A MAGMATIC COMPONENT IN 19TH CENTURY MOUNT BAKER ERUPTIONS?

A 1-3 cm-thick gray, crystal- and glass-rich sandy ash is found near Mount Baker at root level, thins away from the peak, and extends at least 20 km NE. The ash, containing approximately 30 percent juvenile glass, may be a product of magmatic eruptions and a layer of the YP tephra set. YP, correlated with the 1843 eruption from Sherman Crater reported by Hudson's Bay Company traders to have “covered the country with ashes,” has previously been known only as a clay-altered lithic tephra. The gray ash is texturally and stratigraphically distinct from the black, sandy BA tephra produced by Baker's last known magmatic eruption (5,740 14C yrs BP).

The 1843 eruption of Sherman Crater (the active vent below and south of the ice-filled Mount Baker summit crater) was the first in a series of eruptions diminishing in size until about 1880. Several reports of apparent incandescent ejecta with these smaller events, although alternatively explained by lightning, raise the question of whether some or all of them, and especially the large initial eruption in 1843, were phreatomagmatic in nature. The presence of fresh, unaltered glass in the gray tephra, which chemically resembles BA, and of prismatically jointed blocks sitting directly on the Sherman Crater rim, add weight to the probability of a magmatic component to the 19th-century activity.

Sherman Crater is at the center of an area of intensely altered bedrock on the south flank of Mount Baker that has been a repeated source area for flank-collapse-runout lahars, including the Morovitz Creek lahar, the third largest lahar in the history of the volcano that occurred in 1845-1847. The period from 1880 to the mid-20th century was a period of low thermal activity and gas emissions from Sherman Crater. In 1975-1976 activity increased sharply but did not subsequently return to the previous low levels. Evidence for a magmatic component in the continuing gas emissions from Sherman Crater adds to the rationale for definition of the period from 1843 to the present as the Sherman Crater eruptive period, the latest of four eruptive periods of Mount Baker since the latest Pleistocene. The potential for renewed magmatic or increased phreatic activity that will again trigger collapse-runout lahars is the most crucial issue for assessing volcanic hazards in the immediate future of Mount Baker.