Geology Is Messy: The Western Snake River Plain


It’s been a while since WC wrote about geology. And key to understanding the challenges of geology is to realize just how messy it is. WC will approach that challenge with a nearly microscopic look at a tiny part of the puzzle.

The Snake River Plain extends across the bottom third of Idaho in a shallow crescent shape. The eastern two-thirds of Plain is the path of Yellowstone Hotspot, as the North American Plate drifted west southwest over the fixed plume.

Current estimates of the track of the Yellowstone Hotspot, numbers are millions of years ago

Current estimates of the track of the Yellowstone Hotspot, numbers are millions of years ago

You can see that the western third of the Snake River Plain, more or less centered on Boise, isn’t thought to have been in the track of the hotspot. Instead, it’s a graben, an area that subsided between two long, longitudinal faults. Geologists are still arguing over why there’s a large graben, but WC guesses it had something to do with the extensive volcanism in the area.

Because there has been a lot of volcanism. The Snake River has cut a gorge through about 750 feet of volcanic rocks, cinders, ash and other ignimbrites.The gorge wall exposures, where Swan Falls Dam Road drops down to the river, reveals just how messy geology can be.

Welded ash and cinder layers, northerly rim, Snake River Gorge, Swan Falls

Welded ash and cinder layers, northerly rim, Snake River Gorge, Swan Falls

This photo shows about five feet of bedding. Neat layers, sometimes volcanic ash, sometimes cinders, showing a mind-numbing number of eruptions. Redder colors generally mean a little more time between eruptions, allowing rain and snow melt to work on the surface. It’s about as orderly as geology gets.

Faulted layers of cinder and ash, northerly rim, Snake River Gorge, Swan Falls (lizard for scale)

Faulted layers of cinder and ash, northerly rim, Snake River Gorge, Swan Falls (lizard for scale)

But while those layers may be laid down neatly, they don’t stay that way. In this photo, taken just a few yards from the first, you can see the layers have faulted, and the block to the right of the long, vertical crack has dropped about 1.5 inches. It’s an active slippage point – with a geologist’s loupe you could see the dust ground off the two slipping sides – so there will likely be more slippage. It’s starting to get messy.

Geology getting messier, small dike or sill, northerly rim, Snake River Gorge, Swan Falls

Geology getting messier, small dike or sill, northerly rim, Snake River Gorge, Swan Falls

The bottom third of the photo is rhyolite, a kind of volcanic rock, from a major eruption. There was a long lapse of time after the eruption, and top soil, likely wind-blown loess or lake sediments, accumulated on top of the solidified rhyolite. Further eruptions laid down more layers of ash and cinder on the top soil, and while the photo doesn’t show it, those layers are about ten feet deep, and then are capped with a further layer of basalt that’s about 25-30 feet thick.

In a subsequent eruption, as the molten rock moved toward the surface, it found a weakness at the soil-ash boundary. The boundary was weaker than the overcapping layers, so some of the molten rock squeezed sideways, pushing through earlier ash and cinders, forming a sill. The heat from the molten rock re-melted the ash and cinders, cooked the soil layer, and caused contact metamorphism in an aureole around the sill.

The geology of the Western Snake River Plain is further complicated by Lake Idaho. Before the Snake River cut through Hells Canyon, for millions of years there was a lake, maybe an intermittent lake, in the Western Snake River Plain. Volcanism, which always involves extremely hot rock, of course, doesn’t play nicely with water. Water coming into contact with lava results in explosive creation of steam. There are extensive stretches of pillow lava aong the Snake River Gorge, formed when lava emerges in fairly deep water. It’s likely that water played a role in this sequence:

Lake/river sediments, northerly rim, Snake River Gorge, Swan Falls

Lake/river sediments, northerly rim, Snake River Gorge, Swan Falls

This is a macro shot; top to bottom it shows about three inches. The bottommost portion is the usual layers of ash and cinder. The whitish layer with black fragments is ash that fell into calm waters, like a lake. The water chemically altered the ash to a kind of clayey mud, familiar to anyone who has driven in the Owhyee Mountains. The dark gray layer above that is more mysterious. WC speculates it is a mud flow, since the fragments aren’t especially weathered or rounded by erosion. The upper left portion, above the angular break, shows layering, suggesting a stream. Above that is a cinder fall, likely also into water, and then a thicker layer of coarse, cemented ash, probably falling in water. Not in the photo, but above this is a layer of 40-50 feet of basalt.

All of that in an area perhaps fifteen feet wide and ten feet high, along a river canyon 400 miles long across south Idaho and sometimes even deeper than at Swan Falls. Drilling along this part of the Snake River Plain shows the volcanic deposits are at least half a mile deep. Put another way, the Snake River has cut less than a third of the way down through the volcanic layers.

So, yeah, messy.