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The volcanoes are located along the southern limit of the Principe Laredo Fault, a former strike–slip boundary within the Queen Charlotte Basin. They provide a unique opportunity for scientists to unravel the region’s complex geological history since the end of the last ice age, more than 10,000 years ago. 

In the study, scientists collected seismic data and sediment cores from McGregor Cone, a submerged volcano in Milbanke Sound. These data revealed glaciomarine sediments draping the cone’s lower slopes, as well as eroded beach terraces which indicated changing sea levels. 

The sediment cores contained distinct assemblages of fossilized foraminifers, single-celled aquatic microorganisms with carbonate shells. By dating carbon-14 in foraminifers known to occur only at specific depths, the team could constrain a local relative sea-level curve for Milbanke Sound, providing a chronological sequence for the formation of these small volcanoes. USGS Research Geologist Mary McGann, a co-author of the study, led the radiocarbon-dating work.

 

How ice gave way to fire 

The Cordilleran Ice Sheet—a colossal mass that once stretched from southern Alaska down Canada’s western coast into the northwestern U.S.—played a significant role in shaping the region. At the last glacial maximum around 20,000 years ago, the ice sheet was more than one kilometer (.62 mile) thick. 

As it receded, the landscape was transformed. Melting glaciers raised sea level and relieved tremendous pressure on the land itself—the ice sheet’s weight was tantamount to loading and removing a layer of rock 300 meters thick within just a few thousand years. 

Deglaciation also enhances rates of mantle decompression melting and volcanism. Thus, the study authors posit that Milbanke Sound volcanoes may represent deglacially triggered volcanism. The radiocarbon dating, along with geomorphic and geological evidence, show how ice gave way to fire. 

While all volcanoes have their own controlling conditions involving sources, magma chambers, conduits, tectonic settings, local faults, and other details, the study suggests that perhaps some of the glacially influenced volcanic activity along Canada’s western coast shares similarities to the interpretation at Milbanke Sound. 

Understanding the geological history of Milbanke Sound provides critical constraints for comprehending how these small volcanoes formed, offering valuable insights into the broader patterns of environmental change and geological dynamics in the Queen Charlotte Basin since the last ice age. 

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