Topic 4Volcanism and Volcanoes

Topic 4 - Volcanism and Volcanoes

Outline

Introduction

Volcanic Products: Gases, Lava and Pyroclastics

Non-Explosive vs. Explosive Eruptions

Types of Volcanoes

Volcano Distribution

Prediction of Volcanic Eruptions

Index of Volcanic Explosivity

Volcanoes

produce a

variety of

hazards that

kill or injure

people and

destroy

property.

Some

hazards,

such as

landslides or

lahars, may

occur when

a volcano is

not erupting.

Danger

• Toxic gases.

• Explosions.– Nuée ardente…

• Mudflow / lahars.

e.g. Mount Pelée - May 8, 1902

St. Pierre, Martinique, West Indies, Caribbean

28,000 casualties

Death

Only 2 out of the city’s 28,000 residents survived. Nuée ardente also burned ships in the harbor.

Destruction

• Property damage.

• Road destruction.

• Forest blow-down.

Tourism!

IntroductionVolcanic activity on a world wide scale is a

relatively rare process.

Usually occurs in sparsely populated areas.

Therefore these eruptions do not cause

significant damage.

However, if and when volcanic eruptions

occur in densely populated areas, great

catastrophes result.

 

volcanism

Designates the aggregate of processes

associated with the transfer of material from

the Earth’s interior to the surface and the

construction of various structures at the

surface.

volcano

A vent or fissure through which molten and solid material and hot

gases pass upward to the Earth’s surface. Forms a conical mountain.

Many shapes and sizes.

 

A volcano is a relatively high topographic feature. Built as a result of

the addition of volcanic material. A volcano can have a small crater in

the center. Or a larger feature called a caldera.

crater

A circular depression at the

summit of a volcano.

Generally less than 1 km in

diameter.

caldera

A very large volcanic crater. > 1 km in diameter. May form via the

coalescence of several smaller craters, repeated explosions, collapse

or the stoping of surface rocks by a large underground magma

chamber.

  

e.g. Crater Lake, Oregon (mis-named…should be Caldera Lake).

Mount Mazama erupted 6600 years ago. Great volume of material

collapsed and was lost. Depression that remains is filled with water and

is now called Crater Lake. Wizard Island is the new volcanic cone.

  

e.g. Yellowstone is a caldera. Which has swelled up again as new

magma is introduced to the chamber underneath. Supervolcano.

  

Volcanoes and extrusive igneous rocks can build various kinds of

landforms. More on this following the next section.

Events leading to the origin of Crater Lake,

Oregon.

Caldera Formation: Crater Lake

Yellowstone caldera.

Formed approximately

600,000 years ago.

Supervolcano.

Volcanic Products

• Gases

• Lava

• Tephra or

pyroclastics

Gases

• usually first stage of volcanic activity is the release of gases

• major constituent of volcanic gas is superheated steam– constitutes 50-95% of the volume

• some of this steam is:– recycled ground water– from reaction of the hydrogen from

the magma with the oxygen of the atmosphere

– some is juvenile water• water formerly dissolved in molten

rock (magma) deep beneath the Earth’s surface

– Other examples of volcanic gases: CO2, N2, SO2, SO3, CO, HCl,

H2S, CH4

– Early Earth’s atmospheric composition.

• Hazardous gases rarely reach populated areas in toxic concentrations, however:

• SO2 can react in the atmosphere: acid rain downwind.

• Some gases may be absorbed by volcanic ash: which then falls on land. May be incorporated into plants and animals and ultimately humans. e.g. fluorine (as HF).

• Acid gases may produce detrimental effects on vegetation or kill the vegetation. e.g. Kilauea in Hawaii damaged plum and other fruit trees at distances up to 30 miles way.

Trees killed by carbon dioxide rising from beneath Mammoth Mtn volcano, CA.

Dormant volcanoes may emit gases for long periods of time.

e.g. Lake Nyos in Cameroon, 1986.

Gas dissolved and kept in the bottom sediments by the hydrostatic

pressure.

