March/April Geological Society of America Bulletin media highlights

Keywords: Hannegan, caldera, volcanology, geochronology, collapse mechanism, trap door.

Geologic research in the Hannegan Pass area of North Cascades National Park has determined that eroded rock deposits over 3000 feet thick were left by two catastrophic volcanic eruptions around 3.72 million years ago. The rocks, about 44 miles east of Bellingham, Washington, are hardened volcanic ash enclosed in a roughly elliptical collapse area measuring 4.8 by 2 miles (8 by 3.5 kilometers). The study’s authors estimate that over 33 cubic miles of ash were erupted in the cataclysmic blasts. Airborne ash and seering incandescent clouds of gas-propelled ash and rocks flowed over and buried the landscape, certainly obliterating all life for tens of miles around the volcano. Each eruption resulted in collapse of the surface in a manner similar to the formation of the far younger Crater Lake caldera in southern Oregon. In between eruption of the two calderas, a lake formed in the blasted crater. In the time since eruption of the thick piles of ash, erosion has stripped a layer of rock at least a half mile thick from the Cascade landscape. Volcanic activity has migrated from the Hannegan caldera southeastward to Mount Baker in the succeeding millennia. The Hannegan caldera is only the sixth of these colossal volcanoes recognized from the Cascades. It’s discovery serves as a reminder that relatively quiet, snow-covered cones are not the only type of volcano known to erupt in the Cascade Mountains.

Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica
M.J. Flowerdew, British Antarctic Survey, Geological Sciences Division, Cambridge, Cambridgeshire CB3 0ET, U.K.; et al. Pages 275-288.

Keywords: Antarctica, absolute age, U-Pb, Hf, zircon provenance.