Volcanoes and volcanism

• Volcanoes represent venting of the Earth’s interior

• Molten magma rises within the Earth and is erupted either quietly (lavas) or violently (pyroclastics)

Terminology

Magma – molten rock sometimes containing suspended minerals and dissolved gases. Magma forms when temperatures rise sufficiently high for melting to occur in the Earth’s crust or mantle.

Volcano – a vent at the surface in which magma, solid rock, and gases erupt.

Lava – magma that reaches the surface and pours out over the landscape.

A-Characteristics of Magma

Composition –

Controlled by the abundant elements in Earth (Si, Al, Fe, Ca, Mg, Na, K, H, and O).

Most common types of magma are:

basaltic (~50% SiO2), andesitic (60% SiO2) and rhyolitic (70% SiO2)

Magma that solidify on the surface are called extrusive rocks and rocks that solidify below the surface are called intrusive.

~70 - 75% of all magma erupted by volcanoes is basaltic the rest is split between andesitic and rhyolitic.

Rock classification chart - USGSCharacteristics of Magma

Characteristics of Magma

Dissolved Gases

Comprise a small percentage of the magma (0.2 to 3 wt.%). Although not present in abundance these gases strongly influence the eruption style and explosiveness of the magma.

Dominantly H2O and CO2 with small amounts of nitrogen, chlorine, sulfur and argon.

Temperature

Ranges from ~800 C to ~1200 C

Viscosity

a substances resistance to flow

**dependent on temperature and composition

From: Baker D. et al. (2004) J. Geosc. Education

The Viscosities of Foods as Analogs for Silicate Melts

Explosive Eruptions vs. Effusive Eruptions

• Three factors effect the explosivity of a volcano– Temperature of magma

• High-temperature, less explosive

– Composition of magma• Less silica, less explosive

– Gas content of magma• Less gas, less explosive

fluidity

Volcano types

Volcano types: cinder cones

• Cinder cones are volcanoes which erupt only during one episode

• They are explosive, but small in size

• The cone is a pile of pyroclastic debris which piles up at the angle of repose

Volcano types: cinder cones

• The cinders are generally of basaltic composition

• The eruptive activity typically lasts a few months or years

Volcano types: shield volcanoes

• Shield volcanoes are broad, gently sloping volcanoes

• They are composed mainly of basaltic lava flows

• This is a view of Mauna Loa, Hawaii, from the cinder cones of Mauna Kea

Mauna Loa is the tallest volcano on Earth, as measured from the sea floor

Shield volcanoes on Mars

• Other planets also have shield volcanoes

• This is the largest shield volcano in the solar system, Olympus Mons on Mars

• Check out the scale !

Shield volcanoes: Earth vs. Mars

• Red = Hawaiian chain, which is superimposed on Olympus Mons

• this says it pretty well, I think !

Mauna Loa is about here

Volcano types: stratovolcanoes

• Stratovolcanoes consist of alternating layers of lava and pyroclastics

• They are dominantly andesitic in composition

• These volcanoes are typical of subduction zones

Mt. St. Helens (pre-1980)

Volcanic landforms

A large depression generally caused by the removal of large quantities of magma from beneath a volcano causing the ground to collapse into an empty space.

Aniakchak Caldera, Alaska, formed during an enormous explosive eruption that expelled more than 50 km3 of magma about 3,450 years ago. The caldera is 10 km in diameter and 500-1,000 m deep. Subsequent eruptions formed domes, and explosion pits on the caldera floor.

Volcano types: calderas

A-How and where do magmas and volcanoes form ?

What tectonic environment do these volcanoes occur in and why?

Now we need to answer………

How and why do magmas and volcanoes form?

-Global distribution of volcanoes

1-Magma generation at hot spots

• Basaltic magmas at hot spots are derived from deep within the mantle

• the magmas are fed by deep mantle plumes which are stationary relative to the drifting tectonic plates

Intraplate Volcanism

USGS

Hawai’i

• Best example of intraplate volcanism– More lava is extruded here constantly than

anywhere else on Earth!

A Bigger Picture• Looking at Hawai’i, and volcanic seamounts nearby

(underwater volcanic islands)

Current HawaiianIslands

What is a hot spot?

• Some mantle anomaly allows the oceanic or continental lithosphere to melt where it would not normally melt

• The anomaly (usually) stays stationary• The plate(s) moves over it

The Hawai’ian Hot Spot

University of North Dakota

Islands and seamounts get older as you move away from the hotspotIslands and seamounts get older as you move away from the hotspot

Hawaiian island trail

Does the kink represent a change in plate direction?

