endogenic process

VOLCANISMIts Features

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Albert Garcia’s award-winning photo taken on June 15, 1991 was chosen by Time as one of the “Greatest Images of the 20th Century” and also by the National Geographic Magazine as one of “100 Best Pictures” of the 20th Century. Garcia is currently the photo editor of Manila Bulletin.

Catastrophic

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VOLCANISM: Its Features

VOLCANISM?

Volcanism is the eruption of molten rock (magma) onto the surface of a planet. A volcano is the vent through which magma and gases are discharged. Magma that reaches the surface is called lava. Volcanos are named for Vulcan the Roman god of fire!

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How do volcanoes form?

What causes volcanoes to erupt?

Where do most volcanoes occur?

What are the different types of volcanoes?

What determines the degree of violence associated

with volcanic activity?


Definition of termsA volcano is a vent or opening on planet’s surface which allows molten rock called magma, volcanic ash and gas to escape out onto its surface. A volcano gives a look like a mountain from which lava erupts. The hot magma erupting from a volcano is called lava.

ERUPTIONDefinition of Terms

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Most volcanoes are formed by the movement of tectonic plates on the surface of the earth. These plates are basically huge pieces of rock that ‘float’ on the mantle (a layer of the earth that is sort-of liquid rock).

ERUPTIONHow Volcanoes Form

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TECTONIC

PLATES

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ERUPTIONOrigin of Volcanoes

1. Magma rises2. As the magma rises it melts gaps in the surrounding rock3. As more magma rises a large reservoir 4. Pressure from the surrounding rock causes the magma to blast

5. The magma, now called lava, builds up at the vent forming a volcano.6. Often the volcano sides will be higher than the vent forming a depression called a crater

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ERUPTIONWhat Causes Volcanoes to Erupt?

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TECTONIC PLATES

AND SETTINGS

Subducting Plate Margins

Spreading Plate Margins

Intraplate

(Hotspot)

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Again, it’s in the edge of TECTONIC PLATES.

ERUPTIONAreas Where Most Volcanoes Occur

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The Pacific Ring of Fire

ERUPTIONAreas Where Most Volcanoes Occur

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-It is a string of volcanoes and

sites of seismic activity around

the edges of the Pacific Ocean.

-It isn’t quite a circular ring. It

is shaped more like a 40,000-

kilometer (25,000-

mile) horseshoe.

-A string of 452 volcanoes.

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ERUPTIONWhat determines the degree of violence associated


Magma – molten rock or liquid that is mostly silica

Lava – solidified magma

Pyroclasts – from the Greek pyro, “fire”, and clast “broken”

– rock fragments (pyroclastic debris is also called tephra)

Lava flows – fissures in the ground

Extrusive rock - surface rock resulting from volcanic activity (rock formed by solidification of lava)

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ERUPTION!What determines the degree of violence associated


Crater – funnel shaped pit at the top of a volcano. formed when

material is blown out of the volcano by explosions

Caldera – when the volcano

collapses due to an empty

magma chamber

CRATER

CALDERA

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DEVELOPMENT

OF A CRATER

LAKE

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Whether eruptions are very explosive or relatively "quiet" is largely determined by two factors:

(1) the amount of gas in the lava or magma

(2) the ease or difficulty with which the gas can escape to the atmosphere

Viscosity – resistance to flow

PHYSICAL

CHARACTERISTICS

OF LAVA

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FelsicContains more than

65% Silica

TYPES OF LAVA

MaficSilica (Poor) (45-52%)

RholiteBasaltAndesiteIntermediate

Silica

Composition

(53-65%)

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The two most important factors that influence viscosity are the

(1) temperature of the lava relative to the cooler temperature which it solidifies

(2) the silica (Si𝑂2content of the lava)



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ERUPTION!

Composite volcanoes

often form the largest

and tallest volcanoes.

They are the most

explosive and dangerous

of the types of volcanoes.

Types of Volcano

Composite

Volcanoes

Mt. Saint Helens in Washington state.

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ERUPTION!

Usually smaller in size

than composite

volcanoes, and the

eruptions are smaller

also. They form into steep

cone shaped hills.

