Volcanoes

Ms ShaltoLower six

• Volcano - conical or dome-shaped landform built by the emission of lava, gases and solid fragments from a narrow vent in the surface.

• Volcanic materials: include: lava, gases and solid particles - tephra: particles of all sizes, ash, volcanic bombs .

Volcanoes

• A volcano is a place on the Earth’s surface where hot, molten rock (called magma) breaks through.

• There are many different types of volcanoes and material that is erupted.

• However, in general a volcano is classed as “active” if it erupts lava, rock, gas or ash, or if it shows seismic (earthquake) activity.

• A volcano is dormant if it hasn't erupted for a long time (less than 1 million years) but could again one day.

• An extinct volcano will never erupt again.

How and why do volcanoes erupt?• Hot, molten rock (magma) is buoyant (has a lower density

than the surrounding rocks) and will rise up through the crust to erupt on the surface.– Same principle as hot air rising, e.g. how a hot air balloon works

• When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts.

• Large amounts of gas and a high viscosity (sticky)

magma will form an Explosive eruption!– Think about shaking a carbonated drink and then releasing the

cap.

• Small amounts of gas and (or) low viscosity (runny)

magma will form an Effusive eruption– Where the magma just trickles out of the volcano (lava flow).

What causes volcanoes?

What causes volcanoes?

Hot Spots

• Hot mantle plumes breaching the surface in the middle of a tectonic plate

What are Hotspot Volcanoes?

Photo: Tom Pfeiffer / www.volcanodiscovery.com

The Hawaiian island chain are examples of hotspot volcanoes.

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.

The volcanoes get younger from one end to the other.

CONTROLS ON EXPLOSIVITY:Possible interpretive activity

SiO2 MAGMA TEMPERATURE VISCOSITY GAS ERUPTION STYLE

(silica) TYPE (centigrade) CONTENT

~50% mafic ~1100 low low Nonexplosive

~60% intermediate ~1000 intermediate intermediate Intermediate

~70% felsic ~800 high high Explosive

Types of lava The nature of eruption and the resulting volcanic

landforms depend on the nature of lava:

• FELSIC LAVA - high degree of viscosity, resists flow => violent, often explosive eruptions causing steep composite cones.

• Eruptions of felsic lava typically create strato volcanoes, also known as composite volcanoes

• Features of a strato volcano:– steep slopes, - lots of tephra, - violent explosions, -

explosive discharge of gases.

• Examples of composite volcanoes are: Mt. Etna (Italy), Kilimanjaro (Africa), Fujiyama (Japan) Cotopaxi (Equador) Penatubo (Phillipines)

Types of lava

• MAFIC LAVA - highly fluid => relatively quiet eruptions causing lava flows

• Mafic lava (basaltic) produces effusive eruptions that create shield volcanoes with gentle long slopes (e.g. Hawaii)

• lava often flows from fissures - cracks - on the sides of the volcanic dome to form extensive deposits of plateau basalts, or flood basalts

Explosive vs. Effusive

Explosive Eruptions

• Explosive volcanic eruptions can be catastrophic

• Erupt 10’s-1000’s km3 of magma

• Send ash clouds >25 km into the stratosphere

• Have severe environmental and climatic effects

• Hazardous!!!Mt. Redoubt

Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska

Explosive Eruptions

If you release the pressure of a magma chamber (by cracking the surrounding rock or breaking through to the surface) the gas dissolved in the magma will start to exsolve (separate from the melt forming bubbles).

These bubbles, called vesicles, rapidly expand and rise through the magma.

• The rapid escape of gas (volatiles) causes magma to fragment and erupt explosively.

Explosive Eruptions

• Three products from an explosive eruption– Ash fall– Pyroclastic flow– Pyroclastic surge

Pyroclastic flows on Montserrat, buried the capital city.

Ash fall• The fallout of rock, debris and ash from an explosive

eruption column.• An explosive volcanic eruption will propel large volumes of

volcanic rock, ash and gas into the atmosphere. • The larger (most dense) particles will fall out of the air

quickly and close to the volcanic vent. • The smaller particles (ash) can be suspended in the

atmosphere for days to weeks before they fall back to Earth.

• Whilst in the atmosphere the wind can transport the ash particles large distances.

