Thursday, October 30, 2014


If you've only had local experiences, 'rock' and 'granite' are synonymous. It's hard to distinguish something utterly common when you can't imagine a world without it.
Granite boulder
For the layman the question about granite is, "Compared to what?" It's elemental around here, like air and water. To gain perspective you can do what people with big questions have traditionally done, turn to science and religion. Or you can visit other places, then come back and walk around Halibut Point.
Granite landscape, Halibut Point
In these parts granite refers to bedrock, to masonry, to an historic industry. It connotes rugged durability. Quarrying it, shaping it and shipping it all present challenges,  but once it's in place granite outlasts most other materials.

Folly Cove pier,
the shipment point for Halibut Point granite a century ago
Stone in general is hard and heavy, the moreso if like granite it forms deep within the earth where molten minerals, cooling slowly over eons as they move upward into the crust, crystallize into a dense grainy matrix. Granite gets its name from the Italian word for grainy. Its origin is igneous, 'from fire.' 

Chemically similar blends of magma that cool and petrify more quickly will not develop the combination of strength and grain that characterizes  granite. Compare it to the finer-textured rock below, which may have been brought from afar by a glacier, or eroded from a relatively quick-cooled intrusion within the bedrock.
An exotic boulder on the Halibut Point shoreline
Wikipedia affords us amateurs considerable information about mineralogy, which you can follow to the depth of your choice. I learned that silicates comprise ninety percent of the earth's crust and structure all the important components of granite: quartz, feldspar, mica, and amphibole. These particulates are visible in the enlarged photo below, forming the matrix and grains of the granite background stones.
Boulder details
Silicates are compounds of silicon and oxygen. We are very familiar with oxygen in the form O2 in the air we breathe, and as an essential element of organic life with carbon and hydrogen. Quartz, the hardest component of granite, consists entirely of silicon-oxygen molecules SiO4. Oxygen atoms win the versatility sweepstakes.

Feldspar silicates make up much of the mass of granite as well as shaping its particular 'behavior.' It's the lighter-colored substance in the photo. It may take on pink, gray or brown tints according to its chemical individuality and lend specific characteristics to the stone.
All granites are felsic, relatively rich in elements that form feldspar and quartz: silicon, oxygen, aluminum, sodium, and potassium. Granites can be contrasted with mafic rocks, which are relatively richer in magnesium and iron (ferric). Incorporating denser metals that predominate in the earth's core, mafic rocks such as basalt are heavier than granite. Felsic rocks tend to 'float' above them in the earth's crust, forming the continents.
Quarry walls
All manner of other factors go into shaping granite as we encounter it. A significant presence  of iron stained the stone pictured above. Water seeping through fissures dissolved and oxidized the iron, concentrating it around joint surfaces. Local stoneworkers call it 'sap-faced granite.' 

Granite begins as a monolithic formation far below the earth surface. Cracks and seams develop as conditions change. Incomprehensible pressures increase during tectonic collisions. Conversely pressures decrease when the stone is relieved of miles-thick overburden through millions of years of erosion. The accompanying expansions, contractions, and shifts are expressed in the pattern of joints. According to their placement the joints make the stone more or less desirable for quarrying. 

All granite is not equal. The cakey-looking Chelmsford granite utilized these days for curbstones on our Cape Ann streets - cheaper to saw and split - disintegrates at the first rap of a snowplow.
Mastering the challenges and employing the qualities of Cape Ann granite imparts a satisfaction to tradesmen and sculptors, as well as integrating their artisanship to the native terrain with a cohesion for all to enjoy. Nature's vernacular engenders our own.

1 comment:

  1. Wonderfully informative, with a fine selection of photos! I particularly like the one of the boulder in the embrace of the tree. Thanks for sharing.