Glaciers are very large accumulations of ice that slowly flow like rivers. In fact, the definition of a glacier is a river of ice.
What can make ice flow? Lots and lots of snow, combined with time. Glaciers form when more snow accumulates in winter than melts in summer. Over years of accumulating snow, the weight of all that snow presses deeper layers of fluffy snow into dense ice. It takes around 200 feet of built-up snow to begin to form a glacier.
Because the ice is so dense, most of the air bubbles you would find in regular white ice are squeezed out. That makes the ice look blue. As it becomes denser, the ice in a glacier changes character. Instead of being brittle, like ice cubes at home, it acts more like a fluid. Tugged by gravity, the whole glacier moves downhill.
As glaciers slowly flow, big chunks can break off when the edge of glacier reaches the sea. This is called calving. The pieces that fall off become icebergs and float out to sea until they melt.
Glaciers occur in two types: ice sheets which cover large areas of land, and alpine glaciers, which are smaller and occur in mountains. Between these two types, glaciers cover about 10% of all the land on Earth and almost 70% of the world’s fresh water is stored in glaciers.
Ice sheets: then and now
These days, vast ice sheets are limited mostly to Greenland and Antarctica, and some in the high mountain plateaus of the Himalaya Mountains.
But this hasn’t always been true. Earth has experienced five ice ages or glacial periods, when ice sheets were much more extensive. During the most recent ice age (about 20,000 years ago), ice reached its maximum extent, covering approximately one-third of Earth’s land surface. At that time, ice sheets a mile thick covered all of Canada and stretched as far south as New York City. Much of Northern Europe was under ice as well.
It took thousands of years for the ice to build up and cover all this land and then, as the Earth warmed again, to retreat. These big ice sheets were mostly gone by about 14,000 years ago. That may sound like a long time ago. But in some places, land is still slowly rebounding, or rising, from being squashed down by the ice.
We can also still see many landforms created by ice sheets thousands of years ago. Moraines are a good example. Moraines are piles of rock and sand bulldozed into place by moving ice sheets. Moraines form ridges and strips of land. For example, Long Island, New York, Cape Cod, Massachusetts, and many gentle ridges in midwestern states are moraines that were pushed into place by ice sheets as they moved south.
Ice sheets can be very, very old, and very thick. The biggest glacier on Earth, in Antarctica, is 3 miles (4.7 kilometers) thick. In Greenland, parts of the ice sheet may be as much as one million years old and as much as 14,000 feet (4300 meters), or over 2-1/2 miles thick.
Scientists have drilled deep cores into Greenland’s thick ice. The ice cores contain ancient layers of ice. Those layers are like chapters in a book showing events in Earth’s history. In them, scientists sometimes find ash from long-gone volcanoes and forest fires. They also find clues to Earth’s changing climate in tiny gas bubbles trapped in the ice hundreds of thousands of years ago.
Alpine Glaciers: Essential water
Alpine glaciers, also called mountain glaciers, fill high valleys in mountains where lots of snow falls in the winter. These beautiful and dramatic glaciers glitter on peaks like jewels. in cold places such as Alaska, alpine glaciers can reach all the way to the sea.
Even though there are tens of thousands of alpine glaciers in the world’s mountains, they may seem small and remote compared to the great ice sheets. And in fact, they contain much less of Earth’s ice. But alpine glaciers are important for mountain ecosystems. They also play an essential role for us humans. These high glaciers store great amounts of water as snow and ice. In summer, when people, farms, and forests need water the most, alpine glaciers provide it, slowly releasing water as hot weather causes seasonal melt.
In fact, some regions get nearly all their water from glaciers that are hundreds of miles away. As the glacier melts, the melting snow and ice feeds rivers. Those rivers carry this precious glacier water far downstream to farms, villages, and cities where it is badly needed.
Disappearing glaciers
Remember how glaciers grow when more snow accumulates than melts? Well, when the opposite happens, and over the years more snow melts than accumulates, glaciers shrink.
This is exactly what is happening all over the world. As temperatures warm due to climate change, ice sheets and alpine glaciers alike are shrinking. In Antarctica, for example, some of the larger shelves of ice sheets that extend into the sea are breaking off more frequently, and in bigger chunks, than before. The Antarctic Ice Shelf recently calved an iceberg bigger in area than the city of Los Angeles.
Meanwhile, in many mountain regions, precipitation is falling more as rain and less as snow. That means there’s less of the water being stored for summer as snow and ice in the high alpine areas where alpine glaciers form.
Worse, many of these glaciers are shriveling up, or even disappearing altogether, like the Chacaltaya glacier in Bolivia. Some alpine glaciers in the Himalayan mountains may be gone by 2035. When they melt, an important water supply for that region will disappear too. Glacier National Park in Montana has only a quarter of its original ice and the rest of its glaciers are expected to be gone by 2030.
What will happen if glaciers keep melting so fast? All the animals that depend on glaciers would suffer. Humans would suffer too. Besides changing water resources, the melting ice would make sea level rise. For example, 125,000 years ago, when Earth was in a very warm spell, sea level was 18 feet higher than now. If that happened again, many coastal cities would be flooded. And if all the glaciers on Earth were to melt, scientists estimate that sea level would rise 230 feet! But if we all pitch in to slow climate change, we can help save Earth’s glaciers and keep that from happening.
Written by Laura McCamy
Edited by Beth Geiger, MS Geology
Illustrated by Renee Barthelemy