Types of Volcano

Cinder Cone

Volcanoes

Paricutin in Mexico

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ERUPTION!

Shield volcanoes do not

erupt or explode like

composite or cinder cone

volcanoes. Shield

volcanoes have much

smaller eruptions

producing less ash.

Types of Volcano

Shield

Volcanoes

Muana Loa in Hawaii.

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ERUPTION!Pyroclastic flow descending Mayon volcano, Philippines.

-Most of the gas released during

eruption is water vapor

-Carbon dioxide, sulfur dioxide,

hydrogen sulfide, and hydrochloric acid,

are given off in lesser amounts

-Surface water introduced into a

volcanic system can greatly increase the

explositivity of an eruption.

-At high altitudes, the pyroclastics often spread out into a dark, mushroom cloud. The fine

particles are transported downwind by high atmosphere winds. Eventually debris settles

back to earth under gravity's influence as ashfall (or sometimes pumicefili) deposits.

A pyroclastic flow is a mixture of gas and pyroclastic debris that is so dense that it hugs the

ground as it flows rapidly into low areas.

Gases and Pyroclastics

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ERUPTION!Types of Volcanic Eruptions

In a Hawaiian eruption, fluid lava is ejected from a

vent as fire fountains or lava flows. The 1969 eruption

at Mauna Ulu, a vent of Kilauea Volcano in Hawaii,

was a spectacular example of fire fountaining. Photo

by D.A. Swanson, USGS, August 22, 1969.

Hawaiian Eruption

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Short bursts of glowing lava,

created from the bursting of

large gas bubbles at the

summit vent of a volcano typify

a Strombolian eruption. This

photo, taken from the summit

of Stromboli, a volcano in the

Aeolian Islands, Italy, shows a

classic example of this activity.

Photo by Andrew Hague,

Istockphoto.com.

Strombolian Eruption

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Relatively small but violent

explosions of viscous lava create

columns of ash and gas and

occasional pyroclastic flows, as

seen at this eruption of the

Santiaguito volcanic dome

complex in Guatemala. Photo by

Jessica Ball, March 15, 2009.

Vulcanian

Eruption

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The largest and most violent of all explosive

eruptions, Plinian eruptions send columns of

pulverized rock, ash, and gases that rise miles

into the atmosphere in a matter of minutes.

Mount St. Helens in Washington State

experienced a Plinian eruption following a major

flank collapse in 1980. Photo by Austin Post,

USGS, May 18, 1980.

Plinian Eruption

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Lava domes, such as this example in

the crater of Mount St. Helens, are

piles of viscous lava that are too cool

and sticky to flow far. Domes grow and

collapse in cycles, and often form at

volcanoes that also experience Plinian

eruptions. Photo by Lyn Topinka,

USGS, August 12, 1985.

Lava Dome

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Lava erupting through water

creates the dramatic plumes

of scoria and billowing ash-

and-gas clouds of a

Surtseyan eruption. The type

example of this eruption

occurred at Surtsey, a

volcanic island off the coast

of Iceland. NOAA image of

the 1963 eruption.

Surtseyan Eruption

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ERUPTION!Types of Lava Flow

A smooth and continuous

lava crust. Pahoehoe forms

when the effusion rate is

low and consequently the

velocity of lava flow is slow.

Pahoehoe lava flow is

usually at least 10 times

slower than typical aa lava

flow.

Pahoehoe

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Aa lava is a rough rubbly

crust of a lava flow. It is a

major lava flow type.

Aa

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A distinctive lava flow morphology

that has been appearing in our

dredges is pillow lava. Pillow lavas

form when hot lava flows into water

and cools rapidly, creating long

tubes and bulbous pillow-shaped

mounds of rock. Pillow lavas are

found not only in the ocean but

also under glaciers that overlie

volcanoes.

Pillow Lava29



Blocky flows are

common if the silica

content of lavas is

higher (composition

of basaltic andesite to

rhyolite).

Blocky

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ERUPTION!Three Definitions of Volcanoes

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endogenic process

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