Chapter 5 Volcanoes (.pdf)

Chapter 5 

Volcanoes

Mount Rainier

Deane’s Volcano

Deane’s Volcano 

19,000 feet high


19,000 feet high 

Covered with snow year round



Covered with snow year round 

Active, belches ash and pyroclastics year 

round





round 

Lava flows engulf quaint barns with “mail 

pouch tobacco” roofs





round 


pouch tobacco” roofs 

Located where Maryville is, so I can see it 

from my office





round 


pouch tobacco” roofs 

Located where Maryville is, so I can see it 

from my office

Magma 

Simple put, a volcano 

occurs when magma erupts 

onto the surface, either on 

land or the ocean floor 

Magmas originate in the 

mantle at depths between 

50 and 250 kilometers (30 to 

150 miles) below the Earth’s 

surface

Magma from Mantle 

Magma does 

not come from 

the “center” or 

core of the 

Earth 

It comes from 

the upper part 

of the mantle

Source of Magma 

Magmas can reach the 

surface in one of three 

plate tectonic settings: 

Divergent plate boundaries 

Subduction zones 

“Hot spots”

Composition of Magma 

A magma’s chemical composition influences 

its physical properties, which determines 

how it will erupt (quietly or violently) and the 

kind of volcanic structure it will form 

The three major compositional variables of 

magma are the proportions of 

silica (SiO 2) 

iron (Fe) 

magnesium (Mg)

Composition of Magma 

A rock formed from a magma rich in iron and 

magnesium is described as mafic 

A rock formed from a magma rich in silica is 

described as felsic 

There is a gradation from mafic to felsic as 

the proportion of iron/magnesium decreases 

and the proportion of silica increases

Magma and Lava 

The upper mantle is extremely rich in iron and 

magnesium and therefore is of mafic composition 

The magma erupting along the mid-oceanic ridges 

and at oceanic volcanic hot spots is therefore also 

mafic in composition and creates basaltic lavas

Magma and Lava 

For a magma from the 

mantle to erupt on land, 

the magma will have to 

melt its way through the 

crust 

The crust is rich in silicate minerals 

These silicate minerals are melted and become 

part of the magma 

As more and more silicate minerals are added to 

the magma, the magma’s composition 

progressively changes from mafic to felsic

Major Types of Lava 

Basaltic lavas 

Magma has mafic composition, typically erupts at 

1000 o to 1200 o C, flowing basaltic lavas have lowviscosity, 

cools to form basalt 

Andesitic lavas 

Intermediate in composition and viscosity between 

mafic and felsic magmas, cools to form andesite 

Rhyolitic lavas 

Felsic composition, typically erupts at 800 o to 

1200 o C, high-viscosity, cools to form rhyolite

Major Types of Lava 

Basaltic lavas tend to be a dark color, because 

most iron/magnesium minerals are dark 

Rhyolitic lavas tend to be lighter in color, because 

most silica minerals are lighter in color 

Basalt Andesite Rhyolite

Flowing Lava 

The silica-poor mafic 

magmas that produce 

the basalt of the ocean 

floors have low viscosity 

Low viscosity means 

that the lava flows easily 

In contrast, silica-rich 

felsic magmas have high 

viscosity and flow very 

poorly (like cold syrup)

Examples of Lava Flow 

Pahoehoe 

Aa 

Pillow basalts 

Vesicular basalt 

We will look at each type

Aa and Pahoehoe 

Aa: a relatively low 

viscosity basaltic lava 

characterized by a 

sharp, jagged, blocky 

texture 

Pahoehoe: a very low 

viscosity basaltic lava 

characterized by a 

smooth, ropy texture

Aa and Pahoehoe 

A single downhill basaltic lava flow commonly 

has the features of pahoehoe near the source, 

where the lava is still hot and fluid 

While Aa usually 

develops farther 

downstream where 

the cooling lava has 

developed a thicker 

outer layer

Pillow Basalt 

When a basaltic magma 

erupts under the ocean, 

it cools very rapidly and 

forms a “pillow-shaped” 

lava that has a glassy 

texture 

This exposed pillow lava 

was formed under the 

sea and uplifted by 

tectonic forces

Magma contain 

dissolved gases 

which become 

trapped in the 

basalt when the 

lava cools 

The “pitted” 

