Excerpts appear below from U.S. Geological Survey Bulletins, including 2064-ii, "Geology and Stratigraphy of the Challis Volcanic Group and Related Rocks, Little Wood River Area, South-Central Idaho," by Richard F. Sanford, with a section on Geochronology by Lawrence W. Snee.

Available online at http://pubs.usgs.gov/bul/2064/ii/

[begin excerpted text from USGS bulletin 2064-ii]

The Challis volcanic field in central Idaho is the largest of several Eocene volcanic fields in the Pacific Northwest of the United States and Canada....The Challis volcanic field was erupted during the Kamloops-Challis-Absaroka volcanic episode at 51-40 Ma (Armstrong and Ward, 1991).

Two models have been proposed to explain Eocene volcanism in the region. In one model, subduction of a very shallow dipping Farallon plate beneath the continent created a wide volcanic arc (Lipman and others, 1971, 1972; Lipman, 1980). In the other model volcanic activity resulted from rifting as a result of collision between the Pacific and North American plates (Ewing, 1980; Fox 1983; Fox and Beck, 1985) The later model was developed for Eocene rocks of northeastern Washington and British Columbia.

Lynna notes that John McPhee in his book "Annals of the Former World" presents this period of volcanism memorably, and adds his own twist to the story with the help of geologist, David Love. Here are some excerpts from McPhee's book:

     West of Wyoming, in the Eocene, there were no Coast Ranges, no Sierra Nevada. Warm winds off the Pacific brought rains to the Rockies, and a climate similar to the present climate of Florida. In the early Eocene, when the ranges in general looked much as they do today, the mountain building ceased. In the tectonic quiet, erosion of course, continued, and the broad downwarps among the ranges continued to fill. Then came a footnote to the revolution. "In latest early Eocene, fifty-two million years ago, all hell broke loose again," David Love said. From thousands of fissures in northwest Wyoming, lava poured forth by the cubic mile. Torn apart by weather and rearranged by streams, it has since been etched out as the Absaroka Range. "After that, everything went blah," he went on. "In the Oligocene, the tectonic activity was totally dead, and it stayed dead at least until the early Miocene. Thirty million years. Then, in the late Miocene, all hell broke loose again. And all hell has been breaking loose time and again for the last ten million years. This is not a static science."

     During those thirty million years after things went blah, the Rockies were quietly buried ever deeper in their own debris—and, not so peacefully, in materials oozing overland or falling from the sky. Much came in on the wind from remote explosive volcanoes—stratovolcanoes of huge size in Idaho, Oregon, Nevada..."At the end of the Oligocene, only a thousand to four thousand feet of the highest mountains protruded above the aggradational plain. Streams were slow and sluggish and so choked with ash they were unable to erode."...This—in the late Miocene—was the level of maximum fill.

For something began to elevate the region—the whole terrane, the complete interred family of underthrust, upthrust, overthrust mountains—to life them swifltly about a mile...It is known in geology as the Exhumation of the Rockies....(On a relief map, the Absarokas seem to spill out of Yellowstone Park.)...

     In increasing numbers, geologists have come to believe that in a deep geophysical sense Yellowstone is not what is moving. They believe that the great heat that has expressed itself in so many ways on the topographic surface of the modern park derives from a source in the mantle far below the hull of North America. They believe that as North America slides over this fixed locus of thermal energy the rising heat is so intense that it penetrates the plate. The geologic term for such a place is "hot spot."...In Oregon and Washington, in the middle Miocene two hundred and fifty thousand cubic kilometres flowed out within three million years. Having achieved the surface in this form, the plume begins to make its track as the plate above slides by, just as Yellowstone, starting off from the flood basalts of Oregon and Washington, stretched out the pathway that has become the Snake River Plain.

During the past twenty million years, the region that we like to call the Old West is thought to have been passing over not one but two hot spots, which have done much to affect the appearance of the whole terrain. The other one is less intense than Yellowstone, and is at present centered under Raton, New Mexico...Work done in the rock-dating laboratory of Richard Armstrong, a geochemist and geochronologist at the University of British Columbia, showed that Basin and Range faulting began at the western extreme of the region and moved eastward at a general rate of twenty-eight miles per million years—a frame commensurate in time and space with the continent's progress over the hot spots now positioned under Yellowstone and Raton. The Tetons began to rise eight million years ago and are clearly not products of the Laramide Orogeny. They are a result of extensional faulting, and conform to hot-spot theory as the easternmost expression of the Basin and Range...

