Transcript G20-2pow
Ch. 20 Sec. 2 Orogeny Section 20.2 Orogeny Convergence causes the crust to thicken and form mountain belts. Review Vocabulary island arc: a line of islands that forms over a subducting oceanic plate Orogeny - all processes that form mountain ranges also known in geology as orogenic belts I. Mountain Building at Convergent Boundaries Most of Earth’s mountain ranges formed along plate boundaries. Fig 20.7 Pg. 567 Compressive forces • folding • faulting • metamorphism • igneous intrusions A. Oceanic-oceanic convergence 1. Subduction a. Plate melts b. Magma rises Fig 20.8 Pg. 568 2. Island arc complex a. Aleutian Islands (Alaska) b. Lesser Antilles (Caribbean) Section 20.2 Orogeny Hot Spot Island Formation 3. Rock types a. basaltic and andesitic magmas b. sedimentary rock – uplifted or folded against arc (Japan) B. Oceanic-continental convergence Convergence between oceanic and continental plates produces mountain belts that are much bigger and more complicated than island arc complexes. 1. Uplift of continental plate (beginning of orogeny) 2. Compression causes continental crust to fold and thicken 3. Igneous activity and metamorphism are common along boundaries. Fig. 20.9 Pg. 569 C. Continental-continental convergence Intense folding and faulting along continental-continental boundaries produce some of the highest mountain ranges on Earth. Fig 20.10 Pg. 570 Another common characteristic of the mountains that form when two continents collide is the presence of marine sedimentary rock near the mountains’ summits. The marine sedimentary rocks were formed in the ocean basin that existed between the continents before their collision. Orogeny Convergence II. The Appalachian Mountains—A Case Study Geologists have divided the Appalachians into several distinct regions. Each region is characterized by rocks that show different degrees of deformation. Fig. 20. 11 Page 571 Section 20.2 Orogeny Folding Rocks A. The early Appalachians 1. 800 to 700 mya separation from Africa a. two oceans— the ancestral Atlantic Ocean and a shallow, marginal sea b. continental fragment 2. 700 to 600 mya a. Convergence b. Island arc east of North America 3. 500 – 400 mya a. continental fragment thrust over younger rocks b. Blue Ridge Mountains B. The final stages of formation 1. 400 to 300 mya a. Island arc attaches b. Piedmont Province c. Blue Ridge rocks pushed farther west 2. 300 and 260 mya a. Pangaea forms b. Extensive folding and faulting c. Valley and Ridge Province Section 20.2 Orogeny Visualizing the Rise and Fall of the Appalachians The Appalachians formed hundreds of millions of years ago as a result of convergence. Convergence causes the crust to thicken and form mountain belts. Orogeny refers to all of the processes that form mountain belts. Most mountain belts are associated with plate boundaries. Island arc complexes, highly deformed mountains, and very tall mountains form as a result of the convergence of tectonic plates. The Appalachian Mountains are geologically ancient; they began to form 700 to 800 mya. Examine the illustration. What is happening at this mountain belt? Possible answer: An oceanic plate is subducting beneath a continental plate. As the subducting plate sinks, it gets warmer and water is released from minerals that contain it.