surface is 

referred to as 

vesicular 

Vesicular Basalt

Pyroclastic Material 

Pyroclastic material 

is volcanic ejecta 

violently blown out 

of the volcano into 

the atmosphere 

during an eruption 

It can be molten, 

partially molten or 

solid

Pyroclastic Material 

The molten pyroclasts 

can cool and solidify 

as they fly through the 

atmosphere 

Smaller particles can 

be blown 20-25 

kilometers up into the 

stratosphere

Pyroclasts 

Pyroclasts are classified by the size of the 

particle 

Volcanic ash is very fine dust which is “ash-like” 

in consistency 

Cinder is composed of gravel-sized pieces of 

ejecta and usually feels rough to the touch 

Volcanic bombs are large fragments of ejected 

magma which become rounded and cool 

(solidify) as they fly thru the atmosphere

Volcanic Bomb

Volcanic Tuff 

Rocks created from smaller particles 

(such as ash) are called volcanic tuffs

Volcanic Breccia 

Volcanic breccia is composed of large pieces 

of volcanic ejecta solidified into rock

Cinder Cones 

Composite Volcano 

Caldera 

Types of Volcanoes 

Shield Volcano 

Volcanic Dome

Shield Volcano 

Formed mainly of basaltic lavas 

Gentle slopes averaging 2-10 degrees 

Can be huge, up to 120 kilometers wide! 

Long duration of activity, lasting tens of 

thousands of years 

Eruptions usually non-violent 

Long lava flows

Shield Volcano

Shield Volcanoes 

The entire island of Hawaii 

was created by a series of 

shield volcanoes 

The cratered top of Mauna 

Loa is seen covered with 

snow

Composed of rhyolitic and andesitic lavas (felsic 

magma) 

Lava oozes out onto the surface like “thick 

toothpaste” 

Grows slowly 

Volcanic Dome 

Steep-sided and small, rarely more than a couple 

of hundred meters wide 

Typically forms inside an already existing crater

Volcanic Dome

Volcanic Dome 

Novarupta Dome, Alaska 

Mount St. Helens

Cinder Cones 

A volcano formed of cinders and other small-sized 

basaltic pyroclastic material built up around the 

volcanic vent 

Steep sides, with slopes up to 30 degrees 

Relatively small, averaging one kilometer (~1/2 mile) 

in diameter, but can have extensive lava flows 

Short-lived, 

typically a single 

event, lasting for 

a few hours up to 

several months

Cinder Cones

Cinder Cones 

Cerro Negro Cinder 

Cone, Nicaragua 

Amboy Cinder Cone, 

California

Composite Volcano 

Some eruptions may spew out pyroclastic deposits, 

another eruption may consist of andesitic lava flows 

Slopes are intermediate in steepness 

Relatively large, easily 10-15 kilometers in diameter 

Intermittent eruptions over long time span, lasting 

thousands of years 

Eruptions often highly explosive 

An active volcano that can 

erupt with different types of 

material during its life

Composite Volcano

Mount St. Helens 

Mount St. Helens in 

Washington is a composite 

volcano 

On March 16, 1980 the first 

of a series of minor 

earthquakes occurred 

under the volcano 

This marked the beginning 

of a new eruption cycle 

It had been ~350 years 

since the last eruption


On May 18, 1980, at 8:32 am, a 

magnitude 5.1 earthquake 

occurred 1 mile directly under 

the volcano 

10 seconds later, the north 

side of the volcano began to 

collapse followed immediately 

by an explosion that blew the 

top 400 meters (1300 feet) off 

of the peak


“...24 square miles of valley was 

filled by a debris avalanche, 250 

square miles of recreation, timber, 

and private lands were damaged by 

a lateral blast, and an estimated 200 

million cubic yards of material was 

deposited directly by lahars 

(volcanic mudflows) into the river 

channels” 

“Fifty-seven people were killed”


Trees snapped like toothpicks...

Mount St. Helens Ash Fall


Before 

After

Mount St. Helens Today 

Mount St. Helens is 

now a volcanic 

national monument 

and remains active

Caldera 

A large depression (can easily be several miles 

wide) formed by collapse of a volcano into a 

partially drained magma chamber 

The collapse may eject a tremendous amount 

of pyroclastic material into the atmosphere and 

cover very large areas of land with ash and 

debris 

Caldera may have younger domes within it

Caldera

Crater Lake Caldera

Crater Lake Caldera

Crater Lake Caldera

Crater Lake Caldera

Crater Lake Caldera 

50 cubic kilometers of material was 

blown off the top of the mountain

Fissure Eruptions 

A volcanic eruption 

originating along 

an elongate fissure 

rather than a 

central vent

Fissure Eruptions 

Laki Fissure in 

Iceland formed in 

1783, resulting in a 

lava flow that 

covered 13 square 

kilometers

Mid-Oceanic Ridges 

Fissure volcanic activity can occur any where along 

the 60,000 kilometer-long mid-oceanic ridges (much 

more on this when we cover the ocean floor)

