Page 425 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 13 Tectonics, Earthquakes, and Volcanism 389
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Terrane Formation
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North American Terranes
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(a) Wrangell terranes occur in four segments, highlighted in red among the outlines of other terranes along the western margin of North America.
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(b) The snow-covered Wrangell Mountains stretch from east-central Alaska across the Canadian border. Mount McKinley (Denali), the highest elevation in North America at 6194 m, is a granite batholith surrounded by accreted terranes of varying age. McKinley is relatively young by comparison and is still gaining height as a result of plate convergence.
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▲Figure 13.6 North american terranes. [(a) Based on data from USgS. (b) Terra MODIS image, NASA/gSFC.]
contains 50%–75% silica and aluminium. These give the melt a high-viscosity (thick) texture, and therefore it tends to block and plug conduits to the surface.
Bodies of such silica-rich magma may reach the sur- face in explosive volcanic eruptions, or they may stop short and become subsurface intrusive bodies in the crust, cooling slowly to form granitic crystalline plu- tons such as batholiths (see Figure 12.6). As noted ear- lier, their composition is quite different from that of the magma that rises directly from the asthenosphere at sea- floor spreading centres.
Each of Earth’s major lithospheric plates actually is a collage of many crustal pieces acquired from a variety of sources. Over time, slowly migrating fragments of ocean floor, curving chains (or arcs) of volcanic islands, and pieces of crust from other continents all have been forced against the edges of continental shields and platforms. These varied crustal pieces that have become attached, or accreted, to the plates are called terranes (not to be con- fused with terrain, which refers to the topography of a tract of land). Such displaced terranes, sometimes called mi- croplate, or exotic, terranes, have histories different from those of the continents that capture them. They are usually framed by fault-zone fractures and differ in rock composi- tion and structure from their new continental homes.
In the regions surrounding the Pacific Ocean, ac- creted terranes are particularly prevalent. At least 25% of the growth of western North America can be attributed to the accretion of at least 50 terranes since the early Ju- rassic Period (190 million years ago). A good example is the Wrangell Mountains, which lie just east of Prince William Sound and the city of Valdez, Alaska. The Wran- gellia terranes—a former volcanic island arc and associ- ated marine sediments from near the equator—migrated approximately 10000 km to form the Wrangell Moun- tains and three other distinct formations along the west- ern margin of the continent (Figure 13.6).
The Appalachian Mountains, extending from Ala- bama to the Maritime Provinces of Canada, possess bits of land once attached to ancient Europe, Africa, South America, Antarctica, and various oceanic islands. The discovery of terranes, which occurred as recently as the 1980s, revealed one of the ways continents are assembled.
Crustal Deformation
Rocks, whether igneous, sedimentary, or metamorphic, are subjected to powerful stress by tectonic forces, grav- ity, and the weight of overlying rocks. Stress is any force
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