Among the many adjectives for glaciers—spectacular, majestic, awe-inspiring—speed is not usually high on the list. In fact, if one’s pace is described as “glacial,” it’s only slightly faster than “geologic.” However, glaciers are not always as slow as they look. Sometimes they can move quite quickly! Here are a few of those types:

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Jökulhlaups

Pronounced “YER-kel-OIPS,” these are also known as glacial outburst floods, and the word is borrowed from Icelandic, where these can be fairly common. Jökulhlaups occur when water that has been built up behind glaciers or ice caps is suddenly released, either due to the glacier fracturing or a volcanic eruption. In Iceland, the most feared jökulhlaups come from the volcano Katla (KAT-la), which lies beneath the Mýrdalsjökull (MEER-dals-YER-kutt) ice cap. When Katla erupts, the resulting jökulhlaups can reach a flowrate of more than 200,000 cubic meters per second of water. For comparison, the average flowrate of the Amazon river, the largest in the world, is 170,000 cubic meters per second.

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Surge Glaciers

Surge-type glaciers are those that, for reasons not completely understood scientifically, suddenly move forward, advancing several hundred meters or even several kilometers in a few months.  Eyjabakkajökull, a surge-type glacier in Iceland, surged 2 km in 1972/1973, a change that was captured on the first Landsat images acquired of a surging glacier.

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Ice Streams

An ice stream is a fast-flowing part of an ice shelf. Or, in Antarctica, it would be part of the ice sheet itself. But there’s probably a sub-glacial valley, and there’s an acceleration of the ice that’s flowing in that valley compared to the adjacent ice. You can actually see the streams, that is, the flow-lines that help to discriminate these ice streams. Some of them are absolutely enormous in size, and they flow at their own speed, which is usually much faster than the ice that’s adjacent on either side.

So, although they’re not the fastest formations on Earth, glaciers can move with surprising speed. However, running champion Usain Bolt doesn’t need to worry about his Olympic records being broken by a glacier anytime soon.