Granite is a rock that's all over Idaho, but it's not the same all over Idaho. Granite can be hard. Or soft. It can be sparkly. Or dull. It can be difficult to break. Or crumble in your hand. It can be in chunks the size of a house, or as small as sugar granules. When granite sparkles, it's because of minerals in it, usually mica or silica. How can the same kind of rock have so many different characteristics?

The "Idaho batholith" is a geologic formation that covers about 90 percent of the Boise National Forest. The batholith is made up of granite.

How did it get there? About 50 million years ago magma formed under the surface of Idaho. (Magma is melted, liquified rock.) It was powerful stuff. It pushed and shoved upward, creating many of the mountains of Idaho.

Some magma squeezed up to the surface. Most of it stayed 30 to 70 kilometers below the Earth's crust. Eventually, it cooled and hardened. The cooled rock is known today as the Idaho batholith.

Over millions of years, the Earth's crust cracked and eroded and the batholith was exposed so you can see it.

(By the way, scientists often measure big distances in kilometers (km). One km = 1,000 meters, or about six-tenths of a mile.)