     Inevitably, it has been suggested that someday North America may split apart along the Yellowstone perforations of the Snake River Plain. "That gives me a caution," says David Love..."In the Snake River Plain, the volcanics do get older east to west—in a broad sense, yes. But when you get down to details you get down to discrepancies...I would like to see a lot more regional information...The Yellowstone-Absaroka hot spot abruptly terminated at the end of Eocene time. Where the hell did that hot spot go? Twenty-five to thirty million years later, it was reactivated in the same place. What was that plume doing for all those millions of years? How do you reactivate a plume?" [end excerpts from John McPhee's book.]

Necklace pictured below (approximately true-to-size) was created by Steve Howard from PrueHeart Plume Agate.

[Resume excerpted text from USGS Bulletin 2064-ii]

In the Little Wood River area the Challis Volcanic Group includes, in order of decreasing abundance, andesite lava flows and tuff breccia, dacite lava flows and flow breccia, volcaniclastic sedimentary rocks, lithic tuff, nonvolcanic conglomerate, and rhyolite dikes...from oldest to youngest, basal conglomerate, basal andesite, lower volcaniclastic sedimentary rocks, andesite lava flows and tuff breccia, dacite lava flows and flow breccia, upper volcaniclastic sedimentary rocks, lithic-rich tuff, upper mixed unit, dacite intrusion-flow-dome complex, and rhyolite dikes...Modern topographic highs are commonly related to resistant siliceous units within the Wood River Formation. The Challis volcanic Group wraps around these highs...The Challis volcanic Group has a consistent tilt toward the east at an average orientation of N.8º E., 36º E....The euptive history of the Challis volcanic field in central Idaho can be divided into three paarts: (1) early effusive volcanism, dominantly andesitic to dacitic, followed by (2) explosive dacitic to rhyolitic ash-flow tuff eruptions and formation of cauldron complexes, and culminating in (3) intrusion of late-stage dacite to rhyolite domes and plugs. [end excerpted text from USGS Bulletin 2064-ii]

[Begin excerpt from "Geology and Geochemistry of Jasperoid Near Mackay, Idaho" by Anna B. Wilson, Sandra J. Soulliere, Betty Skipp, Ronald G. Worl, and Keith P. Rhea, Idaho Geological Survey Bulletin 27]

There are three major mining districts, Alder Creek, Copper Basin and Lava Creek, and numerous other prospects in the Mackay area. Most of the deposits were discovered during the 1880s when rich silver-lead ores were mined throughout central Idaho. Most production of the late 1800s and early 1900s was from oxidized silver-lead ore. The Empire Mine in the Alder Creek district, active intermittently from 1901 into the 1960s, was a major copper producer. Many other properties in the area have been active intermittently since their discovery, but did not produce substantial amounts of ore. Three types of mineral deposits have been exploited in the Mackay area: skarn deposits, polymetallic veins in Paleozoic sedimentary rocks, and polymetallic veins in volcanic rocks. The skarn deposits are mainly in the White Knob Mountains where Tertiary leucogranite intrudes Paleozoic limestone...The deposits consist of chalcopyrite, pyrite, pyrrhotite, calcite, quartz, magnetite, flourite, scheelite, molybdenite, sphalerite, and specularite in addition to skarn silicate minerals (Umpleby, 1917). [end excerpt from Idaho Geological Survey Bulletin 27] For photos of jasperoids and more info, see Lynna’s Jasperoid blog entry.

Agate slabs for sale at http://www.agateslabs.com, the website of Steve Howard (my brother). Or call Steve at 208.520.2449

Plume agate from the PrueHeart Lode has been carved into some interesting art pieces. See the link below.

http://www.flickr.com/photos/lostsierra/5483544706/in/photostream/

Welcome • Mining • Books • Adventure Travel • Poetry • Great Rift Writers