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

O L E H :HELMU T TODO TU A S IMAMORA

BADAN LING!NGAN "ID!#, #$N$LI%IAN DAN #$NG$&BANGANAB!#A%$N SA&'SI( #(')INSI S!&A%$(A !%A(A 

B$LA*A( %$N%ANG AND$SI%

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 Andesit adalah suatu jenis batuan beku vulkanik  dengan komposisi antara dan teksturspesifik yang umumnya ditemukan pada lingkungan subduksi tektonik di wilayahperbatasan lautan seperti di pantai barat Amerika Selatan  atau daerah-daerah denganaktivitas vulkanik yang tinggi seperti Indonesia. Nama andesit berasal dari namaPegunungan Andes.

Batu andesit banyak digunakan dalam bangunan-bangunan megalitik   !andi dan

piramida. Begitu juga perkakas-perkakas dari "aman prasejarah banyak memakaimaterial ini misalnya# sarkofagus punden berundak  lumpang batu meja batuar!a dll.

$i "aman sekarang batu andesit ini masih digunakan sebagai material untuk nisankuburan orang %ionghoa !obek lumpang  jamu !ungkup&kap lampu taman dan ar!a-ar!a untuk hiasan. Salah satu pusat kerajian dari batu andesit ini adalah'agelang.

Pusat kerajinan dan pemotongan batu Andesit juga terdapat di daerah (irebon dan

'ajalengka )awa Barat. *arena di daerah ini banyak terdapat perbukitan yangmerupakan daerah tambang Batu Andesit. +ntuk batu Andesit di daerah !irebon umumnya bewarna abu-abu dan terdiri dari , )enis utama# Andesit Bintik dan Andesit Polos.

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+lassifiation of Andesites

 Andesite &ˈnd sa tᵻ ɪ  & or &ˈnd "a tᵻ ɪ  &/012 is an e3trusive igneous vol!ani! ro!k  ofintermediate !omposition withaphaniti! to porphyriti! te3ture. In a generalsense it is the intermediate type between basalt and da!ite and ranges from 45 to678 sili!on dio3ide Si9,2 as illustrated in %AS diagrams. %he mineral assemblage

is typi!ally dominated by plagio!lase plus pyro3ene and&or hornblende. 'agnetite "ir!on apatite ilmenite biotite and garnet are !ommon a!!essory minerals./,1 

 Alkali feldspar may be present in minor amounts. %he :uart"-feldspar abundan!esin andesite and other vol!ani! ro!ks are illustrated in ;AP< diagrams.

(lassifi!ation of andesites may be refined a!!ording to the most abundantpheno!ryst. =3ample# hornblende-phyric andesite if hornblende is the prin!ipala!!essory mineral.

 Andesite !an be !onsidered as the e3trusive e:uivalent of plutoni! diorite.(hara!teristi! of subdu!tion "ones andesite represents the dominant ro!k type inisland ar!s. %he average !omposition of the !ontinental !rust is andesiti!./71 Along

 with basalts they are a major !omponent of the 'artian !rust./>1 %hename andesite is derived from the Andes mountain range.

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&AG&A%IS& AND S!BD!+%I'N

 'agmatism in island ar!  regions i.e. a!tive o!eani! margins2 !omes fromthe interplay of the subdu!ting plate and themantle wedge the wedge-shaped region between the subdu!ting and overriding plates.

  $uring subdu!tion the subdu!ted o!eani! !rust  is submitted to in!reasingpressure and temperature leading to metamorphism. ?ydrous mineralssu!h as amphibole "eolites !hlorite et!. whi!h are present in the

o!eani! lithosphere2 dehydrate as they !hange to more stable anhydrousforms releasing water and soluble elements into the overlying wedge ofmantle. <lu3ing water into the wedge lowers the solidus of the mantle material and !auses partial melting./41 $ue to the lower density of thepartially molten material it rises through the wedge until it rea!hes thelower boundary of the overriding plate. 'elts generated in the mantle wedge

are of basalti! !omposition but they have a distin!tive enri!hment of solubleelements e.g. potassium *2 barium Ba2 and lead Pb22 whi!h are!ontributed from sediment that lies at the top of the subdu!ting plate.

 Although there is eviden!e to suggest that the subdu!ting o!eani! !rust mayalso melt during this pro!ess the relative !ontribution of the three!omponents !rust sediment and wedge2 to the generated basalts is still a

matter of debate./61

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%-#I+ALL- '(&$D AND$SI%$

 Andesite is typi!ally formed at !onvergent plate margins but may o!!ur in otherte!toni! settings. Intermediate vol!ani! ro!ks are !reated via several pro!esses#

 <ra!tional !rystalli"ation of a mafi! parent magma.

