Ice Age Glaciers May Be Long Gone, But They’re Still Affecting the Earth (& Will Be for Millennia to Come)
Like a massive hangover after the party of the millennium, Lake Manitoba, Lake Winnipeg and the rest of the province of Manitoba are all still feeling the effects of the last ice age from over 10,000 years ago.
“People somehow assume that these lakes have been the same for thousands of years and will be the same for thousands of years, but they’re not,” said Dr. L. Gordon Goldsborough, an associate professor in the Department of Biological Sciences at the University of Manitoba and the former manager of the university’s now-defunct Delta Marsh Field Station.
In fact, the lakes are currently tilting.
The university biologist offered a nutshell explanation for the phenomenon of isostatic rebound that is occurring to Manitoba and the rest of Canada at this very second.
Around 10,000–20,000 years ago, during the last ice age, Canada was underneath massive sheets of ice anywhere from three to five kilometres thick, Goldsborough said.
These massive sheets of ice pressed down on the land with great force, and once they receded, leaving an enormous lake — that has since been named glacial Lake Agassiz — in their wake, they let the downward force off the land, causing it to rebound. When Lake Agassiz completely drained, it left behind Lakes Winnipeg and Manitoba.
Using his hands to illustrate, Goldsborough noted the north end of Manitoba is rebounding quicker than the south end, causing Lakes Winnipeg and Manitoba to tilt southward.
The tilt, however, is only occurring at a rate of about one centimetre per year, so it’s imperceptible to humans.
Dr. Harvey Thorleifson, director of the Minnesota Geological Survey with the University of Minnesota’s Department of Geology and Geophysics, illustrated isostatic rebound using a sports equipment analogy.
Hudson Bay was the centre point for the glacial ice sheets of the last ice age and “placing the ice sheet on Hudson Bay was like placing a bowling ball on a trampoline,” Thorleifson explained.
Those ice sheets thousands of years ago would have resembled the ice sheets that currently sit on Greenland and Antarctica, the geologist added. He switched to a food analogy to describe the ice sheets as being similar to pancake batter being poured into a pan and merging to form one massive wall of ice that pushed all the water out of Hudson Bay and scraped across the land on their way south to the northern end of the modern-day continental United States.
“Just as the trampoline sinks under the bowling ball, Hudson Bay sank about a kilometre,” Thorleifson said.
Because the subsurface of the earth is fluid, consisting of nearly molten rock at a depth of several tens to hundreds of kilometres, it made the earth’s surface pliable under these massive sheets of ice, causing it to sink as the fluid under the surface was pushed away.
Of course, when one removes that hypothetical bowling ball from the trampoline, the trampoline springs back, which is exactly what is happening to the land as the fluid under the earth’s surface flows back to where it had been displaced by the giant ice sheets. This fluid flowing back to where it had been prior to being squeezed away is what causes the isostatic rebound, albeit at a slightly slower pace than a rebounding trampoline.
“Hudson Bay, after 10,000 years, is only halfway through its spring back,” he revealed.
All the land from Hudson Bay down to North Dakota in the United States is undergoing this isostatic rebound, with the land closest to Hudson Bay rebounding the quickest, as that’s where the heaviest part of the ice sheets were.
“The most rapid land uplift is at Hudson Bay, at about one metre per century,” the geologist said.
Thorleifson said the north end of Lake Winnipeg is rising at a rate of about 60 centimetres per century, while it’s rising about 40 centimetres per century around Gimli, Manitoba and 20 centimetres per century at the south end of the lake.
The vertical shift equals about half a metre of erosion of the shoreline per year at Gimli, he added.
To the west, on Lake Manitoba, Thorleifson noted, that body of water is tilting at an approximate rate of 45 centimetres per century at Fairford on the north end and about 35 centimetres per century at Delta Marsh on the southernmost tip of the lake.
This tilting, along with changes to the climate over the past thousands of years, and expansion due to erosion, has already changed the layout of the lake.
Thorleifson cited studies done around 1980 by Drs. James Teller and William Last, who are also both with the University of Manitoba, that proved the southern basin of Lake Manitoba was once dry land that was overtaken by the lake. The isostatic rebound, which sees the north end rising quicker than the south end, was mostly responsible for the lake’s shift to the south.
And the layout will change further over the next thousand years.
“In a thousand years or so, Lake Manitoba will start to drain to the Assiniboine [River],” Thorleifson said. “At that time, the outlet at Fairford will be abandoned and the lake as a whole will then start to gradually shrink while bearing in mind that the rate of uplift will continue to diminish.”
As the long, slow motion tilt continues, the lake, and Delta Marsh on the southern edge, will continue to creep southward, the geologist said, at an imperceptible pace.
The concept of isostatic rebound was recognized by early scientists based on items such as seashells and other marine fossils being found far inland in places like Scandanavia, around Hudson Bay, and in the Ottawa River Valley, Thorleifson said.
The natural phenomenon is also currently happening in northern Europe and Siberia, which were also covered by the continental ice sheets during the last ice age.
Goldsborough relayed a story about the British explorer Samuel Hearne, who carved his name in a rock near Churchill, Manitoba in 1767, near an iron ring that was pounded into a rock for ships to use for anchoring.
That rock, with its anchoring ring for ships, now sits far inland from the water, which is more evidence of isostatic rebound.
While the rebound does add to the ongoing erosion to the shore of Lake Manitoba, which has been a major concern for several years, the erosion would occur anyway, regardless of the rebounding effect, Goldsborough noted.
“Erosion is going to continue regardless of what the levels of the lake are,” he said.
It’s hard to believe glaciers that have long since ceased to exist can still be having an impact on the land in the northernmost regions of the world, but their impact will still be ongoing for another thousand years, long after we cease to exist.
“Most people don’t think about the effects of the [ice age] glaciers,” Goldsborough mused.
