Chapter 20 – Geological History
CANADA: A YOUNG NATION, BUT AN OLD COUNTRY
The North American Continent was assembled by plate tectonics processes that brought
together many smaller land messes. When fused and locked together, they have created
the geological mosaic of the present-day continent. The process of continental building has
not been a simple one and has taken more than 4 billion years to accomplish.
Construction of North America began at least 4,000 million years ago with the formation
of Acasta Gneiss of the North-west Territories, which now forms part of the Slave
Province of the Canadian Shield.
The building of North American was essentially complete 65 million years ago, although
the modern landscape is the result of geologically recent glaciations that have occurred in
the last 2.5 million years. The last ice sheet left the southern portions of the country only
12,000 years ago, and finally melted in Labrador 6,000 years ago. Remnants of this vast
ice sheet still survive on Baffin Island today as the Penny and Barnes Ice Caps.
WHAT ARE THE MAIN GEOLOGICAL “BUILIDING BLOCKS” OF NORTH
A Geological Jigsaw
The northern part of the continent, in Canada, is underlain by the exposed part of the
ancient core or craton of North America; this exposed part is called the Canadian Shield
and consists predominantly of very old, Archean and Proterozoic rocks (figure 20.3 A,
B). These rocks range in age from 4 billion to approximately 1 billion years old and are
largely devoid of fossils.
The craton is composed of a complex assemblage of several distinct geologic provinces.
Geologic provinces are broad regions of similar rocks, usually covering many thousands
of square kilometers, with characteristics that differ significantly from rock types present
in adjacent areas. Individual geologic provinces have been subdivided into smaller unites
called sub-provinces, which are fault-bounded unites containing similar rock types,
structures and mineral deposits.
Provinces and sub-provinces are now widely recognized to be terranes. Terranes are
discrete fragments of oceanic or continental material that have been added to a craton at
an active margin by accretion.
These likely originated as small continents and remnants ocean-floor crust, each with its
own complex geological history, and were welded together by plate tectonics processes to
form the North American craton.
The full geographic extent of the craton is not immediately apparent from a map of the
geology of North Americas as its outermost margin s are buried by layer of the younger
“cover rocks” that reach thicknesses of more than 10km.