Shaking of ground from a small earthquake, led to a subaqueous land

slide and release of gases from the bottom sediments.

Release of gas, believed to be CO2 (colourless, odorless), killed

approximately 2000 people and numerous animals. CO2 denser than

air, therefore flowed downhill along the surface.

• magma – molten rock material

originating deep beneath the Earth’s surface

– magma rises because it is lighter than the surrounding rock (dissolved gases)

– works its way to the surface along fractures or dissolving rocks in its path

• lava – eventually this molten rock

material may escape through a central vent or elongate fissure at the Earth’s surface… lava

Lava

Lava tube: hollow beneath a lava flow.

Collapsed roof of a lava tube. Can observe the active flow underneath.

2 main types of subaerial basaltic lava flow

Pahoehoe

– flow has a ropy texture and smooth surface

– lower viscosity than aa flows

Aa

– flow is rough and jagged

– angular blocks and fragments are common

• depending on their composition and temperature, magmas differ greatly in their viscosity

Aa flow

Pahoehoe flow

Pahoehoe

lava flow in

Hawaii.

Aa lava

flow

advances

over an

older

pahoehoe

lava flow in

Hawaii.

Pillowed Lava Flow

– lava flow that formed

under water

– interaction of water with

the molten lava forms a

thick glassy crust around

the flowing lava

– form tube structure:

pillowed lava flow

Pillow lava: modern versus ancient examples.

• Thick massive lava flow cooling.

• Columnar jointing:

– heat contraction cracks

– form polygons

– perpendicular to cooling

surface

Tephra or Pyroclastic Debris

Solid material. Solid rock material ejected from volcanoes consists of particles torn from the walls of the volcanic vent (both volcanic and non-volcanic rocks).

Lava clots sprayed into the air cool rapidly and are solid by the time they reach the ground.

 

tephra

A collective term designating all particles ejected from volcanoes. Irrespective of size, shape or composition.

 

pyroclast

An individual particle ejected during a volcanic eruption. Classified according to size.

Pyroclast classification was presented in

the previous lecture on igneous rocks.

Given below, one more time.

 

May range in size from:

dust < 1/16 mm

ash 1/16 to 2

mm

lapilli 2 to 64 mm

blocks or bombs > 64 mm

Blocks are angular (originally were

pieces of rock). Bombs are rounded or

streamlined (originally globs of fluid).

  

Dust ash emitted during a volcanic

eruption may be blown high into the air.

Can be carried great distances by the

prevailing winds.

Lapilli

Volcanic Bomb Ash

e.g. Mount Pinatubo, Philippines (June

15, 1991). Ash was sent into the

upper atmosphere.

Resulted in cooler temperatures. Ash

was visible from airplanes at cruising

altitude (33,000 feet): haze.

Lahar: a mudflow composed of

volcanic material such as ash (has

consistency of wet concrete).

Marella River valley, Philippines.

Tropical storm followed the volcanic

eruption.

Homes partly buried by a lahar.

Mount Pinatubo, Philippines. June 1991.

Lahars inundated Armero, Columbia on November 13, 1985.

23,000 were killed. Second most tragic volcanic event of the 20th century.

(Mount Pelée - May 8, 1902: 28,000 casualties)

Mt. St. Helens

May 18, 1980

(a) Location map.

(b) Mudflows.

(c) Tree blowdown.

(d) Ash cloud.

Nuée ardente – defined as a swiftly flowing, turbulent gaseous cloud, sometimes incandescent, erupted from a volcano, and containing ash and other pyroclastic materials. Density current of pyroclastic flow. “Glowing

cloud”.

Mt. St. Helens8:45 am

May 18, 1980

Car incinerated by hot pyroclastic flows and buried in ash. Note sun-visor remains (i.e. metal frame).

Day turned to night in nearby towns: May 18, 1980.

Yakima, Washington.

85 miles downwind.

Ash clogged air filters – vehicles abandoned.

Shoveling heavy ash (not snow!) – density at least 2.5 x that of snow.