Flood Basalts 

Thick, widespread 

accumulations of basalt 

that cover a large area 

Typically fed by fissures 

An excellent example is 

the Columbia River 

Plateau in Washington 

and Oregon

Columbia Plateau Flood Basalts 

Successive flows of 

flood basalt over a time 

period of 10 to 15 

million years built up a 

plateau 1.8 kilometers 

(6,000 feet) thick and 

covers an area of 

160,000 square 

kilometers (63,000 

square miles)

Large Igneous Provinces 

Areas of extensive flood basalt lava flows are 

found both on land and under the oceans

Deccan Plateau in India 

At the end of the 

Cretaceous period, 

65 million years ago, 

volcanic eruptions 

created the Deccan 

Plateau that covers 

most of southern 

India


After several thousand years, the combined 

thickness of the lava flows were as much as 

2000 meters (6500 feet) and covered an area 

of over 1,500,000 square kilometers


Such staggering quantities of gas were released 

into the Earth’s atmosphere by these continuous 

eruptions, that the chemical composition of the 

entire atmosphere was changed world-wide 

This may have played a major role in the extinction 

of the dinosaurs at the end of the Cretaceous

Diatreme 

When a volcano goes 

dormant, the magma 

and breccia in the 

volcanic vent will cool 

and solidify into rock 

If the volcanic cone is 

eroded away, this plug, 

called a diatreme, will 

be exposed

Shiprock, 

New Mexico 

Diatreme 

Fig. 6.12

Seamounts 

Dotting the ocean floor are submarine volcanoes 

called seamounts 

They can rise above the ocean floor from hundreds 

of meters to even a couple thousand meters 

They are found in all oceans, but by far the 

greatest number occur in the Pacific Ocean

Seamounts 

There are more than a million seamounts in the oceans 

(only a couple thousand are plotted on the map below) 

So there are far more volcanoes under the sea than on 

land

Oceanic Volcanic Hot Spots 

Instability at the coremantle 

boundary 

causes a mantle plume 

to rise, led by a hot, 

turbulent plume head


When the plume 

reaches the base of the 

lithosphere, it flattens 

and decompresses. 

Basaltic magma from 

decompression melting 

erupts as flood basalts


As the plume moves 

over the remains of 

the plume, the plume 

tail, now a hot spot, 

may form a hot spot 

volcano


Continued plate 

movement over the 

hot spot creates a 

chain of volcanic 

islands

Oceanic Volcanic Hot Spots

Hot Spot Volcanism 

Every island in the Hawaiian Island chain was 

created by volcanic eruptions that occurred as 

the Pacific Oceanic Plate crossed over a hot spot 

for millions of years

Hot Spot Volcanism


The Cortes Bank Seamount, 100-miles 

offshore of San Diego, California, is the 

17-mile long remnant of a undersea 

volcanic mountain range that rises to 

within 3 feet of the surface


Fisherman and scuba divers knew about it, 

but it was not on any nautical maps 

The U.S. Navy found it the hard way in 1985 

when the aircraft carrier Enterprise ran 

aground 

Then the surfers found it


“To get the biggest waves at Cortes Bank, you need light winds, low 

tides, and big storm swells from the northwest all at the same time, a 

Pacific surfer's version of the "perfect storm". When it happened on 

January 19, 2001, California big wave riders scrambled to test their 

skill against the biggest, baddest wave ever ridden.”

