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Andesite Rock Type

Andesite is an extrusive volcanic rock. It is an intermediate type between basalt and rhyolite. It is fine-grained in texture and is sodium-rich plagioclase and pyroxene or hornblende. The dominant rock type in island arcs is also known as Andesite. The average composition of the Earth’s crust is andesitic. They are major components of Martian Crust. The name andesite is derived from the Andes mountain range where this rock is found in abundance. 

Andesite Description

A rock that has a medium level of silica and lesser amounts of Alkali metals is Andesite rock. It is also fine-grained. It consists of less than 10% feldspathoid by volume. Andesite is distinguished from basalt by its silica content of over 52%. It is further not possible to determine the mineral composition of volcanic rocks, due to their fine grain-like texture. Andesite is defined chemically as a volcanic rock with a content of 57% to 63% silica. It does not consist of more than 6% alkali metal oxides.


It is usually light to dark grey in color due to its content of pyroxene minerals. It exhibits a wide range of shading. Darker andesite is usually difficult to distinguish from basalt. A common rule of thumb is used, it states when used away from the laboratory, andesite has a color index less than 35.


Porphyritic andesite is another variation that contains larger crystals of plagioclase formed prior to the extrusion in a finer-grained matrix. These minerals have the extreme melting temperatures of the typical minerals that can crystallize from the melting lava. These are usually the first to form solid crystals. Andesite porphyry is the general name that is used for these rocks with two different crystal sizes.


The plagioclase in andesite varies widely in sodium content from anorthite to oligoclase, but it is typically andesine. The mineral included in andesite is augite, pigeonite, or orthopyroxene. 

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Andesite Rock 

Andesite rock description or andesite description type falls under the Igneous rock. The lava  that forms Andesite rock has moderate viscosity forming thick lava flow as well as domes. Andesite is the volcanic equivalent of diorite. It contains silica mainly between 52% - 63%. It is used as an aggregate or to fill. Usually used in the construction and roading industries. It is often not ideal for concrete aggregate because of high silica content. 


Above subduction zones are found Andesite and diorite rocks that have continental crusts. They usually form after an oceanic plate melts. The plate melts during its descent. It produces a source of magma. Diorite forms when magma remains below Earth’s surface and cools slowly. Andesite is a fine-grained rock that forms when magma erupts onto the surface and crystallizes quickly. 


Both Andesite and Diorite have a composition that is intermediate between basalt and granite. The parent magmas formed due to partial melting is the reason behind it.  The partial melting is of the basaltic oceanic plate. This magma receives a granitic contribution by melting granitic rocks. They are ascended or mixed with granitic magma.

Andesite Thin Section

A thin section of rock is prepared by gluing a small piece of rock onto a glass slide. It is then ground down to a thickness of 30 microns. It is done so that light shines through it when examined under the microscope. 

FAQs (Frequently Asked Questions)

Q1. Discuss in Detail the Andesite Meaning.

Ans. A dark, fine-grained, brown, or greyish volcanic rock, commonly found as a constituent of lavas in some areas is known as Andesite. They are found above convergent plate boundaries between continental and oceanic plates. 


Andesite often weathers to various shades of brown. plagioclase feldspar and amphibole minerals are found in abundance in Andesite. Quartz and pyroxene minerals are usually absent or present in small quantities.. It can form at ocean ridges and oceanic hotspots. It varies from partial melting of basaltic rocks. It can also form during eruptions at the continental plate where deep source magma melts. These types of rocks can be found in multiple environments. 


Talking about the volcanic eruptions, when the magma erupts onto the Earth’s surface, the rest of the melt crystallizes quickly. This produces a rock with two different varied crystal sizes. The large crystals are reformed slowly at depth also known as phenocrysts and small crystals are formed quickly at the surface. It is known as groundmass.

Q2. What does the Elusive Definition of Andesite Mean?

Ans. The formal definition of Andesite is diplomatic. Many authors have classified igneous rock based upon their chemical and mineralogical composition.


If we examine a rock that appears to be andesite, but not much confident that it meets the mineralogical and chemical classification of andesite. It is then called an ‘andesitic rock. It means that the rock looks andesite but a proper microscopic examination will give a clear analysis further.

Q3. What are Dissolved Gas and Explosive Eruptions?

Ans. Some magmas that produce explosive eruptions above subduction contain an enormous amount of dissolved gas. The magmas contain several amounts of dissolved gas by percentage. These gases have their various origins. Examples may include the following:

  1. Water vapor is produced when ocean floor sediments on an oceanic plate and are heated in a subduction zone

  2. Water vapor is produced when hydrous minerals dehydrate in the heat of a subduction zone.

  3. Carbon dioxide is produced when rising magma in turns encounters carbonate rocks such as limestone marble or dolomite. 

At extreme depth, these gases can be dissolved in magma freely and smoothly. A rising magma chamber can instantly be pressurized by a landslide, weathering, or lands nearby. The gas underlying erupts exponentially through overlying rock. 


The gas pressure that causes the eruption blows a large number of particles into the atmosphere. These particles known as volcanic ash can be blown high into the atmosphere and is carried much far away due to heavy winds.

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