• For example when Mt Pinatubo (Philippines) erupted in 1991 ,ash was blown all the way around the entire globe

Pyroclastic flows• Are hot, turbulent, fast-moving, high particle

concentration clouds of rock, ash and gas.

• Pyroclastic flows can reach > 100 km from a volcano.

• They can travel 100s km/h and are commonly >400°C.

• They will destroy everything in their path including buildings, agriculture and forests. Although because they contain a high concentration of particles and a low concentration of gas, they are dense and usually are confined to, and flow along, topographic lows (valleys).

• It is extremely important to understand them as they are often the most hazardous component of an explosive eruption.

Direct measurements of pyroclastic flows are extremely dangerous!!!

Types of Volcanoes

Shield volcano

Cinder cone volcano

Composite volcano

Types of Volcanoes: Stratovolcanoes (Composite)

Tall, conical volcanoes with many layers (strata) of hardened lava, tephra and volcanic ash

Characterized by steep profiles and periodic, explosive eruptions

Lava tends to be viscous (very thick)

Common at subduction zones where oceanic crust is drawn under continental crust

Types of Volcanoes: Cinder Cones A cinder cone is a steep conical

hill of volcanic fragments that accumulate around and downwind from a volcanic vent.

The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly.

Cinder cones range in size from tens to hundreds of meters tall. Cinder cones are made of pyroclastic material.

Caldera • A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir.

• Calderas are different from craters, which are smaller, circular depressions created primarily by explosive excavation of rock during eruptions.

• Left: Mount Aso vents a large cloud of steam at Japan's Aso National Park. The mountain's caldera is one of the world's largest.

Effusive Eruptions

• Effusive eruptions are characterised by outpourings of lava on to the ground.

Hawaii

Courtesy of www.swisseduc.ch

Effusive eruption• A volcanic eruption dominated by the passive outpouring of lava

onto the Earth’s surface is called an effusive eruption.• This happens either because there is not enough gas (volatiles) in

the magma to break it apart upon escaping, or the magma is too viscous (sticky) to allow the volatiles to escape quickly.

• Remember: molten rock is called “magma” when it is underneath the ground. It is called “lava” once it has been erupted onto the surface.

• Lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge rate (how fast it comes out of the vent), slope of the ground over which the lava travels, and duration of eruption.

• Although not generally as hazardous as explosive eruptions, lava flows can burn and bury buildings and forests and do pose a danger to people living on or near an active volcano.

Types of Volcanoes: Shield VolcanoesShield volcanoes are

huge in size. They are built up by

many layers of runny lava flows spilling out of a central vent or group of vents.

The broad shaped, gently-sloping cone is formed from basaltic lava which does not pile up into steep mounds.

Volcanic Mountains & Islands

• Submarine volcanoes sometimes form Islands (Hawaii, Philippines, Japan, Sicily).

• Some of the highest mountains in the world were formed by volcanoes –

a) Mt. Fujiyama in Japan, b) Mt. Cotopaxi in Ecuador, South America c) Mt. Kilimanjaro in Tanzania, Africa, d) Mount Shasta in California and Mt. Rainier in Washington state, USA.

Intrusive & Extrusive Volcanic Features

Intr

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volc

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• Dyke: thin vertical veins of igneous rock that form in the fractures found within the crust. Because these intrusive features cool quickly their rocks are dominated with fine mineral grains.

• Sill: horizontal planes of solidified magma that run parallel to the grain of the original rock deposit

• Batholith: large plutonic masses of intrusive rock with more than 100 square kilometers of surface area.

• Volcanic Pipe: if a dyke reaches the surface of the Earth it is then called a volcanic pipe.

Distribution of Volcanoes

Volcanic Hazards

• There are about 500 active volcanoes throughout the world.

• In an average year approximately 50 of these erupt.

• Volcanic hazards create fewer disasters and deaths compared with earthquakes and severe storms.

Primary and Secondary hazard

Volcanoes are composite hazards. There are both primary and secondary

hazards which can be caused by volcanic eruptions.

The primary hazards include pyroclastic flows, air-fall tephra, lava flows and volcanic gases.

The secondary hazards include ground deformation, lahars (mudflows), landslides and possibly tsunamis in ocean floor volcanic eruptions.

