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Idaho, Home of the Batholith

Years ago, there was a bumper sticker that called Idaho the home of the Batholith. It makes a good point and describes what is at the core of central Idaho.

Geologists speak of "provinces" — large areas of land with certain similar features or characteristics — and one of them is the Idaho Batholith province, the amazing heart of rock at the center of the state. The word batholith signifies deep rock. This granite slowly cooled inside the earth rather than on the surface. Then erosion and glaciers exposed and shaped what we see today.

Batholith [Credit: Dave Pahlas]

Geologists believe that the earth's crust is formed of moving tectonic plates. And two of them have been on a collision course for several hundred million years. Idaho exists at that point of collision.

Of course there are repercussions when plates collide. The intense friction of colliding plates melted the continental crust under Idaho, turning rock into molten magma. We see the effects here in central Idaho. The super heated rock formed magma blobs, or plutons, that migrated toward the earth's surface. Being less dense than the surrounding rocks, the magma rose like a marshmallow in hot chocolate.

The blobs that made it to the surface erupted as volcanoes. The ones that didn't — that eventually cooled and crystallized five or ten miles below the surface — we call batholiths.

"We have very large batholiths," says geologist Bill Bonnichsen, "like the Idaho batholith that has a southern lobe, the Atlanta lobe, and the northern lobe, the Bitterroot lobe. Then there's another large batholith called the Kaniksu batholith, that's in the core of the Selkirk range."

Geologist Marty Godchaux describes a batholith this way: "Some people think that the batholith is a huge sheet — and I think there's some merit to that — that that granite magma rises up and finds its level of neutral buoyancy, and then stops there as a giant mass and cools. Other people envision a batholith as being almost beehive-shaped, going down into the deepest part of the crust. I think probably there are more geologists now who would favor the sheet — the batholith as a great, thick, irregular sheet — at a zone of neutral buoyancy in the crust."

Marty Godchaux and Bill Bonnichsen at the Sawtooths [Credit: Bruce Reichert]

So there's this mass of molten rock miles deep in the earth that will eventually become the core of Idaho. But before it becomes visible, it needs to rise up. Then, it needs to shed literally miles of earth before we can see it.

Bill Bonnichsen describes it this way: "The magma of granite composition is going to be less dense than the surrounding rocks, and therefore it is going to attempt to rise; and if you've got a big enough volume of it, then there's really nothing going to stop it. It is going to rise up."

"But countering that is loss of heat," he points out. "So, many batholiths get up to a certain level in the crust where there's a balance between the strength of the rocks and the tendency to rise up, and if they crystallize at that depth, they can still have a certain amount of cover over them. In the case of the Idaho batholith, it came up to a certain level, then did its crystallization — let's say 5 to 9 miles in depth."

This cooling-off period occurred 60 to 100 million years ago, depending upon the batholith. So what exactly happens to the miles of material on top of that now-cooled batholith?

Elephants Perch [Credit: Sauni Symonds]

"Either part of it just physically slides off, or part of it erodes away," explains Bonnichsen. "In the case of central Idaho, both processes have occurred. All those Paleozoic and older rocks that we see over in east central Idaho, I think, to a considerable degree, just basically slid off of the uplifted zone. It took time and it went in pieces, and there's been erosion, and the Idaho batholith is still eroding. That's why you see canyons and mountains there now. It's got topography like that, which indicates it has been eroding for many, many millions of years. It's in the stage of what you'd call mature topography."

Idaho's batholith could also be called an incubator of precious metals. Gold is what catapulted Idaho into the national spotlight back in the 1860's, thirty years before it became a state.

Early prospectors found gold in stream beds and river gravels. Later they realized gold is associated with rocks formed in those tectonic collision zones directly under Idaho. And it's still a pretty safe bet that you'll find gold along what's called the Trans-Challis fault.

"A lot of the gold in Idaho was originally held in quartz veins associated with the Trans-Challis fault zone," says Bill Bonnichsen. "This is a zone of faults and igneous dikes many miles wide extending southwestward across the state from the Challis-Salmon area toward the old gold camps of Idaho City and Silver City. Why nature chose to concentrate gold along that trend is still a good geologic mystery!"

Text by Bruce Reichert