How do we know?
↻ One of the pages with a moving picture
Scientists photograph the same glacier from the same spot, years apart.
- Snow piles up high in the mountains, more than can melt each summer.
- Snow that lasts a summer packs into firn. Buried deeper, it squeezes into ice.
- The heavy ice slowly flows downhill under its own weight.
- It plows up rocks and leaves them behind when it melts back.
Photos of Grinnell Glacier from 1911 and 2016 show how much it shrank.
Glacier National Park in Montana had about 150 glaciers in 1850, and only 26 by 2015.
Glaciers can move fast. Greenland’s Jakobshavn Glacier flowed 9 miles a year by 2010, twice as fast as before.
Wow number
9 miles a year: the speed of Greenland’s Jakobshavn Glacier by 2010, twice as fast as before.
Myth or fact?
Icebergs are frozen seawater. Myth. They are fresh-water ice that broke off glaciers.
Most of Earth’s fresh water is frozen in ice. Fact. Over 68% is locked in ice and glaciers.
Think about it
Glaciers leave rocks behind. What other clues could show a glacier was once there?
Where in the world?
Glacier National Park, Montana, USA which had about 150 glaciers in 1850 and only 26 by 2015. Jakobshavn Glacier in Greenland has a long bay jammed with icebergs — 48.7596° N, 113.787° W.
Where these facts come from
Every number on page 28, and who measured it
This page tells us the park once had about 150 glaciers.
Geology of Glacier National Park — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/geology-and-ecology-of-national-parks/geology-glacier-national-park
We checked it on 31 October 2026
This page tells us 26 named glaciers were still big enough to count in 2015.
Time Series of Glacier Retreat — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/centers/norock/science/time-series-glacier-retreat
We checked it on 31 October 2026
This fact sheet tells us how many glaciers Glacier National Park had in 1850 and in 2015.
NPS History, an online library of National Park Service papers (this one is a U.S. Geological Survey fact sheet)
npshistory.com/publications/glac/glacier-retreat-2017.pdf
We checked it on 31 October 2026
This page explains how snow turns into a glacier.
The National Snow and Ice Data Center, scientists who study ice
nsidc.org/learn/parts-cryosphere/glaciers/science-glaciers
We checked it on 31 October 2026
This page tells us how fast a big glacier in Greenland moves.
The National Snow and Ice Data Center, scientists who study ice
nsidc.org/news-analyses/news-stories/tracking-runaway-glacier
We checked it on 31 October 2026
This page tells us how Glacier National Park's glaciers have shrunk.
Status of Glaciers in Glacier National Park — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/centers/norock/science/retreat-glaciers-glacier-national-park
We checked it on 31 October 2026
This page tells us icebergs are made of fresh water from glaciers.
JetStream Max: Icebergs — NOAA, the U.S. weather and ocean agency
noaa.gov/jetstream/icebergs-max
We checked it on 31 October 2026
This page tells us how much of Earth's fresh water is frozen.
Where is Earth's Water? — The U.S. Geological Survey, the government's Earth scientists
usgs.gov/special-topics/water-science-school/science/where-earths-water
We checked it on 31 October 2026
Has this changed?
Yes, this one changes. Glaciers are always moving, growing or shrinking. The 150 glaciers of 1850 and the 26 of 2015 show how many have been lost.
How glaciers form is not going to change.
Words from this page
- firn
- Snow that has lasted through a summer and packed down, on its way to becoming ice.
- glacier
- A huge, slow river of ice.
- iceberg
- A big chunk of fresh-water ice that has broken off a glacier.
Try it!
Make a slime glacier.
1. Prop one end of a tray on a book. 2. Put a big blob of slime at the top. 3. Set a few dry beans just below it. (Beans are not for mouths!) 4. Check every 5 minutes.
Watch: the slime creeps downhill and pushes the beans, like a glacier moving rocks.
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