Dust and ash from Mt. St. Helens eruption. Erupted Sunday May 18,

1980. May long-weekend in Canada (Monday May 19th - Victoria Day).

Ash

reached

Brandon by

Monday

May 19th.

Street

lights came

on in the

day time

on

Tuesday

May 20th.

View from Brodie Building - looking north.

View from Brodie Building - looking north. Monday May 19, 1980.

Non-Explosive vs. Explosive Eruptions

Type of volcanic eruption depends on whether gases escape easily or are confined. Depends on their composition.

Magmas differ greatly in their fluidity (inverse of viscosity). Magma viscosity is determined mainly by the silica content. As discussed earlier there are three categories of magmas based on SiO2 content.

This relationship is also applicable to viscosity: 

45-52 % low viscosity Hawaii, shield volcanoes, basaltic53-65 % composite volcanoes e.g. Mt. St. Helens> 65 % high viscosity e.g. Mt. Lassen, California

  

Two types of eruptions are the result: “non-explosive” and explosive.

“Non-Explosive” Eruptions

If a magma flows easily (low viscosity), gases that come out of solution

have little difficulty rising through the melt and escaping.

Thus reducing the possibility of a pressure build up and explosion.

Steady escape of gases. Low silica, viscosity. Molten rock flows “quietly”

as fluid lava. Flows may travel up to 100 km/hr down steep slopes.

 

e.g. Hawaiian lava. “Hot-spot” volcanism. Mafic lava. Basalt.

e.g. Iceland lava. Mid-oceanic ridge volcanism. Mafic lava. Basalt.

e.g. “Non-Explosive” Eruptions

Heimaey, Iceland.

First day of the Heimaey, Iceland volcanic eruption: January 23, 1973.

Eruption on Heimaey, Westman-Islands began on January 23, 1973.

Explosive Eruptions

In contrast, the escape of gases from high viscosity magma is hindered.

Gas pressure may build up to a critical level.

Finally released as an explosion.

 

e.g. Mt. St. Helens. Andesite to rhyolite (i.e. dacite).

Moderate to high silica content.

Moderate to high viscosity.

 

At the other extreme, explosive release of trapped gases when the magma

reaches the surface may disintegrate the magma into blebs and clots of

molten magma, partly solidified magma and fragments of the walls of the

volcanic vents are blown into the atmosphere.

Lateral blast. Mt. St. Helens viewed from the northeast: May 18, 1980.

Mt. St. Helens viewed from the south: May 18, 1980.

Mt. St. HelensBefore

After

Types of VolcanoesLava and pyroclastic debris. Accumulate in various proportions around volcanic vents. Builds up various landforms. Five key landforms: Cinder Cones, Shield Volcanoes, Composite Cones or Strato-volcanoes, Lava Dome, Fissure Eruptions.

Volcano Types

Cinder Cones

Mostly pyroclastics.

Cones made as material is blasted out and then settles to the ground.

Composed of ash and cinders.

 

Small to moderate size.

e.g. Springerville, Arizona.

Cinder cone (230 m high) in Lassen Volcanic National Park, CA.

View from the top of the cinder cone in Lassen Volcanic National Park, CA.

Cinder cones in Iceland: Eldfel and Helgafel. Active in 1973.

Shield Volcanoes

Mostly lava. Low relief. Slope of 3.5º near the top of the

volcano. Lava may flow downslope in lava tubes.

 

“Non-explosive”. Basaltic. 45 - 52 % SiO2.

 

e.g. Mauna Loa, Hawaii. Total relief is nearly 34,000

feet. 160 km in diameter.

Equal height above

and below seawater.

e.g. 5 km below sea

level and 4.1 km

above sea level.

(a)Shield volcano. Each

layer consists of

numerous thin basalt

lava flows.

(b)Mauna Loa, active shield

volcano on Hawaii, with

its upper 1.5 km covered

by snow.

(c) Crater Mountain is an

extinct shield volcano in

Lassen County, CA.

Approximately 10 km

across and 460 m high.

Composite Cones or Stratovolcanoes

Interbedded lava and pyroclastics.

Formed from intermittent eruptions.

