Download What do volcano morphologies - Mercer Island School District

• Why are different types of volcanoes different
shapes?
• What do volcano morphologies (shapes) tell us about
their hazards?
• What types of hazards can we expect from volcanoes
in general?
• How do we predict and prepare for volcanic
eruptions?
• How do we predict, evaluate, and plan for the danger
of other hazards from different types of volcanoes?
• What are the dangers from OUR volcanoes?
Basaltic Volcanism:
Chemistry
• High Mg and Fe content
• Low SiO2 content
• Generally low volatile (gas) content
Behavior
• Viscosity is low
• Slope of the volcano is low-angle
(gently sloping)
• Explosivity is low (generally)
• Frequency of eruption can be constant,
for years at a time.
Basaltic eruptions on land occur at different tectonic settings around the
world. Let’s look at 4: Hawai’i, the East African Rift Zone, the Columbia River
Basalts, and Iceland. Where are they, and how do they differ?
7°10°
The morphology of a volcano is strongly controlled by the viscosity of the magma. The
Hawaiian shield volcano shown above is composed of interlayered basalt flows. Slope
angles typcally range between 7°-10° for shield volcanoes.
OCEANIC
CRUST
MANTLE
Shield vocanoes are composed of interlayered basalt flows. Basaltic lava is erupted
along linear vents lying above asthenosphere (upper mantle) hot spots.
Recent pahoehoe basalt flows erupted on the “Big Island” of Hawaii. Mauna Loa
is shown in the background. Note pressure ridges that formed as lava cooled and
was still moving.
The Hawaiian Islands and Emperor Seamount chains formed over a mantle hotspot.
As the Pacific Plated moved to the northwest, new islands form above the hotspot.
The age of the islands becomes progressively older to the northwest. Note that the
plate changed directions ~38 million years ago from a north-south direction to
northwest-southeast direction.
Hawaiian basalt flows can travel great distances from their linear vent because of the
low viscosity of the flow.
As the oceanic plate moves away
from the hotspot the crust begins to
cool causing its density to increase.
The crust subsides causing the island
to submerge relative to sea level.
The volcanic island is subject to
constant wave erosion as well.
In tropical oceans limestone reefs can
form around the island where the
water is shallow. Eventually the
volcano can become completely
eroded leaving a fringing reef island
defined as an atoll.
Over extended periods of time the
island can become completely
submerged and form a seamount (or
guyot).
The nature of volcanic hazards is strongly tied to the viscosity and gas content
of the magma. Basaltic composition eruptions tend to be non-explosive
(effusive).
Basaltic lava is being actively erupted along the East African rift zone. A large
extensional fault is shown in the foreground of the image. What tectonic setting is
this?
The East African rift zone is composed of basaltic lava plateaus. Steam from a
recent eruption is seen in the middle of the image.
Columbia River Basalts (CRBs) are
widespread, covering SE
Washington, Western Idaho, and
Northern Oregon.
They erupted over millions of years.
Plateau basalts form where basalt flows are erupted along linear rifts in terrestrial
settings, such as continental rift zones (e.g., East Africa) or back arc basins (e.g.,
Columbia Plateau).
Miocene basalt flows comprise the Columbia Plateau. The low viscosity flows were
erupted from linear rifts located near the border of Washington, Oregon and Idaho.
Some of these flows made it to the Pacific Ocean near the modern Columbia Gorge.
The deep canyon of Frenchman’s Coulee was eroded “recently” by floodwaters from
the breeched ice dam occupying glacial Lake Missoula 15,000 years ago.
Iceland is situated over the mid-Atlantic seafloor spreading margin. There is
also a hotspot located beneath the island. Basaltic volcanism is prevalent on
this island nation. Because Iceland is located at polar latitudes Arctic Circle
glacial ice presents additional problems when Icelandic volcanoes erupt.
In 1973 a basaltic eruption almost destroyed the important fishing port of
Vestmannaeyjar on the Island of Heimaey.
Groundwater and surface water mixed
with magma produce gas, which resulted
in large amounts of scoria being erupted
during the Heimaey eruption.
