Lecture 8: Volcanoes

Where are most volcanoes located?

Near plate boundaries

• ~65% are found along the ‘Ring of Fire’ surrounding the Pacific Ocean

• Subduction zones (convergent) and mid-ocean ridges (divergent) allow molten rock to reach the surface

Where is magma found?

deep within the crust and upper mantle

Where is lava found?

flowing from an erupting volcano

• essentially magma on the Earth’s surface

magma → in crust / mantle

lava → at surface

What is magma made of?

Silicon + Oxygen —> Silica

• also contains small amounts of gases (water vapour, co2, so2)

What are volcanic rocks names based off of?

the amount of silica present

Types of volcanic rocks from low —> high silica content

1. Basalt → lowest silica content

2. Andesite

3. Dacite

4. Rhyolite → highest silica content

Volcanoes have different shapes based on what two factors of their magma?

1. Chemistry

2. Viscosity

* both based on silica content

What is viscosity?

resistance to flow

Magma viscosity is determined by what 2 things?

1. silica content

2. temperature

Magma with high silica content:

• cooler

• more viscous

• more gases

• more explosive

Magma with low silica content:

• hotter

• less viscous

• fewer gases

• less explosive

What magma types produce the most explosive eruptions?

1. Ryholitic

2. Dacitic

→ highest silica content

What magma types produce the least explosive eruptions?

1. Basaltic

2. Andesitic

→ lowest silica content

Volcanoes are classified based on what?

1. shape

2. appearance

3. eruption style

What are the 4 types of volcanoes?

1. Shield

2. Composite

3. Volcanic dome

4. Cinder cone

Shield volcanoes:

• The largest volcanoes on Earth

• Shaped as broad arcs (like warrior shields) built from lava

• They are associated with basaltic magma

  • least violent eruptions

• Eruptions consist of well-flowing lava (very liquid)

• some eruptions can contain tephra —> pyroclastic deposits/rock

• Common in Hawaii, Iceland, and around the Indian Ocean

What is tephra?

fragmented material blown out during an eruption

• accumulations of tephra are referred to as pyroclastic deposits.

• If compacted together, these deposits are called pyroclastic rock.

Composite volcanoes:

• cone-shaped

• built from a combination of lava flows and pyroclastic deposits

• also called stratovolcanoes

  • term arises from stratified layers of lava and deposits

• Eruptions are more dangerous and explosive but less frequent than shield volcanoes

• common along the west coast from Alaska to Northern California

• Examples: Mt. Rainier, Mt. St Helens

Volcanic domes:

• contain highly viscous rhyolite magma

  • very explosive - high silica

• steep-sided mounds that form around vents

Cinder cone volcanoes:

• relatively small volcanoes composed of small forms of tephra

• they are round to oval shaped and typically contain a crater at the top

• These volcanoes are found in Mexico

What is a maars?

A circular volcanic crater produced by an explosion filled with water

• caused by groundwater encountering magma, creating the explosion

• derived from Latin mare meaning sea - resembles a large lake

What are jokulhlaups and how are they caused?

Floods produced by the melting of huge quantities of ice due to volcanic eruptions beneath or alongside glaciers

• Caused by ice-contact volcanoes

Where are ice-contact volcanoes found?

Iceland & British Columbia

What is a crater?

A depression formed by the explosion of a volcano top

• They can be up to 2 km in diameter

What is a volcanic vent?

An opening on the surface through which lava and pyroclastic debris erupt

• Most vents are circular, but some are elongated cracks called fissures

What is a caldera?

A circular to oval depression formed during the collapse of a volcano

• They can be up to 25 km in diameter

• Eruptions that form calderas are the largest and most deadly eruptions on earth

• formed by the collapse of a magma chamber below a composite volcano during an explosive eruption

What is a geyser?

The release of steam or water produced where groundwater boils in an underground chamber

where are the majority of geysers on Earth?

Yellowstone National Park

What’s the most famous geyser in the world?

Old Faithful

• erupts to a height up to 50m with eruptions lasting for 2-3 minutes

What are super eruptions?

The products of supervolcanoes and are extremely rare events

• they occur when a large volume of magma rises to shallow depths in the continental crust over a hot spot

• The magma is originally unable to break through the crust à pressure continues to build until the crust can longer contain it

Map of past supereruptions

Yellowstone Supervolcano:

• the National Park sits on a massive caldera created from the last eruption of the Yellowstone Supervolcano

• located over a continental hot spot

• Super eruptions occurred there 2.2 Mya, 1.3 Mya, and 640,000 years ago

• Earthquakes in the park are monitored continuously.

