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Unformatted text preview: This document last updated on 08-Feb-2011 EENS 2120 Petrology Prof. Stephen A. Nelson Tulane University Igneous Rocks of the Convergent Margins The convergent plate margins are the most intense areas of active magmatism above sea level at the present time. Most of world's violent volcanic activity occurs along these zones. In addition, much magmatism also has resulted (and probably is resulting at present) in significant additions to the crust in the form of plutonic igneous rocks. Here, we look at this magmatism in terms of the volcanic rocks that appear to be related to subduction. Occurrence The "Pacific Ring of Fire" is often discussed in relation to both earthquakes and volcanoes. It surrounds the Pacific Ocean basin and extends into the Indian Ocean and Caribbean Sea. Active subduction is taking place, along these convergent plate boundaries, as evidenced by the zone of earthquakes, called a Benioff Zone, that begins near the oceanic trenches and extends to deeper levels in the direction of plate motion. Earthquake focal depths reach a maximum of about 700 km in some areas. Volcanism occurs on the upper plate about 100 to 200 km above the Benioff Zone. For this reason, volcanism in these areas is often referred to as subduction-related volcanism. Two situations occur. 1. In areas where oceanic lithosphere is subducted beneath oceanic lithosphere the volcanism is expressed on the surface as chains of islands referred to as island arcs. These include the Caribbean Arc, the Aleutian Arc, the Kurile Kamachatka Arc, Japan, the Philippines, the South Sandwich Arc, The Indonesian Arc, the Marianas, Fiji, and Solomon Islands. Convergent Margins 2/8/2011 Page 1 of 10 2. In areas where oceanic lithosphere is subducted beneath continental lithosphere volcanism occurs as chains of volcanoes near the continental margin, referred to as a continental margin arc. These include the Andes Mountains, Central American Volcanic Belt, Mexican Volcanic Belt, the Cascades, the part of the Aleutian arc on Continental crust, and the North Island of New Zealand. Within these volcanic arcs the most imposing, and therefore most recognized by early workers, features of the landscape are large stratovolcanoes. These usually consist of predominantly andesitic lava flows and interbedded pyroclastic material. But, in the late stages of volcanism more silicic lavas and pyroclastics like dacites and rhyolites are common. Many of these stratovolcanoes pass through a stage where their upper portions collapse downward to form a caldera. These caldera forming events are usually associated with explosive eruptions that emit silicic pyroclastic material in large-volume eruptions. It is the sudden evacuation of underlying magma chambers that appears to result in the collapse of the volcanoes to form the calderas....
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This note was uploaded on 02/01/2012 for the course EENS 2120 taught by Professor Nelson during the Spring '11 term at Tulane.

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converg_margins - This document last updated on 08-Feb-2011...

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