Inspirasi dari kutipan ilmiah yang dituangkan ke dalam suatu kreasi, disusun dan digunakan sebagai referensi pribadi untuk mendukung kegiatan kerja di kantor. Semoga bermanfaat

OL EH :H E L M U T T OD O T UA S I M A M O RA

BADAN LINGKUNGAN HIDUP, PENELITIAN DAN PENGEMBANGAN KABUPATEN SAMOSIR PROVINSI SUMATERA UTARA

BELAJAR TENTANG BATU ANDESIT

Andesit adalah suatu jenis batuan beku vulkanik dengan komposisi antara dan tekstur spesifik yang umumnya ditemukan pada lingkungan subduksi tektonik di wilayah perbatasan lautan seperti di pantai barat Amerika Selatan atau daerah-daerah dengan aktivitas vulkanik yang tinggi seperti Indonesia. Nama andesit berasal dari nama Pegunungan Andes.

Batu andesit banyak digunakan dalam bangunan-bangunan megalitik , candi dan piramida. Begitu juga perkakas-perkakas dari zaman prasejarah banyak memakai material ini, misalnya: sarkofagus, punden berundak, lumpang batu, meja batu,arca dll.

Di zaman sekarang batu andesit ini masih digunakan sebagai material untuk nisan kuburan orang Tionghoa, cobek,lumpang jamu, cungkup/kap lampu taman dan arca-arca untuk hiasan. Salah satu pusat kerajian dari batu andesit ini adalahMagelang.

Pusat kerajinan dan pemotongan batu Andesit juga terdapat di daerah Cirebon dan Majalengka Jawa Barat. Karena di daerah ini banyak terdapat perbukitan yang merupakan daerah tambang Batu Andesit. Untuk batu Andesit di daerah cirebon umum nya bewarna abu-abu dan terdiri dari 2 Jenis utama: Andesit Bintik dan Andesit Polos.

Classification of Andesites

Andesite (/ˈændᵻsaɪt/ or /ˈændᵻzaɪt/[1]) is an extrusive igneous, volcanic rock, of intermediate composition, withaphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals.[2] Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic.[3] Along with basalts they are a major component of the Martian crust.[4] The name andesite is derived from the Andes mountain range.

MAGMATISM AND SUBDUCTION

Magmatism in island arc regions (i.e., active oceanic margins) comes from the interplay of the subducting plate and themantle wedge, the wedge-shaped region between the subducting and overriding plates.

During subduction, the subducted oceanic crust is submitted to increasing pressure and temperature, leading to metamorphism. Hydrous minerals such as amphibole, zeolites, chlorite etc. (which are present in the oceanic lithosphere) dehydrate as they change to more stable, anhydrous forms, releasing water and soluble elements into the overlying wedge of mantle. Fluxing water into the wedge lowers the solidus of the mantle material and causes partial melting.[5] Due to the lower density of the partially molten material, it rises through the wedge until it reaches the lower boundary of the overriding plate. Melts generated in the mantle wedge are of basaltic composition, but they have a distinctive enrichment of soluble elements (e.g. potassium (K), barium (Ba), and lead (Pb)) which are contributed from sediment that lies at the top of the subducting plate. Although there is evidence to suggest that the subducting oceanic crust may also melt during this process, the relative contribution of the three components (crust, sediment, and wedge) to the generated basalts is still a matter of debate.[6]

TYPICALLY FORMED ANDESITE

Andesite is typically formed at convergent plate margins but may occur in other tectonic settings. Intermediate volcanic rocks are created via several processes:

Fractional crystallization of a mafic parent magma. Partial melting of crustal material. Magma mixing between felsic rhyolitic and mafic basaltic magmas in

a magma reservoir

ANDESITIC COMPOSITION

To achieve andesitic composition via fractional crystallization, a basaltic magma must crystallize specific minerals that are then removed from the melt. This removal can take place in a variety of ways, but most commonly this occurs by crystal settling. The first minerals to crystallize and be removed from a basaltic parent are olivines and amphiboles. These mafic minerals settle out of the magma, forming mafic cumulates. There is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust. Once these mafic minerals have been removed, the melt no longer has a basaltic composition. The silica content of the residual melt is enriched relative to the starting composition. The iron andmagnesium contents are depleted. As this process continues, the melt becomes more and more evolved eventually becoming andesitic. Without continued addition of mafic material, however, the melt will eventually reach a rhyolitic composition.

Partially molten basalt in the mantle wedge moves upwards until it reaches the base of the overriding crust. Once there, the basaltic melt can either underplate the crust, creating a layer of molten material at its base, or it can move into the overriding plate in the form of dykes. If it underplates the crust, the basalt can (in theory) cause partial melting of the lower crust due to the transfer of heat and volatiles. Models of heat transfer, however, show that arc basalts emplaced at temperatures 1100 - 1240 °C cannot provide enough heat to melt lower crustal amphibolite.[7] Basalt can, however, melt peliticupper crustal material.[8] Andesitic magmas generated in island arcs, therefore, are probably the result of partial melting of the crust.

