Rather, each erupting vent built a separate, small volcano, and over time a field of numerous overlapping volcanoes was created. Clusters of these vents define the Mount Adams , Indian Heaven , and Simcoe Mountains volcanic fields . The Indian Heaven volcanic field , midway between Mount St. Helens and Mount Adams , is a Quaternary center, chiefly of basalt. All lava flows at Indian Heaven have normal magnetic polarity and so are assumed to be younger than about 0.73 million years, consistent with their morphology and geomorphic relations. The field is dominated by small shield volcanoes surmounted by cinder and spatter cones.
Subglacial vents occur on the northwest flank of the field.
The Indian Heaven volcanic field is midway between Mount St. Helens and Mount Adams , its principal feature is a 30 kilometer long, N10degreesEast-trending linear zone of coalescing, polygenetic shield volcanoes , cinder cones , and flows , with a volume of 100 cubic kilometers. Most of these cones are deeply glaciated, but a few retain characteristic shield morphology. Of the 48 eruptive units in the field, monogenetic cinder cones account for 67 percent, polygenetic shields compose 29 percent, and tuyas and mobergs form 4 percent. Cinder cones have basal diameters ranging from 305 to 1,600 meters, and volumes from 0.9 to 185 cubic meters. Shield volcanoes range from 1.3 to 9.5 kilometers in basal width, with volumes of 60 cubic meters to 11.8 cubic meters.