Current eruption at Etna volcano, Sicily, Italy. Copyright 2002 by Ben Kennedy.

VolcanoesVolcanoesA volcano is a weak spot vent

in the crust or surface of the Earth through which magma and associated gases and ash erupt (come to the surface).

A volcano is also the form or

structure that is produced by the ejected material (usually conical).

Volcanic activity is constructive.

Popocatepetl Mexico - 2001.

Photograph copyrighted and provided by Daniel Hatcher.

When magma reaches thesurface, it is called lava.

Magma or Lava?

Magma is a molten mixture of rock-forming substances, gases and water from the mantle.

Lava released during volcanic activity builds up the surface of the Earth

How do volcanoes form?How do volcanoes form?

• A volcano forms when molten rock erupts or flows as lava from an opening in earth’s surface and builds up a volcanic cone.

• These openings are called vents.• Volcanoes release molten rock, ash, and poisonous gases.

All these products result from melting in the mantle or in the crust.

Like earthquakes, most volcanoes Like earthquakes, most volcanoes occur at occur at plate boundariesplate boundaries. .

• Plate boundaries– Divergent: where

two plates move apart

– Convergent: where two plates move towards each other

– Transform: where two plates move past one another

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part12.html

North America

South America

Australia

Asia

The RING OF FIREhttp://pubs.usgs.gov/publications/text/fire.html

The RING OF The RING OF FIREFIRE

The Ring of Fire is a ring of volcanoes around the Pacific Ocean that result from converging plate boundaries.

Most of the Earth's volcanoes are located around the Pacific Ring of Fire because that is the location of most of the Earth's subduction zones.

• Like earthquakes, most volcanoes occur at plate boundaries. • Along a mid-ocean rift, where two plates move apart, divergent

boundary, a ridge of underwater volcanoes is formed. Examples: Volcanoes in Iceland and the Azores Islands

• At convergent boundaries, where an oceanic plate collides with a continental plate. Examples: Pacific Northwest and the Andes Mountains in South America.

• At convergent boundaries, where an oceanic plate collides with another oceanic plate creating an island arc. Examples: Japan, New Zealand, Caribbean Islands, Philippines, Aleutian Islands.

• Some form at “hot spots” far from plate boundaries. Examples: Hawaiian Islands, Yellowstone

There are about 600 active volcanoes on landand more beneath the sea.

Types of VolcanoesTypes of Volcanoes• The type of eruption determines the type of

volcano.

• Thin, liquid lava spreads far from the vent, building up a gentle slope.

• Thicker, more viscous lava erupts more violently and produces more ashes and cinders that pile up to form a steep cone-shape.

Types of VolcanoesTypes of Volcanoes• Shield Volcanoes

– Kilauea, Hawaii

• Strato Volcanoes– Mt. St. Helens, Washington

• Rhyolite Caldera Complexes– Yellowstone

• Monogenetic Fields– San Francisco volcanic field

• Flood Basalts– Columbia River Basalt province,

Washington to Oregon

• Mid-ocean ridges– Atlantic ridge

http://mordor_unknown_world.tripod.com/id4.html

Features of VolcanoesFeatures of Volcanoes• Magma Chamber

– The pocket beneath the

volcano where magma collects

• Pipe– A long tube through which

magma moves from the magma chamber to Earth’s surface

Features of VolcanoesFeatures of Volcanoes• Vent

– The opening through which molten rock and gas leave a volcano

– Usually there is one central vent at the top of the volcano

– Often times there are additional vents that open on the volcanoes sides

Features of VolcanoesFeatures of Volcanoes• Lava flow

– The area covered by lava as it pours out of a volcano’s vent

• Crater– A bowl-shaped area that

forms around a volcano’s central opening

– Lava collects there

Features of VolcanoesFeatures of Volcanoes• Liquid magma in the

asthenosphere is less dense than the rock in the lithosphere above it, so it flows upward through cracks in the rock– The magma is stored in the

magma chamber

• It continues upward (through the pipe) until it reaches the surface (exiting through a vent) or it is trapped beneath layers of rock.

