Download 04 Chapter 4_Volcanoes and Volcanic Hazards

Engineering Geology
CVL 3315
Chapter 4
Volcanoes and Volcanic
Hazards
Dr. Sari Abusharar
University of Palestine
Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
2nd Semester 2014-2015
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Outline of Presentation
The Nature of Volcanic Eruptions
Materials Extruded during an Eruption
Volcanic Structures and Eruptive Styles
Living in the Shadow of a Composite Cone
Other Volcanic Landforms
Plate Tectonics and Volcanic Activity
Living with Volcanoes
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Mount St. Helens, Washington, USA
Before-and-after photographs show the transformation of Mount St.
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(Saint) Helens caused by the May 18, 1980, eruption
Mount St. Helens on May 18, 1980
Douglas fir trees were snapped off or uprooted by the lateral blast of
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Mount St. Helens on May 18, 1980
The Nature of Volcanic Eruptions
Factors determining the “violence” or
explosiveness of a volcanic eruption
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
The above three factors actually control
the viscosity of a given magma which in
turn controls the nature of an eruption
The Nature of Volcanic Eruptions
Viscosity is a measure of a material’s resistance
to flow (e.g., Higher viscosity materials flow
with great difficulty)
Factors affecting viscosity
• Temperature - Hotter magmas are less
viscous
• Composition - Silica (SiO2) content
- Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
- Lower silica content = lower viscosity or
more fluid-like behavior (e.g., mafic lava
such as basalt)
The Nature of Volcanic Eruptions
Factors affecting viscosity
• 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
Magma Composition vs. Behavior
The Nature of Volcanic Eruptions
Factors affecting viscosity
• In Summary
Lower viscous lavas (e.g., basalt) generally
produce mild eruptions
Highly viscous lavas (e.g., rhyolite or
andesite) produce more explosive eruptions
Materials Extruded during an Eruption
Lava Flows
• Basaltic lavas are much more fluid
• Types of basaltic flows
– Pahoehoe lava (resembles a twisted or ropey
texture ‫)نسيج لزج‬
– Aa lava (rough, jagged blocky texture ‫) نسيج خشن‬
Dissolved Gases
• 1% - 6% of a magma by weight
• Mainly water vapor (H2O) and carbon
dioxide (CO2)
Lava flows
A typical slowmoving, basaltic, aa
flow
A typical fluid pahoehoe
(ropy) lava
Materials Extruded during an Eruption
Pyroclastic materials – “fire fragments”
•
Types of pyroclastic debris
• Ash and dust - fine, glassy fragments
• Pumice - porous rock from “frothy” lava
• Lapilli - walnut-sized material
• Cinders - pea-sized material
• Particles larger than lapilli
– Blocks - hardened or cooled lava
– Bombs - ejected as hot lava
Pyroclastic materials
Bomb is approximately 10 cm long
Pyroclastic materials
Bomb is approximately 10 cm long
Volcanic bombs
Bomb is approximately 10 cm long
Volcanic bombs forming
during an eruption of
Hawaii’s Kilauea volcano.
Ejected lava masses take on
a streamlined shape as they
sail through the air. The
bomb in the insert is about
10 centimeters long
General features of Volcanoes
Opening at the summit of a volcano
– Crater ‫وھدة‬- steep-walled depression at the summit,
generally less than 1 km diameter
– Caldera ‫ كالديرا‬- a summit depression typically
greater than 1 km diameter, produced by collapse
following a massive eruption
Vent ‫ –مخرج‬opening connected to the magma chamber
via a pipe
Fumarole ‫ –داخنة‬emit only gases
Creator vs. Caldera
Crater
Caldera
Creator vs. caldera
General features of Volcanoes
Types of Volcanoes
Shield volcano
Cinder cone
volcano
Composite cone
(Stratovolcano)
General features of Volcanoes
Types of Volcanoes
• Shield volcano ‫بركان درعي‬
– Broad, slightly domed-shaped
– Composed primarily of basaltic lava
– Generally cover large areas
– Produced by mild eruptions of large volumes
of lava
– Example: Mauna Loa on Hawaii
Shield volcanoes
Bomb is approximately 10 cm
built primarily of fluid
basaltic lava flows and
exhibit the shape of a broad,
slightly dome shaped
structure that resembles a
long warrior’s shield.
