Dating lava light

Sunstone occurrences (×) are linked to the local eruptive center of these basalts, Steens Mountain (SM) and Abert Rim (AR).

As the basalts ebbed, new volcanic forces kindled a hot spot in the earth’s mantle, spawning volcanic calderas (C) across Idaho’s Snake River Plain (SRP) to the Yellowstone supervolcano (Y).

Meanwhile, renewed activity (D) moved west across Oregon’s High Lava Plains (HLP) to Newberry Caldera (N).

Illustration by Larry Lavitt, adapted from Long (2009) and Grunder and Meigs (2009).

This superb 2.85 ct sunstone from Sunstone Butte displays the gem’s most valued attributes: a blend of green and red bodycolor, with reflective spangles of native copper glittering in the interior. No discussion of this topic would be complete without mention of the controversy surrounding treated copper-bearing feldspar; Rossman (2011) provides a chronology.

In this view of Lake County’s Dust Devil mine, the local source of the sunstone-bearing lavas is the group of rounded hills (Dudeck Ridge) in the background. Over five days in late July 2013, we visited three important sources of gem-quality sunstone in eastern Oregon: the Ponderosa mine in Harney County, and the Dust Devil (figure 1) and Sunstone Butte mines, both of which are about 120 miles further south, in Lake County.Their studies provided a much better understanding of the dynamics of plate subduction and magmatism under the Pacific Northwest.According to Stewart (1966), the sunstone lavas within the Rabbit Basin are petrographically similar to plagioclase-rich flows within the Steens Basalt.They are a minor but fascinating part of the region’s recent geologic history, likely originating during an episode of widespread basaltic volcanism that occurred 15–17 million years ago (Ma), during the Miocene epoch.Geologists debate whether the volcanic events were caused by melting of subducted oceanic crust or by a hot plume in the earth’s mantle.

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