[Geography Notes] on Andesite Pdf for Exam

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. 

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.