Features of VolcanoesFeatures of VolcanoesA conduit or pipe feeds magma to

the surface.

Near the surface, gas expands and fragments the lava into ash.

Some magma passes through dikes across rock layers to feed vents.

Some magma intrudes parallel to layers to make sills.

http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/parts.html

Types of lava:Types of lava:1. Pahoehoe: hot, thin, flowing, forms

ropes

2. Aa: cooler, much thicker, slow, forms jagged, sharp, blocks.

Types of lava:Types of lava:3. Blocky lava: Cool and stiff, it does not travel

far from the volcano. It usually oozes out after an explosive eruption.

4. Pillow lava: The result of lava erupting underwater. The water causes rapid cooling and results in bubble like pillows.

Types of Types of lavalava::

5. Pyroclastic Flow

The expulsion of ash, cinders, and bombs from a violent volcanic explosion

Volcanic blocks: Solid chunks blasted from the volcano - Volcanic blocks: Solid chunks blasted from the volcano - chunks chunks of the volcano top.of the volcano top.Volcanic bombs: Large blobs of magma that harden in the air -Volcanic bombs: Large blobs of magma that harden in the air -

tear-drop shapes larger tear-drop shapes larger than 64mm in diameter.than 64mm in diameter.Lapilli (La pill ee): Meaning “little stones” - pebble-like pieces Lapilli (La pill ee): Meaning “little stones” - pebble-like pieces from from 2mm to 64mm.2mm to 64mm.Volcanic Ash: Particles less than 2mm in diameter - forms Volcanic Ash: Particles less than 2mm in diameter - forms when when gases force magma to explode into tiny piecesgases force magma to explode into tiny pieces..

Types of Types of lavalava::

• The more silica a lava contains, the thicker and more lightly-colored it will be.

• Rhyolite -

How magma affects a volcanic How magma affects a volcanic eruption.eruption.

• There are three main factors that determine whether a volcano extrudes magma violently or quietly.

• The factors are:– magma’s composition—silica content– temperature– amount of dissolved gases it contains.

Quiet EruptionQuiet EruptionIn general, the hot , basaltic magmas flow easily

and gently.• dark-colored • contains a lot of water• rich in iron and magnesium• cools to form igneous rocks such as basalt. • lava is thin and fluid

The islands of Hawaii and Iceland were formed by many lava flows.

Explosive EruptionExplosive EruptionThicker, cooler magmas • contain more silica• more difficult to force through the vent• may even plug up the vent• causes gases to collect in bubbles and pockets

that increase in size and pressure• increasing pressure ejects the molten rock from

the volcano in a violent explosion.

Ex. Mt. St. Helens, 1980

Composite VolcanoesComposite Volcanoes

Composite volcanoes switch between quiet eruptions of flowing lava and violent eruptions of thick gas-rich lava.

This type of volcano has the most powerful eruptions of all.

•Composite volcanoes are made up of alternating layers of ash, cinders, and lava.

•Their magma is rich in silica and therefore thick. Gases are trapped in the magma, causing eruptions that alternate between flows and explosive activity that produces cinders and ash.

•Composite volcanoes are typically thousands of meters high, with steep slopes.

When Mount St. Helens erupted on 18 May 1980,the top

1,300 ft. disappeared within minutes.

The blast area covered an area of more than 150 sq. miles and sent thousands of tons of ash into the upper atmosphere Mount St. Helens is a stratovolcano.

Image taken on 10/16/94 from the Space Shuttle.

Mt. St. HelensMt. St. Helens• Mt. St. Helens -

before 5/18/80 • Mt. St. Helens -

after 5/18/80

Shield VolcanoesShield Volcanoes

• Shield Volcanoes look like a warrior’s shield. These broad, slightly dome-shaped volcanoes are the world’s largest.

• Volcanoes with broad, gentle slopes and built by eruption of fluid basalt lava are called shield volcanoes.

• Basalt lava tends to build enormous, low angle cones because it flows across the ground easily.