Primary Volcanic Hazards: Pyroclastic Flows

• Pyroclastic flows are fast-moving, avalanche-like, ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that can travel at speeds in excess of 150 km per hour.

Pyroclastic Flow - direct impact

Courtesy of www.swisseduc.ch

Pyroclastic Flow - burial

Pyroclastic Flow - burns

Primary Volcanic Hazards: Lahars

• Lahars, also known as mud flows or debris flows, are slurries of muddy debris and water caused by mixing of solid debris with water, melted snow, or ice.

Lahars

• Heavy rain after an eruption or hot volcanic activity melting snow and ice will provide a large volume of water that will flow down the sides of the volcano. This water picks up the newly erupted material forming fast flowing torrents of water, mud, ash, rock and debris.

• Lahars can flow great distances and be very destructive. The bottom photo shows a lahar knocking down a concrete bridge.

Primary Volcanic Hazards: Tephra

Tephra (ash and coarser debris) is composed of fragments of magma or rock blown apart by gas expansion.

Tephra can cause roofs to collapse, endanger people with respiratory problems, and damage machinery.

Tephra can clog machinery, severely damage aircraft, cause respiratory problems, and short out power lines up to hundreds of miles downwind of eruptions.

Primary Volcanic Hazards: Gases

The concentrations of different volcanic gases can vary considerably from one volcano to the next.

Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide.

Other principal volcanic gases include hydrogen sulfide, hydrogen chloride and hydrogen fluoride.

A large number of minor and trace gases are also found in volcanic emissions, for example hydrogen, carbon monoxide, halocarbons, organic compounds, and volatile metal chlorides.

Primary Volcanic Hazards: Lava Flows

• Lava flows are generally not a threat to people because generally lava moves slowly enough to allow people to move away; thus they are more of a property threat.

Lava Flow - Heimaey, Iceland

• Iceland, January 23,1973.

• Large fissure eruption threatened the town of Vestmannaeyjar.

Lava Flow - Heimaey, Iceland

• The lava flows caught the inhabitants by surprise

• Before the eruption was over, approximately one-third of the town of Vestmannaeyjer had been destroyed

Lava Flow - Heimaey, Iceland

• However, the potential damage was reduced by spraying seawater onto the advancing lava flows.

• This caused them to slow and/or stop, or diverted them away from the undamaged part of the town.

Secondary Volcanic Hazards: Flooding

Drainage systems can become blocked by deposition of pyroclastic flows and lava flows. Such blockage may create a temporary dam that could eventually fill with water and fail resulting in floods downstream from the natural dam.

Volcanoes in cold climates can melt snow and glacial ice, rapidly releasing water into the drainage system and possibly causing floods.

Secondary Volcanic Hazards: Famine

Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree Fahrenheit for periods of one to three years.

Tephra falls can cause extensive crop damage and kill livestock which may lead to famine.

How do we minimize the risk of active volcanoes?

Volcano Monitoring

Volcano Observatories are set up on all active volcanoes that threaten the human population. These are designed to monitor and potentially to predict the eruptive behaviour of the volcano in question.

Volcano Monitoring

• Seismicity• Deformation• Gas Output

– (on volcano and remote sensing techniques)

These three things are the most important

precursors to an eruption.

Seismic Activity

• Earthquake activity commonly precedes an eruption– Result of magma pushing up towards the surface– Increase volume of material in the volcano shatters the rock– This causes earthquakes

Seismic Activity

• Earthquake activity is measured by Seismographs– Seismographs are stationed on the flanks of the volcano– These record the frequency, duration and intensity of

the earthquakes and report it back to the volcano observatory.

Deformation Monitoring

• “Tiltmeters” are used to measure the deformation of the volcano– The tiltmeters measure changes in slope as small

as one part per million. A slope change of one part per million is equivalent to raising the end of a board one kilometer long only one millimeter!

Deformation Monitoring

• Tilltmeters can tell you when new material enters the magma chamber.

Note the presence of earthquakes in relation to the deformation. Often it is a combination of events that fore-warns of an eruption.

A

B

Gas Monitoring

• Commonly gas output from a volcano increases or changes composition before an eruption.– As magma rises to the surface it releases (exsolves) much of its

gas content.– This can be measured

Gas Monitoring

• Gas samples are collected from fumaroles and active vents.

• Gas levels may also be monitored by remote sensing techniques