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Idaho Geology, in About a Minute

By Bill Bonnichsen (Bio)

Once upon a time, but only about 200 million years ago, Future Idaho was lying there, dumb and happy, in the form of very old metamorphic rocks folded up under layers of Precambrian and Paleozoic marine sediments, while the last of the mid-continental seaway was vanishing from North America and as the continent was breaking up with the rest of the Pangaean supercontinent.

Then, right out of the Pacific, the Wallowa microcontinent crashed into, and scraped along, the west side of Future Idaho. Wham, bam, thank-you ma'am!! Well the consternation that followed included squeezing bits of oceanic seafloor between the two masses and starting the subduction of the Pacific seafloor under the continent.

Dorsey Creek rhyolite [Credit: Bill Bonnichsen]

As you might imagine, things really began to get hot, as huge volumes of granitic magma were melted out of the sinking seafloor, and rose up into the continent's edge to form the gigantic Idaho, Kaniksu, and other batholiths between about 100 and 65 million years ago.

The bulging up during this time caused most of the Precambrian and Paleozoic sedimentary layers to slide eastward from over the batholith, becoming complexly folded and faulted and forming eastern Idaho's overthrust belt.

But wait, the hot times were only beginning. Continued melting down below between about 55 and 40 million years ago let more magma rise up and erupt explosively to form the Challis volcanic field, with its gigantic calderas and accompanying Eocene granite plutons that pierce the Idaho batholith.

Challis volcanics [Credit: Bill Bonnichsen]

After only a few million years of rest, the lower regions provided another humungous, and even hotter, supply of magma. Starting about 17 million years ago basalt lavas erupted through swarms of dikes, to coalesce into the Columbia River basalt fields of western Idaho and eastern Washington and Oregon.

By this time Future Idaho was starting to look like a battleground, with old volcanoes scattered around, fault-block mountain ranges forming and eroding, rivers that were cutting deep canyons, and giant, but temporary, lakes dotting the landscape.

But still no rest for Future Idaho — somehow the Yellowstone hotspot formed down deep near where Idaho, Oregon, and Nevada meet about 14 million years ago. Hot basaltic magma coming up from the hotspot melted big zones of the crust where the batholiths had previously been, making large pools of rhyolitic magma that erupted as a series of supervolcanoes — gigantic calderas — which line up SW to NE across southern Idaho to the site of current activity at Yellowstone.

Then basalt lavas erupted in the supervolcano track to form the Eastern Snake River Plain about as it is now. But wait, while this was going on, the Western Snake River Plain region was stretched in a SW-NE direction to form a big deep basin. This was filled with Lake Idaho, a Lake Ontario-sized body of water about 1500 feet deep, that lasted from about 12 to 2 million years ago, and only drained away once Hells Canyon was cut to its present depth.

Council Mountain [Credit: Bill Bonnichsen]

Then, out of the north, came the final big act — the Pleistocene Ice Ages. Glaciers formed on many mountain ranges and eroded high peaks into their present forms. And in northernmost Idaho, a tongue of the continental glacier flowed down the Purcell Trench to form a large ice dam, impounding the huge glacial Lake Missoula in western Montana.

Each time this lake rose too high it breached its ice dam to let torrents of water race across northern Idaho, filling in the Rathdrum Prairie with gravel, and coursing across the basalt fields eroding Washington's channeled scablands.

Not to be outdone, about 17,500 years ago, Utah's Lake Bonneville also got so full that it overflowed into southeastern Idaho. This flood followed the courses of the Portneuf and Snake Rivers on its way to the ocean, leaving behind interesting deposits like the Melon Gravel boulder fields.

Folded sediments along the Salmon [Credit: Bill Bonnichsen]

But the geological good times haven't yet ended, as volcanic activity certainly will happen again — the youngest flows at Craters of the Moon are only about 2000 years old — and mountains are still rising, as indicated by the most recent major earthquake near Borah Peak in 1983.

So, who knows what the next 200 million years might bring, but I bet it will be exciting! The Earth is a dynamic planet and Idaho clearly proves it!