Andesite

Igneous petrology

1. ANDESITES

2. CONTENTS INTRODUCTION HISTORY OF ANDESITE TEXTURE MINERALOGY & CHEMISTRY TYPES OF ANDESITES GENESIS OCCURRENCE USES CONCLUSION REFERENCE

3. INTRODUCTION Andesite is an extrusive igneous, volcanic rock of intermediate composition, with Aphanitic to Porphyritic texture. The mineral assemblage is typically dominated by plagioclase plus pyroxene & or hornblende . Magnetite, Zircon, Apatite, Ilmenite , Biotite & Garnet are commen accesory minerals Andesites seen in continental margins,especially in belts overlying Benioff Zones the most abundant volcanic rocks of island arcs The name Andesite is derived from the ANDES mountain range

4. HISTORY OF ANDESITE The word “ Andesite " is derived from the Andes Mountains, located along the western edge of South America , where andesite rock is common. ANDESITE , a name first applied by C. L. VON BUCH to a series of lavas investigated by him from the Andes , which has passed into general acceptance as the designation of a great family of rocks playing an important part in the geology of most of the volcanic areas of the globe

5. MINERALOGY & CHEMISTRY A dark-colored, fine-grained extrusive igneous rock with about 52 to 63 percent by weight of silica (SiO 2 ). Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende . At the lower end of the silica range, andesite lava may also contain olivine The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals .

6. TEXTURE Texturally, most andesites are porphyritic rocks with prominent phenocrysts of both plagioclase and mafic minerals. These phenocrysts are typically enclosed in a piilotaxitic groundmass .

7. TYPES OF ANDESITES There are different types of Andesites namely:- Olivine Andesites Pyroxene Andesites Hornblende & Biotite Andesites

8. Olivine Andesite Olivine is a typical phenocryst mineral in the more basic andesites having a silica or Si0 2 in 52—55%. Olivine-bearing basaltic andesites are prominent in the “circum-Pacific belt lava”. The modal composition of this rock is phenocrystic olivine (5), hypersthene (4), and augite (1); labradorite microlites (50); dark glass with refractive index approximately 1.54 (40). The Si0 2 content is approximately 55%. Other olivine andesites of Paricutin carry olivine in much the same proportions, but many of them contain less glass, and microlites of plagioclase accompanied by a good deal of finely crystalline hypersthene are correspondingly more abundant. olivine PLAG Microlites

9. Pyroxene Andesites Andesites in which the sole or dominant mafic components are pyroxenes are especially common on composite volcanoes in orogenic belts . Most of them carry abundant phenocrysts of zoned plagioclase with cores as calcic as anorthite and rims mainly of oligoclase, Plagioclase microlites generally are andesine or oligoclase. Most of the hypersthene in pyroxene andesites occurs as phenocrysts OLIVINE

10. Hornblende & Biotite Andesites These generally form thick, short flows, steep-sided domical protrusions, or intrusive plugs and dikes. They are usually more siliceous and alkaline than pyroxene andesites , and hence grade into dacites and latites. Their hornblendes are seldom fresh and green except in quickly chilled, glass-rich types; otherwise they are brownish or reddish oxyhornblendes or are partly or completely replaced by granular mixtures of augite and magnetite. Their biotites likewise are often reddened by oxidation and more or less replaced by granular opaque oxides . Fresh pyroxenes almost always accompany the hornblende and biotite, and occasionally a little olivine is to be seen. The plagioclase of these rocks tends to be somewhat more sodic than that of the olivine andesites and pyroxene andesites, and potassic feldspar tends/to be more abundant.

11. Genesis Via magma mixing : Basaltic magma may also mix with rhyolitic magma. This usually occurs in continental arc areas such as the Andes, where the high geothermal gradient above the subducted plate, and hydrothermal flows within the mantle wedge may create an underplate of softened, partially molten continental crust of intermediate or felsic composition. Basaltic magmas intruded into this anomalously hot zone will prompt partial melting of the crust, and may mix with these melts to produce intermediate compositions, typically andesite to trachyte in composition. In 2009, researchers revealed that andesite was found in two meteorites (numbered GRA 06128 and GRA 06129) that were discovered in the Graves Nunatak Ice field during the US Antarctic Search for Meteorites 2006/2007 field season. This possibly points to a new mechanism to generate andesite crust.  

12. OCCURENCE Andesites are the most abundant volcanic rock of island arcs & mobile continental margins, especially in belts overlying BENIOFF ZONES and are regionally associated with Tholeiitic basalts & Rhyolites or both. Around the pacific ocean today for examples Andesite , Predominates in composite volcanoes of the Andes, central America, the North Western United States and the Aleutian, Japanese, Javanese, South Western Pacific Island Arcs BENIOFF ZONE S

13. Indian Occurrence of Andesites Whole-rock compositions range from 50.7 to 59.8 wt.% SiO2, and thus span the range from basalt to andesite. All samples contain phenocrysts and microphenocrysts of olivine (plus spinel), plagioclase, and clinopyroxene. A notable textural feature in many samples from all age groups is the presence of abundant (to 40 vol.%), large (to 4 mm) phenocrysts of plagioclase. The central cinder cone of Barren Island in vigorous activity, on 30 March 2009, photographed from the western shore of the island, 1 km away. In the foreground are the a flows of the recent (1991–2006) eruptions. The summit crater is ~500 above the sea and the viewer.

14. USES As ornamental stone for historical buildings. As slabs for various purpose. Most imposing architectonic complexes inherited from the Incans/Mayans Society is precisely Saqsaywaman

15. CONCLUSION Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende . Basaltic magmas intruded into the anomalously hot zone will prompt partial melting of the crust, and may mix with these melts to produce intermediate compositions, typically andesite to trachyte in composition. Andesites have been studied extensively and there are known occurrences of gold with it. Andesites are seen in continental margins, especially in belts overlying Benioffzones.The most abundant volcanic rocks of island arcs and mobile Andesites are also used as ornamental stones for building as well as slabs.

16. REFERENCE Charles M. Gilbert & Francis J. Turner, Text book on Igneous & sedimentary petrology. Edition 1984, pp 138- 149 Nockolds S.R., R.W.O’B.Knox , Chinner G.A., PETROLOGY FOR STUDENTS. Edition 1979, pp 96- 101 & 94 www.wikipedia.org www.hawaiianuniversity-andesite_ign peterology.co.in www.answer.com

17. T H A N K Y O U