In continental arcs, such as the Andes, magma often pools in the shallow crust creating magma chambers. Magmas in these reservoirs become evolved in composition (dacitic to rhyolitic) through both the process of fractional crystallization and partial melting of the surrounding country rock. Over time as crystallization continues and the system loses heat, these reservoirs cool. In order to remain active, magma chambers must have continued recharge of hot basaltic melt into the system. When this basaltic material mixes with the evolved rhyolitic magma, the composition is returned to andesite, its intermediate phase.[9]

WHAT IS ANDESITE

Andesite is the name used for a family of fine-grained, extrusiveigneous rocks that are usually light to dark gray in color. They often weather to various shades of brown, and these specimens must be broken for proper examination. Andesite is rich inplagioclase feldspar minerals and may contain biotite, pyroxene, or amphibole. Andesite usually does not contain quartz or olivine.

Andesite is typically found in lava flows produced by stratovolcanoes. Because these lavas cooled rapidly at the surface, they are generally composed of small crystals. The mineral grains are usually so small that they cannot be seen without the use of a magnifying device. Some specimens that cooled rapidly contain a significant amount of glass, while others that formed from gas-charged lavas have a vesicular or amygdaloidal texture.

WHERE DOES ANDESITE FORM?

Andesite and diorite are common rocks of the continental crust above subduction zones. They generally form after an oceanic plate melts during its descent into the subduction zone to produce a source of magma. Diorite is a coarse-grained igneous rock that forms when the magma did not erupt, but instead slowly crystallized within Earth's crust. Andesite is a fine-grained rock that formed when the magma erupted onto the surface and crystallized quickly. 

Andesite and diorite have a composition that is intermediate between basalt and granite. This is because their parent magmas formed from the partial melting of a basaltic oceanic plate. This magma may have received a granitic contribution by melting granitic rocks as it ascended or mixed with granitic magma.

ANDESITE DERIVES

Andesite derives its name from the Andes Mountains of South America. In the Andes it occurs as lava flows interbedded withash and tuff deposits on the steep flanks of stratovolcanoes. Andesite stratovolcanoes are found above subduction zones in Central America, Mexico, Washington, Oregon, the Aleutian Arc, Japan, Indonesia, the Philippines, the Caribbean, and New Zealand, among other locations.

Andesite can also form away from the subduction zone environment. For example, it can form at ocean ridges and oceanic hot spots from partial melting of basaltic rocks. It can also form during eruptions at continental plate interiors where deep-source magma melts continental crust or mixes with continental magmas. There are many other environments where andesite might form. 

Water vapor produced when a rising magma chamber encounters groundwater.

At depth, these gases can be dissolved in the magma like carbon dioxide dissolved in a can of cold beer. If that can of beer is shaken and suddenly depressurized by opening the can, the gas and the beer will erupt from the opening. A volcano behaves in a similar manner. A rising magma chamber instantly depressurized by a landslide, faulting, or other event can produce a similar but much larger explosive eruption.

Many volcanic plumes and ash eruptions occur when gas-charged andesitic magmas erupt. The gas pressure that causes the eruption blows large amounts of tiny rock and magma particles into the atmosphere. These particles can be blown high into the atmosphere and carried long distances by the wind. They often cause problems for aircraft operating downwind from the volcano.

Catastrophic eruptions like Mount St. Helens, Pinatubo, Redoubt, and Novarupta were produced by andesitic magmas with enormous amounts of dissolved gas under high pressure. It is difficult to imagine how a magma can contain enough dissolved gas to produce one of these eruptions.

DISSOLVED GAS AND EXPLOSIVE ERUPTIONS

Some magmas that produce volcanic eruptions above subduction zones contain enormous amounts of dissolved gas. These magmas can contain several percent dissolved gas by weight. This gas can have several origins, examples of which include the following:

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

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

Carbon dioxide produced when rising magma encounters carbonate rocks, such as limestone, marble, or dolomite.

 

ANDESITE PORPHYRY

Occasionally, andesites contain large, visible grains of plagioclase, amphibole, or pyroxene. These large crystals are known as "phenocrysts." They begin forming when a magma, which is cooling at depth, approaches the crystallization temperature of some of its minerals. These high-crystallization-temperature minerals begin forming below the surface and grow to visible sizes before the magma erupts.

When the magma erupts onto the Earth's surface, the rest of the melt crystallizes quickly. This produces a rock with two different crystal sizes: large crystals that formed slowly at depth (known as "phenocrysts"), and small crystals that formed quickly at the surface (known as "groundmass"). "Andesite porphyry" is the name used for these rocks with two crystal sizes. 

THE ELUSIVE DEFINITION OF ANDESITE

The formal definition of andesite is problematic. Many authors have classified igneous rocks based upon their chemical and mineralogical compositions. However, none of these classifications are in perfect agreement. 

For a fine-grained rock like andesite, these classifications are impossible to use precisely when in the field or the classroom. They require chemical or mineralogical analyses that are usually not available, affordable, or practical. 

If you examine a rock that appears to be andesite, but you are not confident that it meets the mineralogical or chemical classification of andesite, you can properly call it an "andesitoid" rock. That means that while the rock looks like andesite, a microscopic examination or chemical testing might prove you wrong!

Figure

Thank You

Dedicated to :1. My loving wife , Hutapea Olga Y.V, dr;2. My loving daughter :a. Simamora Michelle Renata Robertina;b. Simamora Helga Martha Davina;

Also :3. Environment, Research and Development Agency of Samosir Regency Governmentof North Sumatera Province;2. People of Samosir Regency3. All of You

Alumni : PSMIL – Universitas Padjadjaranat Bandung