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Unformatted text preview: 1 Lab 5 Andesites, Latites, Trachytes, Dacites and Rhyodacites In this lab, we will examine volcanic rocks that are more "differentiated" than basalts. The main objective of this lab is to train you to classify these rocks, and to be able to tell them apart, which is not always easy. In addition, you will answer several "process - oriented" questions for each sample, which are aimed at emphasizing some of the concepts that you learnt in lecture, and will help you understand some of the processes involved in the formation of these rocks. Andesites most commonly occur at convergent plate boundaries (e.g. island arcs and continental margins). They are also common in oceanic islands (e.g. Hawaii), although there, they tend to be phenocryst - poor, and commonly belong to the Hawaiite - mugearite series. Andesites span a relatively wide range of compositions ranging from basalts to trachyandesites (latites) and trachytes. Andesites that are compositionally close to basalts are therefore best termed "basaltic andesites". Trachytes are the Na- and K- rich equivalents of andesites. They therefore contain more modal sanidine, anorthoclase or orthoclase than andesites and trachyandesites (latites). Sodic varieties (best termed "alkali trachytes") may contain aegerine, riebeckite or rare fayalite, and are considered the differentiation products of AOB's. Dacites and Rhyodacites: Unlike andesites and trachytes, dacites and rhyodacites are silica - oversaturated. They therefore commonly contain phenocrysts of quartz or one of its high T polymorphs (e.g. Tridymite or cristobalite). Because of their higher SiO 2 content, they tend to contain more glass compared to andesites and trachytes, and are...
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.
- Fall '11