Steep-sided volcanic cone (30º). This equals the angle of repose.

Therefore can not get any steeper for pyroclastic deposits.

 

Produced by alternating layers of pyroclastic debris and lava flows.

Interbedded pyroclastics and intermediate silica lavas (» 60 % silica).

 

Produce majestic peaks in the Andes, Cascades, Alaska, B.C. and

Japan. e.g. Mt Fuji, Mt. St. Helens, Mt. Rainier. Composite volcanoes

are responsible for most volcanic hazards.

Why are composite volcanoes responsible for most

volcanic hazards? Explosive eruptions. Common

along convergent plate boundaries. Usually near

coastal areas: densely populated.

Mayon volcano, Philippines

Mount Shasta, California

Mount Shasta, California

Composite Volcanoes or Stratovolcanoes

Nyiragongo, Democratic Republic of Congo, Africa. Composite volcano.

Mount

Pinatubo,

Philippines

Composite

volcano.

June 1991

Caldera Formation

Lava Dome

Dome shaped. Steep sided. Made up of very viscous

lava.

High silica content (> 65 %). Rhyolitic composition.

Very explosive and dangerous: plug the volcanic vent.

 

e.g. Mt. Lassen,

California.

Chaos Crags, Lassen

Volcanic National Park,

California. Four lava domes.

Steep-sided lava dome, Alaska.

Active Fissure

Fissure Eruptions

Lava erupted through long fissures are termed “fissure eruptions”.

More common in the geological past: Miocene and Pliocene (5-17 mya).

Form “basalt plateaus”.

 

e.g. Columbia River Basalts in Oregon and Idaho.

Up to 1000 m thick.

 

 

Most recent example in Iceland.

930 A.D. and 1783 A.D.

Extensive basalt flows.

Fissure eruptions.

Columbia River basalts (> 20 lava flows) exposed in the canyon of the

Grand Ronde River, Washington.

Pyroclastic flows from Mount

Pinatubo (June 1991) filled this

valley to a depth of up to 200 m.

Volcano DistributionPrinciple active volcanoes of the Earth are located at the boundaries of lithospheric plates. Formation of new crustal material. Takes place between separating crustal plates by volcanic processes. This helps to explain the distribution of some active volcanoes. Much volcanic activity in zones of plate separation occur beneath the sea. But in places, such as Iceland, it is above sea level.

 

Approximately 80% of active volcanoes occur in the Circum Pacific Belt (“Ring of Fire”). Extending along western S.A., western N.A., Japan, Indonesia, etc.

Note: no volcanoes along the collision zone between India and Asia.Continent-to-continent convergent boundary. Note: Africa to Europe collision zone does have some volcanoes.Ocean crust is being subducted.

As part of the “Ring of Fire” belt

in the U.S., a number of

volcanoes form part of the

Cascade Range, California,

Oregon and Washington.

Many of these volcanoes have

erupted in historic times,

examples include:

 Lassen Peak, California 1914, 1915

Mt. Baker, Washington 1843, 1854, 1858, 1870

Mt. St. Helens 1843, 1980

Mt. Rainier 1820, 1843, 1846, 1854,

1858, 1870, 1894

Mt. Gaschaldi 10,000 years BP

Mt. Meager 2500 years BP

Stikine Belt, B.C. 200, 1340 years BP

Yukon, White River Ash 1200, 1500 years BP

Mudflows are a

significant hazard for

the next eruption of Mt.

Rainer: Seattle and

Tacoma, Washington.

Tacoma

Seattle

Mid Ocean Ridges

Basaltic magma

derived from the

mantle.

Shield volcanoes

produced.

e.g. Iceland.

Mid Atlantic

Ridge.

Volcanoes occur along divergent and convergent plate boundaries. Also occur (more rarely) in intraplate areas (e.g. Hawaii). Three main areas of distribution:

- Mid Ocean Ridges

- Subduction Zones

- Intraplate Volcanism

Subduction Zones

Andesitic volcanoes. Magma is mixed with both oceanic and continental

crust. Intermediate silica composition. Pacific Rim of North America:

California, Oregon, Washington, B.C. Related to the subduction of the

Juan de Fuca plate.