2-Magma generation at mid-ocean ridges

• In these zones, the mantle rises and melts, producing magma of silicate composition

• the magma continues to rise, and erupts mainly as basaltic lava flows

**volcanism and earthquakes are separate issues

This rifting process is dramatic on Iceland

• Iceland is literally being torn apart by rifting of the two plates…

• yet its center is continually renewed by new magma from the mantle…

• the same thing is going on under the ocean

One result of these processes

• Krafla volcano erupts frequently, producing spectacular fountains of fluid lava

3-Magma generation at subduction zones

• During subduction, the subducted oceanic plate is heated as it plunges into the mantle

• At a depth of 80-120 km, melting begins, and volcanoes are produced which parallel the subduction zone

Andesitic to Dacitic magmas are typical of these volcanoes

Stratovolcanoes

Volcanic landforms

Mt. Fuji, Japan

-explosive eruptions

-viscous lava

-built of interlayered lava and pyroclastic material

-usually andesitic in composition

Volcanoes and Plate Tectonics

Stratovolcano

Stratovolcano eruptions

Pinchincha, Ecuador Anak Krakatau, Indonesia

Indonesia

Population: 215 million World’s fourth most populous nation. 60% on island of Java

Volcanoes: 79 active - 20% of the world total -600 eruptions since 1800

Krakatau

August 27th, 1883

Height 2575 ft (785 m)

KrakatauCirca 1880

• 18 Cubic Km material ejected

Krakatau Eruption

• >36,000 people killed• Blast 10,000 greater than at Hiroshima

The explosion blew away the northern two-thirds of the island and it was almost instantaneously followed by the collapse of the unsupported volcanic chambers which formed the huge underwater caldera

Other Features: calderas

NOW, WHAT ABOUT RHYOLITIC VOLCANISM?

• SiO2 contents are even greater than is the case in andesitic magmas, therefore....... viscosities are even greater.

• So, Incredible resistance to flow!

NOW, WHAT ABOUT RHYOLITIC VOLCANISM?

• In addition, rhyolitic magmas tend to be richer in H2O, because they form by partial melting of the crust, and melting is only possibly there if H2O is present.

• In other words, rhyolitic magmas exsolve more H2O (more bubbles form as the magmas rise), yet the bubbles cannot expand owing to the high viscosity of the magma (they must expand as pressure decreases, i.e., as the magmas rise through the crust)

• Recipe for a major disaster!!!

Yellowstone National Park, Wyoming,offers an excellent example of rhyolitic volcanism

Yellowstone National Park, Wyoming,

• The volcanic eruptions, as well as the continuing geothermal activity, are a result of a large chamber of magma located below the caldera's surface.

• The magma in this chamber contains gases that are kept dissolved only by the immense pressure that the magma is under.

• If the pressure is released to a sufficient degree by some geological shift, then some of the gases bubble out and cause the magma to expand.

• This can cause a runaway reaction. If the expansion results in further relief of pressure, for example, by blowing crust material off the top of the chamber, the result is a very large gas explosion.

Geysers and fumaroles!

The H2O is of near-surface origin, but the heat is due to a batholith (still partially

molten) not far below the surface

Life Cycle of a CalderaInitial eruptive stage

along ring fractures

Collapse alongside

eruption

Remaining lava

extruded

Resurgent dome

formsCalderas can host lakes

-Active hydrothermal systems

Uplift and ring

fracture formation

Smith and Bailey

• Calderas are primarily rhyolitic – largest explosive eruptions are caldera-related– lava is cool and viscous, rises slowly, allowing pressure to build up– gas percolates slowly through the viscous magma, does not have an

easy way to vent

• Stratovolcanoes are dacitic-andesitic– have eruptions of intermediate explosivity– can undergo lava flows if lava is mafic enough and hot enough– lava domes if lava is more felsic and cooler– gas does vent, but slowly, through fissures

• Shield volcanoes are usually basaltic– lava is very hot and fluid– gases easily pass through magma to be released into the atmosphere– experience gentle, effusive activity

• fountaining if pressure builds, usually at the start of an eruption

SUMMARY

Volcanic activity

• In the following slides, I will give you some examples of volcanic activity:

– lava flows, including flood basalts

– lava domes

– pyroclastic falls and pyroclastic flows

– lahars and debris avalanches

– volcanic gases

Volcanic activity: lava flows

• This is a basalt lava flow in a channel

• Due to its low silica content and high temperature, it is quite fluid (but stickier than maple syrup)

• Yet lava usually flows fairly slowly

Pahoehoe lava

This is a Hawaiian term for smooth, ropy lava

It generally exhibits fluid-like textures

Do you want to walk on pahoehoe ?

Aa lava

• This type of lava is quite blocky on the surface, and comparatively cool

• Yet below the surface, the lava is fairly massive and much hotter

• Do you want to walk on aa ?

