What is the Canadian Shield?

Please refer to display rock 2 for information on gneiss.

Gneiss-3

 Photo by Julie Lantos.

Display rock 3 is Precambrian age gneiss with near-vertical foliation.  Foliation is the segregation of different minerals into layers.

When you take a close look at this display rock, you’ll see wide bands created by the separation of the felsic layers, with pink feldspar, from the dark mafic bands.  Under intense heat and pressure, the gneiss was folded into “books” . The bands formed perpendicular to the direction of pressure.  This rock was formed deep in the roots of an ancient mountain chain.

Spec-3
Diagrams of folding courtesy of the New Mexico Geological Survey.

The shape of the continents we see today is the result of billions of years of geological activity. Continental and oceanic crust has been both created and destroyed by the constant motion and jostling of the tectonic plates that make up the Earth’s crust.

Spec3-a
Public domain image of the United States Geological Service via Wikimedia Commons.

The North American craton is the ancient core of the continent. 

Shield
Public domain image of the United States Geological Service via Wikimedia Commons.

The ancient cores of the world’s continents are coloured pink and orange on this geological map.

Cratons are made up of older continental fragments that were fused together. The Canadian Shield is the exposed portion of the North American craton.  The history of its seven geological provinces is a complex story of microcontinent collisions and mountain building over the past 3.6 billion years that slowly enlarged the ancestral landmass that would become North America.  Deep within the roots of the mountain chains, under conditions of intense heat and pressure, 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.

Shield-2
Public domain image of the United States Geological Service via Wikimedia Commons.

The Precambrian age rocks of the Canadian Shield that encircle Hudson Bay are coloured orange and ochre on this geological map.

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