By Dave Tucker, October 19, 2012
Mount Baker, like most volcanoes, erupts bits of magma and rocky debris into the atmosphere. When this material lands on the ground, the deposit is called ‘tephra’, the Greek word for ‘ash’. However, in volcanology, ‘ash’ is a size category for tephra fragments less than 2 mm across. Larger tephra fragments up to 64 mm are called ‘lapilli’ (Italian, ‘little stones’) . Anything larger than that is a ‘bomb’ ( if it was obviously hot when it landed) or a ‘block’ (cold when it fell, or at least no evidence that there was any molten component remaining).
The last geologically verified eruption of magma from Sherman Crater occurred 6600 years ago. An ash cloud deposited ash to the north and east. This sandy ash layer, preserved all over the northern Cascades, is called the ‘BA tephra’, for ‘black andesite’. Just downwind from Sherman Crater, deposits on Boulder Ridge are 15+ cm thick; ash preserved in the soil in Heather Meadows is 7-10 cm thick. The ash plume carried about 1 cm of ash to the north end of Chilliwack Lake in British Columbia. A mere 3 mm fell onto charcoal in a cooking fire pit at Cascade Pass. The wind was blowing pretty strongly out of the west on the day of the eruption, since there is almost no BA ash west of Mount Baker. A repeat of the eruption in the same weather conditions would dose Marblemount and maybe Rockport with between 1 and 3 cm of sandy ash, enough to be a real nuisance and a temporary health hazard from silica in the dust, but not enough to collapse buildings. By the time the ash cloud covered the 37 km (23 miles) through the atmosphere to Marblemount, the ash particles would be cooled so they would likely not be a burn hazard.
You can easily find BA deposits around Mount Baker. Meadows are the best places, since there has been less disruption by tree roots. Look in banks and trail cuts just below the organic layer at the base of the vegetation; the BA is a sandy layer a few cm thick. The ash is gray when it has dried out, or nearly black if moist.It often washes out of trail cuts and forms a black sandy layer on trail surfaces. The most easily reached spot is a 6-8 foot road cut along Mount Baker Highway in Heather Meadows, on the west side of the road 100 yards north of the parking lot for the Austin Pass Picnic Area. A thin white layer is the most conspicuous- this is much finer grained ash that came all the way from the Crater Lake caldera in southern Oregon! The BA is just above it. The 8 foot bank is not the easiest place to scramble
Use your ever-present hand lens to examine clasts in the ash deposit. You should be able to spot nearly clear plagioclase feldspar crystals up to 1 mm, and perhaps black, dark green or honey-colored pyroxene crystals. All these grew in the magma befor it erupted. Many of the particles are ‘lithics’, or fragments of cold rock that were torn out of the volcano’s throat as the magma blasted past.