Fire fountaining

• Sometimes, basaltic lava can contain lots of gas

• Then, small explosive eruptions form fire fountains

• As partially liquid drops fall back to the ground, they may coalesce to form a lava flow

Flood basalts

• The previous examples represent small-scale activity

• But basaltic eruptions can be huge, forming lava plateaus

• These huge outpourings may occur quickly (1-3 Ma) and may contribute to mass extinctions

Global distribution of large igneous provinces (LIPS)

Mainly flood basalts

Lava domes

Mt. Unzen, Japan

Unzen began growing a lava dome in mid-1991. The dome complex continued to grow until 1995

Lava domes at Unzen

• This dome was the first to be erupted, in May 1991

• The lava is silica-rich and thus highly viscous (sticky) and cannot easily flow

• Thus it tends to form steep-sided domal structures

Volcanic activity: lava domes

• By early 1995, the dome complex had grown substantially and was highly oversteepened

• As pieces of the dome broke off, they would fragment, creating pyroclastic flows

Volcanic activity: pyroclastic falls

• During explosive volcanic eruptions, ash falls downwind of the volcano

• In the case of very large eruptions, the ash may be deposited over a vast area

Ash fall-out from the Mt Pinatuba eruption

Volcanic activity: pyroclastic flows

• Pyroclastic flows are suspensions of hot pyroclastic material, air, and gas which descend under the influence of gravity

• Their velocity is generally very high (50-500 km/hr)

• This example is a flow from Mt. St. Helens

Volcanic activity: pyroclastic flows

• This is another example, descending the slopes of Unzen volcano after part of the dome has collapsed

• The flow has a dense core which is hidden by the billows of ash which are rising

Unzen, 24 June 1993

Ash fall Hazards

Aviation safety

Death

Structural damage

Contaminated drinking water

Climate change

Volcanic activity: lahars

• Lahar is an Indonesian word for volcanic debris flow

• Lahars are flows of water and loose volcanic debris

• They are especially prevalent at snow-clad and ice-clad volcanoes

Lahars• Lahars are volcanic mudflows• They are triggered in one of three ways

– Volcanic activity melting snow and ice on the volcano• Or a summit crater lake rupturing and draining due to volcanic activity

– Torrential rainfall providing the water source• Does not require an active volcano• Rainwater mixes with existing ash to create the lahar

– Rocks and ash from a landslide entering an existing drainage route• Can destroy dams, unleashing more water

• As they progress, lahars can undergo a process called “bulking”– Incorporate material from the area they flow over

• Erode sides of existing drainage channel• As they progress, they are also diluted by existing river water

– A dilute lahar is called a hyperconcentrated flow• Following a river valley, these fast-moving flows can transport ash and

sediments over 250 km away from the source

Impact of Lahars

• During bulking, large objects like rocks, cars, houses can be picked up– These often end up destroying bridges as the lahar

progresses

• People living in river valley drainage systems of volcanoes are at risk

• Sediment load from the volcano can affect the way a river flows, fish populations, etc.

Lahars

(Castatela 1980) – Mt. St. Helens

(Castatela 1980) – Mt. St. Helens

Volcanic activity: debris avalanches

• Sometimes a volcanic edifice is weakened

• Wholesale collapse of part of the volcano may ensue

• During collapse, a debris avalanche occurs, and a scalloped scar remains

Unzen volcano, with the 1792 scar in the foreground

Volcanic activity: gases

• Volcanic gases are typically highly acid

• Major constituents include H2O, CO2, HCl, SO2, and HF

• This photo shows gas emission from Masaya volcano in Nicaragua

Volcanic activity: gases

• This is also Masaya volcano…

• but this photo was taken from the space shuttle

• it shows the gas plume being blown out over the Pacific Ocean

Volcanic activity: gases

• About 15 km downwind from Masaya, the coffee crop is adversely affected by the acid gases

Volcanic gases

• Don’t forget that powerful explosive eruptions can inject large amounts of gas into the stratosphere

• The impact is climate change...

Sizes of volcanic eruptions

• The Volcano Explosivity Index (VEI) is similar to the Richter scale for quakes

• It is logarithmic

• It emphasizes the degree of explosivity of eruptions

VEI – Volcanic Explosivity Index

• Eruptive styles– Hawaiian eruptions can produce lava, but not much ash– Strombolian eruptions produce bombs and lapilli and more ash– Vulcanian eruptions have an ash plume and only minor blocks– Plinian eruptions have lots of ash, minimal lapilli and blocks

Low sio2

High SiO2

Volcanic hazards of Canada

Canada has “active” volcanoes (black triangles) which pose a potential threat in B.C.

