Page 406 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

370 part III The Earth–Atmosphere Interface
F cus Study 12.1 Natural Hazards Geomorphic Belts and Exotic Terranes in Western Canada
The Western Cordillera is subdivided into five roughly parallel geomorphic belts aligned with a northwest– southeast trend (Figure 12.1.1). Distinc- tive combinations of bedrock geology and landforms characterise each belt. Moving from east to west, the belts
are Foreland, Omineca, Intermontane, Coast, and Insular. Contained within each belt are various exotic terranes that have been captured by continen- tal North America. (Exotic terranes are pieces of crust that have a history dif- ferent from the continent that captured them; this concept is explained further in Chapter 13.)
The Rocky Mountains represent the Foreland Belt. The belt is underlain by an immense thickness of Precambrian and Phanerozoic sedimentary rocks, depos- ited offshore of the old North American craton that, during mountain building between 100 and 55 million years ago, were folded and thrust eastward for at least 150 km onto the edge of the old continent.
The Omineca Belt derives its name from the Omineca Mountains of central British Columbia. This belt also includes the Purcell, Selkirk, Columbia, Monashee, and Cariboo mountain ranges of south- ern British Columbia. Metamorphic rocks
with lesser amounts of granitic rocks dominate the bedrock ge- ology of the Omineca Belt. The metamorphic rocks are com- plexly folded and faulted, and represent the roots of an ancient mountain chain that formed between 180 and 60 m.y.a.
High plateaux and deep
river valleys characterise
the landscapes of the Intermon- tane Belt. This belt is underlain by a variety of Paleozoic and Mesozoic volcanic, granitic, and sedimentary rocks. The pres- ence of volcanic and granitic rocks in the Intermontane Belt, the intense metamorphism of these same rocks represented in the Omineca Belt, and the folding and thrust faulting of sedimentary rocks in the Fore- land Belt, together record the initial phase of mountain build- ing (180 to 100 m.y.a.) associated with the accretion of exotic terranes along the western margin of the North American craton.
The Coast Belt includes the Coast and Cascade mountains and the Fraser Lowland near
Alaska
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Beaufort Sea
Great Bear Lake
Northwest
Territories
Great Slave Lake
OMINECA BELT
INTERMONTANE BELT COAST BELT
I
N
F
O
S
R
U
E
L
L
A
A
R
N
B
D
E
B
L
E
T
L
T
The features that form this “ring” are caused by the subducting edge of the Pacific plate as it thrusts deep into the crust and mantle and produces molten mate- rial that makes its way back toward the surface. The up- welling magma forms active volcanoes along the Pacific Rim. Such processes occur at similar subduction zones throughout the world.
Hot Spots
As mentioned, volcanic activity is often associated with plate boundaries. However, scientists have found an
estimated 50 to 100 active sites of upwelling material that exist independent of plate boundaries. These hot spots (or hot-spot volcanoes) are places where plumes of magma rise from the mantle, producing volcanic activity as well as thermal effects in the groundwater and crust. Some of these sites produce enough heat from Earth’s in- terior, or geothermal energy, to be developed for human uses, as discussed in Focus Study 12.2. Hot spots occur beneath both oceanic and continental crust. Some hot spots are anchored deep in the lower mantle, tending to remain fixed relative to migrating plates; others appear to be above plumes that move by themselves or shift with
Georeport 12.4 Spreading Along the East Pacific Rise
The fastest rate of seafloor spreading on Earth occurs along the East Pacific Rise, which runs roughly north–south along the eastern edge of the Pacific plate from near Antarctica to North America. Spreading is occurring at a rate of 6 to
16 cm per year, depending on location. For perspective, human fingernails grow at a rate of about 4 cm per year.
60
55
PACIFIC 50OCEAN
130
Yukon
▲Figure 12.1.1 Five principal geomorphic belts of western Canada. [Map adapted courtesy of LITHO- PROBE Secretariat, The University of British Columbia.]
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250 500 KILOMETRES
British Columbia
Alberta