Week 8: Volcanoes

Volcanoes

Most volcanoes are located near plate boundaries.

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Subduction zones and mid-ocean ridges allow molten rock to reach the surface

Approximately _ */_* of all volcanoes are found on the ‘Ring of Fire’ surrounding the Pacific Ocean.

2/3

Types of molten rock

Magma

* it is found deep within the crust and upper mantle

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Lava

* it is found flowing from an erupting volcano

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Essentially, lava is magma on the Earth’s surface.

Magma

The most abundant elements in magma are __**silicon**__ and __**oxygen**__; when combined they are referred to as silica.

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Volcanic rocks are named based on the amount of silica present

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**Types of volcanic rocks:**

1 = low silica

4 = high silica


1. basalt,
2. andesite,
3. dacite,
4. rhyolite

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basalt - common in Hawaii

Viscosity

Magma also contains small amounts of gases (water vapour and carbon dioxide).

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Volcanoes have different shapes based on the chemistry and *viscosity* of their magma.

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Magma viscosity is determined by ^^silica content and temperature^^

Magma + Silica content

**Magma with high silica content:**

* cooler, more viscous, more gases

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**Magma with low silica content:**

* hotter, less viscous, fewer gases

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Volcanoes with ==high silica magma== produce the ==most explosive== eruptions.

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As magma approaches the surface, the pressure lowers allowing gases to bubble up and escape.

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What types of magma produce explosive eruptions?

Rhyolitic and dacitic magmas

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\*Basaltic and andesitic magmas produce eruptions that tend to flow rather than explode

Types of volcanoes

Volcanoes are classified into 4 different types.

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The classification is based on their shape, appearance, and style of eruptions.

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Types of volcanoes:

* shield
* composite
* volcanic dome
* cinder cone

Shield volcanoes

These are the largest volcanoes on Earth and are shaped as broad arcs (like warrior shields) built from lava.

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They are associated with ==Basaltic magma==

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Eruptions are non-explosive and consists of gentle flows

What is tephra - definition

Fragmented material blown out during an eruption

Tephra

Some eruptions cancontain *tephra*.

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Accumulations of tephra are referred to as @@pyroclastic deposits.@@

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

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These volcanoes are common in Hawaii, Iceland and around the indian Ocean

Composite Volcanoes

These volcanoes are cone-shaped and are built from a combination of lava flows and pyroclastic deposits.

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They are also called stratovolcanoes; this term comes from the layers of lava and deposits.

Composite Volcanoes

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

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These volcanoes are common along the west coast from Alaska to Northern California

Most well known composite volcanoes in N. America?

* Mt. Rainier, Mt. St. Helens (both in Washington State)
* Mt. Adams (in Oregon)

Volcanic domes

These volcanoes contain highly viscous rhyolite magma.

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They are steep sided mounts that form around vents

Cinder cone volcanoes

These are relatively small volcanoes composed of small pieces of tephra

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They are round to oval-shaped and typically contain a crater at the top

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These volcanoes are found in Mexico

Maars

__definition:__

* A circular volcanic crater produced by an explosive eruption and filled with water.

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They are caused by groundwater encountering magma creating an explosion

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Maar derives from the latin mare meaning sea and resembles a large lake

Ice contact volcanoes

Some volcanoes erupt beneath or against glaciers.

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These eruptions melt huge quantities of ice producing floods known as jökulhaups.

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When lava contacts glaciers, it quickly cools to form pyroclastic rock.

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Ice-contact volcanoes are found in Iceland and British Columbia

Mount Garibaldi - Ice contact

Evidence of the Mt. Garibaldi eruption 12,000 years ago in British Columbia is preserved in currently exposed rock.

Volcanic features

__**Crater**__

* A depression formed by the explosion of a volcano top. They can be up to 2 km in diameter.

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__**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*.

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__**Caldera**__

* A circular to oval depression formed during the collapse of a volcano.
* can be up to 25 km in diameter
* Eruptions that form calderas are the largest and most deadly eruptions on Earth.

Formation of Calderas

Calderas form by the collapse of a magma chamber below a composite volcano during an explosive eruption.

Hot springs and geysers

Heated groundwater can discharge at the surface as a __**hot spring**__

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Groundwater that boils in an underground chamber to periodically produce a release of steam or water is called a *geyser*.

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There are are approx 1000 geysers on earth and nearly half are in yellowstone national park.

Old faithful

This is the most famous geyser in the world.

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It erupts to height up to 50m with eruptions lasting for 2-3 minutes.