• The largest volcanoes on Earth are Shield Volcanoes.

Cinder ConesCinder Cones• A cinder cone volcano is

built up from ashes, cinders, and rocks that burst from Earth during violent eruptions.

• The rocks fall back to Earth near the opening.

• At the same time, hot ash covers a larger surrounding area.

• Cinder cones erupt for a very short amount of time.

Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings.

Craters and CalderasCraters and Calderas

• Craters: A funnel-shaped pit at the top of the central vent of a volcano.

•Caldera: A large hole formed when the magma chamber empties and collapses.

Lava PlateauLava Plateau

• Instead of forming mountains, some eruptions form high, level plateaus. – First, lava flows out of several long cracks in an area. – The thin, fluid lava travels far before cooling and

solidifying. – Again and again, floods of lava flow on top of earlier

flows. – After millions of years, these layers form high

plateaus. – Example: Columbia Plateau (pictured here)

Volcanic NeckVolcanic Neck• Formed when magma hardens in a

volcanic pipe. The softer rock around the pipe wears away, exposing the hard rock of the volcanic neck.

BatholithBatholith

• A batholith is a mass of rock formed when a large body of magma cools inside the crust. Example: Pike’s Peak

Dome MountainDome MountainA dome mountain is formed when rising magma

is blocked by horizontal layers of rock. The

magma forces the layers of rock to bend upward

into a domed shape. Eventually the rock above

the dome wears away leaving it exposed.

• Not all of Earth’s volcanoes form at the edges of the plates.

• Sometimes a volcano forms when a narrow column of hot molten rock breaks through the lithosphere in the middle of a plate. This narrow column of magma is called a hot spot.

• A hot spot does not move. However, the plate above it moves forming a chain of volcanoes.

• The Hawaiian Islands were formed by a hot spot under the Pacific Plate.

• Today, only the island of Hawaii, located directly over the hot spot, has active volcanoes.

Volcanic ActivityVolcanic Activity• Volcanoes are rather

unpredictable . Some erupt regularly, others have not erupted in modern history.

• Scientists classify them as active, dormant or extinct.

Active Active VolcanoesVolcanoes

• An active volcano is one that erupts wither continually or periodically such as Mount Katmai in Alaska and Mount St. Helens in the Cascade Range.

Dormant VolcanoDormant Volcano A volcano that has been known to erupt

within modern times but is now inactive is classified as a dormant volcano. Mount Rainier in Washington state are example of dormant volcanoes in the United States.

Extinct VolcanoExtinct Volcano

A volcano not known to have erupted within modern history is classified as an extinct volcano.

They have been worn away almost to the level of their magma chamber.

Scientists can be wrong. Mount St. Helens was considered to be dormant but erupted after long periods of inactivity.

Hot SpringHot Spring

• A pool formed by groundwater that has risen to the surface after being heated by a nearby body of magma

GeyserGeyser

• A fountain of water and steam that builds up pressure underground and erupts at regular intervals

Geothermal Geothermal EnergyEnergy

• Energy from water and steam that has been heated by magma

• Geothermal energy is produced by drilling a well into the ground where thermal activity is occuring.

• Once a well has been identified and a well head attached, the steam is separated from the water, the water is diverted through a turbine engine which turns a generator.

• Usually the water is injected back into the ground to resupply the geothermal source.

• different from predicting a flood or an earthquake

• volcanoes usually shows signs of "awakening" before they erupt

• give geologists time to monitor the precursory events 

Predicting Eruptions

Predicting EruptionsPredicting EruptionsGeologists use sensitive instruments to monitor an active

volcano to determine if magma is approaching the surface.

Signs that a volcano is about to erupt include:

• Changes in tilt caused by magma movement underground using tiltmeters & laser-ranging devices

• increasing temperature of hot springs near the volcano

• changes in the shape of the volcano or surrounding land

• an increase in hot gas and ash from the vent

• small earthquakes around the volcano

Photograph by Gudmundur E. Sigvaldason, Nordic Volcanological Institute, Reykjavik, Iceland