Bedrock in the Bitterroot Mountains is mostly granite. If Lewis and Clark ever cracked a chunk to expose a fresh surface, they saw a very pale rock glittering in the sunlight that reflected off a crystal surface. If they cracked a great many chunks, they would eventually have noticed that most of the granite is very pale gray. Almost white, in fact, and composed of crystals generally smaller than a pencil eraser. That rock has a faintly streaky look, like the grain in a piece of wood. The rest of the granite is composed mostly of crystals larger than a pencil eraser, and not even faintly streaky. Those granites are also pale, but generally pink, or at least pinkish.
The story of the gray granite begins about 200 million years ago, when the western edge of North America first squashed against the floor of the Pacific Ocean as the continent began to move west. The crumpling contintental margin rose into a range of monster mountains that may well have reach elevations of about 20,000 feet. That estimate rests fairly firmly on quite a variety of independent lines of evidence too detailed to consider in this brief discussion.
Courtesy, Donald W. Hyndman
A specimen of gray granite from the Idaho batholith. Note the white crystals among the black streaks. The penny is for scale.
Heat rising from the depths accumulated in those early mountains until the rocks ten miles or more deep within them began to melt into granite magma—a pasty molten rock. Enormous volumes of that magma, thousands of cubic miles, had accumulated by sometime before 80 million years ago. That excessively high mountain range became so engorged with molten granite, and lost so much mechanical strength that it collapsed. Enormous slabs tens of thousands of square miles in extent, and as much as ten miles thick, sheared off the top of the range and slide east into western Montana on a slippery banana peel of molten granite magma. Some of them moved at least 90 miles. Enormous volumes of the lubricating granite magma erupted from the leading edges of the moving slabs to make volcanoes in western Montana. They produced much of the maze of mountains and broad valleys that the expedition threaded on its way through western Montana and the eastern edge of central Idaho. Most of the magma, however, remained behind in Idaho, where it crystallized into granite when its heavy cover slid off. That is the pale gray granite that is at the surface in most of the country between Boise and Missoula.
The earth's crust of western Montana sank under the weight of the displaced slabs, so the heights of the mountains are only a fraction of the thickness of the displaced slabs. Meanwhile, the earth's crust of central and northern Idaho rose after it was relieved of all that weight. That raised the Bitterroot Mountains and exposed the pale gray granite. The erosion surface that began to develop when the gray granite was exposed has since become the dramatic landscapes of the Bitterroot Mountains—with some help from further geological events.
Those events happened between about 890 and 70 million years ago, during late Cretaceous time. Dinosaurs still walked the earth, but vanished at the end of Cretaceous time, 65 million years ago, when Tertiary time began. That brings us to the murky story of the pink granite.
Supported in part by a grant from the Idaho Governor's Lewis and Clark Trail Committee