Geologists recognize three major classifications of rock. One category is igneous rock; formed from molten material from deep beneath the earth’s surface, known as magma. Examples of igneous rock would include granite, basalt, and pumice.
Sedimentary rocks like limestone, sandstone, conglomerate and mudstone are formed when sediment is deposited in deep layers or beds. Usually these beds are formed by deposits gathered by water, but may also be built by wind and ice activity. Sedimentary beds are the medium in which almost all fossils are found.
Gneiss is one of the planets most abundant rocks, forming many of the Earth’s enormous rock shields, an example of which is the Canadian Shield which covers much of the arctic. The oldest rocks on Earth, at least on the surface where they can be studied, are also Gneiss formations, like those found in northern Greenland. Gneiss is an example of the third category of rock known as metamorphic rock.
Metamorphic means simply “changed in form”, and that perfectly describes rocks like gneiss and schist and the other metamorphic rocks, they have been changed into their present form from igneous or sedimentary rock, or now and then from pre existing metamorphic rocks. These raw materials which in the end become metamorphic rock are known as protoliths.
The agents for change are heat and pressure. When heat acts on rock at temperatures over 200 C and this continues over time and pressures of over 1500 bars join in, chemical changes begin to occur that change the essential nature of rock. Combinations of pressure and heat lead to different forms of metamorphoses and therefore to different kinds of rock, but the heat can never be hot enough to actually melt the protoliths or the result will be merely another igneous rock.
There are grades of metamorphoses. Gneiss is a high grade metamorphic, formed under very high temperatures and pressures. Schist, another common metamorphic rock is mid grade metamorphic, formed under less dramatic circumstances and slate, which is formed from shale, is formed under relatively low heat and pressure for a geologic process and is considered a low grade metamorphic. Low grade metamorphic rocks may still contain remnants of fossils if the original sedimentary rock contained them, but they will be distorted and generally useless for study.
Heat and pressure cause different forms of crystallization in metamorphic rocks, in the case of slate these crystals line up in layers easily split from one another creating flat, smooth surfaces, making this stone very valuable not only for pool table to but for a myriad of building applications including roofing, paving and flooring.
Gneiss needs great heat and pressure for the necessary chemical and crystallization processes to take place. There are three processes that can bring these conditions about.
A pluton is an intrusion of molten rock or magma rising to the surface from great depth. As it rises it heats the rock above it and adjacent to it while also adding to pressure as it forces its way through the protolithic materials, the pre existing sedimentary, igneous or older metamorphic rocks. Rocks in immediate contact with the pluton will usually melt and become once more an igneous rock. But rocks at the correct distance will alter and recrystallize.
Very often this recrystallization produces a coarsely grained rock showing layers of quartz, feldspar, mica and amphiboles. This is a classic description of gneiss and frequently this is how gneiss beds are formed.
The great gneiss shields like the Canadian Shield require heat and pressure on a continental scale, and this is provided by plate tectonics. The collision of two massive tectonic plates often results in the subduction of one. Forced beneath the other plate the subducted plate becomes exposed to great heat and enormous pressure, resulting in metamorphoses; huge gneiss beds are created to reemerge perhaps a billion years in the future as some of the oldest rocks on the planet.
The third method by which metamorphics in general and gneiss in particular may be formed occurs when over vast periods of time beds of sedimentary or igneous rocks are pressed deeper and deeper by the weight of continued sedimentation or volcanic flow, or both. At some point the correct combination of heat and pressure will develop and a metamorphic bed will be created and perhaps transition through slate, schist and then to gneiss as the process upgrades.
Gneiss may be formed from a number of protoliths and by a variety of processes, but the result will always be a strong, coarsely grained rock with pronounced bands of light and dark minerals; the feldspars, micas and so forth mentioned earlier. While the banding is generally flat distortion may alter the bands into lively swirls, when polished these surfaces become quite attractive.
Since there is an abundance of gneiss it is fortunate that there are many commercial applications for it. Gneiss is used for road building, monuments, building structure, facades, interior facing, flooring, gravestones, counter tops and work surfaces, to name just a few applications. The Washington monument is built of granite, gneiss and marble, for example. Born in heat and pressure, gneiss is one of our planets more useful and versatile stones.