 Partial melting of !rustal material.

  'agma mi3ing between felsi! rhyoliti! and mafi! basalti!  magmas in a magma

reservoir

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 AND$SI%I+ +'&#'SI%I'N

 %o a!hieve andesiti! !omposition via fra!tional !rystalli"ation a basalti!magma must !rystalli"e spe!ifi! minerals that are then removed from the melt.%his removal !an take pla!e in a variety of ways but most !ommonly this o!!urs

 by !rystal settling. %he first minerals to !rystalli"e and be removed from a basalti! parent are olivines and amphiboles. %hese mafi! minerals settle out of

the magma forming mafi! !umulates. %here is geophysi!al eviden!e fromseveral ar!s that large layers of mafi! !umulates lie at the base of the !rust.9n!e these mafi! minerals have been removed the melt no longer has a basalti!!omposition. %he sili!a !ontent of the residual melt is enri!hed relative to thestarting !omposition. %he iron andmagnesium  !ontents are depleted. As thispro!ess !ontinues the melt be!omes more and more evolved eventually

 be!oming andesiti!. @ithout !ontinued addition of mafi! material howeverthe melt will eventually rea!h a rhyoliti! !omposition.

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 Partially molten basalt in the mantle wedge moves upwards until it rea!hes the base of the overriding !rust. 9n!e there the basalti! melt !an either underplate the !rust !reating a layer of molten material at its base or it !an move into theoverriding plate in the form of dykes. If it underplates the !rust the basalt !anin theory2 !ause partial melting of the lower !rust due to the transfer of heat

and volatiles. 'odels of heat transfer however show that ar! basalts empla!edat temperatures 00 - 0,> ( !annot provide enough heat to melt lower!rustal amphibolite./51 Basalt !an however melt peliti!upper !rustal material./C1

  Andesiti! magmas generated in island ar!s therefore are probably the resultof partial melting of the !rust.

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 In !ontinental ar!s su!h as the Andes magma often pools in the shallow !rust!reating magma !hambers. 'agmas in these reservoirs be!ome evolved in!omposition da!iti! to rhyoliti!2 through both the pro!ess of fra!tional!rystalli"ation and partial melting of the surrounding !ountry ro!k. 9ver timeas !rystalli"ation !ontinues and the system loses heat these reservoirs !ool. In

order to remain a!tive magma !hambers must have !ontinued re!harge of hot basalti! melt into the system. @hen this basalti! material mi3es with theevolved rhyoliti! magma the !omposition is returned to andesite itsintermediate phase./D1

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 /"A% IS AND$SI%$

 Andesite is the name used for a family of fine-grained e3trusiveigneous ro!ks thatare usually light to dark gray in !olor. %hey often weather to various shades of

 brown and these spe!imens must be broken for proper e3amination. Andesiteis ri!h inplagio!lase feldspar minerals and may !ontain biotite pyro3ene or

amphibole. Andesite usually does not !ontain :uart" or olivine.

 Andesite is typi!ally found in lava flows produ!ed by stratovol!anoes. Be!ausethese lavas !ooled rapidly at the surfa!e they are generally !omposed of small!rystals. %he mineral grains are usually so small that they !annot be seen

 without the use of a magnifying devi!e. Some spe!imens that !ooled rapidly

!ontain a signifi!ant amount of glass while others that formed from gas-!harged lavas have a vesi!ular or amygdaloidal te3ture.

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 /"$($ D'$S AND$SI%$ '(&0

 Andesite and diorite are !ommon ro!ks of the !ontinental !rust abovesubdu!tion "ones. %hey generally form after an o!eani! plate melts during itsdes!ent into the subdu!tion "one to produ!e a sour!e of magma. $iorite is a!oarse-grained igneous ro!k that forms when the magma did not erupt butinstead slowly !rystalli"ed within =arthEs !rust. Andesite is a fine-grainedro!k that formed when the magma erupted onto the surfa!e and !rystalli"ed

:ui!kly.

 Andesite and diorite have a !omposition that is intermediate between basalt and granite. %his is be!ause their parent magmas formed from the partialmelting of a basalti! o!eani! plate. %his magma may have re!eived a graniti!!ontribution by melting graniti! ro!ks as it as!ended or mi3ed with graniti!

magma.

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 AND$SI%$ D$(I)$S

 Andesite derives its name from the Andes 'ountains of South Ameri!a. In the Andes it o!!urs as lava flows interbedded withash and tuff  deposits on the steepflanks of stratovol!anoes. Andesite stratovol!anoes are found above subdu!tion"ones in (entral Ameri!a 'e3i!o @ashington 9regon the Aleutian Ar!)apan Indonesia the Philippines the (aribbean and New Fealand among

other lo!ations.

