ASK-A-GEOLOGIST by Jeff Wynn

MORE on rocks, the building blocks (pun intended) of Geology

The name “Metamorphic” derives from Greek, and was used first sometime before 1810. It comes from a word meaning “after” or “beyond” (meta), plus a word meaning “shape” (morph). Old-time miners described these rocks as looking “stewed and cooked” — because they sort of look that way, like molasses and water stirred together. They form from igneous, sedimentary, or even other metamorphic rocks during deep burial in the Earth, where pressure and heat can cause the rocks to deform plastically. A “gneiss” is one example — a coarse-grained rock that frequently has the original sedimentary layers still crudely visible in it… if you step back far enough. A “schist” on the other hand may be finely layered from original freshwater mud or seafloor dead-creature organic goop, and muscovite (white mica) can glitter in the sunlight from all the tiny grains that form in the smeared-sheared, compressed material during the pressure-heat cycle.

Metamorphic rocks are usually classed by the degree of metamorphism — how much the original rock has been compressed, stewed, cooked, and sheared — with gneiss found on the extreme far recrystallized end, and something called “greenschist” on the near, relatively weakly metamorphosed end. Metamorphic rocks are usually moderately ugly-looking — but interesting to the discerning eye of an experienced geologist. They commonly draw the eye to their curious character. Below is a photo of a copper ore sample in gneiss from Alaska.

For more on metamorphic rocks see https://en.wikipedia.org/wiki/Metamorphic_rock.

Twice I climbed Mount Roraima in SE Venezuela. It is a “tepuy” or “tepui”– a cliff-sided mountain on the Brazil-Guyana-Venezuela frontiers that was the inspiration for several movies including “Aracnophobia” and “Avatar.” The 3-km/9,400-ft tall, flat-topped mountain is 95% quartz in the form of quartzite — ancient mostly-quartz sand that has been compressed and welded into an ultra-hard metamorphic rock that looks for all the world like irregular sugar cubes when held in your hand. Most of the plant life on the top is endemic — found nowhere else in the world. Much of that plant life is predatory: Venus fly-traps and forms of Pitcher plant designed to trap insects in this otherwise nourishment-poor sea of quartz.

There is a place in the Catalina Mountains north of Tucson, Arizona, where from the city you can see the original sedimentary layers that make up the front (south end) of the range. If you get up close, however, you can’t see this very well at all — you just see the coarsely recrystallized gneiss, the final product of the metamorphism. There is an even more final product: as you move northward in the range, the depth of burial of this uplifted and tilted block of sediment came up from ever-deeper depths, until the gneiss grades smoothly into a granite. A graduate student (later the geology department chair) was mapping this granite complex and once came across the shape of a crinoid — a 400 million-year-old fossil from the original sedimentary rocks. For geologists, to see all this happening in one place as a continuum of change (or metamorphism) is indescribably cool.

Chalcopyrite ore (the gold color) in a metamorphic gneiss from Alaska.