glaciers have a length-to-width ratio of more than 1, which means they have a greater length than width. Such glaciers are usually confined to narrow valleys. The rock mass supply (talus) comes from the steep and rocky highlands nearby. A prominent feature is that the rock mass is only added to the head of a tongue-shaped rock glacier resulting in vertical growth of rock mass.


Lobate glaciers have a length-to-width ratio of less than 1. Such ice masses have a greater width than length. The rock is often derived from the valley walls that enclose the glacier. Since it has a wider size, the rocks can join in the glacier from multiple sides, adding more to the glacier’s width. 

Rock glaciers that do not fall into the above two categories are called complex rock glaciers

Bottom Line 

Rock glaciers are an interesting geological feature that often appears as land masses and move downwards in response to gravity. While the gravitational pull is a significant factor, how the landmass moves depends upon several factors, with the deforming ice beneath the surface being the most prominent one. Weather conditions, how the rock glaciers were formed, and the source of rocks (talus) are also major determinants of the glacier’s movements.