Mauna Loa is one of five shield
volcanoes that collectively make up
the island of Hawaii.
General features of Volcanoes
Types of Volcanoes
• Cinder cone volcano ‫بركان مخروط حمم فتاتي‬
– Built from ejected lava fragments, mainly
cinder-sized (pea-sized )
– Steep slope angle
– Rather small size
– Frequently occur in groups
Cinder cones volcano
Cinder cones are built
from ejected lava
fragments (mostly cinders
Bomb is approximately 10 cm long
and bombs) and are
usually less than 300
meters (1000 feet) in
height
Cinder cone near Flagstaff,
Arizona, USA
General features of Volcanoes
Types of volcanoes (continued)
• Composite cone (Stratovolcano ‫) بركان طباقي‬
– Most are located adjacent to the Pacific
Ocean (e.g., Fujiyama, Mt. St. Helens)
– Large, classic-shaped volcano (300’s of
meters high & several miles wide at base)
– Composed of interbedded lava flows and
layers of pyroclastic debris
– Most violent type of activity (e.g., Mt.
Vesuvius)
A composite volcano
A size comparison of the three
types of volcanoes
A. Profile of Mauna Loa, Hawaii, a typical shield volcano
B. Profile of Mount Rainier, Washington, a typical composite cone
C. Profile of Sunset Crater, Arizona, a typical steep-sided cinder cone.
Active volcanoes in Western US
Of the 13 potentially active
volcanoes, 11 have erupted in the
past 4000 years and 7 in just the
past 200 years. More than 100
eruptions, most of which were
explosive, have occurred in the
past 4000 years. Mount St.
Helens is the most active volcano
in the Cascades. Its eruptions
have ranged from relatively
quiet outflows of lava to
explosive events much larger
than that of May 18, 1980.
Volcanic Hazards
Nuée ardente ‫(السحابة المتوھجة‬fiery or glowing
cloud) – A deadly pyroclastic flow
– Fiery pyroclastic flow made of hot gases infused
with ash and other debris
– Also known as glowing avalanches (‫)انھيارات ثلجية‬
– Move down the slopes of a volcano at speeds up
to 200 km per hour
– In general, flows that are fast and highly
turbulent (‫ )مضطربة بشكل كبير‬can transport fine
particles for distances of 100 kilometers or more
A nueé ardente on Mt. St. Helens
Pyroclastic flows
Japan’s Unzen volcano
engulfed
and
burned
hundreds of homes and
moved cars as much as 80
meters (250 feet).
St. Pierre, Mount Pelée, 1902
St. Pierre as it appeared shortly
after the eruption of Mount
Pelée, 1902
St. Pierre before
the eruption
The Roman city of Pompeii, Italy
Ruins of Pompeii. Excavation
began in the 18th century and
continues today
Plaster casts of several
victims of the AD 79
eruption of Mount
Vesuvius.
Volcanic Hazards
Lahar ‫ –تدفقات طينية‬volcanic mudflow
– large composite cones volcano may generate a
type of very fluid mudflow referred to by its
Indonesian name lahar
– Mixture of volcanic debris and water
– Move down stream valleys and volcanic slopes,
often with destructive results
Lahars - mud flows
When Mount St. Helens erupted in 1980, several lahars were
generated
Volcanic landforms
Calderas ‫كالديرا‬
•
•
•
•
Steep-walled depressions at the summit
Size generally exceeds 1 km in diameter
Produced by collapse
Caldera Types:
-Crater Lake – Type
- Hawaiian – Type
- Yellowstone – Type
‫ يبلغ قطره عدة كيلومترات أو‬، ‫ منخفض بركانى كبير يشبه حوض الترسيب‬‫ يعتقد أنھا تتكون نتيجة‬، ‫ يتكون بعد ثورة بركان‬، ‫ له جوانب رأسية‬، ‫أكثر‬
. ‫انھيار سقف غرفة الصھارة الخالية‬
Formation of Crater Lake, Oregon
Crater Lake and Wizard Island
Volcanic landforms
Pyroclastic flows
• Associated with felsic & intermediate
magma
• Consists of ash, pumice, and other
fragmental debris
• Material is propelled ‫ تندفع‬from the vent at
a high speed
Other volcanic landforms
Fissure eruptions ‫انبثاق طولي‬and lava plateaus
‫ھضاب البية‬
• Fluid basaltic lava extruded from crustal
fractures called fissures
• e.g., Columbia River Plateau
Basaltic fissure eruption
Lava fountaining from a
fissure and formation of fluid
lava flows called flood basalts
Columbia river Plateau
The Columbia River
basalts cover an area of
nearly 200,000 square
kilometers (80,000 square
miles).