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The average interval between eruptions is 70 minutes

Super eruptions

These are the products of *supervolcanoes* and are extremely rare events.

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

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The magma is unable to break through the crust; pressure builds until the crust can no longer contain it.

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A massive amount of land was covered by ash fall from super eruptions.

Yellowstone supervolcano

Mya, 1.3 Mya Yellowstone National Park sits on a massive caldera created from the last eruption of the Yellowstone Supervolcano.

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This area is located over a continental hot spot

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Super eruptions occured 2.2 Mya, 1.3 Mya, 640,000 years ago

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Yellowstone Supervolcano

Earthquakes in the park are monitored continuously.

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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.

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

Yellowstone national park

The park is continuously monitored for geologic activity.

Volcanoes in Canada

They are generally restricted to British Columbia and southern Yukon.

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Mt. Baker in Washington state poses the greatest risk to Canada

Mt. Baker

An eruption of Mt. Baker would eject large amounts of ash over a densely populated region of Canada.

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It would cause landslides and melt glaciers thus causing floods and lahars.

effects of volcanoes

On average, 50 to 60 volcanoes erupt each year.

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Over the past century, 100,000 pople have been killed by eruptions.

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^^Ash and gases reflect sola radiation causing a cooling.^^

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Notable eruptions impacting climate:

__**Mt. Tambora (1815)**__

* Indonesia 1816 is known worldwide as “the year without a summer”

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__**Mt Pinatubo (1991)**__

* Philippines 1992 is one of the coolest years worldwide in the 20th century

Lava Flows

. These occur when magma flows out of a central crater or a fissure on the side of a volcano.

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==**Pahoehoe basaltic lava:**==

* Low viscosity (a few km per hour), high temp, when hardened, it has a smooth texture

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@@**Aa basaltic lava:**@@

* High viscocity (a few meters per day), lower temp when hardened, it has a rocky temperature

Lava Flow examples

Lateral blasts

Definition:

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

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Example:

* A lateral blast from Mt. St. Helens flattened forests for over 20km

Pyroclastic flows

Pyroclastic flows are avalanches of ash, gas, and rock fragments that travel down the slopes of a volcano during an explosive eruption.

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

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More people have been killed by pyroclastic flows than any other volcanic eruption.

Ash fall

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

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Hazards of ash fall:

* destroys vegetation
* contaminated surface water
* health hazards to people and animals
* causes aircraft engine failure

Poisonous gases

Volcanoes emit numerous gases at high quantities.

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

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A type of smog known as vog can be produced: this can induce asthma attacks and respiratory problems

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High amounts of carbon dioxide released at once can kill animals and vegetation.

Sector collapse

The flank of a volcano can collapse at any time.

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As magma travels up the channel, the volcano can inflate, and its slopes can become over-steepened.

Lahar

__**Definition:**__

A large amount of material that has become saturated with water and moves downslope.  It is an Indonesian word.

* Lahars are also referred to as mudflow

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Knowledge of local topography and history of past events can aid in determining the recurrence intervals of lahars.

Mt. st. Helens

Before its eruption, the volcano had been dormant for 120 years.

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* In march of 1980, small explosions were evident due to groundwater contacting magma.

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A bulge began growing on the flank of the mountain.

Mt. st. Helens eruption

On May 18, 1980, a M 5.1 earthquake caused the area of the bulge to break off and fall downslope.

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

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

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It killed 57 people, mainly from pyroclastic flows

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

Mt. St. Helens eruption graphic

Natural service functions

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

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Internal heat from volcanoes can produce renewable geothermal energy.

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Volcanic landscapes attract tourism and recreation

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Eruptions have created new land (Hawaii, Iceland)

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Volcanic soils can be good for agriculture since they are porous, well-drained, and contain nutrients.

Minimizing the volcanic hazard

An eruption forecast is a statement of the probability that a volcano will erupt within a defined time.

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Forecasts are based on information provided by:

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__**1. Monitoring seismic activity**__

* shallow earthquakes can precede eruptions
* short warning times from this information are a concern

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__**2. Thermal and hydrologic monitoring**__

* an accumulation of magma changes properties of the rock and soil
* increased heat may also melt snow or glaciers

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__**3. Land surface monitoring**__

* Involves monitoring for growth of bulges, swelling, and opening of cracks

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__**4. Monitoring volcanic gas emissions**__

* Increases in CO2 OR SO2 may indicate magma is moving toward the surface

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__**5. Understanding local geologic history**__

* mapping of volcanic rocks
* dating of pyroclastic deposits