By Larry Larason
It’s not unusual to hear the sounds of construction or renovation in my neighborhood during good weather. But a few years ago the sound of a jackhammer lured me outside to see what was going on. My neighbor was having a tree planted, and the workmen had encountered bed rock. They were chipping away at it with the jackhammer.
Over the fence my neighbor told me, “They hit granite!” Well, I have the same rocks in my yard and had chipped at the same layer to plant things. So, feeling pedantic, I explained to him, “There’s no granite here in Gallup. It’s just tough sandstone.”
I suppose some people like my neighbor assume that granite just means any hard rock. But to a geologist the term is much more specific. Granite is an igneous rock that cooled and crystallized below the surface of the Earth and consists of a mix of quartz, feldspars, and mica. There may be other minerals, as well, including hornblende and zircon, but the first three are the most important ones.
Where does granite come from? Granite is about 70 per cent silica; that means it formed as magma from melted crustal rocks. The magma cools slowly beneath the surface [intrusive], allowing mineral crystals to form. If the magma erupts at the surface [extrusive], it cools too fast to crystallize and can take various forms, which we classify by texture. The same sort of magma can create rocks with quite different appearance, although, chemically, they are the same.
Pumice is volcanic froth. When it erupted it was full of gas that escaped as it rapidly cooled. Often the resulting stone, which resembles a sponge, is light enough to float on water, at least until it becomes water logged. The stone is glassy and brittle, and is easily abraded to sand sized particles.
Volcanic glass is called obsidian. It probably erupted slowly, sort of like a flow of molasses. Obsidian contains little water or other volatile components, so it is compact. Obsidian is usually black to dark gray. It was highly prized during prehistory for making tools and weapons. A chipped blade of obsidian has a sharper edge than a steel medical scalpel, and a few surgeons use obsidian blades. However, such blades are not approved by the FDA because they are brittle and can fracture under lateral pressure. Also obsidian scalpels cannot be mass produced.
If obsidian holds water at the time of eruption, it becomes the light gray rock known as perlite. Perlite is mined for several industrial uses, including insulation. Because it is hydrated, it can be popped like popcorn. This expanded perlite is used by gardeners. As a soil amendment it aids moisture retention and prevents soil compaction. Perlite was mined on Grants Ridge during the 1990s. The deposit there contained blobs of obsidian called Apache tears; rockhounds search the field at the base of the ridge to pick up those that eroded out. Another major perlite mine in New Mexico is near Socorro.
The next form of extrusive granitic magma is rhyolite. The silica in the magma makes it very viscous, so it tends to explode rather than flow. When it explodes the debris forms tuff. The homes carved into cliffs at Bandelier National Monument were worked in rhyolite tuff, which resulted from an explosive eruption of Valles Caldera. Another explosive eruption in the Four Corners happened about 27 million years ago. The La Garita Volcano created a caldera roughly 22 x 47 miles in size between Creede and Pagosa Springs, Colorado. Over the course of 3 to 4 days La Garita ejected 1,200 cubic miles of rhyolitic ash, which drifted as far east as the Atlantic Ocean.
When rhyolite flows, it is often pretty, with swirly patterns. Such stones are occasionally used in jewelry. Orbicular rhyolite is one of my favorites. It has spherules that are usually multi-colored and give it the name bird’s eye rhyolite. No one is quite sure how the spherules form. It doesn’t polish as well as agate, but it is quite striking.
Granite forms deep in the Earth. As it slowly cools, minerals begin to separate out of the melt and form crystals. These crystals bump into one another, so they are not perfectly shaped. The minerals are quartz [clear or white], two feldspars, which give the stone most of its color, and spots or specks of black mica [biotite] and/or another dark mineral, such as hornblende. All of these minerals are silicates and contribute to the high silica makeup of granite.
If you watch HGTV you know that granite counter tops are very much in vogue these days. Much of what is called granite in home decoration is not truly granite. It may be some other variety of crystalline igneous rock, or even, in some cases, a sedimentary stone. The other night on the International House Hunters show I saw a realtor point out a “marble counter,” but what he was directing attention to was clearly granite. [Marble is metamorphic limestone]. We have nice small-grained pink granite in the Zuni Mountains, but you won’t see it on any kitchen countertops, because during the two uplifts the Zunis have experienced most of the granite became fractured. It would be hard to find solid pieces large enough to cut into countertop size. It could be cut into tiles, but someone would have to quarry it.
The last in the series of granitic rocks is pegmatite. This is where things get weird and wonderful! Pegmatite cools very slowly and the individual crystals grow to half an inch or larger. Most pegmatite occurs in veins or dikes, but sometimes it forms “pods,” usually within granite or metamorphic rocks. The process is poorly understood, but some sort of chemical enrichment occurs as the pods form. Minerals that might be only a speck in granite appear large in pegmatites, and some of the crystals are huge. One example: A record crystal of spodumene, a rare lithium aluminum silicate, nearly 50 feet long was found in a mine in the Black Hills of South Dakota. In the Harding Pegmatite Mine near Dixon, New Mexico spodumene crystals in the range of 20 to 30 feet in length appear to be stacked like jack straws in the mine’s wall. The Harding Mine may be best known for its pink and purple “sissy rocks” [muscovite and lepidolite]. These pods are economically important because of the many minerals found in them, including several precious gems. More than 65 different minerals were found in the Harding pegmatite, including massive deposits of beryl.