Another major hazard is ashfall from explosive eruptions of Cascade volcanoes in Washington state

Case history: Mount St. Helens, Washington state, USA

Tectonic setting of Mt. St. Helens

Previous eruptions of Mt. St. Helens

• The volcano is the most active of all the Cascades volcanoes

• Based on previous activity, there was a fairly high probability that the volcano would again erupt before the millenium

Stages at Mt. St. Helens

• Stage 1: precursory activity, 20 March - 18 May 1980

• Stage 2: the climactic eruption of 18 May 1980

• Stage 3: post-climactic activity, 1980-present

Stage 1: Precursory activity; eruptions

• The first phreatic eruption occurred on 27 March 1980

• then explosions continued…the volcano was preparing itself

• The eruptions of 13, 18 April consisted of steam and ash

27 March 1980 eruption

Precursory activity: seismicity

• A magnitude 4.1 earthquake was recorded on 20 March 1980 under the volcano

• By 1 April, harmonic tremor was observed (continuous seismic signal of similar wavelength)

• this is a pretty good indication that magma is involved

A seismogram from seismic station RAN on 2 April showing the occurrence of harmonic tremor

Harmonic tremor

Precursory activity: deformation

• A bulge was first detected on the NNE flank of the volcano on 19 April 1980

• Deformation was as high as 5 feet/day !

• This was an indication that magma had moved into the volcano itself

• At the same time, eruptive activity decreased during 14-23 April

bulge28 September 1979 17 May 1980

Development of the bulge

Precursory activity: gas emissions

• Although some sulfur dioxide was detected, not very much was actually being emitted

• This suggests that the volcano had somehow sealed itself, and that pressure was building

• This interpretation is consistent with (a) the bulge and (b) decreased eruptive activity

Stage 2: The climactic eruption, 18 May 1980

• At 0832 local time, a M 5.1 earthquake struck the volcano

• A large portion of the volcano slid away

• This simultaneously developed a debris avalanche and a lateral blast

Sliding...

Sliding and depressurizing...

…and blastingAnd blasting

The blast

• The lateral blast was comparatively cool at 100-300° C

• But its speed approached 500 km/hr

• It was therefore devastating to a very large area (180°, up to 20 km distance)

Tree blowdown by the blast

Note trees still standing

Sector collapse and the debris avalanche

• The sector collapse reduced the height of the volcano substantially

• A horseshoe-shaped amphitheatre was formed

• The avalanche deposit was emplaced in the Toutle River valley

Lahars

• The mixing of melted snow and ice with loose pyroclastic material created the perfect conditions for lahars

• Also, the debris avalanche deposit was water-saturated, producing lahars

Debris carried by lahars

Note the “high-water” mud marks on the trees

Monitoring and forecasting lahars

• To monitor lahars, a series of acoustic flow monitors (AFMs) have been installed

• They measure the ground vibrations from lahar movement

• They are insensitive to ground motion from quakes, which occur at a lower frequency

Hazards associated with volcanoes

Final costs from the 18 May devastation

• 35 people dead (it was a Sunday)

• 22 people never found

• $ 2.7 billion US in damage

• contrast this with the $ 10-20 billion in losses from the 1989 Loma Prieta and 1994 Northridge earthquakes

Would you call this event a disaster?

Stage 3: Post-climactic activity

• Subsequent activity consisted of generally diminishing explosive eruptions

• By mid-late 1980, lava domes began to grow in the amphitheatre

• these were repeatedly destroyed by small explosive eruptions

Explosive eruptions

Explosive activity diminished gradually over the next several years

Pyroclastic flows

Spectacular pyroclastic flows were observed forming from the crater

These are some of the best examples you will ever see

Pyroclastic flows - note big rounded pumices

Dome growth-destruction cycles

Dome growth-destruction cycles

Early lava dome

Dome growth-destruction cycles

Late lava dome

Volcanic ash and aviation safety

• Many of the world’s civil air routes cross active volcanoes

• Ash erupted by a volcano frequently reaches aircraft altitudes (9-12 km)

• Aircraft encounters with ash are potentially fatal

• Ash can clog engines, causing them to fail

Global air routes

North Pacific air routes

Volcanic ash and aviation safety

• All this shows that good communication among volcanologists, meteorologists, and aviation people (especially pilots!) is essential

Volcanoes - web

• Four good starting points for volcanoes:

• http://gsc.nrcan.gc.ca/volcanoes/index_e.php

• http://vulcan.wr.usgs.gov/home.html

• http://www.geo.mtu.edu/volcanoes/

• http://volcano.und.nodak.edu

Volcanoes - reading

• Francis, P., 1993. Volcanoes a planetary perspective. Oxford, Clarendon Press.

• Sigurdsson, H., B.F. Houghton, S.R. McNutt, H. Rymer, and J. Stix, eds., 2000. Encyclopedia of volcanoes. San Diego, Academic Press.

• Smith, W.S., 1983. High-altitude conk out. Natural History, v. 92, no. 11, November 1983, pp. 26-34.

• Tilling, R.I., 1998. Volcanoes. http://pubs.usgs.gov/gip/volc/text.html