Andesite is an extrusive igneous rock of intermediate composition typically dominated by plagioclase feldspar with pyroxene and/or hornblende. It forms from the mixing of basaltic and crustal magmas in continental margins above subduction zones. Andesites are the most abundant volcanic rocks in island arcs, occurring in belts above Benioff zones. They have been used historically as ornamental stone for buildings.

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

3.  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.  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.  A dark-colored, fine-grained extrusive igneous rock
with about 52 to 63 percent by weight of silica (SiO2).
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. 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.  There are different types of Andesites namely:-
 Olivine Andesites
 Pyroxene Andesites
 Hornblende & Biotite Andesites

8. Olivine is a typical phenocryst
mineral in the more basic
andesites having a silica or Si02
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 Si02
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.  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.  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.  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. 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. 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.  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.  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.  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