Continental Volcanic Hot Spots 

Yellowstone Caldera Chain formed as the North 

America Plate moved to the west southwest


The ages of the Yellowstone Calderas formed 

over the hot spot range from 16 million years 

ago to the present


The youngest 

caldera is 80 

kilometers long 

and 55 kilometers 

wide (50 by 35 

miles) or roughly 

the size of Knox 

County 

It lies within the 

Yellowstone 

National Park


The caldera is large enough so that you cannot 

obviously realize that you are in a giant crater


Currently, volcanic activity is exhibited only via 

numerous geothermal vents, including Old 

Faithful Geyser, but within the past two million 

years, it has undergone three extremely large 

explosive eruptions, up to 2,500 times the size 

of the 1980 Mount St. Helens eruption


The most recent explosion occurred 630,000 

years ago, 

1000 cubic kilometers of pryoclastic material 

was blown into the atmosphere 

Most of the United States was covered with ash


The brown area marks the extent of the ash fall

Volcanic Hazards

Lava Flows 

Lava erupts from the flank of the Barcena Volcano 

on the Revillagigedo Islands, Mexico in the Pacific 

Ocean causing little damage

Lava Flows 

Lava set fire to the brand new Wahaùla Visitor 

Center in Hawaiì Volcanoes National Park when an 

1989 eruption of Pu‘u ‘O‘o–Kupaianaha completely 

covered the area with thin pahoehoe lava flows

Lava Flows of Parícutin 

The Parícutin eruption began as a fissure in a 

cornfield owned by farmer Dionisio Pulido on 

February 20, 1943 in the Mexican state of 

Michoacán 

Pulido, his wife 

and son, all 

witnessed the 

initial eruption of 

ash and stones 

first-hand as they 

plowed the field

Lava Flows of Parícutin 

The villages of Paricutín and San Juan 

Parangaricutiro were both buried by lava 

and ash

Eruption Cloud 

So much smoke and 

ash was released by 

the Chaiten Volcano 

in southern Chile 

during the 2008 

eruption, that the 

eruption cloud 

stretched across 

South America from 

the Andes 

Mountains to out 

over the Atlantic 

Ocean

Gases 

Volcanoes can 

also eject great 

quantities of 

gases and steam 

which can be 

mixed with the 

pyroclasts

Gases 

Sulfur dioxide (SO 2) 

Hydrogen sulfide (H 2S) 

Carbon dioxide (CO 2) 

Hydrogen Chloride (HCl) 

Hydrogen Fluoride (HF)

VOG 

Noxious sulfur dioxide gas and other pollutants 

emitted from Kilauea Volcano on the Island of 

Hawaiì react with oxygen and atmospheric 

moisture to produce volcanic smog (vog) and 

acid rain

VOG 

Sulfur dioxide, a pollutant that is also generated 

by burning coal and oil, can cause asthma and 

other respiratory illnesses and aggravate lung 

and heart disease 

Mixed with atmospheric moisture, it produces 

acid rain

Lahar 

Lahar is an Indonesian word that describe 

fast moving volcanic mud flows 

They kill by drowning people in mud that is 

too thick and too heavy to swim in

Lahar 

Lahars are formed by the sudden mixing of 

large volumes of pyroclastic material, such 

as ash, with water 

For example, when volcanic activity melts a 

glacier on a mountain top, or because of 

heavy rain, or the draining of a lake in a 

crater

Lahar 

“Sunday 18 March 

2007 4:59 GMT 

Wellington, New 

Zealand – 

A massive lahar, or 

volcanic mudflow, 

swept down New 

Zealand's 2,797metre 

high Mount 

Ruapehu on Sunday 

after its steaming 

crater lake burst its 

banks...”

Lahar 

Lahars can flow very fast, at speeds faster than 

100 kilometers per hour (60 miles per hour) 

They can flood large areas and flow for long 

distances in river channels 

Lahar in the Belham Valley, Montserrat, in the 

Caribbean, during heavy rain, 19 July 2007

Lightning 

Lightning storm at the Chaiten Volcano, Chile

Nuée Ardente 

Nuée ardente is a poetic-sounding French 

word introduced in 1904 to describe the 

terrible pyroclastic flow that destroyed the 

entire city of St. Pierre on the island of 

Martinique in 1902 

It translates to “glowing incandescence”

Nuée Ardente 

Nuée ardente are extremely fast moving 

fluidized bodies of very hot gas, ash and rock 

that hug the ground as they flow down the 

volcanic slope 

Temperatures can be as high as 800 o C

Nuée Ardente 

Mayon Volcano, Philippines, 1984

Martinique is an island 

formed by volcanoes in 

the Caribbean 

It was a French colony 

1902 Martinique 

The dormant Mount Pelée 

volcano began a new 

series of eruptions on 

April, 25, 1902

With each passing day, 

the eruptions became 

worse 

The French governor 

refused to evacuate the 

city of St. Pierre and 

surrounding smaller 

cities at the base of the 

volcano 


On Wednesday, May 7, 1902, the governor with his 

family and entourage visited St. Pierre 

There was a gala ball that night


The next day at 8:02 am, 

the city of St. Pierre was 

destroyed by a pyroclastic 

flow that roared down the 

slopes of Mount Pelée at of 

670 kilometers per hour 

(415 mph)

Over 29,000 people 

were burned alive in 

seconds 


The entire town was 

completely destroyed


Supposedly, there was one survivor, ex-convict 

Louis-Auguste Cyparis (aka Ludger Sylbaris), 

who was in jail either for murder or being drunk 

He joined the circus


Don’t worry! 