Ocean plate has

to be subducted

for melting to

occur.

Note: this can

occur at ocean-

ocean or ocean-

continent

boundaries, not

continent-

continent.

Intraplate Volcanisme.g. Hawaii. “Hot spots”. Basaltic lavas. Shield volcanoes. Hawaiian Islands are anomalous. They lie within, rather than at the margin of the Pacific Plate. Seem to be related to a “hot spot” in the underlying mantle. Forms a string of volcanoes as the Pacific plate migrates northwest.

Tectonic Settings and Volcanic Activity

Prediction of Volcanic EruptionsGeneralization: volcanic eruptions are predicted with a much higher

success rate than earthquakes.

 

If volcanic eruptions can be predicted then it may be possible to

significantly reduce the loss of life or property damage associated with

an eruption. People can be evacuated from a threatened area and

moveable property taken way from the area.

 

Predictions of the time at which a volcanic eruption is likely to occur are

of two types: General vs. Specific.

 

A general prediction consists of a statement that a volcano is likely to

erupt in the near future. “Near future” may be from hours to years.

Does serve to alert the population to the potential hazard.

However, there are many possible ramifications. Loss of business (e.g.

tourism). Volcanologist under extreme pressure to get the right answer.

Loss of life, if the evacuation is not successful.

 

Predictions of volcanic eruptions can be based on several lines of

evidence:

Past History

Behaviour of Fumaroles

Magnetic Properties of the Rock

Temperature

Swelling

Earthquakes

Past History

Where a long enough record of past history for a particular volcano

exists it may be found to have a more or less regular period of eruptive

cycles. In that case, once a period of average quiescence is completed,

another eruption can be expected in the near future.

 

Probably not a very accurate predictive tool since the length of quiet

periods for individual volcanoes varies greatly. Historical records are

not very good.

Also, the magnitude of the volcanic eruption also needs to be

considered. Magnitude varies. Might be a longer quiet period after a

huge blast because the pressure is greatly relieved.

 

Is this a good method for predicting volcanic eruptions?

Behaviour of Fumaroles

Not much detailed information. Lack of consistency.

Fumarole is defined as a volcanic vent from which gases and vapours

are emitted. Occur along fissures. Some poisonous gases.

 

But for some volcanoes, the following have been noted prior to an

eruption:

Temperature of fumarole gas is found to increase ( T).

Volume of gas increases ( gas volume).

Composition of the gas changes prior to a volcanic eruption.

 

A change in volcanic gas chemistry preceded a later eruptive phase for

Mt. St. Helens: composition, volume, and temperature changes.

Magnetic Properties of the RocksAs magma accumulates beneath a volcano and begins to rise towards the surface preceding an eruption, the heating of the surrounding rock reduces the magnetic properties of the rocks. In fact the rocks may be heated above the Curie temperature ( above which thermal agitation prevents spontaneous magnetic ordering) and they could lose their magnetism.  TemperatureA possible method of detecting a forth coming volcanic eruption is to detect increases in the temperature by aerial infrared photography repeated at regular intervals. Heavy forest cover can effect the results. Valuable in areas which do not have too heavy a forest cover. Russians have used this technique in the Kurile Islands and Kamchatka.They have suggested that satellites could be used for this purpose. This technique was used on Mt. St. Helens. With some success prior to the main eruption in May 1980.

Swelling

When magma pushes its way up beneath a volcano it causes the

structure of the volcano to swell. This swelling is detected by

measurements of the tilting of the ground surface.

Measured using lasers or tiltmeters. Tiltmeters can measure movement

as little as 1 mm in 1 km. Two tiltmeters placed at right angles to one

another are all that is required to define the direction and amount of tilt.

 

e.g. Kilauea, Hawaii between 1964-1966. North-south and east-west

tiltmeter data predicted eruptions prior to their occurrence.

Eruption of the Kilauea volcano has occurred at various stages of tilting.

