Page 436 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

400 part III The Earth–Atmosphere Interface
▲Figure 13.16 Landscape at the northern end of the Appalachian Mountains. The Long Range Mountains in Newfoundland. The upland plateau surfaces represent ancient erosion created by intense weath- ering and fluvial erosion, most likely under the warmer and moister climates of the late Cretaceous and early Tertiary Periods. River valleys dissect the margins of the plateaux. The valleys were initiated as the plateaux were uplifted in preglacial times and progressively deepened by fluvial and glacial erosion. [STRLSW/CP Images.]
and North America (250–300 million years old). The Ap- palachians contrast with the higher mountains of west- ern North America (35–80 million years old). As noted, the complexity of the Alleghany orogeny derives from at least two earlier orogenic cycles of uplift and the accre- tion of several captured terranes.
Each orogenic event was separated by long periods of time during which rock weathering and erosion gradually stripped material from the mountain summits. The end result of these processes was the production of upland plateaux with elevations equal to or greater than 700 m above sea level throughout much of Atlantic Canada, ex- cluding Prince Edward Island.1 Fluvial and glacial ero- sion has created steep-sided valleys that dissect the edge of the upland plateaux (Figure 13.16).
Structure and composition link the mountain ranges separated by the Atlantic Ocean and the Pangaea colli- sion and separation. In fact, the Lesser (or Anti-) Atlas Mountains of Mauritania and northwestern Africa were connected to the Appalachians at some time in the past, but the mountains embedded in the African plate rafted apart from the Appalachians.
The Western Cordillera
The rugged topography that characterises much of this region in western North America developed in response to volcanism and tectonism accompany- ing the accretion of exotic terranes throughout the Mesozoic and early Tertiary Periods (180 to 45 m.y.a.). Mountain peaks commonly exceed elevations of 3000 m above sea level. Mount Logan, its peak the highest point in Canada at an elevation of 5959 m above sea level, occurs within the St. Elias mountain range. The terrain
1A. S. Trenhaile, Geomorphology: A Canadian Perspective (Oxford University Press, 2003).
in this region is composed of a variety of intrusive and extrusive igneous rocks, metamorphic rocks, and sedi- mentary rocks that have experienced folding and faulting.
Pleistocene alpine glaciation widened and deepened river valleys, creating mountain passes and deep fjords. These important breaks in the mountain ridges greatly influenced European exploration of the region in the 18th and 19th centuries. The routes selected for several transcontinental railways in the latter part of the 19th century were guided by this topography.
The Innuitian Mountains
This region occupies the northeastern margin of the Queen Elizabeth Islands and Baffin Island in Nunavut, Canada. Folding and thrust faulting of sedimentary rocks during the Ellesmerian orogeny in the late Paleozoic Pe- riod (Devonian-Pennsylvanian) initiated mountain build- ing. This orogeny also involved the eruption of basaltic lava and the intrusion of granite plutons. Volcanism and folding and thrust faulting of sedimentary rocks during the mid-Cenozoic period Eurekan orogeny characterised a later phase of mountain building. Mountains in this re- gion reach elevations up to 2400 m above sea level.
World Structural Regions
Examine the first two maps in this chapter (the map opening the chapter and Figure 13.3) and you note two vast alpine systems on the continents. In the Western Hemisphere, the Cordilleran system stretches from Tierra del Fuego at the southern tip of South America to the peaks of Alaska, including the relatively young Rocky and Andes Mountains along the western margins of the North and South American plates. In the Eastern Hemisphere, the Eurasian–Himalayan system stretches from the European Alps across Asia to the Pacific Ocean and contains younger and older components.
These mountain systems also are shown on a struc- tural region map as the Alpine system (Figure 13.17). The map defines seven fundamental structural regions that possess distinctive types of landscapes, grouped because of shared physical characteristics. Looking at the distribution of these regions helps summarize the three rock-forming processes (igneous, sedimentary, and metamorphic), plate tectonics, landform origins and construction, and overall orogenesis. As you examine the map, identify the continental shields at the heart of each landmass. Continental platforms composed of younger sedimentary deposits surround these areas.
Earthquakes
Crustal plates do not glide smoothly past one another. Instead, tremendous friction exists along plate boundar- ies. The stress, or force, of plate motion builds strain, or deformation, in the rocks until friction is overcome and the sides along plate boundaries or fault lines sud- denly break loose. The sharp release of energy that occurs at the moment of fracture, producing seismic