Meltwater can freeze
on its way down.
Meltwater travels down
cracks in the ice.
Sediment or rock
Sectional vie w
sciencemag.org SCIENCE 1122 1 DECEMBER 2017 • VOL 358 ISSUE 6367
NEWS | FEATURES
A glacier unleashed
Glaciers gain mass in their upper reaches, where snowfall is heavier,
and lose it at their snouts, where the ice breaks up and melts (right).
Most glaciers flow steadily, but some get stuck and accumulate
mass (center), then release it in a surge. A surging glacier
can race down a valley or mountain, growing thinner
and longer (left). Then, anywhere from days to
years later, the glacier’s speed ebbs and
it begins thickening again.
Surging glaciers are riddled with crevasses, especially in their lower reaches.
When the surge ends, meltwater that built up under the glacier before the
surge may sweep mud and debris from its snout.
In a “normal” glacier,
meltwater drains efficiently
from its base, carrying away
heat and leaving the ice
anchored to its bed.
Buildup to a surge
If drainage is poor or
meltwater can accumulate
under a glacier, warming
the ice and lifting it off
Once the surge releases
the meltwater, the glacier
subsides onto its bed, and
the cycle begins again.
Meltwater plays a key role in triggering surges. Pooling on the glacier’s surface,
it can seep down into crevasses. There it can refreeze, releasing heat that softens
the ice; it can also pool at the base of the ice.