Excerpts from Jim Conrad's
from the May 1, 2005 Newsletter issued from the Sierra Nevada foothills
somewhat east of Placerville, California, USA
We have plenty of rocks here, and they're doozies. As I type these words I look out the window and see white, rounded boulders the size of wash tubs appearing to graze the green open spaces. Walking downslope to a ledge just below the house I can find outcroppings over which to hang my legs. Often those rocks are mantled with dark green mosses, lichens and ferns. Sometimes you see blackish rocks but the handlens shows that the blackness is black crustose lichen encrusting white rock. Way, way below, the river courses through a disorienting jumble of white, house-size boulders and so far I haven't found a way to get to the water's edge without scooting down the sides of those tilting-toward-the-river boulders a good distance on my butt, all the while wondering if I'll be able to get back up.
These rocks are classic granite and you can't get a much more serious rock than a granite one.
Whenever I sit for a rest and there's a granite rock nearby with a freshly broken face, I like to use my handlens to see the rock's crystalline structure. It's easy to identify the quartz, feldspar, mica and other minerals comprising the rock.
Individual quartz crystals are hexagonal and transparent. You can see a nice one my Natchez friend Karen found in an Arkansas mine at the top of the page at www.backyardnature.net/g/minerals.htm . In granite rocks usually quartz shows up as a flat, shiny surface of clear, glasslike substance. The flat surface is a crystalline face. In some granites the face might be as large as a thumbnail or larger. Orthoclase feldspar appears as an opaque, white mass. If you see tiny, very thin, black layers like black book-pages stuck together, it's biotite mica. Identifying the other black minerals is harder. You can see a close-up of a granite rock at the bottom of my page at www.backyardnature.net/g/rox-ign.htm
Granite is considered an " intrusive igneous rock." That means that once it was molten magma deep within the earth, but it never erupted to be lava. By definition, magma becomes lava when it's erupted. When magma or lava cool, the minerals making up the molten soup begin crystallizing. The slower a magma cools, the larger its crystals grow. Thus when you see a granite with relatively small "grain" you know that it cooled fairly fast. It must have been a thin layer intruded between colder strata, or close to the earth's surface. Granite with very large crystal faces and bodies of feldspar were surely formed deep within the earth where it took millions of years to cool off.
The name for very coarse-grained granite is pegmatite. Some pegmatites may have grains several feet across. The granite around the house is fairly fine grained.
The Sierra Nevadas display some magnificent granitic domes of a type seldom found elsewhere in the world. I hope to be writing about them later when the snowfields above us melt and I can strap on my backpack and wander a while.