 Andesite !an also form away from the subdu!tion "one environment. <ore3ample it !an form at o!ean ridges and o!eani! hot spots from partial meltingof basalti! ro!ks. It !an also form during eruptions at !ontinental plate interiors

 where deep-sour!e magma melts !ontinental !rust or mi3es with !ontinental

magmas. %here are many other environments where andesite might form.

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 @ater vapor produ!ed when a rising magma !hamber en!ounters groundwater.

 At depth these gases !an be dissolved in the magma like !arbon dio3idedissolved in a !an of !old beer. If that !an of beer is shaken and suddenlydepressuri"ed by opening the !an the gas and the beer will erupt from theopening. A vol!ano behaves in a similar manner. A rising magma !hamberinstantly depressuri"ed by a landslide faulting or other event !an produ!e asimilar but mu!h larger e3plosive eruption.

'any vol!ani! plumes and ash eruptions o!!ur when gas-!harged andesiti!magmas erupt. %he gas pressure that !auses the eruption blows large amountsof tiny ro!k and magma parti!les into the atmosphere. %hese parti!les !an be

 blown high into the atmosphere and !arried long distan!es by the wind. %heyoften !ause problems for air!raft operating downwind from the vol!ano.

(atastrophi! eruptions like 'ount St. ?elens Pinatubo Gedoubt andNovarupta were produ!ed by andesiti! magmas with enormous amounts ofdissolved gas under high pressure. It is diffi!ult to imagine how a magma !an!ontain enough dissolved gas to produ!e one of these eruptions.

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DISS'L)$D GAS AND $1#L'SI)$ $(!#%I'NS

Some magmas that produ!e vol!ani! eruptions above subdu!tion "ones !ontainenormous amounts of dissolved gas. %hese magmas !an !ontain severalper!ent dissolved gas by weight. %his gas !an have several origins e3amplesof whi!h in!lude the following#

 @ater vapor produ!ed when o!ean-floor sediments on an o!eani! plate areheated in a subdu!tion "one.

 @ater vapor produ!ed when hydrous minerals dehydrate in the heat of asubdu!tion "one.

(arbon dio3ide produ!ed when rising magma en!ounters !arbonate ro!kssu!h as limestone marble or dolomite.

 

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 AND$SI%$ #'(#"-(- 

9!!asionally andesites !ontain large visible grains of plagio!lase amphiboleor pyro3ene. %hese large !rystals are known as Hpheno!rysts.H %hey beginforming when a magma whi!h is !ooling at depth approa!hes the!rystalli"ation temperature of some of its minerals. %hese high-!rystalli"ation-temperature minerals begin forming below the surfa!e and grow to visible si"es

 before the magma erupts.

 @hen the magma erupts onto the =arthEs surfa!e the rest of the melt!rystalli"es :ui!kly. %his produ!es a ro!k with two different !rystal si"es# large!rystals that formed slowly at depth known as Hpheno!rystsH2 and small

!rystals that formed :ui!kly at the surfa!e known as HgroundmassH2. HAndesiteporphyryH is the name used for these ro!ks with two !rystal si"es.

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%"$ $L!SI)$ D$INI%I'N ' AND$SI%$

%he formal definition of andesite is problemati!. 'any authors have !lassifiedigneous ro!ks based upon their !hemi!al and mineralogi!al !ompositions.?owever none of these !lassifi!ations are in perfe!t agreement.

<or a fine-grained ro!k like andesite these !lassifi!ations are impossible to usepre!isely when in the field or the !lassroom. %hey re:uire !hemi!al ormineralogi!al analyses that are usually not available affordable or pra!ti!al.

If you e3amine a ro!k that appears to be andesite but you are not !onfident

that it meets the mineralogi!al or !hemi!al !lassifi!ation of andesite you !anproperly !all it an HandesitoidH ro!k. %hat means that while the ro!k looks likeandesite a mi!ros!opi! e3amination or !hemi!al testing might prove you

 wrong

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<igure

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Terima Kasih

 Thank

 You

Dedicated to :

1. My lovin !i"e # Huta$ea Ola Y.%# d&'

(. My lovin dauhte& :

a. Si)a)o&a Michelle Renata Ro*e&tina'

*. Si)a)o&a Hela Ma&tha Davina'

Al+o :

,. Envi&on)ent# Re+ea&ch and Develo$)ent Aency o" Sa)o+i& Reency -ove&n)ent

o" o&th Su)ate&a /&ovince'

(. /eo$le o" Sa)o+i& Reency

,. All o" You

Alu)ni :

/SMIL 0 Unive&+ita+ /adada&an

at 2andun