The Idaho Batholith is a vast chunk of granite that dominates central Idaho. It’s some 15,400 square miles of rock. People talk about it as if it’s a gigantic, homogenous lump of stone. It’s not. Geology is complicated.
Most, but not all, geologists agree the Batholith was formed when the Pacific Plate subsumed under the ancient coast of the North American Plate, which was pretty much along the Idaho-Oregon border then. When denser, oceanic crust sinks below comparatively lighter continental crust it gets pushed down far enough that it melts. Some of the blobs of molten rock find their way to the surface, erupting in volcanic chains like the Cascade Mountains. Others never reach the surface, but congeal part way to the surface. In this case, as more crust got shoved under the North American Plate, the whole edge of the plate was uplifted. Erosion gradually cut down to the blob of congealed rock and exposed it. And that became the Idaho Batholith.
That’s the big picture. The details are much messier, much more complicated. It wasn’t one giant blob of molten rock that rose up from that melted, subsumed Pacific Ocean crust. It was a number of blobs. Those blobs are called plutons. Pockets of the native rock, the stuff through which the plutons rose, are trapped between those plutons. That trapped rock was altered – metamorphosed – by the heat and pressure. The plutons themselves, especially at the margins, melted some of the native rock, mixing those older rocks’ chemistry with their own.
And the plutons themselves solidified at different rates, the margins cooled comparatively quickly but the interiors much more slowly. Some of the plutons are a dozen miles across. It takes millions of years for a lump of basalt that large to congeal.1
While all that was going on, other geology was happening, too. The Challis Eruptions, areas where that molten rock did reach the surface, sent sheets and dikes of molten rocks through the cracks and faults in the cooling granite. Superheated water percolated through other cracks as well, depositing veins of quartz and other rocks.
And the chemistry of the ocean bottom that was shoved under the North American Plate wasn’t uniform. Marine sediments, sediments washed off of the North American Plate, still different sediment washed off of the encroaching Wallowa Terrane; all went down under the North American Plate to melt and mingle with the molten plate itself.
The result is a real hodge-podge of batholith. The Idaho Batholith at the western edge is older and chemically different from the Idaho Batholith of the Eastern edge. You don’t have to study the microscopic crystals or perform istopic testing to tell the difference. In many place, it looks quite different. Here’s a photo from Lick Creek Summit, in the heart of the Atlanta Lobe of the Idaho Batholith.
What you are seeing is mostly the kind of granite called granodiorite, and it weathers in oblate, curved surfaces. You can see those curved shapes in the rocks in the right foreground, and in the overall shapes of the ridges in back.
This photo also gives you a sense of the size of the original batholith, and the effects of 40 million years of weathering. The tops of all the ridges are about the same altitude, with deeply eroded valleys, a sure indication this was originally one, more-or-less level chunk of rock.
By contrast, the granite of the eastern edge of the Atlanta Lobe, along the famous Sawtooth Mountains, is visibly different.
You can see the mountaintops and ridges here are sharper and more angular. The granite in the Sawtooth portion of the batholith contains more silicon, as well as other odd elements like beryllium and molybdenum. It has more feldspar as well, giving it a pinkish cast. In fact, the pluton that created the Sawtooths pushed up through the other, older pluton of central parts of the batholith. Oddly, the easterly portions of the Idaho Batholith tend to be younger, sometimes as much as 40 million years younger, than the westerly portions. Geology is messy.
At Grandjean, Idaho, on the westerly side of the Sawtooths, metamorphosed outcrops of the original native rock are exposed. You can see the bedding in the central part of the photo, just above the treetop. The pressure of the rising pluton not only metmorphosed the sedimentary deposits; it lifted the altered native rock to a 70 degree angle.2
All of this is probably far more than you wanted to know about the geology of the Idaho Batholith. And WC has barely scratched the surface, as it were. And oversimplified; as you read this, Real Geologists are furiously emailing WC, accusing him of grossly distorting what really happened.
But here’s the takeaway from this blog post: geology is messy. The Idaho Batholith is messy. It’s beautiful, it’s rugged, and it’s a geological mess. Because geology is messy.
- Hence, the incredible number of hot springs in central Idaho. 40 million years later, the rock is still hot enough to boil water. ↩
- There’s a stretch on Idaho Highway 21, through the middle part of the South Fork of the Payette Canyon, where the road runs through a stretch of that tortured metamorphic rock. It suffers more rock fall and landslides than likely any other stretch of roadway in Idaho. The metamorphosed rock weathers along those old sedimentary layers, and slides down the 70 degree hillside in slabs. ↩