Enigmatic boulder trains, supraglacial rock avalanches, and the origin of “Darwin's boulders,” Tierra del Fuego

[摘要] Charles Darwin considered himself to be a geologistand published extensively on many geologic phenomena. He was intrigued with thedistribution of erratic boulders and speculated upon their origins. In his accountsof the voyage of the HMS Beagle,Darwin described crystalline bouldersof notable size and abundance near Bahía San Sebastian, south of the Strait of Magellan, Tierradel Fuego. Influenced by Charles Lyell?s reflections upon slow, vertical movements of crust,submergence, and ice rafting to explain drift, Darwin proposed that the bouldersof Bahía San Sebastian were ice-rafted. Benefiting from 170 years of subsequentstudy of the glacial history of Tierra del Fuego, petrography, and terrestrial cosmogenicnuclide measurements, we revisit the origin of “Darwin?s Boulders” at Bahía SanSebastian. We suggest that they, as well as another train of boulders to the west,at Bahía Inútil, represent rock falls of Beagle-type granite from the CordilleraDarwin onto glacial ice flowing into the Bahía Inútil–Bahía San Sebastian lobe.These supraglacial rock avalanche deposits were subsequently elongated into bouldertrains by glacial strain during transport and then deposited upon moraines. Thecosmogenic nuclide exposure dates support the correlation of Andean glaciationswith the marine oxygen isotope record and the glacial chronologies recently proposedfor Tierra del Fuego.

[学科分类] 地质学

[关键词] Fire safety;fatal fires;smoke alarms