Maine State Stone
Gneiss is a hard, tough, crystalline metamorphic rock. It varies in composition depending on the parent rock that melts or partially melts to form it; the main types, orthogneiss (metamorphosed igneous rocks) and paragneiss (metamorphosed sedimentary rocks), both occur in Maine.
Gneiss is commonly mistaken for granite, and quarries often sell it as granite. Because the commercial applications of the two rocks are nearly identical, the United States Geological Survey also routinely records production of gneiss under the category "granite." At times, granite intergrades into gneiss, making identification challenging. However, unlike granite, which is massive (uniform in composition) and relatively low in mica, gneiss has larger amounts of mica and is foliated, or oriented in plates. The resulting cleavage planes make gneiss easy to quarry in slabs. Another result of the foliation is a tendency for gneiss to exhibit attractive color striations.
A STATE SYMBOL
While Maine has not designated an official state rock, gneiss has shaped the state’s physical, economic and cultural environment in significant ways. A quick look at a bedrock map of Maine reveals that gneiss is both common and widely distributed in the state. Anyone who enjoys outdoor recreation or visiting historic sites in Maine has seen the local gneiss either in natural settings or in buildings, walls, and paving stones.
Quarrying has long been a major industry in Maine: in 1894, the state’s quarries sold "granite" (much of it really gneiss) valued at $1.55 million (in 1894 dollars). Only Massachusetts brought in more income from granite/gneiss. Maine shipped its rock products coastwise to build cities on the Atlantic seaboard. About 40 percent of the quarries’ product at the time was paving stones, so the Maine gneiss was underfoot in many cities at the turn of the century.
Gneiss, like all metamorphic rocks, forms under the high temperatures and pressures characteristic of mountain building. The same events that built the Appalachian Mountains are responsible for most of the gneiss in Maine. Half a billion years ago, the varied landmasses that would become Maine were still south of the equator, some of them still undersea, some part of other continents that later broke apart. When the tectonic plates carrying these land masses collided, the Appalachian Mountains were thrust up, in the process squeezing and heating many of the original sedimentary, volcanic, or plutonic rocks so that they metamorphosed.
Gneiss is one of the most common rock types on earth, occurring all over the world in places where tectonic collision or subduction has pushed mountains up. Gneiss is the dominant rock in Norway and much of northern Europe, including the Pyrenees and the southern Alps.
USES OF GNEISS
Gneiss is one of the most favored stones for building and monument construction because of its load-bearing capacity, hardness, and resistance to weathering. Its toughness makes it useful for breakwaters, riprap (shoreline protection), and aggregate for industrial applications such as roadbeds.
Stone quarrying remains a major industry in Maine, although the purposes have changed. While we tend to think of stone in its older uses as "dimension stone," cut to shapes for buildings and paving, concrete has largely supplanted this use. Only one quarry in Maine currently supplies cut stone. On the other hand, seven quarries in the state produce an annual 2.2 million tons of crushed "granite" (much of it, of course, really gneiss) valued at $18 million (2006 figures).
Gneiss has been used throughout the world for millennia in monuments and ornamental objects. At least since the third millennium BCE, the vast Chepren’s Quarry in the Nubian Desert furnished Egyptian artisans with gneiss for ornamental objects, funerary vessels, and statues, including a famous life-sized figure of King Chepren.
Chemical Formula: There are a wide variety of chemicals actually found in gneiss
Color: White, colorless, cream, light yellow, light blue, light green, pale red, light brown, gray
|Author: World Trade Press|