Glacier Heaven - Southeast
In Southeast Alaska, maritime climate and coastal mountains work together to create favorable conditions for glaciation. The icefields straddle the Coast Mountain Range on the United States-Canadian border, directly in the path of the Pacific Ocean's prevailing winds. Moist air flows toward the mountains, rises, cools, and releases snow and rain. Annual snowfall on the Juneau Icefield exceeds 100 feet, and mild Southeast summers assure that winter snow accumulation exceeds summer snowmelt at higher elevations.
As the Earth Turns - Changes
Glaciers, perennial accumulations of ice, snow, sediment, rock and water, respond to changes in temperature, snowfall and geologic forces. Several components make up a glacial system: the ice and sediment contained in the glacier; the valleys, fiords and rock features it flows over, on, or around; and the deposits left by its retreat or advance.
New snow layers create pressure on existing layers of snow and ice. This process, "firnification", changes snow to firn, a dense granular snow (like corn snow). After the first season's melt, snow becomes firn. As it is compressed further, firn becomes ice.
As the snow collects over many years, an ice field forms. Ice flows down the valleys and slopes of the mountains to the lower elevations, and glaciers are born.
Anatomy of a Glacier (87 KB illustration)
Coerced by gravity, ice pursues the
path of least resistance. Ice depth and bedrock angle influence
the rate of glacial flow. Glaciers contain two zones of ice flow.
The zone of plastic flow, ice closest to the bedrock,
experiences extreme pressure from the weight of the ice above
and conforms to the anomalies in the bedrock. The zone of
brittle flow, the upper 150 feet of glacial ice, lacks this
pressure and reacts inelastically to the bedrock features, forming
elongated cracks called crevasses which fluctuate with
the glacier's flow. Tubular chutes or moulins drain surface
meltwater, and formidable spires of ice called seracs reach skyward.
Ice plummets over particularly steep terrain creating ice falls.
One theory suggests that differences in seasonal flow rates over
an icefall create the convex bands called ogives at the
base of the falls, which undulate down glacier. The erosive power
of glacial flow changes the landscape and scrapes much of the
soil and rock from the valley walls that channel its irrepressible
Glaciers - Master Carvers of Landscapes
The landscape around a glacier clearly illustrates the effects of Pleistocene and Holocene glaciation. Ice excavates the bedrock, forming bowl-shaped cirques, pyramidal horns, and a series of jagged spires called arête ridges that separate glacial valleys. As glaciers carve U-shaped valleys, rocks plucked from the bedrock and frozen in the ice etch grooves and striations in the bedrock. Rocks scoured from surrounding valley walls create dark debris lines called lateral or medial moraines along the edges and down the center of glaciers. Pulverized rock called rock flour, ground by the glacier to a fine powder, escapes with glacial meltwater producing the murky color of glacially fed rivers and lakes. Glacial recession unmasks trimlines, slightly sloping changes in vegetation or weathered bedrock on the valley walls that indicate a glacier's height at its glacial maximum. Meltwater transports glacially eroded material to the outwash plain, an alluvial plain at the edge of retreating glaciers. Icebergs break away or calve from the faces of glaciers ending in lakes or the ocean.
Cracked pieces of rock, plucked or torn from the bedrock, are carried with other debris in and on the glacier. This debris scrapes the valley walls and floors, leaving grooves and striations. Rock debris is crushed and ground into fine grains, called rock flour.
a tidewater glacier advances, it pushes a mound of debris called a moraine
shoal in front of its terminus, protecting it from deep tidal water.
If climate or glacial dynamics force the glacier's terminus to retreat
from its moraine shoal, the deeper water behind the shoal causes the
glacier to calve, rapidly producing many icebergs and triggering its
retreat. Once the glacier retreats to a stable position, calving slows,
and the glacier advances again, gradually rebuilding its moraine shoal.
Why the Pretty Colors?
Perhaps inter-glacial warming trends will prevail. The icefields may continue to melt as glacial meltwater trickles among the debris, and plant and animal communities ultimately reclaim the land. Maybe the next Ice Age waits just around the corner, and the icefields will again advance. Modulating climate and astronomical forces may trigger glaciation, and the ice would once more scour the bedrock, destroying all life within its reach and forcing animal communities to find new homes.
will happen in the centuries yet to come? The neo-glaciation that created
the coastal icefields started only 3,000 years ago, a mere blink in
geologic time. Also youthful by geologic standards, the Holocene's climatic
warming and glacial events began in Alaska just 10,000 to 15,000 years
ago, and the history of the Great Ice Age stretches back almost two
million years in time. Although clues from the past illuminate today's
observations, the future of glaciation provides a perplexing question
for scientific research. Regardless of advance or retreat, melt or accumulation,
one factor on the icefields will remain constant. Change will persevere.
The Big Shuffle
As the soil is replenished and the time since
the last glacial advance continues to pass, additional species repopulate
the land. Each episode of glacial advance and retreat renews the cyclic
tug-of-war between ice and vegetation.
Flightseeing, helicopter tours, charter boats and kayaking are among the many options available to the visitor wishing to see the icefields. Contact the ranger district nearest the glacier of your choice for more information and a list of available flightseeing and charter operators.
Keep plenty of distance between yourself and large wildlife - a close approach for a photograph may be interpreted by the animals as aggression. Bear bells and bear repellent use are recommended.
The icefields and glaciers of the Tongass are part of a spectacular,
un-spoiled wilderness which belongs to you. Help protect them.
Enjoy your visit but please take special care to preserve all
aspects of the environment so future visitors may enjoy it as
Updated July 10, 2002
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