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[TGS7401]

Dr. Hill. Gendoet Hartono

Semester 4, 2017-2018

2 SKS teori

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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, Karakter & produknya Indek Letusan Gunung Api, Bentang Alam & Material Lahar, Gas Gunung Api & Awan Abu

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

Lahar, gas gunung api & awan

abu

la·harˈlähär/Noun Geologynoun: lahar; plural noun: lahars1. a destructive mudflow on the slopes of a volcano.

A lahar ( /ˈlɑːhɑːr/, from Javanese: ꦮ꧀ꦭꦲꦂ , translit. wlahar) is a type

of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.

lahar/ˈlɑːhɑː/ noun 1. a landslide of volcanic debris mixed with water down the sides of a volcano, usually precipitated by heavy rainfall

LaharsLahar is an Indonesian term that describes a hot or

cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys.

When moving, a lahar looks like a mass of wet

concrete that carries rock debris ranging in size fromclay to boulders more than 10 m in diameter.

Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow

several tens of meters per second--much too fast forpeople to outrun.

Diagrammatic portrayal of stages in formation of a cohesive debris flow or lahar, where the failed mass consists of similar amounts of hard, coherent rock and weak, readily disaggregated material.

The principal components of volcanic gases are water vapor (H2O), carbon dioxide (CO2), sulfur either as sulfur dioxide (SO2) (high-temperature volcanic gases) or hydrogen sulfide (H2S) (low-temperature volcanic gases), nitrogen, argon, helium, neon, methane, carbon monoxide and hydrogen.

Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action.The sources of volcanic gases on Earth include:• primordial and recycled constituents from the Earth's mantle,• assimilated constituents from the Earth's crust,• groundwater and the Earth's atmosphere.Substances that may become gaseous or give off gases when heated are termed volatile substances.

Wikipedia, the free encyclopedia

Magma contains dissolved gases that are released into the atmosphereduring eruptions.

Gases are also released from magma that either remains below ground (for example, as an intrusion) or is rising toward the surface.

In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems.

Volcanic gases

At high pressures deep beneath the earth's surface, volcanic gases are dissolved in molten rock.

But as magma rises toward the surface where the pressure is lower, gases held in the melt begin to form tiny bubbles.

The increasing volume taken up by gas bubbles makes the magma less dense than the surrounding rock, which may allow the magma tocontinue its upward journey.

Volcanic eruptions happen when lava and gas are discharged from a volcanic vent. ... The most dangerous type of volcanic eruption is referred to as a 'glowing avalanche'. This is when freshly erupted magma forms hot pyroclastic flow which have temperatures of up to 1,200 degrees.

A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that moves away from a volcano reaching speeds of up to 700 km/h (430 mph). The gases can reach temperatures of about 1,000 °C (1,830 °F).Pyroclastic flows are a common and devastating result of certain explosive eruptions; they normally touch the ground and hurtle downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.

Pyroclastic Rocks

Vitric shards

Crystal Fragments

Pyroclastic Deposits

Pyroclastic flows Pyroclastic flows are fluidized masses of rock fragments and

gases that move rapidly in response to gravity.

Pyroclastic flows can form in several different ways.

They can form when an eruption column collapses, or aslavathe result of gravitational collapse or explosion on a

dome or lava flow.

These flows are more dense than pyroclastic surges and

can contain as much as 80 % unconsolidated material.

Pyroclastic flowsThe flow is fluidized because it contains water and gas

from the eruption, water vapor from melted snow andice, and air fromdownslope.

the flow overriding air as it moves

Pyroclastic Flows

• Pyroclastic flows are gravity-driven surface flows of debris which travel as a high particle density solid-gas dispersion. • They can be thought of as a slurry with gas instead of liquid water.

Pyroclastic Flows

• Emplaced hot (not usually molten).

• Restricted to topographic lows.

Pyroclastic Flows

• Emplaced hot (not usually molten).

• Restricted to topographic lows.

pumice

lithics

Pumice flows = ignimbrites

Pyroclastic Flows

ground surge

Fossil fumaroleCarbonised wood

Pyroclastic Flows: Evidence for Heating

Welded Pyroclastic Flows

Dark fiamme make up the eutaxitic texture

Pyroclastic Flows

Pyroclastic Flows

Vent erosion causes increase in mass of plume

Pyroclastic flows often found at the top of the sequence prior to eruption of lavas

Pyroclastic Deposits: Air Fall (Tephra)

Pyroclastic air fall deposits (tephra) are poorly sorted

(except at large distances i.e. distal deposits)

Thickness and grainsize of air fall decrease away from vent.

Agglomerate close to vent, through lapilli to ash.

Pyroclastic Deposits: Air Fall (Tephra)

Pyroclastic Deposits: Air Fall (Tephra)

Bomb sags in bedded ash/lapilli.

Ballistic ejecta

Pyroclastic Deposits: Air Fall (Tephra)

Ballistic ejecta

Stratification due to pulsing of an eruption observed closer to the ventReverse grading occurs due to increasing vent diameter due to erosion

Increase in lithics

Air Fall

Pyroclastic Deposits: Air Fall (Tephra)

In more distal units layers may represent individual discrete eruptions.

Pyroclastic Surges

Base surge

Low particle density particle/gas suspension flows

Pyroclastic Surges

Base surge

Climbing dune forms

Pyroclastic Surges

Base surge

Cross bedding

Climbing dune forms

Epiclastic Deposits

Poorly consolidated volcaniclastic deposits are rapidly reworked by runoff to form epiclastics.

Flood plain

Epiclastic Deposits

Volcaniclastic deposits are often reworked to become epiclastic sediments.

Lahar Deposits

Mt St Helens, 2003

Lahar deposits caused by melting of ice and snow in 1981 eruption.

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