Mendenhall Glacier Visitor Center
Frequently Asked Questions
Mendenhall Glacier Moves and Shapes the Mendenhall
Valley
Why do glaciers form?
Is the glacier retreating?
What happens after the glacier retreats?
What evidence do glaciers leave behind?
What wildlife lives near the glacier?
Why is the ice blue?
Who was Mendenhall?
Why do glaciers form?
In Southeast Alaska, maritime climate and coastal mountains create favorable
conditions for glaciation. Moist air flows toward the mountains, rises
and releases snow and rain. Average annual snowfall on the Juneau Icefield
exceeds 100 feet. Mild Southeast Alaskan summers cause winter snow accumulation
to exceed summer snowmelt at higher elevations. Year after year, snow
accumulates, compacting underlying snow layers from previous years into
solid ice. Mendenhall Glacier is one of the 38 large glaciers that flow
from the 1500 square mile expanse of rock, snow and ice known as the
Juneau Icefield. As glacial ice continues to build, gravity pulls the
ice down slope. The glacier slowly scours the bedrock and grinds down
its 13-mile journey to Mendenhall Lake.
Is the glacier retreating?
A neo-glaciation period began 3,000 years ago and ended in the mid-1700s.
At this time, Mendenhall Glacier reached its point on maximum advance,
and its terminus rested almost 2.5 miles down valley from its present
position. Mendenhall Glacier started retreating in the mid-1700s because
its annual rate of melt began to exceed its annual total accumulation.
The icefield's snowfall perpetually creates new glacial ice for Mendenhall
Glacier and this ice takes 200-250 years to travel from the Juneau Icefield
to Mendenhall Lake. Water depth at the glacier's terminus is 220 feet.
At this rate, the glacier would take several centuries to completely
disappear. For Mendenhall Glacier to advance, the icefield's snowfall
needs to increase, the glacier's rate of melt needs to decrease, or
both. Glacier advance would require a reversal of the current warming
trends.
What happens after the glacier retreats?
As Mendenhall Glacier retreats and uncovers bare rock, the wind carries
seeds and spores of moss and lichen onto barren land. Alder, willow
and cottonwood systematically grow in degalciated landscapes. Glacier
debris, poor in nutrients, depends on flowering lupine and alder to
fix nitrogen in the soil, and all species add organic matter to the
soil as they are overtopped and shaded out by other species. Spruce
and hemlock ultimately rise to close the forest canopy, eventually creating
an old growth forest. Encompassing almost 350 years, this sequence of
plant succession provides habitat for an increasing number of plants
and animal species.
What evidence do glaciers leave behind?
The base of Mendenhall Glacier works like a giant piece of sandpaper.
As the ice flows towards Mendenhall Lake, the glacier plucks rocks that
become imbedded in the ice from the valley floor. The glacier scrapes
these rocks across the bedrock creating grooves and striations. The
glacier's erosive power changes the landscape and scrapes much of the
soil and rock from the valley walls. Rocks scoured from the surrounding
valley walls create dark debris lines called moraines on the edges and
down the center of the glacier. As the glacier continues its path towards
Mendenhall Lake, it grinds rock to a fine powder call rock flour that
escapes with glacial melt water and creates the lake's murky color.
Mendenhall Glacier's retreat exposes its trimline, slightly sloping
changes in vegetation on the valley walls that indicate the glacier's
height at its point of maximum advance. As the glacial ice melts or
calves icebergs, the glacier drops geologically misfit rocks called
erratics that its ice either quarried further up the valley or that
fell onto the ice from rock walls above the glacier. These granite boulders
can be seen lying on the metamorphic rock around the visitor center.
What wildlife lives near the glacier?
Coyote, porcupine, squirrel, snowshoe hare, and short-tailed weasel
build homes on the valley floor, and migrating songbirds build nests
in the deciduous shrubs in the young forest. In Steep Creek, beavers
work to create ponds while spawning sockeye and coho salmom provide
a food source for black bears and eagles. Loons, gull and Arctic terns
nest around Mendenhall Lake, and mountain goats favor the rocky terrain
and alpine meadows on the surrounding peaks.
Why is the ice blue?
Glacial ice appears blue because it absorbs all colors of the visible
light spectrum except blue, which it transmits. The transmission of
this blue wavelength gives glacial ice its blue appearance. Glacier
ice may also appear white because some ice is highly fractured with
air pockets an indiscriminately scatters the visible light spectrum.
Who was Mendenhall?
Appointed by President Harrison, Thomas Corwin Mendenhall (1841-1924)
served as Superintendent of the U.S. Coast and Geodetic Survey from
1889 to 1894. A noted scientist, Mendenhall also served on the Alaska
Boundary Commission that was responsible for surveying the international
boundary between Canada and Alaska. In 1892, this glacier was renamed
to honor Mendenhall. Naturalist John Muir first named the glacier Auke
Glacier in 1879 after the Aak'w Kwaan of the Tlingít Indians.
![horizontal line](https://webarchive.library.unt.edu/eot2008/20090114185539im_/http://www.fs.fed.us/r10/tongass/images/hruler04.gif)
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