Page 433 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 13 Tectonics, Earthquakes, and Volcanism 397
LIECHTENSTEIN SWITZERLAND
FRANCE
AUSTRIA
SLOVENIA
CROATIA
GERMANY
▲Figure 13.15 European alps. The Alps are some 1200 km in length, occupying a crescent of 207 000 km2. Complex overturned faults and crustal shortening due to compressional forces occur along convergent plates. Note the snow coverage in this December image. [Terra MODIS image, NASA/gSFC.]
Types of Orogenesis
Three types of tectonic activity cause mountain building along convergent plate margins (illustrated in Geosystems in Action, Figure GIA 13.2). As discussed in Chapter 12, an oceanic plate–continental plate collision produces a subduction zone as the denser oceanic plate dives beneath the continental plate, Figure GIA 13.2a. This convergence creates magma below Earth’s surface that is forced up- ward to become magma intrusions, resulting in granitic plutons and sometimes volcanic activity at the surface. Compressional forces cause the crust to uplift and buckle. This type of convergence is now occurring along the Cor- dilleran mountain system that follows the Pacific coast of the Americas and has formed the Andes, the Sierra Madre of Central America, and the Rockies (Figure GIA 13.1).
An oceanic plate–oceanic plate collision can pro- duce curving belts of mountains called island arcs that rise from the ocean floor. When the plates collide, one is forced beneath the other, creating an oceanic trench. Magma forms at depth and rises upward, erupting as it reaches the ocean bottom and beginning the construction of a volcanic island. As the process continues along the trench, the eruptions and accumulation of volcanic mate- rial form a volcanic island arc (Figure GIA 13.2b). These processes formed the chains of island arcs and volcanoes that range from the southwestern Pacific into the west- ern Pacific, the Philippines, the Kurils, and on through portions of the Aleutians. Some of the arcs are complex, such as Indonesia and Japan, which exhibit surface rock
deformation and metamorphism of rocks and granitic intrusions.
These two types of plate collisions are active around the Pacific Rim, and each is part thermal in nature be- cause the diving plate melts and migrates back toward the surface as molten rock. The region of active volca- noes and earthquakes around the Pacific is known as the circum-Pacific belt or, more popularly, the Ring of Fire.
The third type of orogenesis occurs during a conti- nental plate–continental plate collision. This process is mainly mechanical, as large masses of continental crust are subjected to intense folding, overthrusting, faulting, and uplifting (Figure 13.2c). The converging plates crush and deform both marine sediments and basaltic oceanic crust. The European Alps are a result of such compres- sion forces and exhibit considerable crustal shortening in conjunction with great overturned folds, called nappes.
The collision of India with the Eurasian landmass, producing the Himalayas, is estimated to have shortened the overall continental crust by as much as 1000 km and to have produced telescoping sequences of thrust faults at depths of 40 km. The Himalayas feature the tallest above- sea-level mountains on Earth, including all 10 of Earth’s highest peaks.
The Appalachian Mountains
The old, eroded, fold-and-thrust belt of southeastern Canada and the eastern United States has origins dating to the formation of Pangaea and the collision of Africa
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