Sea Ice: Formation & Types

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll be learning about the different types of sea ice that cover the North and South poles. By the end of the lesson, you'll be able to explain what polar ice caps, icebergs, and glaciers are and how they form.

The Frozen Ocean

Picture the poles of the Earth. Covered in white snow and thick sheets of ice, penguins and polar bears roam the surface. Skirting around the patches of ice, seals zoom through the icy water. These patches of frozen ocean are called sea ice. Although you might think of slick, thick sheets of sea ice, there are actually many different types depending on the conditions of the ocean and climate. Sea ice is only present in cold, polar environments where temperatures are low enough to freeze the salty ocean water. Today, we'll look at how sea ice forms and the different types of sea ice.

Arctic sea ice
sea ice


Glaciers are large sheets of compressed snow that, over years, have frozen into ice. Covering about 10% of the globe, glaciers exist in the most Northern and Southern poles.

Glaciers start forming when snow falls and does not melt in the warmer months. Each year, more and more snow accumulates. Over time, the compressed snow starts to form ice crystals. The ice crystals grow as the snow compacts and air pockets are forced out, making the ice more dense. After two years of this process, the snow and ice become firn, an intermediate between snow and true glacier ice. The firn keeps compressing, forming larger and larger crystals. Over hundreds of years, glaciers form.

Glaciers breaking in Alaksa

Despite the dense, frozen nature of glaciers, they actually move, sliding across the Earth like a frozen river. Since glaciers are so dense and heavy, they deform under their own weight, sliding across the rock that is much more sturdy. As they move, glaciers may also grow larger, or advance, or shrink, called retreating. If snow accumulation is great, the tip of the glacier will advance. If there is a lot of evaporation, or high temperatures, the tip of the glacier may retreat. The glacier still can move forward, even if it is retreating. It would be moving, but just getting smaller.

There are different types of glaciers depending on their location and features. For example, mountain glaciers form at high altitudes, like glaciers in the Himalayas and the Andes in South America. In this lesson, we'll be looking at one type of glacier in particular, polar ice caps.

Aerial view of a mountain glacier
mountain glacier

Polar Ice Caps

Ice caps are a type of glacier that cover less than 50,000 square kilometers, which is over 19,000 miles across. Polar ice caps exist in both the North and South pole, hence the name 'polar'. These large sheets of ice are found in flat regions of high elevation. Ice caps usually have a slightly raised center, with ice sloping downwards into a flat plane.

Satellite photo of ice caps in Iceland
ice caps

Ice caps form like glaciers. Snow accumulates year after year, eventually compressing into firn. The firn accumulates more snow and larger crystals grow inside the ice cap. Over many years, the snow is eventually compressed entirely into glacier ice. The glacier ice flows over everything around it, covering mountains, planes and other geological features. Although the exterior of the polar ice caps are harsh environments, the seas below are rich in aquatic organisms.

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