The Canadian Shield–also called the Laurentian Plateau, or
Bouclier canadien (French)–is a massive geological shield
covered by a thin layer of rocks that forms the nucleus of the
North American or Laurentia craton. It is an area mainly covered by
igneous rock which relates to its long volcanic history. It has a
deep, common, joined bedrock region in eastern and central Canada
and stretches North from the Great Lakes to the Arctic Ocean,
covering over half of Canada; it also extends south into the
northern reaches of the United States. Population is scarce, and
industrial development is minimal, although the region has a large
hydroelectric power potential.
The Canadian shield is a physiographic division, consisting of
five smaller physiographic provinces, the Laurentian Upland, Kazan
Region, Davis, Hudson, and James. The shield extends into the
United States as the Adirondack Mountains (connected by the
Frontenac Axis) and the Superior Upland. The Canadian Shield is
U-shaped, but almost semi-circular, which yields an appearance of a
warrior's shield or a giant doughnut, and is a subsection of the
Laurentia craton signifying the area of greatest glacial impact
(scraping down to bare rock) creating the thin soils.
The Canadian Shield is a collage of Archean plates and accreted
juvenile arc terranes and sedimentary basins of Proterozoic age
that were progressively amalgamated during the interval 2.45 to
1.24 Ga, with the most substantial growth period occurring during
the Trans-Hudson orogeny, between ca. 1.90 to 1.80 Ga. The Canadian
Shield was the first part of North America to be permanently
elevated above sea level and has remained almost wholly untouched
by successive encroachments of the sea upon the continent. It is
the Earth's greatest area of exposed Archaean rock. The metamorphic
base rocks are mostly from the Precambrian Era (between 4.5 billion
and 540 million years ago), and have been repeatedly uplifted and
eroded. Today it consists largely of an area of low relief 300 to
610 m (980 to 2,000 ft) above sea level with a few monadnocks and
low mountain ranges (including the Torngat and Laurentian
Mountains) probably eroded from the plateau during the Cenozoic
era. During the Pleistocene epoch, continental ice sheets depressed
the land surface (see Hudson Bay), scooped out thousands of lake
basins, and carried away much of the region's soil.
When the Greenland section is included, the Shield is
approximately circular bounded on the northeast by the northeast
edge of Greenland, with Hudson Bay in the middle. It covers much of
Greenland, Labrador, most of Quebec north of the St. Lawrence
River, much of Ontario including northern sections of the southern
peninsula between the Great Lakes, the Adirondack Mountains of
northern New York, the northernmost part of Lower Michigan and all
of Upper Michigan, northern Wisconsin, northeastern Minnesota, the
central/northern portions of Manitoba away from Hudson Bay, the
Great Plains, northern Saskatchewan, a small portion of
northeastern Alberta, and the mainland northern Canadian
territories to the east of a line extended north from the
Saskatchewan/Alberta border (Northwest Territories and Nunavut). In
total, it covers approximately 8,000,000 km2 (3,088,817 sq mi). It
covers even more area and stretches to the Western Cordillera in
the west and Appalachians in the east, but the formations are still
underground. The underlying rock structure does include Hudson Bay
and the submerged area between North America and Greenland.
The multitude of rivers and lakes in the entire region is caused
by the watersheds of the area being so young and in a state of
sorting themselves out with the added effect of post-glacial
rebound. The Shield was originally an area of very large mountains
(about 12,000 metres or 39,000 ft) with much volcanic activity, but
over the millennia the area was eroded to its current topographic
appearance of relatively low relief. It contains some of the most
ancient volcanoes on Earth. It has over 150 volcanic belts (now
deformed and eroded down to nearly flat plains) that range from 600
to 1200 million years old.
Each belt probably grew by the coalescence of accumulations
erupted from numerous vents, making the tally of volcanoes in the
hundreds. Many of Canada's major ore deposits are associated with
Precambrian volcanoes.
The Sturgeon Lake Caldera in Kenora District, Ontario is one of
the world's best preserved mineralized Neoarchean caldera
complexes, which is some 2.7 billion years old. The Canadian Shield
also contains the Mackenzie dike swarm, which is the largest dike
swarm known on Earth.
Mountains have deep roots and float on the denser mantle much
like an iceberg at sea. As mountains erode, their roots rise and
are eroded in turn. The rocks that now form the surface of the
Shield were once far below the Earth's surface.
The high pressures and temperatures at those depths provided
ideal conditions for mineralization. Although these mountains are
now heavily eroded, many large mountains still exist in Canada's
far north called the Arctic Cordillera. This is a vast deeply
dissected mountain range, stretching from northernmost Ellesmere
Island to the northernmost tip of Labrador. The range's highest
peak is Nunavut's Barbeau Peak at 2,616 metres (8,583 ft) above sea
level. Precambrian rock is the major component of the bedrock.
The North American craton is the bedrock forming the heart of
the North American continent and the Canadian Shield is the largest
exposed part of the craton's bedrock.
The Canadian Shield is part of an ancient continent called
Arctica, which was formed about 2.5 billion years ago, during the
Neoarchean era. It was split into Greenland, Laurentia, Scotland,
Siberia, East Antarctica and is now roughly situated in the Arctic
around the current North Pole.
To claim credit for this cache you will need to answer the
following questions:
- Using and analog GPS (ie a compass), hold it my the large rock
and record its behavior. What do you think causes this?
- Estimate the dimensions of the rock.
- What is your current elevation?
- What evidence of glaciation do you see in the surrounding
lands?
- Post a picture at ground zero.