A French government review panel 

concluded that the island governor 

was correct in not evacuating the city

Phreatic Explosion 

When water mixes 

with magma below 

the surface, you can 

get a tremendous 

steam explosion 

These can be the 

greatest explosions 

on Earth 

Nisino-sima Volcano 

in the Pacific Ocean

Santorini 

Santorini is a circular group 

of islands in the Aegean 

Sea and is part of Greece 

The central lagoon is a 

volcanic caldera 12 by 7 

kilometers (8 by 4 miles) in 

size

About 3500 years ago 

there was a tremendous 

volcanic eruption 

30 cubic kilometers of 

the island was blown as 

high as 36 kilometers 

into the atmosphere 

This is believed to be 

the largest volcanic 

explosion in recorded 

human history 

Santorini

Atlantis? 

The eruption, ash fall and 

tsunamis are believed to 

have destroyed the Minoan 

civilization at the height of 

its wealth and power 

This may be the source of 

Plato’s story of Atlantis

Santorini 

Akrotiri is an ancient city on Santorini that was 

buried under hundreds of feet of volcanic ash 

from this tremendous explosion 

It is an active archaeology dig that is open to the 

public

Santorini 

The volcano is still active as recent as 1950

How Bad Can It Be? 

1815 Indonesia 92,000 killed 




1902 Martinique 29,025 killed 

1902 Guatemala 6,000 killed 


1951 New Guinea 2,942 killed 

1982 Mexico 1,700 killed 

1985 Columbia 23,000 killed


Cumulative deaths due to volcanic 

eruptions over the past 500 years


Causes of volcanicrelated 

deaths Over 

the last 2,000 years

Mount Tambora 

Mount Tambora (or Tomboro) is an active 

stratovolcano on Sumbawa island, Indonesia 

It was thought to be 14,000 feet (4,300 meters) high 

when it blew up in April 1815 

The death toll was at least 71,000 people (some 

estimate 92,000 people were killed), of which 

11,000 to 12,000 were killed directly by the eruption

Mount Tambora 

The explosion was heard on Sumatra island 

2,000 kilometers (1,200 miles) away 

The red areas outlines the heavy ash fall

Mount Tambora 

The eruption ejected immense amounts of 

volcanic dust into the upper atmosphere and 

created global climate anomalies in the 

Northern hemisphere 

The entire world experienced dusty skies and 

deep red sunsets 

(Dusty sunsets 

in Hong Kong 

from 1992 

Mount Pinatubo 

eruption)

Year Without a Summer 

1816 became known as the Year Without a 

Summer and the Poverty Year and Eighteen 

hundred and Froze to Death 

Agricultural crops failed and livestock died in 

much of the Northern Hemisphere, resulting in 

the worst famine of the 19th century 

Europe, still 

recuperating from 

the Napoleonic 

Wars, suffered 

from food 

shortages

Year Without a Summer 

Food riots broke out in 

Britain and France and grain 

warehouses were looted 

The violence was worst in 

landlocked Switzerland, 

where famine caused the 

government to declare a 

national emergency 

Huge storms, abnormal rainfall with flooding of 

the major rivers of Europe (including the Rhine) 

are attributed to the event, as was the frost in 

August 1816

Future Eruptions?

Future Eruptions?

Future Eruptions?

Mount Vesuvius 

The Mount 

Vesuvius 

Volcano is 

surrounded by 

the city of 

Naples, Italy 

Vesuvius is 

called the most 

dangerous 

volcano in the 

world


In A.D. 79, an 

eruption buried 

the cities of 

Pompeii and 

Herculium in ash 

in a manner of 

minutes 

Perhaps 15,000 

to 25,000 people 

lived in these 

cities at the time 

of the eruption


Over 1500 bodies have been 

found in the ruins of Pompeii 

The bodies were instantly 

incinerated by the hot ash


Today, Naples 

has a population 

of 3,000,000 

people 

A new eruption 

would probably 

devastate the 

entire city...

Chapter 6 

Weathering and Soil

Chapter 5 Volcanoes (.pdf)

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