UNDERSTANDING VOLCANOS
Dr. Walter Hays,
Global Alliance For
Disaster Reduction
ESSENTIAL KNOWLEDGE FOR
SOCIETAL SUSTAINABILITY
A FRAMEWORK FOR
UNDERSTANDING THE IMPACTS
OF VOLCANIC ERUPTIONS ON
PEOPLE AND THEIR COMMUNITIES
GLOBAL DISTRIBUTION OF 1,500
ACTIVE VOLCANOES
ERUPTIONS OF MOUNT
MERAPI
May 15, and June 6 - 8, 2006
LOCATION IN CENTRAL JAVA
MOUNT MERAPI RECEIVED CLOSE
ATTENTION ON APRIL 18
INDONESIA’S MOUNT MERAPI
ERUPTED ON MAY 15, 2006
 Mount Merapi, a
stratovolcano,
emitted lava,
debris, and a
pyroclastic flow
(or cloud) on
May 15.
MAY 15, 2006 ERUPTION
 Hot ash
released.
MAY 15, 2006 ERUPTION
 Volcanic ash
turned
everything
white
MAY 15, 2006 ERUPTION
 School children
wore masks to
counter adverse
health effects of
breathing
volcanic ash.
MAY 15, 2006 ERUPTION
 Volcanic
ash covered
crops and
vegetation.
MAY 15, 2006 ERUPTION
 Volcanic ash
covered
automobiles
and affected
jet airline
traffic.
MOUNT MERAPI ERUPTED AGAIN
ON JUNE 6-8, 2006
 Mount Merapi
volcano emitted
lava, debris, and
pyroclastic flows
(superheated
clouds of gas) on
Tuesday, June 6
and Wednesday,
June 7.
EXPLOSIVENESS OF JUNE 8
ERUPTION SENT 15,000 FLEEING
EVACUATION
 11,000 from
three districts
evacuated to
schools and
otheren “safe
haven” emergency shelters.
MANY CHOSE TO EVACUATE
 Many citizens
chose to
evacuate.
 Evacuation was
ordered.
 Villagers
remembered the
1994 disaster.
MANY CHOOSE NOT TO
EVACUATE
 Many citizens
chose not to
evacuate
because shelters
are boring and
they wanted to
provide for
livestock and
tend crops.
ESSENTIAL KNOWLEDGE
ON VOLCANOES
Plate tectonics and
volcanic activity
 Global distribution of volcanoes
Most volcanoes are located within
or near ocean basins
Basaltic rocks: in oceanic and
continental settings
Granitic rocks: in continental
settings
The nature of
volcanic eruptions
 Three physical characteristics of a magma
control its viscosity, the physical property
that ultimately determines the “violence” or
explosiveness, of the eruption:
Composition
Temperature
Dissolved gases
The magma affects the severity
of a volcanic eruptions
In summary
–Basaltic Magmas =
mild eruptions
–Rhyolitic or Andesitic
Magmas = explosive
eruptions
The nature of volcanic eruptions
 Viscosity is a measure of a
material’s resistance to flow
 Factors affecting viscosity
Temperature - Hotter
magmas are less viscous
Composition - Silica (SiO2)
content
The nature of volcanic eruptions
–Higher silica content = higher
viscosity (e.g., felsic lava such
as rhyolite)
–Lower silica content = lower
viscosity (e.g., mafic lava such
as basalt)
The nature of volcanic eruptions
Dissolved gases
–Gas content affects magma
mobility
–Gases expand within a magma as
it nears the Earth’s surface due
to decreasing pressure
–The violence of an eruption is
related to how easily gases escape
from magma
Materials extruded
from a volcano
 Lava flows
 Basaltic lavas exhibit fluid behavior
 Types of basaltic flows
–Pahoehoe lava (resembles a twisted or
ropey texture)
–Aa lava (rough, jagged blocky texture)
 Dissolved gases
 1% - 6% by weight
 Mainly H2O and CO2
Materials extruded
from a volcano
 Pyroclastic materials – “fire fragments”
• Types of pyroclastic debris
–Ash and dust - fine, glassy
fragments
–Pumice - porous rock from
“frothy” lava
–Cinders - pea-sized material
Materials extruded
from a volcano (continued)
 Pyroclastic materials – “fire fragments”
• Types of pyroclastic debris
–Lapilli - walnut-sized material
–Particles larger than lapilli
Blocks - hardened or cooled
lava
Bombs - ejected as hot lava
Volcanoes
 General features
 Opening at the summit of a volcano
–Crater - summit depression < 1 km
diameter
–Caldera - summit depression > 1 km
diameter produced by collapse following
a massive eruption
 Vent – surface opening connected to the
magma chamber
 Fumarole – emit only gases and smoke
Volcanoes
 Types of volcanoes
Shield volcano
–Broad, slightly domed-shaped
–Generally cover large areas
–Produced by mild eruptions of
large volumes of basaltic lava
–Example: Mauna Loa on Hawaii
Volcanoes
Cinder cone
–Built from ejected lava
(mainly cinder-sized)
fragments
–Steep slope angle
–Small size
–Frequently occur in groups
Volcanoes
Composite cone (stratovolcano)
–Most are located adjacent to
the Pacific Ocean (e.g.,
Fujiyama, Mt. St. Helens)
–Large, classic-shaped volcano
