Baker bombs from the sky!

This 10 oz (280 gram) hot bomb landed on Dobbs Cleaver, 3.2 miles (5 km) north of Sherman Crater. OUCH! The surface cracks are ‘breadcrust texture’. Click to enlarge.

Not all tephra from Mount Baker is ash-sized. Dozens of volcanic bombs litter rock slabs on Dobbs Cleaver, north-east of Hadley Peak and 5 km (3 miles) due north of Sherman Crater. The largest are at least 10 cm (4 inches) across- this would have been an exciting but dangerous place to be when these hot stones fell from the sky! The Dobbs Cleaver bombs have a ‘breadcrust’ surface, meaning that the blobs of lava erupted into the atmosphere were incandescent and magmatic gases were still expanding. A thin surface crust quickly hardened in the cold air, but the expansion of gas in the bomb cracked that surface, just like a good thick bread crust. These are presumably

Cracked breadcrust on Galician rye bread. From the Apple Pie, Patis and Pate website.

ballistic bombs blown out of Sherman Crater in the last magmatic eruption at Baker, 6600 years ago. That eruption showered the northern Cascades with ash of the BA tephra (described here on this website).

A vesicular, scoriaceous tephra bomb from the summit of Mount Baker, on the north rim of Sherman Crater. Enlarge to see the many small holes (vesicles) left by escaping gases.

Dobbs Cleaver is at top center of this 7.5′ topo map.

This shower of volcanic bombs was discovered and collected in 1999 by Dave Tucker and Tim Boyer during an attempt to sample the spire of crumbling volcanic breccia near the summit of The Bastile. One of the larger bombs was submitted for whole-rock, X-ray diffraction chemical analysis. The composition is andesitic (57.9 weight % SiO2), very close to BA bombs around Sherman Crater.

This excellent video shows ballistic bombs erupting at Eyjafjalljökull in Iceland, 2010. Note the people standing on the ash-covered promontory of lava, and the bomb craters all around them. Great vantage point- but not too smart.



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