• A super eruption could last for weeks and spread ash over half of the U.S. 

• Ash fall would be over 1000 times that released by Mt. St. Helens.

• Millions of people would die from ash suffocation and the U.S. agriculture economy would be destroyed from a super eruption

Where are volcanoes found in Canada?

• British Columbia

• Southern Yukon

Which volcano poses the greatest risk to Canada?

Mt. Baker in Washington State

Climatic effects of volcanoes:

• ash and gases reflect solar radiation, causing a cooling

2 notable eruptions impacting climate:

1. Mt. Tambora (1815, Indonesia) —> “the year without a summer”

2. Mt. Pinatubo (1991, Philippines) —> caused one of the coolest years worldwide in the 20th C

What is a lava flow?

When magma flows out of a central crater or a fissure along the side of a volcano

Pahoehoe basaltic lava:

• low viscosity (a few km per hour)

• high temp

• when hardened, has a smooth texture

Aa basaltic lava:

• high viscosity (a few m per hour)

• lower temp

• when hardened, has a blocky texture

What is lateral blast?

An eruption directed away from a volcano where materials are blown parallel to the surface

• Example: a lateral blast from Mt. St. Helens flattened forests for over 20 km

What is pyroclastic flow?

An avalanche of ash, gas, and rock fragments that travels down the slopes of a volcano during an explosive eruption.

• Speeds can reach 150 km/h and the flow can travel up to 30km from the source

• More people have been killed by pyroclastic flows than any other volcanic phenomenon

What is ash fall?

Particles of ash can be carried downwind hundreds of kilometres from an eruption site

What are some hazards of ash fall?

• Destroys vegetation

• Contaminates surface water

• Health impacts to people and animals

• Causes aircraft engine failure

Poisonous gas hazard of volcanoes:

• Volcanoes emit numerous gases at high quantities.

• Eruptions containing sulphur dioxide can burn holes in leaves and lead to the formation of acid precipitation.

• A type of smog knows as vog can be produced; this may induce asthma attacks and respiratory problems.

• High amounts of carbon dioxide released at once can kill animals

What is sector collapse?

When the flank of a volcano collapses - can happen at any time

• As magma travels up an inner channel, the volcano can form a bulge where its slopes have become over-steepened

What is lahar?

A large amount of material that has become saturated with water and moves downslope. 

• an Indonesian word

• also referred to as mudflows

Mt. St. Helens eruption:

• before its eruption, it was dormant for 120 years

• in March 1980, small explosions occurred due to groundwater contacting magma

• A bulge began growing on the flank of the mountain

• On May 18, 1980, a M 5.1 earthquake (at convergent boundary) caused the bulge to break off and fall downslope.

• A lateral blast occurred from the area of the former bulge and the entire north slope was destroyed.

• Ash was ejected from the central crater, reached heights of 19km, and travelled around the world

• It killed 57 people, mainly from pyroclastic flows

• The eruption left behind a barren landscape that is slowly reforesting naturally.

What is the deadliest aspect of volcanic eruptions?

Pyroclastic flows

Natural service functions of volcanoes:

• Ancient volcanoes provided the gases that now form the atmosphere and sustain life on Earth.

• Internal heat from volcanoes can produce renewable geothermal energy (ex. geysers).

• Volcanic landscapes attract tourism and recreation

• Eruptions have created new land (Hawaii, Iceland)

What is an eruption forecast?

a statement containing the probability that a volcano will erupt within a given timespan

Forecasts are based on information provided by what 4 things?

1. Monitoring seismic activity

2. Thermal and hydrologic monitoring

3. Land surface monitoring

4. Monitoring volcanic gas emissions

5. Analyzing the local geologic history

1. Monitoring seismic activity

• Shallow earthquakes can precede eruptions

• Short warning times from this information are a concern

2. Thermal and hydrologic monitoring

• An accumulation of magma changes properties of the rock and soil

• Increased heat may melt snow or glaciers above

3. Land surface monitoring

Involves checking for the growth of bulges, swelling, and opening for cracks

4. Monitoring volcanic gas emissions

Increases in carbon dioxide or sulphur dioxide may indicate magma is moving toward the surface

5. Analyzing the local geologic history

• Mapping of volcanic rocks

• Dating of pyroclastic deposits