(1000’s of ft. high and several
miles wide at base)
Mt. St. Helens – prior
to the 1980 eruption
Mt. St. Helens (after
the 1980 eruption)
Volcanoes
Composite cone (stratovolcano) continued
–Composed of interbedded lava
flows and pyroclastic debris
–Most violent type of activity
(e.g., Mt. Vesuvius)
Volcanic Hazards
Nuée ardente –
A Fiery pyroclastic flow made of
hot gases infused with ash and
other debris
Also known as “glowing
avalanches”
Move down the slopes of a
volcano with velocities
approaching 200 km/hour
Volcanic Hazards
Lahar – volcanic landslide or
mudflow
Mixture of volcanic
debris and water
Move down slopes of
volcano and stream
valleys with velocities of
30 to 60 miles/hour
volcanic hazards
 Pyroclastic flow
Felsic and intermediate magmas
Consists of ash, pumice, and other
debris
Material ejected at high velocities
Example: Yellowstone plateau
Other volcanic landforms
 Caldera
Steep-walled depressions at the
summit
Generally > 1 km in diameter
Produced by collapse
Example: Crater Lake, Oregon
Other volcanic landforms
 Fissure eruptions and lava plateaus
Fluid basaltic lava extruded from
crustal fractures called fissures
Example: Columbia River Plateau
 Lava domes
Bulbous mass of congealed lava
Associated with explosive eruptions of
gas-rich magma
Other volcanic landforms
 Volcanic pipes and necks
Pipes - short conduits that
connect a magma chamber to the
surface
Volcanic necks (e.g., Ship Rock,
New Mexico) - resistant vents
left standing after erosion has
removed the volcanic cone
Shiprock, New Mexico
Intrusive igneous activity
 Most magma is emplaced at
depth in the Earth
Once cooled and solidified, it
is called a pluton
Intrusive igneous activity
 Nature of plutons
Shape - tabular (sheetlike) vs.
massive
Orientation with respect to
the host (surrounding) rock
–Concordant vs. discordant
Intrusive igneous activity
 Types of intrusive igneous features
Dike – a tabular, discordant
pluton
Sill – a tabular, concordant
pluton (e.g., Palisades Sill in
New York)
Intrusive igneous activity
 Types of intrusive igneous features
Lacolith
–Similar to a sill
–Lens or mushroom-shaped mass
–Arches overlying strata upward
Intrusive igneous activity
 Intrusive igneous features continued
Batholith
–Largest intrusive body
–Surface exposure > 100+ km2
(smaller bodies are termed
stocks)
–Frequently form the cores of
mountains
Plate tectonics and
igneous activity
 Global distribution of igneous activity
Most volcanoes are located within
or near ocean basins
Basaltic rocks: oceanic and
continental settings
Granitic rocks: continental settings
Plate tectonics and
igneous activity
 Igneous activity at plate margins
Spreading centers
–Greatest volume of volcanic
rock is produced along the
oceanic ridge system
Plate tectonics and
igneous activity
 Igneous activity at plate margins
Mechanics of spreading
Decompression melting of the
mantle occurs as the lithosphere
is pulled apart
Large quantities of basaltic
magma are produced
Plate tectonics and igneous activity
Subduction zones
–Occur in conjunction with deep
oceanic trenches
–Location of partial melting of
descending plate and upper mantle
–Rising magma can form either
An island arc if in the ocean
A volcanic arc if on a continental
margin
Plate tectonics and
igneous activity
Subduction zones are associated
with the Pacific Ocean Basin
The region representing
the Pacific Rim is known as
the “Ring of Fire”
Location of majority of
world’s explosive volcanoes
Plate tectonics and igneous activity
 Intraplate volcanism
 Occurs within a tectonic plate
 Associated with mantle plumes
 Localized volcanic regions in the overriding
plate are called a hot spot
–Produces basaltic magma sources in
oceanic crust (e.g., Hawaii and Iceland)
–Produces granitic magma sources in
continental crust (e.g., Yellowstone Park)
Volcanoes and climate
 The basic premise
Explosive eruptions emit huge
quantities of gases and finegrained debris
A portion of the incoming solar
radiation is reflected and
filtered out
Volcanoes and climate
 Past examples of volcanism
affecting climate
Mount Tambora, Indonesia –
1815
Krakatau, Indonesia – 1883
Volcanoes Can Impact Regional
And Global Climate
 Modern examples
Mount St. Helens, Washington 1980
El Chichón, Mexico - 1982
Mount Pinatubo, Philippines - 1991