Curious by Nature: What is glacial flour?

Empire reader Mackey Migel wrote a few weeks ago with a simple question: What is glacial flour?

The short answer is that it’s ground up rock. And no, you can’t cook with it (it’s ground up rock).

The long answer has to do with the mechanical action of glaciers and the strength of ice.

University of Alaska Southeast professor Eran Hood studies watershed-scale biogeochemistry, snow and alpine hydrology — to put it simply, Hood knows a lot about water in all its states. Glaciers are frozen rivers and they grind down the rock underneath them, Hood explained. The massive weight of the glacier creates tiny bits of rock called glacial flour, also known as rock flour or glacial silt.

“It’s basically just bedrock that has been mechanically ground up to a fine silt-size particles,” Hood said.

Its small size sets it apart from other small rocks, sand and pebbles.

“What distinguishes it in terms of looking at a glacial river is the silt is fine enough material that it can stay in suspension in the water column. … It has that grey color if you look at Mendenhall River or any of the other glacier rivers,” Hood said.

The Mendenhall Glacier produces a lot of glacial flour every year. Hood and his colleagues measure the amount of suspended sediment in the Mendenhall River with scientific equipment on Back Loop Road. That allows them to estimate how much glacial flour the Mendenhall Glacier produces.

On average, Hood calculates, the Mendenhall River exports about 114 million pounds of glacial flour to the ocean every year. That’s about 1.3 million cubic feet of material, or about 14.8 olympic swimming pools full of glacial flour.

“There’s deposits of it everywhere,” Hood said. If you stand on the edge of Mendenhall Lake, or near the water at Eagle Beach State Recreation Area, you’re likely standing on glacial flour.

And the Mendenhall Glacier isn’t even the biggest producer of glacial flour in the Juneau area. Much of the glacial flour the Mendenhall Glacier produces settles out in Mendenhall Lake. Glaciers that don’t feed into lakes, like Herbert Glacier, north of Juneau on the road system, send far more glacier flour into the ocean.

“The sediment that is washed out from under Mendenhall gets trapped in the lake so glaciers that do not have lakes in front of them (like Herbert Glacier) can export far more glacier flour out to the ocean,” Hood wrote in an email to the Empire.

What effect does this have on the environment? About half of the water in the Gulf of Alaska comes from glacial melt, current estimates hold. In Southeast, about 30 percent of all the water flowing from land to sea is glacier melt water.

It’s not yet known what animals prefer glacial melt water and which don’t. An ongoing study which the Empire wrote about last fall could add to that understanding.

Hood said that one thing is known: glacial flour adds more iron to the environment. Chemical reactions with water slowly dissolve the iron in rock, he explained. With larger rocks, that happens very slowly. That’s because there’s not much surface area in a large boulder compared the weight of rock.

But the small size of glacial flour increases this weight to surface area ratio, speeding up the rate at which iron dissolves into water. It’s the same principle at work with ice. Small cubes of ice dissolve quicker in water compared to large cubes.


• Contact reporter Kevin Gullufsen at kevin.gullufsen@juneauempire.com, 523-2228 or on Twitter @KevinGullufsen.


More in Neighbors

Tari Stage-Harvey is the pastor of Shepherd of the Valley Lutheran Church. (Photo courtesy of Tari Stage-Harvey)
Living and Growing: Keeping a faith that makes our hearts bigger

Art, ideas or politics aren’t Christian because they quote the Bible or use Christian symbols.

David Corenswet is Superman in "Superman." (Promotional image courtesy DC Studios)
On the Screen: ‘Superman’ a bold vision of hope, kindness

The film dares to say that kindness is “punk rock.”

Photo by Charles Whittlesey
Tatiana McWethy demonstrates her restoration technique on the icon of St. Nicholas.
Living and Growing: Historic Russian-American icons receive 1st restoration in 130 years

When that rare Juneau sunlight hit the canvas, St. Nicholas came to life, and swam in it

Photos by Bill Glude
About 50 people attended a rally promoting Canada-United States friendship on the international border near Skagway on July 5.
About 50 people from the Yukon and Skagway attend border rally

Aim of rally in the White Pass was the promotion of Canada-United States friendship and good relations

The old Forest Service administration bunkhouse building pictured here was built during the boom of the logging industry and has sat empty for decades. Now, it may be repurposed as a cultural healing center in Kake, Alaska. Photo by Tyler Bell.
Salmon State: A healing center for Kake

The center will be open to attendees from all Southeast Alaska communities, especially rural ones.

Toasted nuts top this shortbread cookie bar.
Cooking for Pleasure: Three layers of sweetness

These bar cookies combine layers or shortbread, chocolate and toasted nuts

Jensen-Olson Arboretum is seen in this undated photo. (Michael S. Lockett / Juneau Empire File)
Free art lessons for children to be hosted at arboretum July 26

Registration opens July 16 at noon and is limited to 20 students.

Photo courtesy Scott Burton
The first Resilience Circles cohort gathers in Juneau for a cohort retreat.
Woven Peoples and Place: Bridging knowledge systems

Across Southeast Alaska, partners are advancing the co-production of knowledge

Dave Ringle, special projects coordinator at St. Vincent de Paul, is photographed Thursday, June 19, 2025. (Natalie Buttner / Juneau Empire)
Living and Growing: Speaking our values with action

Service changes when there is a relationship involved

Most Read