By: Edward C. Appleyard
When the monolith recently placed in the atrium-to-be of the new Centre for Environmental and Information Technology (CEIT) finally can be viewed in its full glory, there will, no doubt, be many passers-by who will puzzle about its name. Is it pronounced "neece", or "G-neece", or "G-niss", or what? Metamorphic rock terminology is nowhere near as complicated as that of igneous rocks but nevertheless does possess a number of important terms derived from classical language roots or 19thC European sources that are not immediately intelligible. The term gneiss is one of the latter - and by the way, it is pronounced "nice" as the pun in the title indicates.
The majority of metamorphic rock names comprise a textural descriptor, in this case "gneiss", and a compositional descriptor, often something like "granitic" or possibly "migmatitic". There is a problem though, and that is that not everyone uses the term "gneiss" in the same way. (Dumb metamorphic petrologists!) It was apparently first used in 1561 by Agricola to describe the country rocks around the mineral-rich Erzgebirge district in Germany and Czechoslovakia. The problem resides in whether compositional layering of light-coloured (felsic) minerals and dark-coloured (mafic) minerals into alternating bands is an essential property. Those who would deny that mineral segregation layering is essential to a gneiss would apply the term to any foliated, medium to high grade metamorphic rock that is not a schist, i.e. a low to medium grade rock particularly enriched in well-foliated micaceous minerals.
My predilection is that historical usage favours the term "gneiss" for a foliated rock, of high metamorphic grade, consisting of alternating mineralogically distinct (usually felsic and mafic) layers. The recrystallization process which results in the minerals segregating into these layers is invariably the result of strong deformation (strain) resulting from viscous flow of the rock under conditions of very high temperature and pressure. If it is possible to discern the nature of the pre-metamorphic parent rock it is permissible to distinguish "orthogneiss" - derived from an igneous parent, from "paragneiss" - derived from a sedimentary parent. Because the monolith is still cocooned in a protective wrapping as the building rises around it, I can't study it closely enough right now to say to which of these two classes it belongs.
The compositional modifier is easier to explain. "Granitic", used sensu lato, means an assemblage predominantly of quartz and feldspar with much smaller amounts of mafic minerals, especially biotite mica, amphibole, maybe pyroxene, and frequently very minor amounts of minerals like garnet, sphene, etc. The term "migmatitic", on the other hand, is derived from the Greek word for "mixture" and is used for rocks in which a metamorphic-looking component is combined intimately with an igneous-looking component. These rocks are most commonly the result of partial melting of the original rock under conditions of very high metamorphic temperatures. The igneous-looking phase is usually "granitic", so our granitic gneiss may also be migmatitic.
During regional metamorphism such as occurs typically in mountain belts during the mountain-building episode (orogeny) there is always intensive deformation. Under the high temperatures and pressures such as exist within the core of such belts, the rocks deform, not as rigid, brittle objects that we associate them to be under surface PÐT conditions, but rather by flowing in a viscous fashion. The layering and foliation that exists in the gneisses will commonly be deformed subsequently. Thus we may observe folds, sometimes of quite complex form; nice examples are present in the CEIT building monolith.
Now, what about this particular monolith? What do we know about its geological history?
Its home for the past billion years (more-or-less!) has been with rocks that now are exposed in an outcrop north of the French River in Bigwood Township, south of Sudbury. This is an area that occurs within the Grenville Tectonic Province of the Canadian Precambrian Shield. The Grenville Province comprises the roots of a collisional orogenic (mountain) belt that is exposed along the southeastern margin of the Canadian Shield from the east coast of Georgian Bay as far as the Labrador coast. In reality, the Grenville rocks go much farther than that and have been identified in southern Norway and southwestern Sweden before they dive under younger rocks again. In the other direction, they are known to extend in the subsurface at least as far as the Sierra Madre mountains in Mexico. Some folk even contend that the same fold-belt can be found in Antarctica and possibly even on the southeastern margin of Africa. The ponderous pavane of plate tectonics certainly frees up the imagination!
The Grenville mountains were formed around a billion years ago when some other wandering continent collided with the eastern flank of the Laurentian Craton, i.e. proto-North America. The latest idea is that this bulldozing bully may have been the western flank of what later became South America! As a consequence of this head-butting, the Grenville mountains were thrust up in a series of stages into a mountain range that would have resembled, perhaps even rivaled, the contemporary Alpine/Himalayan chain. This happened between 1.300 Ga (Giga-years - i.e. 1.3 billion years ago) and 0.950 Ga (i.e. 950 million years ago).
The French River gneisses appear originally to have been part of the margin of the Laurentian craton (read continental core) and had been emplaced during an even earlier magmatic/metamorphic event sometime between 1.900 and 1.450 Ga (billion years ago). We would have to date our monolith radiometrically to determine when within this rather large range our rock was born. However, its rest was not peaceful for once the Grenville collision occurred it was heated and deformed under the combined pressure of the colliding continent and the weight of perhaps as much as 25 km of superincumbent rocks. It was during this protracted orogenic stage that our gneiss became, gneissic, migmatitic and folded, all more-or-less at the same time.
To human eyes young mountain ranges appear as prominent upward thrusting elements of topography, but like an iceberg their greatest mass is really below the surface and extends downward as a great root into the upper mantle and, to use another analogy, when the superstructure of the range is reduced by weathering, erosion and transport of sediment away from the range, the whole belt, root and all, rise buoyantly, like a ship being emptied of its cargo. Thus, over perhaps hundreds of millions of year, the Grenville rocks rose inexorably upward in response to surface unloading and the French River gneisses came closer and closer to the surface, cooling all the while. It would appear that they were not far from their present crustal level as the Precambrian Eon turned into the Phanerozoic and the pages on the calendars flipped over to the Cambrian period. The final event, perhaps, was the passage overhead of the great Pleistocene ice-sheets which scraped away the final few centimetres off the surface and the gneisses finally saw the light of day.
So, should you ever pass through the atrium of Earth Science's soon-to-be new home, pause and cast your eyes at the oldest member of the department, standing there proudly. Tip your hat to it and say, "What a nice gneiss - and have a good day!".