petrologi [TGS7208] 2 SKS teori 1 SKS praktikum Semester 2, 2017-2018 by: hill. gendoet hartono

UTS UAS Siklus Batuan, Kontrak Kuliah, Kelulusan, & Praktikum Magma, Diferensiasi, Viskositas, Gas dan Komposisi Batuan Beku, Intrusi Dalam & Karakteristiknya Batuan Gunung Api, Intrusi Dangkal & Karakteristiknya Batuan Gunung Api, Lelehan, Letusan & Karakternya Klasifikasi Batuan Beku & Gunung Api Batuan Alterasi UTS Batuan Sedimen, Jenis & Proses Pembentukan Batuan Sedimen Silisiklastika Batuan Sedimen Karbonat Klasifikasi Batuan Sedimen Batuan Metamorf, Jenis & Proses Pembentukan Batuan Metamorf Derajat Rendah, Menengah & Tinggi Klasifikasi Batuan Metamorf UAS

mula jadi MAGMA

mag·ma ˈmaɡmə’ noun noun: magma; plural noun: magmata; plural noun: magmas hot fluid or semifluid material below or within the earth's crust from which lava and other igneous rock is formed by cooling. Dictionary

Magma (from Ancient Greek μάγμα (mágma) meaning "thick unguent") is a mixture of molten or semi-molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals, dissolved gas and sometimes gas bubbles. Magma often collects in magma chambers that may feed a volcano or solidify underground to form an intrusion. Magma is capable of intruding into adjacent rocks (forming igneous dikes and sills), extrusion onto the surface as lava, and explosive ejection as tephra, or fragmented rock, to form pyroclastic rock. From Wikipedia, the free encyclopedia

Origin of Magma Where does the heat come from that melts rocks? Formation of Earth Heat from the decay of radioactive elements

Factors that Affect Magma Formation Temperature Increases with depth Pressure Water Content Decreases melting point Mineral Composition Different minerals, different melting points

Sources of Magma

Origin of Magmas MORBs Subduction Continental

Karakteristik Magma Slushy Mix of molten rock, gases, and mineral crystals Common Elements Oxygen (O) Silicon (Si) Aluminum (Al) Iron (Fe) Magnesium (Mg) Calcium (Ca) Potassium (K) Sodium (Na) Compounds in Magma Silica ( Si02) Most abundant Greatest effect on Magma Characteristics Effects melting temp Viscosity of Magma Types of Magma Based on amount of Silica Basaltic Andesitic Rhyolitic

Types of magma are determined by chemical composition of the magma Types of magma are determined by chemical composition of the magma. Three general types are recognized: Basaltic magma --  SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na Andesitic magma --  SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K Rhyolitic magma --  SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na

DENSITY Density controlled by magma composition: FeO wt% most important. In general, basalts are richer in Fe, Ca, and Ti than rhyolites; rhyolites are richer in Na, Al, and Si than basalts: Basalt magma: 2.65 to 2.80 gm/cm3 Andesite magma: 2.45 to 2.50 gm/cm3 Rhyolite magma: 2.18 to 2.25 gm/cm3 Density is also controlled by Temperature and Pressure. Higher Temperatures cause melts to Expand ==>> Lower Density Higher Pressures cause the melts to Compress == >> Higher Density

VOLATILES H2O most abundant volatile in most magmas CO2 next most abundant volatile In general, Basalt magmas are DRY, i.e. H2O < 0.5 wt% MORB = 0.25% H2O Hawaiian Tholeiite = 0.5% H2O Alkali Olivine Basalt = 0.9% H2O Andesites, Rhyolites, Granites: Higher Water Contents Paricutin Andesite = 2.2% H2O at 1100oC Granites/Rhyolites wide range H2O: 0.5% to 7% H2O by weight. Water lowers viscosity: OH- ions act as Network Modifers, substitute for O2 in tetrahedra. Water lowers solidus temperature: Effect greater at higher pressures

VISCOSITY Viscosity is the resistance to flow (opposite of fluidity).  Viscosity depends on primarily on the composition of the magma, and temperature. Viscosity decreases with increasing temperature and high H2O, CO2. Some Natural Viscosities In Poise {= gm/cm-sec} H2O at 20oC 0.01 poise Glycerin 15 poise Erupting Hawaiian Basalt 3 x 10 3 poise @ 1150o-1200oC Rhyolite Magma 108 to 1011 poise @ 700-750oC Asthenosphere 1022 poise @ 1400-1600oC

Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma). Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma). Thus, basaltic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to 100,000 times more viscous than water.  Rhyolitic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water.  (Note that solids, even though they appear solid have a viscosity, but it is very high, measured as trillions time the viscosity of water).  Viscosity is an important property in determining the eruptive behavior of magmas. Viscosity has a control on magma flow rates, volcano morphology, rates of gas escape, ratesof convection, rates of crystal settling or flotation, rates of diffusion and crystal growth.

FLUIDITY OF MAGMA Fluidity or Viscosity of magma depends on content (%) of Silica Silica Rich known as Acidic magma More viscous, so do not spreads and piles up at one place Silica poor -Known as Basic magma -Less viscous, moves faster and occupies larger area However, the viscosity of magma is considerably influenced by temperature too. When temperature is less-more viscous and when high temperature- less viscous.

Pahoehoe aa

Gases in Magmas At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface.  This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles.   Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced.  The composition of the gases in magma are: Mostly H2O (water vapor) with some CO2 (carbon dioxide) Minor amounts of Sulfur, Chlorine, and Fluorine gases The amount of gas in a magma is also related to the chemical composition of the magma.   Rhyolitic  magmas usually have higher dissolved gas contents than basaltic  magmas.

Ringkasan Magma Type Solidified Rock Chemical Composition Temperature Viscosity Gas Content Basaltic Basalt 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 oC 10 - 103 PaS Low Andesitic Andesite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 oC 103 - 105 PaS Intermediate Rhyolitic Rhyolite 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na. 650 - 800 oC 105 - 109 PaS High

Igneous rocks are classified as felsic, mafic, intermediate, and ultramafic, depending upon their mineral compositions. Felsic rocks such as granite are light- colored, have high silica contents, and contain quartz and feldspars. Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium. Intermediate rocks have moderate silica levels. Ultramafic rocks have low silica contents and very high levels of iron and magnesium. Igneous groups can be further identified by crystal size and texture.

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