Back when WC was a sullen teenager, he worked one summer on the University of Alaska Institute of Marine Science’s R/V Acona, a research ship. In the Acona’s skiff, we made a practice run northwards up Gastineau Channel to Auke Bay, measuring water depth and sampling bottom sediments. Today, some 54 years later, much of the way you’d have to get out of the skiff and walk, and you’d sample sediments with a shovel.
Gastineau Channel separates Douglas Island from mainland Alaska, the City of Juneau from the community of Douglas. When Juneau was founded, ocean-going vessels could circumnavigate Douglas Island via Gastneau Channel. Today, you can charitably call it a wetland, and in a few places only at high tides.
Partly the Channel is filling with sediment washing out via the Mendenhall River and Mendenhall Glacier. Partly it’s mass wasting, as glacial sediments slough down the steep, glacier-carved walls of the Channel. But mostly, it’s isostatic rebound. During the Wisconsin Glaciation, all of southeastern Alaska was covered with an immensely thick layer of glacial ice, as deep as 3,000 feet in places. That ice weighs a lot: 57-plus pounds per cubic foot. All that weight has the effect of depressing the continental crust under it, forcing it downwards into the semi-plastic mantle. Areas beyond the edge of the ice are forced up, although typically less than the displacement under the ice. The technical term is glacial isostatic adjustment (GIA).
The ice mostly melted of southern Alaska 8,000 years ago, although like most geology, it’s more complicated than that. The Little Ice Age – itself an exceptionally complex phenomenon, and not an ice age at all – from about 1300 AD to 1750 AD partially re-filled some area in southern Alaska with ice and modified the whole process. But, at the risk of oversimplifying, since about 6000 BC, and certainly since 1750 AD, glacial isostatic adjustment has been causing southern Alaska’s land masses to rise.
GIA was first discovered in southern Sweden in eighteenth century, where it was initially thought that sea levels were falling. On the initiative of Anders Celsius a number of marks were made in rock on different locations along the Swedish coast. By 1765 it was possible to conclude that it was not a lowering of sea levels but an uneven rise of land. In 1865 Thomas Jamieson came up with a theory that the rise of land was connected with the ice age that had been first discovered in 1837. The theory was accepted after investigations by Gerard De Geer of old shorelines in Scandinavia published in 1890. WC’s Swedish ancestors would be proud.
The amount and rate of isostatic rebound varies depending upon the depth of the overlying ice, the thickness of the underlying continental crust, applicable plate tectonics, when the ice actually melted, and probably other factors not yet understood.
But GIA is raising the northern half of southeast Alaska and the Alaska Gulf Coast at eye-popping rates. At 12 mm a year at Juneau – call it half an inch – over those 54 years since we took the Acona’s skiff the length of Gastineau Channel, that’s more than two feet of rebound. Geology in real time. At the north end of Lynn Canal ninety miles north of Juneau, scientists calculate there has been 5.7 meters of uplift since 1750, nearly nineteen feet, clocking along at 28 mm a year. And still rising.
Isostatic rebound, and especially glacial isostatic adjustment, is re-shaping the southern quarter of Alaska.
(WC thanks reader tomdurk for asking the question that led to this blogpost.)