For much of the year, glaciers are a wonderland of white. Their blanket of snow reflects every color of the visible spectrum, but nothing penetrates that surface. Then the summer thaw comes, and all that surface snow is ripped open by streams eroding into older, denser ice beneath. When that happens, glaciers bleed torrents of raging meltwater and reveal their bluish guts.
These glacier-powered erosion processes are a source of life-sustaining mineral and nutrient elements that find their way into ecosystems around the globe. “Glaciers are a big driver of biogeochemical cycles, both on a local scale in Patagonian fjords and across the planet’s polar ice sheets,” says Ohio State University geochemist David Hawkings. In a 2021 paper in the journal Global Biogeochemical Cycles, his team demonstrated how glaciers release silicon and iron from bedrock to water flowing down their sides, then disperse that material throughout an entire basin or fjord system. And in a study published last year in Nature Geosciences, Hawkings and colleagues found that glaciers can also spread mercury into aquatic ecosystems, in part by transporting it through surface erosion and then depositing it on the ice sheet floor.
But as rising temperatures continue to shave the ice caps from high mountains worldwide, those precious glacial resources are becoming less and less available. And it’s not just an environmental problem: Melting glaciers will contribute about a foot to global sea level rise by the end of this century.
The ice that caps mountain ranges like Mount Kilimanjaro and Antarctica is undergoing dramatic changes. Over the past 50 years, many of these glaciers have retracted at a rate eight times faster than they did in the 1970s.
Thompson studies glaciers by collecting samples from them and analyzing their components, including air bubbles trapped in the ice. These reveal bits of the atmosphere of thousands of years ago, which lets scientists reconstruct past climates. But even the smallest glaciers are constantly changing, and it’s hard to know how much of what they contain is natural versus manmade.
As a glacier forms, air bubbles are trapped in the layers of crystals that form it. Over time, these bubbles compress against the crystals to compact them. This compaction is a source of the glacier’s flow, known as its “pressure gradient.” A high pressure gradient means that melting glacial water (which is less dense than the ice) tends to flow downwards and outwards, in valley glaciers and continental ice sheets.
The only thing that keeps a real glacier from being like a pan of semi-connected ice cubes floating in water is that the balance between production and drainage (recall that liquid is more dense than ice) usually favors drainage over production. The exception is when the ice is under great stress, as in a valley glacier under a large glacier tongue. Then the ice may break up and flow in several directions, or calve off into individual bergs. It’s important to understand these flows because they tell us about the glacier’s history, its dynamics and its response to changes in its environment.
