Thursday, April 6, 2017

The Granite Industry, Part 1 of 6 - Granite

By exposing a window into the rocky crust of the earth, the actions of ocean, glaciers, and industry have made Halibut Point fascinating to geologists and casual visitors alike. Who can resist the sense of titanic forces composing the ground we stand on?

The Babson Farm Quarry, Halibut Point
For all this to be possible requires an immense story over a span of time that we can enumerate but not comprehend. In similar terms we regard the longevity of stone itself.

On Cape Ann, if it's stone, it's granite. The granite solidified deep in the earth's crust from melted minerals. It was gradually brought to the surface over hundreds of millions of years by a combination of (1) the erosion away of miles-thick stone crust above it, and (2) the drifting, collision, and subduction of entire continents over the same molten core from which the granite, as well as other igneous stones, was forged. The field of geology endeavors to explain why various types of stone exist and why granite predominates here almost exclusively. *

Sheets of granite along the Halibut Point shoreline
Man's utilization of granite on Cape Ann began with the accessible boulders sitting above ground, relatively easy to split and move for local building purposes. Commercial potential expanded with the improvement of tools in the nineteenth century, with the exploitation of exposed ledges, and with the capability to ship bulky cargos over water. These conditions existed at many places along the shoreline. Natural sheeting and jointing made work easier but over-exposure to sunlight, frost, and wave forces could compromise the strength of the stone.

Halibut Point ledges drilled to produce dimension stone
When flat, durable paving stones began to replace rounded cobblestones on city streets in the 1840s, Cape Ann was in a good position to meet the market. Small-scale operators cut blocks from ledges, divided them into paving units, and carted them to protected shipping points.

Natural jointing, supplemented by drilling at the quarry
Cape Ann granite formed under tremendous heat and pressure miles below the earth's surface. Relatively recently it experienced glaciations with the crush of two thousand feet of ice. As these weights were relieved by erosion and climate change the compressed stone expanded upward, cleaving into horizontal layers. Tectonic movements like continental drift and earthquakes added further joints along lines of vertical shearing.
Quarrymen developed the ability to drill and blast downward in quest of larger pieces of useful stone. The natural fractures facilitated removal of material for certain dimensions and purposes. But to obtain massive blocks of high quality granite they usually had to excavate at least twenty or thirty feet in depth.

At its best granite features straight, consistent grain desirable for strength and workability. Cape Ann granite's relatively high quartz content and large crystal size - the result of a long, slow cooling of the magma deep in the earth - impart great resistance to compression and abrasion, the forces of street traffic. These qualities make it less ideal for monument carvers who prefer fine-grained stone. And Cape Ann granite is not economical for the sawing and hydraulic splitting operations of today's mass-produced, softer curbstone. Granite deposits may be chemically similar but vary in their structure because circumstances gave them greater or lesser time to form crystals.

A felsic dike revealed in the quarry wall
Silica, the predominant element, unites with oxygen, aluminum, potassium and sodium to form quartz, feldspars and mica minerals that characterize granite. Many factors relating to source material, reactivity and temperature govern these processes. Calcium and metals such as iron and magnesium prevalent at the earth's core with higher melting points, define the composition of other igneous rocks. If added in to granitic magma they create 'hybridizations' or 'impurities' that make mineralogy interesting. 

Exotic elements may intrude either while the rock is still plastic (warm) or through a fracture in solidified stone. A noticeable example exists on the western wall of the Babson Farm Quarry, where a dark gray band contrasts with its surrounds. Both are felsic granite, meaning feldspar-based, with traces of other minerals accounting for color differences. The intrusion of the thin zone of secondary magma into the already-cooled granite meant that it crystallized fairly quickly, resulting in its finer-grained texture.

In a 1920s brochure the Rockport Granite Company described its product as The King of Rocks resembling in composition "the Egyptian granite of which the ancient obelisks and sarcophagi were built. It is strong beyond all possible requirements..." The company offered various shades of gray; 'sea green' especially beautiful in polished finish; and 'Moose A Bec Red' from its quarry in Jonesport, Maine. It also supplied rusty shades stained by dissolved iron running through joints in the stone. "Unlike most seam face granites, the color runs deep into the stone and it can therefore be used for headers, sills, steps, etc. as well as for ashlar surfaces."
The Luxor Obelisk, Paris 2016
Ancient Egyptian rulers desiring the most impressive and permanent monuments directed obelisks of granite weighing up to 120 tons. Three millennia later the inscriptions are still legible. How they were able to quarry, transport, carve and erect these enormous monoliths stupefies modern minds. But aiming at immortality, they chose the best material. 

* Dr. Martin E. Ross provides useful explanations and field guides in Cape Ann, Its Physical and Environmental Geology, 2015.

For another glimpse of the geologic complexities of Cape Ann see online:
THE CAPE ANN PLUTONIC SUITE: A FIELD TRIP FOR PETROLOGY CLASSES,by John B. Brady, Smith College, and John T. Cheney, Amherst College

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