hewhocaves wrote:http://vulcan.wr.usgs.gov/Volcanoes/Ida ... _moon.html Initially these eruptions are very violent and produce a lava known as rhyolite. Huge calderas of up to 30 miles in diameter are formed when these devastating eruptions take place. Later a more fluid lava known as basalt flows onto the surface and covers the rhyolitic flows. Yellowstone National Park, the area where the hot spot is believed to be located at this time, is the place where catastrophic rhyolitic eruptions last occurred 600,000 years ago. Craters of the Moon represents the second stage of the eruptions where fluid basaltic lava covered the landscape as recently as 2,000 years ago.
rhyolite is a extrusive (quickly cooling version of granite - essentially a light color) basalt is darker.
Basalt has more iron, magnesium and manganese, and more dark feldspars, and less quartz. What are called mafic minerals are usually dark black, to dark green in color. That being said, rhyolite is *not* necessarily light-colored. Ash fall (just what you think it is) unaltered rhyolites are often powdery, and light colored (lavender, pink, yellow, cream) as well as lightweight. Yellowstone rhyolite is um--yellow. However, don't judge a rhyolite by its color. Most of the Missouri rhyolite porphyries are black to dark purple or maroon, with white or pink feldspar flecks. These are generally ashflows--ash explodes very hot, comes to earth, forms thick hot layers, remelts, and flows down the mountain, recooling slow enough to grow the crystal flecks.
as for why the mineral composition varied - the off-the-cuff reaction is that its near a subduction zone (to the west) and so the magma coming upwards is essentially the melted continental plate which would have quite a vareity of elements associated with it. However, there is probably a more specific / altenate explanation to it involving the Yellowstone hot spot. but regardless, the point is that there was a mix of materials which changed over time forming the different colors.
Making a grand assumption that Idaho is not unlike Washington State or Yellowstone, you've got basically three varieties of volcanic magmas-- mafic (deep crustal origin, heavy-thin basaltic--like Hawaii) felsic ( shallow crustal origin,lots of quartz and feldspar, rhyolitic thick, explosive--like all the Mt. St. Helens flows except the one Ape Cave is in) and intermediate (halfway between the two-- rocks like andesite, diorite--lots of light feldspars, some dark heavy minerals, not a lot of quartz--similar to the Sierra Nevada.)
There is also a process called magma fractionation-- basically a magma with a lot of all components in it will settle like a cooling stew, with the light minerals rising to the top like fat, intermediate minerals staying in the middle like broth, and heavy minerals sinking like the meat carrots and potatoes. If you drain off one of the separated components sideways down a fracture you get different lavas. Or, if the light stuff blows off, then all that remains for the next reheating is the heavy stuff.
The process of fractionation is how and why you can get rhyolite, basalt and andesite flows very close to each other, or even on top if each other over time.
Olivine, peridotite and similar minerals will give you green and black colors.
Iron can give you a suite of colors from black to yellow. Black, brown, orange, maroon, purple, lavender, bluish, green, yellow...
Manganese gives you black.
Magnesium can give you pink or purplish.
Sulfur-- bright yellow or orange.
Metals and semi-metals like lead give a variety of golds, silvers, gray and brass.
Copper blue, green, bright brown or orange.
Calcium and sodium give white. Potassium--purple or pink.
Quartz--clear, white or gray if pure.
There are others, but these are the main rock coloring elements and minerals.