Ancient glaciers have contributed to the formation of some of the most spectacular landforms on Earth
while modern examples provide data to help us figure out changes to our climate.
In today’s video, we will start with the basics and discuss some of the most common features of two major types of glaciers.
A glacier is a long-lasting mass of slow-moving ice and snow.
For a glacier to form, we need an environment where cold temperatures dominate for most of the year.
Consequently, most glaciers are either found in polar regions or at high elevations on the continents.
We recognize two basic types of glaciers. Most of the world’s ice is preserved in two massive ice sheets found in polar regions.
In contrast, high mountains on most continents are home to smaller alpine glaciers also known as mountain or valley glaciers.
We’ll begin with a closer look at ice sheets. Let’s start with the obvious.
Ice sheets are huge, extending for millions of square kilometers.
Such large volumes of ice and snow can only accumulate and remain where temperatures are very low.
For example, Antarctica is the coldest place on Earth.
Average annual temperatures in the high interior of the continent are about -57 degrees Celsius.
Even areas along coast rarely get above freezing during the year.
To cover such a large area, ice sheets must also be very thick.
There are several kilometers of ice sitting on top of Greenland and Antarctica,
consequently the ice sheet covers every landform with the exception of some particularly tall mountains.
Glacial ice flows from the thick interior of the ice sheet to the thinner margins.
You will have noticed that Antarctica is surrounded by ocean. A glacier that ends in the ocean may be known as a tidewater glacier.
There are many glaciers that flow off Antarctica and terminate in the ocean,
where the ice can form floating ice shelves or can break off to create ice bergs.
Ice sheets contain ice formed recently but also have layers of ice that were formed up to hundreds of thousands of years ago.
For example, some of the ice layers at the base of the Greenland ice sheet may be more than 100,000 years old.
The thicker ice on Antarctica extends even farther back in time.
The European Project for Ice Coring in Antarctica has drilled the oldest existing ice core
which contains 800,0000 year old ice from a location in Antarctica known as Dome C.
Scientists can interpret the ice properties to figure out how temperature has fluctuated on Antarctica throughout that time.
The record reveals a sequence of long cold periods separated by shorter warm episodes.
For more information about the relationship between the global distribution of ice sheets
and variations in climate,watch our Ice Ages and Climate cycles video.
Outside of polar regions, alpine glaciers form at high altitudes in major mountain ranges on every continent except for Australia.
Alpine or valley glaciers typically begin in a permanent snow field at high elevations.
For example, the Aletsch Glacier is the longest glacier in Europe
and begins at an elevation of just under 4000 meters in the Swiss Alps.
Let’s take a closer look at some of the features of this glacier.
More than 2000 meters below the summit, temperatures are much warmer and cause the glacier to melt
and supply water that then fills this substantial reservoir.
Traveling about 5 kilometers up the valley we come to the end of the glacier
where solid ice is buried under sediment at the glacier terminus.
This sediment, a mix of large boulders, sand and clay-sized particles,
is largely derived from material breaking off or eroding from the valley walls.
We can follow this sediment a little further along where we notice these dark bands .
known as medial moraine, that transport sediment along the glacier
Notice the “bathtub ring” like feature along the valley sides indicating that the glacier used to be
larger and thicker and occupy more of the valley.
Zooming in on the surface of the glacier we can see cracks in the ice known as crevasses
that form above locations where the base of the glacier moves over a step in the valley floor.
Continuing along we see that the glacier we have been following is actually made up of ice
from several smaller tributary glaciers.
These smaller glaciers flow out of steep-sided hollows known as cirques that form just below the ridge crest
and serve as the supply source for the glacier.
Notice that, unlike the rest of the glacier, the cirques are snow-covered.
The overlying ridge is topped by a couple of pointed horn-shaped peaks
characteristic of alpine glacial landforms.
Drawing back we can see the full extent of the glacier, from its beginnings in the high cirques
to its terminus 24 kilometers farther down the valley.
Alpine glaciers are much shorter and thinner than ice sheets, they tend to move more slowly, and the age of
the ice flowing through these glaciers is more likely to be measured in hundreds of years
rather than hundreds of thousands.
Not all alpine glaciers are as impressive as the Aletsch glacier.
Some are much smaller like these cirque glaciers in the Chugach Mountains of Alaska.
Some glaciers expand as they exit a valley to form wider Piedmont Glaciers, like the Malaspina glacier in Alaska.
We had three learning objectives for today’s lesson,
how confident are you that you could successfully respond to each objective?