Indian Heaven Volcanic FieldThe Indian Heaven volcanic field, midway between Mount St. Helens and Mount Adams, is a Quaternary center, chiefly of basalt. Paul Hammond has worked here for many years but has published few detailed results (Hammond et.al., 1976; Hammond and Korosec, 1983; Hammond, 1984, 1987). He graciously allowed some of this work to be summarized, mainly from and informal field guide he wrote in 1985.
About 60 eruptive centers lie on the 30-kilometer-long, N10degreesEast-trending, Indian Heaven fissure zone (Hammond, 1984; Hammond et.al., 1976). The 600 square kilometer field has a volume of about 100 cubic kilometers and forms the western part of a 2000-square-kilometer Quaternary basalt field in the southern Washington Cascades, including the King Mountain fissure zone along which Mount Adams was built (Hammond et.al., 1976; Hildreth et.al., 1983).
All lava flows at Indian Heaven have normal magnetic polarity (Hammond et.al., 1976) and so are assumed to be younger than about 0.73 million years, consistent with their morphology and geomorphic relations. Two K-Ar ages (Hammond and Korosec, 1983; Hammond, 1987) suggesting ages between 3 and 4 million years are probably too old (Korosec, 1987a,b); they are based on trace contents of radiogenic argon, and no evidence exists for a long hiatus or marked erosion to explain the lack of reversely magnetized flows in the section. The youngest eruption produced Big Lava Bed about 8,200 carbon-14 years B.P. (before present).
The field is dominated by small shield volcanoes surmounted by cinder and spatter cones. Subglacial vents occur on the northwest flank of the field (Hammond, 1987). The overall shape of the field is that of a north-south elongate shield, with basal diameters of 30 kilometers and 10-15 kilometers, whose center rises about 1 kilometer above its base. Hammond (1984) noted that most of the flows erupted from the center of the field. Hence Indian Heaven can be interpreted as a large, complex shield volcano, fed by a central reservoir, that supports numerous flank vents.
Pahoehoe and a'a typify the basalt flows; some of the andesite is block lava. Tephra production was minimal, although the basaltic tephra accompanying extrusion of the Big Lava Bed flow has a volume of about 100,000 cubic meters. Big Lava Bed is known for its tubes and for the remarkable microtopography on its surface, which mostly results from inflation of the flow when its feeder tubes clogged.
The field is dominantly basaltic (Smith, 1984; Hammond, 1984), but basaltic andesite and andesite also occur. Mann Butte, a rhyolite dome or plug cast of the field was once considered to be Pleistocene (Hammond et.al., 1976) but now is interpreted as Tertiary (Hammond, oral commun., 1986). About half of the basaltic andesite and andesite was erupted on the flank of the field, possibly form fractionated stranded magma bodies, and the rest in the central part of the field (Hammond, 1984). Both low-K, olivine-normative tholeiitic basalt and high-Al, hypersthene-normative calc-alkaline basalt occur (Smith, 1984; Hammond, 1984), as well as transitional types. In this regard the basalt at Indian Heaven is representative of basalt elsewhere in the southern Washington Quaternary field (Smith, 1984).
The field occupies a 100-mgal gravity low (Hammond et.al., 19876; Williams et.al., 1988) that may reflect light Tertiary rocks below the field, a hydrothermally altered or fracture zone, or a shallow magma reservoir as yet undetected by other means.
The elongate field follows the direction of regional extension and faulting as well as overall volcanism in the Cascades. A few north-trending normal faults are mapped just northeast and southwest of the field (Walsh et.al., 1987), chiefly on the basis of relations within the Tertiary section. Little is known of their ages, not surprising in view of the overall lack of detailed knowledge of the Tertiary rocks in southern Washington.
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