Field Notes: The Late Lake Idaho, Part 1


There are so many mysteries about Lake Idaho. How did it last an incredible 7.8 million years? Where did it go? Why did it go? Geologists are slowly teasing out some of the answers, but as in the case of most good science, each answer just raises more questions. This is Part 1 of 2 looking at what’s known about vanished Lake Idaho.

Lake Idaho and the Snake River Volcanic Province
The dark and medium blue is Lake Idaho; the light blue is the track of the Yellowstone Hot Spot; the blue in ovals are major volcanic centers along the track of the Snake River Volcanic Province

Contrary to appearances, the Western Snake River Plain – the stretch of the Snake River Plain from Mountain Home to the northwesterly end – is not the track of the famous Yellowstone Hot Spot. The Western Snake River Plain curves up to the northwest; the track of the hot spot is well documented and is a nearly straight southwest-northeast line from the McDermitt eruptive center in north central Nevada to Yellowstone National Park, and the dating of the many eruption centers along the line tracks nicely the path of the hotspot.1

Rather, the Western Snake River Plain is a graben, a rift valley, created starting about 17 million years ago. The central part of the Plain sagged down along faults along the northerly and southerly edges. As it sank, water outflow (probably to the south) was cut off, and the graben slowly filled with water. It seems to have initially been a fresh water lake, which is known to have existed starting at 11 million years ago, quite possibly earlier. At its largest, Lake Idaho was about the size of the modern Lake Ontario, extending from modern day Hells Canyon east to Twin Falls, and from Boise south to the Jordan Valley.

Fossil lake bed layers, Shoofly Oolite Formation, Idaho
Lake Idaho lakeshore layers, Shoofly Oolite Formation, Idaho. Mrs. WC and Tali the Geology Dog, for scale.

At some point about halfway through its existence, Lake Idaho began to lose water, likely through reduced inflow and evaporation. Saline deposits have been found in the eastern end and oolites in the southerly side, implying high mineral concentrations, something like the present Great Salt Lake. In some areas, those lake sediments are overlain by a basalt flow that’s about 9.3 million years old. And then another 1,800 feet of lake sediments are deposited on top of that basalt flow. So there seems to have been two Lake Idahos, in the same basin, at different times.

We know more about the more recent lake fill. The water level rose to about 3,800 feet above sea level, about 1,200 feet deep at Boise. The historic depth is determined by basalt flows on both the northerly and southerly sides of the Snake River Plain that are deposited as pillow lavas, magma that flowed into water before hardening. All during this time, the graben floor – the lake bottom – continued to sink downwards, at least 1,200 feet. So one mystery is solved: the lake didn’t fill up with sediment because the lake bottom kept descending along the faults on the northerly and southerly sides. There’s a modern example of this effect in Russia’s Lake Baikal, which is also located in a rift valley.

Exposed Lake Idaho bottom, eroded into the “Pillars of Rome.” Note the layer of welded tuff on top of the mud, sands and gravel, implying the lake bottom was already exposed when the hot ash tuff fell. The “pillars” are about 150 feet high, less than 10% of the total lake bottom deposits

Why did the graben develop? One theory is that the Western Snake River Plain, submerged under more than a thousand feet of water, was “stretched,” pulled northwest and southeast. Another question raised and not yet definitively answered. Certainly there has been been a lot of volcanic activity in the area: the earlier passage of the Yellowstone hot spot, the immense Columbia Group Basalt flows and the basin and range activity.

That “stretching” thinned the continental crust in the area. That, in turn, permitted significant volcanic activity along the floor of the lake bottom. There were both submerged, underwater volcanoes and other volcanoes that emerged above the water. Impressive amounts of basalt and rhyolite were spread across the lake bottom and the bounding mountains. The relationship of those 3-4 million years of volcanic activity to the passage of the Yellowstone hot spot to the south and the flood basalts of to the north is still unknown. Answer one science question, and it raises still more.

Continued in tomorrow’s post.


1 Technically, the Yellowstone hotspot or plume hasn’t moved at all. Rather, the North American Plate is being shoved west-southwest, and the hot spot is punching through the crust as the North American Plate passes over the hot spot. WC, like Real Geologists, talks about the trace of those events as “the path” of the hot spot.

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