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Secrets of the Snake River Plain Revealed
A Photo Essay by Bill Bonnichsen (Bio)
The Snake River Plain is a recent chapter of Idaho's geological story. It started about 16 million years ago, just after the major pulse of Columbia River basalt volcanism. The first part of Snake River Plain development was not even in the Snake River Plain, but in the Owyhee Plateau area where Idaho, Oregon and Nevada meet. There, massive volcanism commenced as the Yellowstone hot spot began to develop. As time passed and North America moved southwestward, the hot spot cut a swath across southern Idaho and into the northwest corner of Wyoming, where the most recent massive volcanism occurred, less than a million years ago. During the continent's trip over the hot spot, an adjacent zone developed as the Earth's crust was pulled apart, forming a large topographic depression. This became the western Snake River Plain, and for a long time it held a giant lake, which geologists have named Lake Idaho.
Two fundamental types of volcanism were involved in the formation of the Owyhee Plateau and the Snake River Plain, as the wave of volcanism swept across southern Idaho. The first was the eruption of rhyolite from magmas with the same composition as granite, being rich in silica and alkali elements. These magmas mainly were materials melted out of the Earth's crust, so its no wonder they have a granitic composition. The other type of volcanism was the eruption of basalt. These magmas came from deeper in the Earth, from the region geologists call the mantle, and were hotter than the rhyolite magmas. Basalt is much richer in iron and magnesium and poorer in silica than rhyolite.
Besides the volcanism, the deposition of sediments in the western Snake River Plain during the existence of Lake Idaho, and the cutting of canyons after it had drained were two major events occurring as the Snake River Plain landscape evolved. The final episode impacting the landscape along the Snake River was the Bonneville Flood. When Utah's Lake Bonneville overflowed about 17,500 years ago, the deluge swept along the course of the river and modified the landscape in many interesting ways in its wake.
This photo essay [click on the thumbnails for larger images] should give a bit of flavor and add a few more details to these various major facets of the Snake River Plain's evolution: the volcanism, Lake Idaho, canyon cutting and the Bonneville Flood.
Blackrock Escarpment: Several welded tuff layers are magnificently exposed in this 1500-foot-high cliff in Bruneau Canyon near the Nevada border. Each layer is rhyolite that erupted explosively from the Bruneau-Jarbidge region, an early active portion of the Yellowstone hot spot. The layers range from 12.7 million years old at the base of the sequence to 10.3 million at the top. They make up the Cougar Point tuff, a volcanic unit widely distributed along the margins of the central Snake River Plain. In a geologic instant some of these enormous explosions erupted many hundreds of cubic miles of rhyolite magma as tiny molten particles that sped across the land buoyed up by hot gasses, blanketing areas more that 100 miles across. Welded tuff layers like these are believed to lie beneath much of the Snake River Plain and Owyhee Plateau.
Balanced Rock Rhyolite: There are many rhyolite lava flows, like the 8-million-year-old Balanced Rock flow near Castleford, within the Snake River Plain and Owyhee Plateau. Most are younger than the huge welded tuff layers, but are older than most of the basalt flows. These lavas form from rhyolite magmas with very low gas contents that just oozed out of the earth rather than erupting explosively. Low gas contents make rhyolite magmas very viscous, so they form flows that may be hundreds of feet thick and take years to cool. As they cool, prominent vertical fractures form in their interiors which, when exposed in canyon walls, typically guide the development of large vertical pillars.
Basal Vitrophyre: Due to its chemical makeup rhyolite magma not only is viscous but also is slow to crystallize when cooled. This leads to the situation seen at the base of most rhyolite bodies, both welded tuff layers and rhyolite lava flows, where the most rapidly cooled bottom material didn't have a chance to crystallize, but remained in a glassy state. This glassy form of rhyolite, called vitrophyre by geologists, is black in color, in sharp contrast to more slowly cooled interior parts of rhyolite sheets that did crystallize and which typically are brown to red in color. Thus, even with the strong color contrast, both parts of a unit, like the one in this photo, formed at the same time from the same magma batch.
Shoshone Falls: This beautiful waterfall on the Snake River is carved into the upper part of the 6-million-year-old Shoshone Falls rhyolite lava flow. A nearby drill hole indicates the flow to be about 600 feet thick. This waterfall will be sustained for a long time because rhyolite is very hard and resistant to erosion, in contrast to the overlying, thin and pervasively jointed basalt flows from which blocks readily are broken out by natural processes. Shoshone Falls likely was developed into its present form during the Bonneville Flood, about 17,500 years ago, and has not changed much since then.
Buck Mountain Volcano: About 11.6 million years ago this small rhyolite volcano, in the Owyhee Mountains south of Homedale, was built during one of the eruptions that led to the formation of the Jump Creek rhyolite. In this view one is looking into the volcano's crater area. A small rhyolite lava flow was extruded from the volcano, whereas the volcano's slopes are an accumulation of small blobs of spatter that coalesced into the original conical form. When only small volumes of rhyolitic magma are erupted small volcanoes like Buck Mountain are built, whereas large volumes of rhyolitic magma typically erupt through long fissures along which the surface subsides to form calderas.
