The Barry Arm fjord to the east of Anchorage, Alaska, in Prince William Sound is ripe for disaster. At the end of the fjord, a melting glacier climbs up a mile-long slope to the mountains. Melting ice is currently destabilizing the glacier, and a team of scientists believes that a glacial landslide is likely. So, in this case, a massive slope failure would send millions of tons of ice and rock cascading into the sound’s waters, creating a tsunami. Glacial tsunamis thus pose a threat to coastal towns.
One of the more famous landslide tsunamis occurred in July of 1958 at Lituya Bay on the southeast Alaskan coast when over 80 million tons of rock, destabilized by an earthquake, tumbled into the bay. The tsunami generated by this landslide rose to 1,720 feet. In 2015, another landslide at Taan Fjord, Alaska, generated a 600-foot-high tsunami.
The massive amount of ice in the Barry Arm glacier will generate a tremendous amount of energy if the entire one-mile slope fails.
Glacial landslides are an increasingly recognized hazard in the Anthropocene world of climate change and global warming. These landslides create glacial tsunamis when they spill into the ocean. Like any landslide, the root-cause is slope instability. One common cause of instability is the removal of the base (or toe) of a slope. To visualize this, stack a pile of balls and remove the first row of balls along the base. Then a layer of balls upslope will come cascading down.
Another common cause is a loss or reduction of friction. Fluids like water lubricate the contacts between layers of rock or soil. Thus, the loss of friction between the two layers will cause the upper layer to slip downslope.
As the climate warms, glaciers face both of these problems. The Barry Arm glacier is currently retreating from the fjord. The ice at the toe of the slope is disappearing, removing upslope support. At the same time, meltwater lubricates the base of the glacier. Meltwater from glaciers moves through cracks and crevices to the base of the ice mass. Then, once the water reaches the base, it flows downslope at the interface between the glacier and the underlying rock. Satellite imagery confirms the glacier slipped 600 feet downslope between 2009 and 2015.
When and how much?
Key questions of when a glacial landslide could occur and how much ice and rock would slip into the ocean cannot be precisely answered. If there was a complete, catastrophic collapse of the entire glacier, then about 500 million cubic meters of material would crash into the waters below, generating a tsunami several hundred feet high. This tsunami threat translates into a thirty-foot wave inundating the town of Whittier about twenty minutes after the actual landslide.
The most likely causes for initiating a glacial landslide into the Barry Arm fjord are an earthquake, prolonged heavy rains, or a heatwave. The shake energy from an earthquake is a well-known trigger for landslides. The threat from prolonged heavy rains exists because the rain delivers excess water to the glacier, and as the water seeps to the base, it further lubricates the contact between ice and rock. Likewise, a heatwave has the same effect as rain only the excess water is from meltwater, not rain.
Glacial landslides are real and increasing in frequency as the climate warms. When these landslides spill into surrounding bodies of water, then the resulting glacial tsunamis may become more dangerous than the actual landslides.
Glacial landslides (Source: ArcheanWeb) – https://archeanweb.com/2020/05/04/glacial-landslides/ Also:
Volcanic tsunamis (Source: ArcheanWeb) – https://archeanweb.com/2020/01/13/volcanic-tsunamis/ Also:
‘It Could Happen Anytime’: Scientists Warn of Alaska Tsunami Threat (By Henry Fountain; New York Times) – https://www.nytimes.com/2020/05/14/climate/alaska-landslide-tsunami.html Also:
Feature Image: Coxe Glacier – Barry Arm (Modified) – By Frank K. from Anchorage, Alaska, USA – Ice that broke off of Coxe Glacier, Barry Arm, PWS, Alaska (IMG_6824a), CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=4442222