| Popis: |
On 10 March, 1975, large amounts of steam were first sighted coming from the Sherman Crater of Mt. Baker, long the site of mild fumarole activity. To document and evaluate the new activity, we conducted a series of aerial and ground-based observations and experiments including the collection of samples within the crater, installation of 35-mm automatic sequence cameras at Park and Anderson Buttes, and overflights with a mapping camera and infrared scanner. Significant changes characterizing the new activity are: development of a new, energetic fumarole in an area of earlier mild activity; growth of crevasses within and concentric to the crater walls; collapse of a central 70 m diameter plug of ice to form a warm lake; and ejection of tephra rich in analcite and sulfides from the new fumarole. Although the tephra contains minor amounts of unaltered glass, it does not differ in this or other respects from material collected from the west crater wall, where fumaroles have long been active. This suggests that the glass is older tephra derived from the fumarole vent walls. Weather and lighting conditions were such that the Park Butte sequence camera provided acceptable images of the crater area on 27% of the frames for the period 1825 Pacific Daylight Time June 5 to 1825 PDT August 2. Of these, 8% show a significant plume (>300 m above the vent). Large plumes occur during favorable weather conditions of low wind and high relative humidity, and all appear white. While there are abundant manifestations of increased heat emission from Sherman Crater, there is as yet no sign of a fundamental change in the style of activity, or that the rise of fresh magma is the cause. |
