UNDERSTANDING VOLCANOS
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Transcript UNDERSTANDING VOLCANOS
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