China Hat Dome: China Hat, a geologically young landmark near Soda Springs in the Blackfoot lava field, stands nearly 1000 feet high and is 1.4 miles long. It formed during a small rhyolite eruption only about 57,000 years ago, and is the largest of three domes lined up above a buried fissure that fed magma to the surface. The rhyolite magma was so viscous it couldn't flow away from the vent; it just accumulated in a big pile. Since the volcanic activity in the Blackfoot lava field was so recent, the area is of considerable interest for geothermal energy. Domes similar to China Hat are a common type of volcanic landform that are built where small batches of viscous magma erupt.
Bruneau Canyon: Bruneau Canyon is about 800 feet deep and only a few hundred feet wider at this scenic viewpoint south of Bruneau. The river rapidly cut down through many layers of basalt, ranging from 7 or 8 million years old at the bottom to 4 or 5 million years old at the canyon rim, as Lake Idaho drained away and afterwards. Once the lake was drained, the river's base level became much lower; previously the high lake level had prevented the river from cutting down through the basalt.
Basaltic Tuff: About a million years ago, just after Lake Idaho had drained away, the near-lake region may have been swampy and certainly had a high groundwater table. When basalt magma rose into this wet environment, as happened many places in the western Snake River Plain, large amounts of steam formed and erupted explosively, blowing out fragments of the basalt magma and the surrounding lake sediments. Such explosions are called phreatomagmatic eruptions; from them are deposited colorful layers of tuff like the ones illustrated from along the Snake River near Grand View. Also shown is the end of a thin basalt flow that was deposited during the phreatomagmatic eruptions. The thinness of this flow shows the basalt lava was very fluid, in contrast to the extreme stiffness of rhyolite lavas that permit only very thick flows to form.
Pillow Basalt: When a basalt flow runs into a lake or river it commonly breaks up into small blobs of lava, typically from a few inches to a few feet across, which cool rapidly into partly glassy basalt. Geologists call these rounded lava blobs pillows. The pillow basalt layer shown in the photo is in the Jerome Golf Course basalt flow, exposed at the Snake River Canyon rim a few miles downstream from Perrine Bridge. These pillows were formed tens of thousands of years ago when the basalt flow, which was spreading northward, ran into the Snake River. Clearly this happened before the river had eroded its canyon, since the layer of pillows is at the canyon rim. Pillow basalts occur at many other places in the Snake River Plain, especially where Lake Idaho once stood.
Glenns Ferry Formation: Layers of sand, interbedded with silt and sometimes gravel, commonly are deposited near the margins of large lakes, such as Lake Idaho. In this photo, taken a few miles south of Bruneau, the near-shore deposits of Lake Idaho are part of the Glenns Ferry formation. These sediments were deposited in the lake during the latter part of its existence, from about 6 to 2 million years ago. The curious elongate structures sticking out of the sand layer in the middle of the photo are concretions, in which the sand has been cemented by minerals from the groundwater. The elongation of these concretions likely indicates the direction water was flowing through the sand.
Chalk Hills Formation: Typically, at locations far from shore out in the middle of large lakes, silt and clay, as well as volcanic ash that falls from the sky, are deposited in fairly uniform layers. Such is the case for the sediments shown in this photo of a locality south of Oreana. These layers are part of the Chalk Hills formation and were deposited during the earlier part of Lake Idaho's existence, probably between about 10 and 6 million years ago. The rocks in the photo's foreground are at the top of a 10- to12-million-year-old rhyolite lava flow, from which the sediments were eroded after the lake was drained.
Teepee Rocks: These hoodoos, which resemble a row of teepees or an assembly of giant garden gnomes, are carved in a thick layer of volcanic ash known as the tuff of Ibex Peak exposed along Trapper Creek, southwest of Oakley. This ash accumulation, which in places is hundreds of feet thick, collected in a lake system along the Idaho-Utah border area between about 17 and 10 million years ago. The ash came from numerous large caldera-forming rhyolite eruptions at various places in the Owyhee Plateau and Snake River Plain in the early stages of the development of the Yellowstone hot spot. The holes in the teepees are the result of freeze/thaw action and wind erosion.
Plunge Pools: During the Bonneville Flood large volumes of water poured into Snake River Canyon in the area upstream from the city of Twin Falls, eroding out and enlarging the canyon in the process. In this photo taken at the canyon rim near Dierkes Lake, one can see little plunge pools that were excavated in the basalt by water falling over the canyon rim, and tearing out blocks of basalt in the process. The pools now are spring-fed because irrigation has led to a shallow water table in that area.
Melon Gravel: During the Bonneville Flood, blocks of basalt, some larger than 10 feet across, bounced along the bottom of the flood channel, continually bumping into one another. The blocks became partially rounded in the process, and when guided by the flood into areas where the water velocity was lower, accumulated into large bars, similar to gravel bars in normal rivers, but of a much larger scale. These partly rounded boulders, located in a field near Hagerman, are a good example of the Melon Gravel, which is the name geologists have used for these flood deposits. This bar contains basalt boulders from many different basalt flows exposed upstream in the walls of Snake River Canyon.