Friday, January 9, 2015

What a Geologist Sees - Part 7


To many geologists, rock quarries are playgrounds, especially if there is more than one type of rock present. Unfortunately, because of today's litigious society, it is difficult to gain access to quarries and to do it without the constraints of a tour group is even more difficult.

My Dad and I initially visited this local "road gravel" quarry about 1975, when you could just drive in there on a Sunday and as long as you stayed away from the machinery and the vertical quarry walls, you were OK.

By the time I went back to do some field work for my "undergrad thesis", during the Summer of 1976, I had to beg for a release form and then I was only allowed about 50 minutes, covering two different visits. Nowadays, the quarry is a very popular place for tours (every Thursday) and because of the popularity with school groups, you have to make reservations about 6 months in advance and for some reason, they don't allow photographs. Hmm. [This photo was taken in 1976.]

The light-colored rock is a metamorphosed granite, called a "gneiss" and it consists of quartz, two different feldspars, and two different micas, along with a host of minor accessory and trace minerals. When rocks are metamorphosed over a large area, this is called "regional metamorphism".

The black colored rock is a diabase dike. Diabase is a dark-colored, fine-grained igneous rock, similar to basalts that one sees in lava flows in Hawaii, Iceland, New Mexico, Idaho, and elsewhere. It is largely composed of calcium plagioclase feldspar, pyroxene, and olivine.

An igneous dike is a tabular (flat) body of rock that intrudes and cuts-across pre-existing rock and local structures. A tabular body of igneous rock that is parallel to local structures is a sill.

The relationship between the two rock bodies is termed a "cross-cutting relationship", wherein the dike cuts-across the pre-existing rock, thus the dike is the younger rock, even if we do not know the absolute (radiometric) ages of either rock body. In the 1700's, James Hutton recognized this concept.

When this dike (about 3 feet wide) was intruded into the gneiss "country rock" (presumably related to the fracturing of the crust during the rifting of Pangea), the heat of the intrusion triggered some minor changes to the gneiss through "contact metamorphism". This was a "dry intrusion", i.e., it didn't contain much pressurized water, so the "zone of contact metamorphism" in the gneiss - adjacent to the dike - is only about 5 - 6 inches wide. Pressurized water helps ions move around, triggering more mineral changes.

So, with the small width of the intrusion and the paucity of pressurized water, the changes to the gneiss were rather minor. These diabase dikes are common in the Piedmont of Georgia, South Carolina and North Carolina. They can range in width from 3 inches wide in this quarry to more than 1,000 feet in South Carolina. Virtually all of them are oriented NW-SE, cutting across the rest of the regional geologic structures. They are all presumed to have been intruded during the Triassic and Jurassic Periods of the Mesozoic Era (the age of the dinosaurs).

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