The Rise and Rise of Southeast Alaska
Sailing up Alaska’s Icy Strait in 1794, British Captain George Vancouver passed the tongue of a glacier that rose 4,000 feet from the sea. Today, after retreating 65 miles, that glacier is gone. In its place are the scraped-rock cliffs and deepwater fjords of Glacier Bay, the beautiful evidence of the fastest glacial retreat ever recorded. The loss of ice in Glacier Bay and other nearby areas may be why northern Southeast Alaska is one of the fastest-rising regions on Earth.
When the land bounces back after centuries under the oppressive weight of a glacier, scientists call it “post-glacial rebound.” The land around Lynn Canal and Glacier Bay in Southeast Alaska is rising as fast as human fingernails grow (about an inch each year). That’s one of the fastest rates in the world, according to Chris Larsen of the Geophysical Institute.
Larsen, who works on the project with Juneau-based researcher Roman Motyka, is a graduate student who spends his summers boating and hiking the wilderness shorelines of the northern panhandle of Southeast Alaska. Larsen, Motyka and others have installed 70 global positioning system (GPS) sites in a shotgun pattern around the area. The GPS allows them to track how fast the land is rising and also shifts to the north, south, east or west. For a historical perspective not available with the GPS, they also use tide records available since 1925 and the changes in shoreline recorded in tree rings.
When tons and tons of glacier melt, Earth’s crust bounces back like a mattress, first getting a bit thicker and then beginning a gradual rise. The rebound, which scientists are also studying in Hudson Bay in Canada, takes thousands of years. In Hudson Bay, a glacier many times the size of Alaska’s Glacier Bay melted at the end of the last ice age, about 10,000 years ago. Since then, the land has risen about 375 feet in some places. Larsen said Southeast Alaska probably won’t rise as high as Hudson Bay, but areas of Southeast are gaining elevation at an impressive rate. Sullivan Island, between Juneau and Haines, has risen more than 18 feet in the last 250 years.
Scientists have puzzled over the cause of Southeast Alaska’s rapid rise for decades because glacial rebound isn’t the only force at work in the area. The nearby Fairweather Fault is where two of Earth’s vast plates—the Pacific and North American—are grinding past one another. Movement of the plates near the fault could be compressing the crust and forcing it upward in places. The difference between uplift caused by plate movement and glacial rebound is almost impossible to quantify, but the latter idea is supported by the observations of people like Vancouver and naturalist John Muir. They both helped document the birth of Glacier Bay after the disappearance of billions of tons of ice.
One of the best remaining clues to the mystery of Southeast’s rising lies in the dense forest above the water’s edge in bays and inlets surrounding Glacier Bay. There, Larsen and Motyka found the weathered evidence of an old shoreline that may tell them how far the land was pressed down by the weight of the ice. By counting the rings of trees that colonized the beaches after they rose from the sea, Larsen and Motyka determined that the land began a rapid uplift around 1850.
The timing of the trees’ invasion of newly available shoreline matches the rapid retreat of Glacier Bay ice, which seems to support the notion that Southeast is rising because of glacial rebound but doesn’t rule out the movement of Earth’s plates. With GPS providing accurate readings of ground lift, researchers within the next few years hope to understand the full story of Southeast’s sprint to the sky.
