Photo by Brent Eades.
Display rock 2 is metamorphic gneiss of Precambrian age that was delivered to our area by the last glacier.
A long time ago.
Over billions of years, continental collisions created mountain chains which slowly enlarged the landmass that would become North America. Deep within the mountain roots, volcanic and sedimentary rocks were transformed into metamorphic rocks, and intruded by magma. As the mountains eroded away, their ancient roots were exposed at the Earth’s surface. During the planet’s five ice ages, glaciers broke off some of these ancient rocks and carried them far from their original locations. Along the way they were shaped by mechanical abrasion. Display rock 2 was deposited nearby when the last glacier melted from our area about 11,500 years ago.
What is a gneiss? And how on earth do you pronounce it?
Gneiss is a metamorphic rock that shows a distinct banding but, unlike a schist, has little tendency to split along these planes. The word gneiss comes from an old German word meaning bright or spark: it is pronounced “nice” with a silent g. The composition of gneiss can vary greatly – in fact gneiss essentially refers to the texture of the rock, rather than its composition. Gneiss appears banded, or striped, because of the different proportions of minerals in each band, and the separation of mafic (dark) and felsic (light) minerals. Banding can also be caused by differing grain sizes of the same minerals.
Photo by Julie Lantos.
Take a stroll all around this display rock and look for two kinds of banding. The dark (mafic) minerals (amphibole, mica) have separated from the light (felsic) minerals (feldspar, quartz).
What is a metamorphic rock?
A metamorphic rock is a rock that has been changed by heat and/or pressure (or the introduction of chemically active fluids) but without melting into liquid. This is called a solid-state change. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Metamorphism occurs between 200°C and ~850°C. The original rock (the protolith) may be sedimentary, igneous, or even metamorphic, and can often be identified in spite of these changes.
Marble (limestone), schist (shale), slate (shale) and quartzite (sandstone) are all examples of metamorphic rocks.
Under extreme metamorphism a rock known as migmatite can be formed. It is created when a metamorphic rock such as gneiss partially melts, and then that melt recrystallizes into an igneous rock, creating a mixture of the un-melted metamorphic part with the recrystallized igneous part.
The minerals found within a metamorphic rock can be used as an indicator of the pressure and temperature under which the rock was formed.
How are gneisses formed?
Public domain image of the United States Geological Survey via Wikimedia Commons.
When continents collide mountains are produced. Gneiss is formed deep in the mountain roots.
Gneiss makes up the most of the Earth’s deep continental crust. If you were to drill straight down on pretty much any continent, you would eventually find gneiss. This makes sense, because gneiss is formed by regional metamorphism, during which deeply buried sedimentary or igneous rocks are subjected to high temperatures and pressures. Over the 4.5 billion year life of the earth, tectonic plate movements have caused continents to collide, over and over again. These collisions throw up mountains, such as the present day Himalayas. The high temperatures and pressures under which solid rock can be transformed are created by the tectonic forces that push the continents together, and the tremendous weight of the ensuing mountains.
Where are gneisses found?
As well as deep within the Earth’s crust, gneiss can also be found near the Earth’s surface, including here in Lanark County. This is possible because continental collisions have occurred over and over again, lifting these ancient rocks from their deep burial and exposing them by erosion. Gneiss and other metamorphic rocks make up much of Canada in the geographic area we call the Canadian Shield. (See display rock #3 for further information on the Canadian Shield, and how it formed.)