Mount A won't blow

Although Mount Ashland has a volcanic shape, it won't erupt any time soon. It isn't and never was a volcano.

Millions of years of erosion removed remnants of any volcanic structure, exposing the magma (molten rock) source that cooled 5.7 miles beneath the earth's surface about 163 million years ago. This slowly cooled magma is called, appropriately enough, a pluton, after Pluto, the Roman god of the underworld.

How do we know it cooled within the earth and not on the surface? Do we use a geologic Ouija board? No, we look at the size of crystals, which allows a good estimate of cooling rate.

If magma freezes before crystals are generated, glassy obsidian forms. If it erupts to the surface (lava), it cools more slowly, yet small crystals have time to form, giving the rock a dull cast with mineral speckles seen when the rock is rotated in the light.

However, if it cools beneath the surface, heat is retained for a longer period of time, which allows crystals to grow to larger sizes. Compare the sizes of crystals in lava (found along roads east of the valley) to those at Mount Ashland. Which are larger?

How do we know the depth at which the magma cooled? No need to contact the Psychic Network. We look at research done on minerals by geologic lab rats. These pasty-colored people run experiments at controlled pressures and temperatures then make calculations regarding the composition of minerals under controlled conditions. Finding the same minerals in rocks of Mount Ashland allows a reasonable estimate of crystallization conditions.

You don't need a magnifying lens to identify the large minerals in Mount Ashland granite. The gray, semi-glassy mineral is quartz; the pale, salmon-colored mineral is orthoclase; plagioclase is white; the shiny, flaky, black mineral is biotite; and the dull, harder, black mineral is hornblende. Interestingly, the biotite, when slightly weathered, takes on a golden sheen that is often mistaken for flat gold flakes seen in streams.

If a party starts getting dull, a piece of Mount Ashland granite placed on a coffee table makes a good conversation starter. You'll also appear quite intelligent (always advantageous) when identifying the minerals for astounded and completely enthralled guests.

Most of all, get out and look around. Let the Earth speak to you. And, by all means, ask questions.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.


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