Other volcanic landforms
Lava Domes
• Bulbous (‫ )بصلي الشكل‬mass of congealed lava ‫البة‬
‫متجمدة‬
• Most are associated with explosive
eruptions of gas-rich magma
Lava Domes
This lava dome began to
develop following the May,
1980, eruption of Mount St.
Helens.
Other volcanic landforms
Volcanic pipes and necks
• Pipes are short conduits that connect a
magma chamber to the surface
• Volcanic necks (e.g., Ship Rock, New
Mexico) are resistant vents left standing
after erosion has removed the volcanic
cone
Shiprock, New Mexico
This structure, which stands over 420 meters
(1380 feet) high, consists of igneous rock that
crystallized in the vent of a volcano that has
long since been eroded away
Plate tectonics and volcanic
activity
Global distribution of volcanic activity is
not random
• Most volcanoes are located within or near
ocean basins
• Basaltic rocks are common in both
oceanic and continental settings, whereas
granitic rocks are rarely found in the
oceans
Distribution of some of the
world’s major volcanoes
Plate tectonics and igneous activity
Igneous activity along plate margins
• Spreading centers
‫مراكز االنتشار‬
– The greatest volume of volcanic rock is
produced along the oceanic ridge system
– Mechanism of spreading
– Lithosphere pulls apart
– Less pressure on underlying rocks
– Results in partial melting of mantle
– Large quantities of basaltic magma are
produced
Divergent plate volcanism
(Oceanic ridge)
Divergent plate volcanism
(Continental rifting ‫)تصدع‬
Plate tectonics and igneous activity
Igneous activity along plate margins
• Subduction zones ‫أنطقة االندساس‬
– Occur in conjunction with deep oceanic
trenches
– Descending plate partially melts
– Magma slowly moves upward
– Rising magma can form either
– An island arc if in the ocean
– A volcanic arc if on a continental margin
Convergent plate volcanism
(Island arc)
Convergent plate volcanism
(Continental volcanic arc)
Plate tectonics and igneous activity
• Subduction zones
– Associated with the Pacific Ocean Basin
– Region around the margin is known as the
“Ring of Fire” ‫حلقة النار‬
– Most of the world’s explosive volcanoes
are found here
Plate tectonics and igneous activity
Intraplate volcanism ‫تبركن داخل اللوح‬
• Activity within a tectonic plate
• Associated with plumes of heat in the
mantle ( ‫تيار صاعد يحمل الحرارة والمواد المنصھرة من األجزاء السفلى‬
‫ ويعزى النشاط البركانى داخل اللوح بعيدا عن‬، ‫للوشاح إلى األجزاء العليا‬
. ‫)حوافھا إلى ھذه البلومات‬
• Form localized volcanic regions in the
overriding plate 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)
Intraplate volcanism
(Oceanic)
Intraplate volcanism
(Continental)
Volcanism on a tectonic
plate moving over a hot spot
A rising mantle
plume ‫تيار صاعد‬
with large bulbous
head and narrow
tail.
Rapid decompression
melting of the head of a
mantle plume produces
vast outpourings of
basalt to generate the
oceanic plateau.
Later, less voluminous
activity caused by the
rising plume tail produces
a linear volcanic chain on
the seafloor
Volcanoes and climate
Explosive eruptions emit huge quantities
of gases and fine-grained debris into the
atmosphere which filter out and reflect a
portion of the incoming solar radiation
Examples of volcanism affecting climate
• Mount Tambora, Indonesia – 1815
• Krakatau, Indonesia – 1883
• Mount Pinatubo, Philippines - 1991
End of Chapter 4