Glaciers are huge masses of ice that move and flow over land. They form over many years in places where snow builds up faster than it melts. As snowflakes accumulate and are packed together tightly, the glacier grows.

As a glacier grows vertically, its increasing weight makes it deform and flow outward along its edges – or advance – due to gravity. As it spreads outward, it covers more of the landscape. Though they are always changing, glaciers stick around. Once formed, a glacier remains in an area for a very long time, and it takes decades, or even centuries, to completely melt away. When a glacier melts faster than it accumulates snow, it retreats.

What sank the Titanic?

An iceberg! When glaciers reach the edge of land and extend onto the ocean, pieces sometimes break off and fall into the sea. This process is called calving. These chunks of ice can be big or small, and often drift out into the wide ocean. The Titanic collided with one of these large pieces, called an iceberg, that is thought to have broken off of Sermeq Kujalleq (Jakobshavn Glacier), a fast-moving glacier on Greenland’s west coast.

Glaciers vary in size. How small can a mass of ice be and still be called a glacier? It has to be large enough to move under gravity. And that turns out to be about 150 feet thick and 25 acres in area – that’s 22 times the size of the Museum of the Earth!

The largest glacier today is much bigger: covering most of Antarctica and 13,000 feet thick in places – that’s 2.5 miles tall. This massive ice sheet holds about 60% of Earth’s fresh water.

Small flakes, big changes

Glaciers are made up of countless snowflakes, packed together over hundreds and thousands of years. The immense pressure of being buried in layers of snow compresses the flakes into ice crystals, trapping air bubbles between the crystals.

Preserved inside these bubbles are clues to Earth’s history, such as the composition of the atmosphere in the past. This includes past levels of greenhouse gases – like carbon dioxide and methane. Not just air, but dust and volcanic ash are frozen within the ice. Scientists decode these clues to see how Earth and its climate have changed.

What color is a glacier?

Underneath a glacier’s surface, or where the surface has recently broken, the ice is sometimes a brilliant blue. Why? Glacial ice becomes tightly compacted over time, squeezing air bubbles and increasing ice crystal size. As a result, the ice absorbs all colors of light but blue, which is reflected back to our eyes.

Did you know that a glacial ice sheet, called the Laurentide Ice Sheet, covered Ithaca until about 18,000 years ago? It may have been as much as one mile thick, from top to bottom.

The image to the right is part of an ice core from the Greenland ice sheet. What can it tell us?

Annual layers: See the light lines and the dark lines? These tell us about the seasons. Summer and winter layers differ in Greenland because in the summer, the surface warms, giving the snow a different texture than that which accumulates during the cold winter. Chemical analysis of the ice itself can tell us about past temperatures.

Dust: Dust particles that settle on the ice sheet get trapped in the glacial ice. Scientists study these particles to learn about the climate and environmental conditions from the time when the dust fell. For example, high dust concentrations can indicate high volcanic activity.

Air bubbles: Within the glacial ice are trapped air bubbles. These contain gases such as carbon dioxide and methane. Scientists analyze these gases to determine the chemical composition of Earth’s atmosphere at the time the bubbles formed.