Glaciology

Glaciology as a subject begins in the 19th century, with the proposal of the Glacial Theory by Agassiz. For a long time it remains the domain of geography, and it is only in the 1950's that glaciology becomes more quantitatively based. In addition, glaciology has more recently lost its innocence, and has become an integral part of the story concerning the past and future climate of the planet.

Surging glaciers

One of the most dramatic phenomena in glaciology is that of the glacier surge. This is a (sometimes rapid) advance of a glacier which occurs repeatedly, more or less periodically. A prime example is the Variegated Glacier in Alaska which advances by some six kilometres every twenty years or so, over a period of a year. This represents a temporary increase in velocity of up to a hundredfold.

Surges of this type are relaxation oscillations, thought to be driven by an instability associated with a hysteretic switch between different subglacial drainage mechanisms, and a consequent switch in basal sliding velocity. Other surges, such as those of Trapridge in the Yukon or Bakaninbreen in Svalbard, indicate less violent oscillations.

While surging glaciers are found in many parts of the world, there are none, or none left, in the European Alps, where glaciology was born. The last surging glacier in the Alps appears to have been Vernagtferner, which surged in 1601 and 1772 (as is known from paintings), possibly around 1844, and again (a dying spasm) in 1898. It has not surged since, and indeed the glacier tongue has more or less disappeared because of the warming climate. The surge period appears to have been about 80 years, with some decline towards the end.

Subglacial floods

Jökulhlaups (or subglacial floods) are periodic floods which emerge from beneath glaciers. Some of the best known occur in Iceland, where subglacial lakes can form due to the presence of subglacial volcanoes, such as that at Grimsvötn. These floods occur every five to ten years, and can have peak discharges up to 10,000 cubic metres a second, about one hundred times the normal discharge. Larger floods can occur following volcanic eruptions, but these are dwarfed by the massive floods which apparently occurred during the last ice age, such as the Missoula floods which carved out the channelled scablands of Eastern Washington State.

Ice streams

Drainage of ice in ice sheets such as Antarctica and Greenland does not occur uniformly, but through huge outlet glaciers or ice streams. In regions of Antarctica such as the Siple Coast, multiple ice streams flow into the Ross Ice Shelf, and are separated by ice ridges which are much more slowly moving. The mechanism for this streaming is not well understood, although it is likely to be similar to the mechanism of surging, and caused by a positive feedback involving speed of flow, frictional heating, basal water production, and lubricated sliding. There is evidence that streaming is time-dependent, and that the shut-off of an ice stream causes ponding of water, and is a possible formative mechanism for sub-glacial lakes, of which there are many in the Antarctic.

Subglacial bedforms

When the ice sheets are removed at the end of an ice age, they leave their imprint on the landscale in the form of a variety of undulatory bedforms. Drumlins, ribbed moraine and mega-scale glacial lineations are all examples of the sorts of corrugated landscapes which are produced. It is thought that they arise through an erosional instability in a somewhat similar way to dunes, but crucially involve the dependence of the sediment erosion rate on the basal water pressure.