III. Volcanic Deposits

A. Lava: effusive depositsNon-explosive1. Basalt2. Andesitic Lava

B. Pyrolcastic depositsExplosive deposits1. Andesitic2. Rhyolitic

Viscous: it tends to cool underground or explode violently.

Introduction to Volcanoes

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3. Pyroclastic Flows (nuée ardente)

French for “glowing cloud” or glowing avalanches

Airborne material that rushes down the sides of a volcano.

“Floats” on a layer of trapped air and magmatic gases, so remains buoyant.

Speeds exceeding 100 miles per hour, even on flat land (up to 125 mph)

Still hot when they stop flowing—fuse together to form welded tuff

Welded tuff = solid form of tephra

Introduction to VolcanoesIII. Volcanic Deposits

Pyroclastic Flows (nuée ardente)

Introduction to Volcanoes

Pyroclastic Flows (nuée ardente)

Introduction to Volcanoes

Pyroclastic Flows (nuée ardente)

Introduction to Volcanoes

Pyroclastic flow: High-speed avalanches of hot ash, rock fragments, and gas move down the sides of a volcano during explosive eruptions or when the steep edge of a dome breaks apart and collapses.

These pyroclastic flows, which can reach 800°F and move at 100-150 mph, are capable of knocking down and burning everything in their paths.

Introduction to VolcanoesI. Factors controlling types of

Volcanic Eruptions

A.Viscosity—resistance to flow

1. Temperature—heat lowers viscosity (heating syrup)

2. Magma composition—directly related to silica content—chains even before crystallization begins

B. Gases: 1-9% of magma

II. Types of Volcanic Eruptions

1.Effusive (non-explosive): lava flows out fairly easily

a. Lava (basalt, andesite)

2. Explosive / Pyroclastic a. pyroclastic deposits w/ different types of Tephra

3. Lahars: volcanic mud flows

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Question of the week

1. What two factors control the types of volcanic eruptions?

III. Volcanic deposits

C. Lahars

Indonesian for mudflow.

Volcanic debris become saturated with water, massive mudflows

Form and race down valleys (usually follow gullies and stream valleys)

Water from snow melting, or rain after a volcano erupts (can happen after the eruption)

Consists of debris to ash to 100-ton boulders

Travels tens of kilometers per hour.

Introduction to Volcanoes

W. W. Norton

Introduction to Volcanoes

III. Volcanic deposits

C. Lahars

Mt St Helens, water was 4 meters above flood stage—not so populated (killed 6 people)

Deposited 3.4 million kg of debris into Columbia River. Ships couldn’t navigate for a week

Introduction to Volcanoes

III. Volcanic deposits

C. Lahars

1985: Nevado del Ruiz in Andes – Armero, Columbia--lahar killed 23,000 people. Río Lagunillas, former location of Armero.

Introduction to Volcanoes

Within four hours of the beginning of the eruption, lahars had traveled 100 km and left behind a wake of destruction: more than 23,000 people killed, about 5,000 injured, and more than 5,000 homes destroyed along the Chinchiná, Gualí, and Lagunillas rivers.

Mt. Rainier

Dormant volcano whose glacial load exceeds that of any other volcano in the coterminous US. 4393 meters, 14410 feet to the summit of Mt. Rainier

Lahars, along Mt. Rainer, pose the greatest volcano hazard in the Cascades Range.

Hazards from tephra and pyroclastic flows relatively minimal.

The number of pyroclastic deposits relatively small, probably because they are converted to lahar-type deposits as they flow over the abundant glaciers and snow-fields.

5700 years ago, the Osceola mudflow originated from the summit of Rainier.

Largest known lahar from Mt. Rainier (past 10 ka). Began when part of the volcano collapsed, turned into a lahar or mudflow.

It was the product of a large debris avalanche composed mostly of hydrothermally-altered material, and may have been triggered as magma forced its way into the volcano.

Covers an area of 550 sq km or 212 sq miles.

Extends all the way to Puget Sound

If this mudflow moved as quickly as one that happened in Columbia about 10 years ago, it would have been emplaced within 3 hours.

Osceola deposits extend at least as far as the Seattle suburb of Kent, and to Commencement Bay, now the site of the Port of Tacoma.

Region now home to many people in Buckley, Enumclaw, Pacific, Auburn, etc.

Schematic diagram of a strato volcano, illustrating the different layers of different materials that comprise them.

The purple colors represent ash layers, either the products of fall-out from big eruption clouds or the products of pyroclastic flows.

These ash layers are thin but widespread.

The orange colors represent lava flows, and note that some of them have cinder cones associated with them at the vent.

Electron Lahar

Occurred about 600 years ago, and has not been correlated with an eruption.

More than 30 m (100 feet deep).

Made it all of the way to Puget Sound.

Risk- more than 100,000 people live on lahars associated with Mt. Rainier.

The risk that a structure will be affected by a lahar is about the same as by a fire.

Frequency of lahars about 1 every 500 years.  May not have advance warning!

III. Volcanoes

D. Gases—(volatiles)—held in magma by confining pressure, when pressure drops then gases escape. Gases from within the Earth - volcanoes provide an escape mechanism

Gases that come out of volcanoes greatly influence the composition of our atmosphere

1-9% of total weight of magma70% water15% CO2

5% N2

5% SO2

5% Cl2, H2 and Ar

SO2 gas can be especially nasty In the atmosphere it combines with water vapor, producing H2SO4. This may remain suspended in the atmosphere for years producing acid rain.

Increasing the acidity of local, regional and global waters

Environmental Geoscience

Environmental Geoscience

When Mount Pinatubo erupted in the Philippines June 15, 1991, ~ 20 million tons of sulfur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere.

Gases and solids injected into the stratosphere circled the globe for three weeks.

Volcanic eruptions of this magnitude

•Impact global climate

•Reduce the amount of solar radiation reaching the Earth's surface

•Lower temperatures in the troposphere

•Change atmospheric circulation patterns

Climate change

Gases injected up into the atm (stratosphere-10-50km of atm) many remain for years & affect climate

Large sulfuric-acid droplets (aerosols)

Reflects solar radiation coming from sun back into space (also absorbs)

Lowers troposphere temp (0-10 km)June 1991 Pinatubo eruption in Philippines

volcano lowered global temp ~ 0.9F

Climate change:

1815 eruption of Indonesia’s Tambora “Year without a summer”

Unusually cold spring and summer and an early fall (lowered temp 2-5F)

Caused shortened growing season and crop failures - famine in some regions

Snow in upstate New York in June! Eruption caused two days of darkness 400 miles around the volcano.

Ash column reached 43 km.

Spectacular sunsets inspired Byron - wrote the poem Darkness

Mary Shelley wrote a short story that eventually led to her writing of Frankenstein.

F in high quantities can be lethal to animals and plants

CO2 contributes to global warming although volcanoes only contribute a small proportion to this:

Volcanoes release 110 million tons of carbon dioxide into the atmosphere

Human activities release 10 billion tons into the atmosphere