Courtesy of the National Park Service Glacier Bay Web Site
Enter Glacier Bay, and you cruise along shorelines that were completely covered by ice just 200 years ago. Explorer Captain George Vancouver found Icy Strait choked with ice in 1794, and Glacier Bay was a barely indented glacier. That glacier was more than 4,000 feet thick, up to 20 miles or more wide, and extended more than 100 miles to the St. Elias Mountain Range. But by 1879, naturalist John Muir found that the glacier had retreated 48 miles. By 1916, the Grand Pacific Glacier headed Tarr Inlet 65 miles from Glacier Bay's mouth. Such rapid retreat is known nowhere else. Scientists have documented it, hoping to learn how glacial activity relates to climate change.
Worldwide, the glacial facts are staggering. Glaciers and polar ice store more water than lakes and rivers, groundwater, and the atmosphere combined. Ten percent of our world is under ice today, equaling the percent being farmed. If the world's ice caps thawed completely, sea level would rise enough to inundate half of the world's cities. The Greenland and Antarctic ice caps are two miles thick. Alaska is four percent ice.
Glaciers form because snowfall in the high mountains exceeds snowmelt. The snowflakes first change to granular snow -- round ice grains -- but the accumulating weight soon presses it into solid ice. Eventually, gravity sets the ice mass flowing downslope at up to seven feet per day. The park includes some 12 tidewater glaciers that calve into the bay. The show can be spectacular. As water undermines some ice fronts, great blocks of ice up to 200 feet high break loose and crash into the water. John Hopkins Glacier calves such volumes of ice that it is seldom possible to approach its ice cliffs closer than about two miles. The glaciers seen here today are remnants of a general ice advance -- the Little Ice Age -- that began about 4,000 years ago. This advance in no way approached the extent of continental glaciation during Pleistocene time. The Little Ice age reached its maximum extent here about 1750, when general melting began. Today's advance or retreat of a glacier snout reflects many factors: snowfall rate, topography, and climate trends. Glacial retreat continues today on the bay's east and southwest sides, but on its west side, several glaciers are advancing.
The snowcapped Fairweather Range supplies ice to all glaciers on the peninsula separating Glacier Bay from the Gulf of Alaska. Mount Fairweather, the range's highest peak, stands at 15,320 feet. Near Johns Hopkins Inlet, several peaks rise from sea level to 6,520 feet within just four miles of shore. The great glaciers of the past carved these fjords, or drowned valleys, out of the mountains like great troughs. Landslides help widen the troughs as the glaciers remove the bedrock support on upper slopes.
Huge icebergs may last a week or more, and they provide perches for bald eagles, cormorants, and gulls. Close by, kayakers have heard the stress and strain of melting: water drips, air bubbles pop, and cracks develop. Colors betray a berg's nature or origin. White bergs hold many trapped air bubbles. Blue bergs are dense. Greenish-blackish bergs may have calved off glacier bottoms. Dark-striped brown bergs carry morainal rubble from the joining of tributary glaciers or other sources. How high a berg floats depends upon its size, the ice's density, and the water's density. Bergs may be weighed down, submerged even, by rock and rubble. A modest-looking berg may suddenly loom enormous -- and endanger small craft -- when it rolls over. Keep in mind that what you see is "just the tip of the iceberg."
Glaciers grew in response to climatic severity. When glaciers dominated the land, they tended to worsen harsh climatic conditions by reflecting heat rather than absorbing it. This generated cold high-pressure cells in the atmosphere that held the warm oceanic air at bay. By contrast, today with glacial ice at a relative minimum, we live in a period of greater than average mildness. The large, low pressure systems sweeping off the Gulf of Alaska are now the dominant force in the climate, bringing with them the abundant moisture for which southeast Alaska is infamous. Since oceanic currents in the Gulf of Alaska are mostly from the south, southeast Alaska is bathed by warmer waters than are customary for its latitude, so temperatures are particularly mild on average.
The sea's moderating and humidifying influence pervades the region as a result of the penetration of marine channels and bays, with the greatest influence being along the outer coast. Closer to the continental interior, especially near major passes, periodic influences of continental air reduce the average rainfall and bring more extreme seasonal temperature variations.
The Glacier Bay region's extreme topography indicates a landscape driven by immense energies, which derive from the area's position astride the active suture between the North American and Pacific plates. For over 100 million years, North America has been plowing obliquely into the Pacific plate, presently at a rate of several centimeters per year. Generally, the Pacific plate has been forced under North America, but occasional bits -- island arcs, pieces of sea floor, fragments of continental margin -- have been scraped off one plate or the other, shattered, and smeared along the leading edge of North American plate. Four such terranes have accumulated in a largely northwest-southeast pattern to form the Glacier Bay region.
The seam between the outboard-most terrane and the present continental margin remains active. Frequent earthquakes dramatically illustrate that plate motions continue to this day. The resulting compressional motion has forced some rocks upward to form mountain chains. Others are forced downward and melted in the process. Molten rock then moves volcanically through the shatter zone, where it cools and welds together one of the world's most complex geological jigsaw puzzles.
Highlands forming by this process intercept the predominantly onshore flow of Pacific air, wringing out its abundant moisture in the form of rain and snow. For at least seven million years, snows have accumulated in the uplands to form glacial ice, which has invaded the lowlands many times as the climate has periodically cooled. During the height of the most recent of these Great Ice Ages about 20,000 years ago, an ice sheet covered all of the Glacier Bay region except the highest peaks and certain headlands. Then it would have been possible to walk from Glacier Bay to Cape Cod without ever getting off the ice.