Long periods of increased tilting do not always lead to an eruption.

This indicates that the magma can recede or it begins to intrude into the

flanks of the volcano.

Problem is that at the present time, tilt

measurements are only made at a few

volcanoes around the world.

 

Distance measurements can augment

tiltmeter information. Use lasers to

measure distances and combine this

information with tiltmeter data in order

to get an accurate representation of the

magnitude of swelling.

 

Swelling occurred in Mt. St. Helens

prior to the March 1980 eruption. On

the north side of the mountain. Bulge

moved at 1.5 m per day!

Led to the lateral blast. Landslide triggered by the swelling reduced the strength of the mountain wall and directly led to the lateral blast.

Could predict with confidence that an eruption was going to occur at some point in time in the future, however were unable to predict the exact date or type.

Most geologists felt the eruption would be vertical based on a previous analysis of Mt. St. Helens eruption history. Nobody predicted a lateral blast despite the prominent bulge on the northern margin.

Lateral blast was not predicted. 

Tragic that a geological post was set up and “manned” on the north side of Mt. St. Helens.Post was several km north of the mountain.Same side as the swelling.USGS geologist was killed (David Johnson).

Mount St. Helens, Washington: Debris Avalanche and Eruption

Earthquakes

Volcanic eruptions are commonly preceded by earthquakes.

Earthquake foci will move upwards towards the surface. As the magma

makes its way to the surface. At Kilauea, shallow earthquakes

numbering in the thousands per day may precede volcanic eruptions.

Called “Harmonic Tremor”.

 

Harmonic tremor is defined as a more or less continuous oscillating

ground motion. With a frequency of 0.5 to 10 Hz.

 

Harmonic tremors may be generated by the drag of the magma against

the walls of the conduit.

At Kilauea it indicates the eruption will follow in a few hours. These

shallow earthquakes occur during the period of swelling and continue

for periods of several days to several hours, commonly stopping with

the beginning of an eruption.

Similar swarms of earthquakes may also occur however during times of

magma recession when the top of the volcano is sinking. So shallow

earthquakes can only be used to indicate impending eruptions when the

character of ground tilting is also known.

 

Major earthquakes preceded the main Mt. St. Helens eruption and

increased earthquake activity occurred just hours before a later

(August) eruption. Similar pattern before the Mt Pinatubo eruption.

Evacuated a US air force base in the Philippines.

 

 

Pattern of Earthquake Behaviour:

(i) earthquakes increased in frequency

(ii) earthquakes increased in magnitude and intensity

(iii) earthquakes (foci) became shallower

Mt. St. Helens

Chronology of events.

Predictions and

hazard analysis.

Hazard zone

designations.

April 30, 1980.

Approximately 2½ weeks

before the eruption.

Note the prediction of a

vertical, symmetrical

blast, and the position of

USGS geologists.

Actual hazard zones.

May 18, 1980.

Tree-blowdown area.

Pyroclastic flows.

Mudflows.

Downed trees near headwaters, south fork, Toutle River: May 19, 1980.

Ash fall and downed trees (parallel orientation of trees).

National Guard identifying victims of Mt. St. Helens.

Hazard zone

designations:

1978.

Mudflow hazard

prediction.

Vertical blast

prediction.

Hazard zone

designations:

April 1, 1980.

Mudflow hazard

prediction.

Vertical blast

prediction.

Actual hazard zone

designations:

May 18, 1980.

Mudflow hazard

prediction.

Mudflows travelled at 50 km/hr and crested 8 meters above normal river level.

Logging trucks thrown around like toys by massive log and mudflows.

Index of Volcanic Explosivity

Volcanic Explosivity Index (VEI) is a semi-quantitative measure of the

destructiveness of a volcanic eruption.

VEI depends on: cloud height (km) and volume of tephra (km3).

Volume of lava, fatalities, and property damage are not considered.

e.g. Nevado del Ruiz eruption (1985) in Columbia killed 23,000 people,

yet only had a VEI = 3.

e.g. Tambora eruption (1815) in Indonesia had a VEI = 7.