G G L E O O G I A P I U U N N G [TGS7401] 2 SKS teori Semester 4, 2017-2018 G E O O G I G A P I U U N N G [TGS7401] 2 SKS teori Dr. Hill. Gendoet Hartono

UTS UAS Geologi Gunung Api, Kontrak Kuliah, Kelulusan, & Praktikum Tataan Tektonik & Gunung Api, Rejim Tektonik & Struktur Bumi Magma & Lava, Komposisi Kimia Batuan Pijar Tipe Erupsi & Bentuk Gunung Api, Letusan & Lelehan Gunung Api Bawah Permukaan Air Indek Letusan Gunung Api, Bentang Alam & Material Lahar, Gas Gunung Api & Awan Abu UTS Geologi Gunung Api Di Pacific Ring Of Fire Geologi Gunung Api Di Spreading Center Geologi Gunung Api Di Hot Spot Geologi Gunung Api Moderen Geologi Gunung Api Purba Bencana Gunung Api, Mitigasi & Monitoring Evolusi Gunung Api UAS

Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism. On Earth, magma forms by partial melting of silicate rocks either in the mantle, continental or oceanic crust. Magma is a molten and semi-molten rock mixture found under the surface of the Earth. On the rare occasions when magma breaks the surface, as in a volcanic eruption, it is called lava. Wikipedia, the free encyclopedia

Magma exists as pockets and plumes beneath the surface of the Earth

Magma Type Chemical Composition Temperature (degrees C) Viscosity Gas Content Basaltic 45-55% SiO2; High in Fe, Mg, Ca; Low in K, Na. 1000 - 1200 Low Andesitic 55-65% SiO2; Intermediate Fe, Mg, Ca, Na, K 800-1000 Intermediate Rhyolitic 65-75% SiO2; Low in Fe, Mg, Ca; High in K, Na 650-800 High

Magma Basaltic Low viscosity means that mafic magma is the most fluid of magma types. It erupts non-explosively and moves very quickly when it reaches Earth’s surface as lava. This lava cools into basalt, a rock that is heavy and dark in color due to its higher iron and magnesium levels. Basalt is one of the most common rocks in Earth’s crust as well as the volcanic islands created by hot spots. The Hawaiian Islands are a direct result of mafic magma eruptions. Steady and relatively calm “lava fountains” continue to change and expand the “Big Island” of Hawaii.       

Magma Andesitic As a result of its higher viscosity and gas content, intermediate magma builds up pressure below the Earth’s surface before it can be released as lava. This more gaseous and sticky lava tends to explode violently and cools as andesite rock. Intermediate magma most commonly transforms into andesite due to the transfer of heat at convergent plate boundaries. Andesitic rocks are often found at continental volcanic arcs, such as the Andes Mountains in South America, after which they are named.  

Magma Rhyolitic Thick, viscous felsic magma can trap gas bubbles in a volcano’s magma chamber. These trapped bubbles can cause explosive and destructive eruptions. These eruptions eject lava violently into the air, which cools into dacite and rhyolite rock. Much like intermediate magma, felsic magma may be most commonly found at convergent plate boundaries where transfer of heat and flux melting create large stratovolcanoes. 

MAGMA/LAVA → ERUPTIONS Non-Explosive Eruptions: Fluid lava flows easily allows gases to bubble away

Explosive Eruptions: Viscous lava traps the gases until large pressures build up & the system explodes Pyroclastic flow (ash, rock fragments) flow out of vent

There are three main types of lava flow PAHOEHOE – has a shiny, smooth, glassy surface. It tends to be more fluid (lower viscosity), hence flows more quickly and produces thinner flows (typically 1-3 m). AA – a rubbly flow, with a molten core, with higher viscosity (but same composition) which, therefore, tends to move more slowly and produce thicker flows (typically 3-20 m). BLOCKY – similar to Aa, but even thicker (>20 m), with a blocky rather than rubbly surface. Andesites, dacites and rhyolites tend to form blocky flows.

RHEOLOGY – fluid dynamics of lava flows RHEOLOGY – fluid dynamics of lava flows. Viscosity and Yield Strength are the two most important factors that influence Surface morphology (flow type) Size and shape of the flow Flow thickness Eruption rate Flow velocity Length of lava flows Many of these factors are important for evaluating the hazard potential of a flow. Also useful in remote sensing and planetary studies.

VISCOSITY – is probably the most single important factor and is a measure of how strongly the melt is polymerized. (Silicic melts are more polymerised than basaltic melts, hence are more viscous) Low viscosity High viscosity Shear stress Strain rate (flow rate) Newtonian Fluids X Strain rate = dx/dt (flow rate) Shear stress Viscosity ( ƞ ) = Shear stress/ Strain rate

Effects of composition and temperature on viscosity rhyolite dacite andesite basalt komatiite Temperature oC 1600 1200 800 1 100 104 106 108 Viscosity (Pa.s) Pa.s = Pascal Seconds = kg/m/sec 1 Pa.s = 10 poise (water = 10-3 Pa.s) Dissolved water reduces viscosity as does higher pressure

Tipe Lava

Pahoehoe: Lava with a ropelike surface texture due to partial cooling as the lava flowed. Relatively hot, low viscosity lava.

Aa: Blocky, rough lava flow Aa: Blocky, rough lava flow. Due to high viscosity lava that flowed pushing chunks of solid and semi-solid blocks.

Pillows: A form of closed lava tube (with a bulbous end) that forms when a lava flows into water (e.g., a lake or ocean) and cools very rapidly.

Lava tube formation Lava tube: A tube formed by cooling and solidifying of the lava walls while fluid lava continued to flow inside.

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