What is a Glacier?
A glacier is a large mass of snow and ice, with ice at the base, resting on land, long-lived and capable of movement.
Below is an air-photo that shows some glaciers in Greenland coalescing into a larger glacier mass.
The tributary glaciers are of a type called “alpine” glaciers; meaning that they flow out of mountain valleys.
Greenland happens to be one of two places where massive continental glaciers are also present today (the other is Antarctica).
Continental glaciers (as the name implies) are huge features that cover thousands of square miles and are exceptionally thick, upwards of 10-15 thousand feet in some cases! In contrast, alpine glaciers are constrained to valleys, and rarely more than a couple kilometers in width (although can be quite long), and are at most a few thousand feet thick.
Aerial Photo Glaciers in Greenland
This is glaciated terrain in the Indian Peaks Wilderness, about 50 miles northeast of Denver, Colorado. In this case, “glaciated” means that there were once major Pleistocene glaciers here (appr. 20ky ago) but the snow in these photographs are not glaciers!
On the flanks of Mt Toll here, we see what are best called “snowfields.”
A snowfield may persist, even through the summer months, but they don’t show movement and the classic features of glaciers.
We’ll see more of the features of Glacial Erosion (mountains and valleys and ridges) in a later section.
Glacial Ice
Really, a lot like a metamorphic rock!
Below, a piece of glacial ice that has “calved” off of a coastal glacier (ice breaking off and floating into the water).
From– https://en.wikipedia.org/wiki/Blue_iceberg
Glacial ice starts out as snow.
Compaction leads to recrystallization as ice crystals, and over time these flow and even
grow into each other.
Just like a metamorphic rock– recrystallization and re-growth of mineral grains.
(Solid H2O, if natural, meets the definition of mineral.)
USGS photomicrograph of glacial ice. Each grain is a little under a millimeter in diameter.
Glacial Movement
A feature of real glaciers, like this Argentinian one on the left, are the fractures into the ice called crevasses. A typical crevasse can be relatively shallow, or hundreds of feet deep.
Crevasses form because the upper part of glaciers are “brittle.”
Deep in the glacier, the ice moves in a plastic kind of manner.
It deforms more like silly-putty or toothpaste! This is much like the region of the mantle called the asthenosphere, which you may recall is a plastic zone, atop which the more brittle lithosphere moves (hence, plate tectonics!).
Check out the cross sectional look into a glacier, below.
The differential speed of movement with depth is what causes the brittle deformation into
Above image from S. Earle, Physical Geology, OER text
Below is an interesting animation of glacial movement and crevasse formation.
Note that crevasse formation is enhanced by a glacial flexure, a bend as it goes from shallow to steeper gradient– as would be expected with a brittle material.