Indian Heaven Volcanic Field, Washington

Indian Heaven Volcanic Field

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.

• Trip Mileage 0.0 - Junction Roads 99 and 25, go south on Road 25.

• Lone Butte

  Trip mileage 40.7 - from Road 30: ... View ahead of 430-meter-high, 0.3 cubic kilometer Lone Butte tuya (Hammond, 1987). According to Hammond, the lower 230 meters of tuya is palagonitized hyaloclastite, overlain by 40 meters of hyaloclastic surge deposits at the eastern end of the tuya and 40 meters of foreset-bedded pillow breccia at the western end (exposed in the quarry visible from here). These subaqueous deposits are capped by a 145-meter-thick remnant of a subaerial scoria cone, whose western flank is overlain by 65 meters of subaerial basalt flows. Four dikes cut the hyaloclastite. At least 3 hours hiking time are needed to see these relations.

• East Crater

  Trip mileage 47.9 - from Road 65: ... Roadcut in basalt of Indian Heaven, diktytaxitic and highly plagioclase- and olivine-phyric (Hammond, 1987). Flow was erupted from west vent of 120-meter-high East Crater in the Indian Heaven Wilderness between 29,000 years ago and Evans Creek time (Hammond, 1987). The basalt has about 50.2% SiO2 and 0.45% K2O (P.E. Hammond, unpub. data, 1985).

• The Wart

  Trip mileage 53.3 - from Road 65: ... At 9 o'clock is The Wart, a 145-meter-high cinder cone that erupted a small basaltic andesite flow just prior to Evans Creek time.

• Red Mountain

  Trip mileage 53.3 - from Road 65: ... Also visible is top of 1,514-meters high Red Mountain (with lookout tower), a vent complex that erupted andesite, basaltic andesite, and olivine basalt several times, most recently between the Hayden Creek and Evans Creek stades (P.E. Hammond, written commun., 1983; [Wise, 1970]).

• Sheep Lakes

  Trip mileage 54.9 - from Road 60: ... Small cut in basalt of Sheep Lakes, erupted form east base of Red Mountain just before eruption at The Wart (P.E. Hammond, written commun., 1983).

• Big Lava Bed

  Stop 52: Trip mileage 56.7 - on Road 60: ... Glaciated cinder cone covered by 3 meters of Big Lava Bed tephra deposit along edge of Big Lava Bed flow. The cinder cone is oxidized, weathered, and clay-rich. It is locally overlain by 5 meters of Evans Creek Drift (P.E. Hammond, written commun., 1983).

  The Big Lava Bed tephra deposit was erupted from a 295-meter-high cone about 3.5 kilometers east of here. This cone is the source of the tube-fed basalt flow that forms Big Lava Bed itself. The tephra blanket flow is oval, extends northeastward toward Trout Lake, covers about 200 square kilometers, and contains about 100,000 cubic meters (P.E. Hammond, written commun., 1985). The Big Lava Bed flow moved west from the cone before turning south, eventually reaching the Little White Salmon River nearly 17 kilometers from its source (Korosec, 1987b). It consists of many thin flow units fed by lava tubes. Pressure ridges and plateaus testify to inflation of the flow when lava poured into blocked tubes. A platform at the west base of the source cone was apparently uplifted about 60 meters during the eruption (P.E. Hammond, written commun., 1985). The Big Lava Bed flow contains about 0.9 cubic kilometers, covers 59 square kilometers (Korosec, 1987b), and is 8,200 +/- 100 Carbon-14 years B.P. (Hammond and Korosec, 1983).