Water pumped from the ocean was used to
quench lava flow and divert it towards the
sea.
Property damage may be extensive from effusive basaltic eruptions, but loss of
life is rare.
Two weeks after the eruption ceased the pyroclastic material (ash and cinders) were
cleaned from the buildings and streets. Heimaey was saved.
Andesitic Volcanism:
Chemistry
• Moderate Mg and Fe content
• Moderate SiO2 content
• Generally high volatile (gas) content
Behavior
• Viscosity is high
• Slope of the volcano is high-angle
(tall and pointy, or irregular)
• Explosivity is high
• Frequency is 100s of years.
• Mountains can be built in 10,000s of years.
25-35°
The slope angle of the of strato (composite) volcanoes is controlled by the angle of
repose (25-35°) for unconsolidated pyroclasts and the viscosity properties of the
silica-rich andesite and dacite flows. The volcanoes shown above are part of the
Aleutian Island, Alaska.
Strato (composite) volcanoes form along subduction zones where partially melted
ocean crust, marine sediments and water-enriched mantle rock rise to the surface.
The image shown above is a cross-section of the Japanese subduction zone.
Strato (composite) volcanoes form from interlayered pyroclastic and intermediate
(andesite or dacite) lava flows. Strato volcanoes can become larger over time with
subsequent eruptions.
Strato (composite) volcanoes
become large with upbuilding.
Eruptions along the flank can
occur where extensional cracks
can develop as magma upwells
within the magma chamber.
Over time magma can become
more silica-rich through
fractionation and the volcano’s
life cycle can end with a
cataclysmic eruption and
emptying of the magma
chamber.
Cascade Volcanism:
Calderas, and stratocones, and lahars…
Oh MY!
The Cascade volcanoes are typical strato volcanoes with slope angles
between 25°-35°. Mount Rainier is shown in the foreground with Mt. St.
Helens lying to upper right.
Volcanism along the Cascadia Subduction Zone is a potential hazard to those
residents living close to the volcanoes or adjacent to river valleys that originate from
the volcanoes.
Mt. Rainier has ~90% of all the glacial ice in the Cascade Mountains. Even small
eruptions can cause catastrophic melting and result in lahar flows (volcanic
mudflows).
Lahar flows are generated when snow or
glacial ice is melted by hot pyroclastics. The
slurry of mud and debris will tend to flow
down the slope of the volcano within preexisting drainages.
Lahar flows are very dangerous and more
difficult to predict their magnitude as they can
be generated by relatively small eruptions.
Lahar flows are the number one potential volcanic hazard generated from Mount
Rainier. There have been several large lahar flows over the past 6000 years.
Over the past 6000 years
several major lahar flows
have inundated river
valleys on Mt. Rainier.
The Oceola lahar occurred
5300 years ago and flowed
as far as Tacoma and
southern Puget Lowland.
The Electron lahar
occurred ~500 years ago
and reached the town of
Puyallup.
Why are the areas of
highest risk farther up the
river valleys? What factors
are important to consider
when you think of risk
factors? (Freqency and
Magnitude of eruption or
event).
Mt. Rainier lahar deposits appear as hummocky topography within the Puyallup
River valley. Lahar flows would reach the towns of Puyallup and Orting in less
than 1 hour.
A new development being constructed on a Mt. Rainier lahar flow (lobe outlined by
white dashed lines) near Enumclaw, Washington.
Large lithic fragments are preserved in the Paradise Lahar deposits.
Lahar deposits may contain large volcanic clasts incorporated during an
eruption and or reworking exising sediment.
Prior to the 1980 eruption Mt. St. Helens stood at nearly 10,000 feet above sea
level.
Prior to the culminating eruption on May 18th, 1980, seismic activity was frequent as
the magma moved upward beneath the volcano. Why does movement of magma
produce seismic activity (minor earthquakes)?
Mt. St. Helens was truly the gem of the Cascades prior to its recent eruption. Note
the beautiful symmetric cone prior to the eruption.
Mt. St. Helens was predisposed to landsliding on the north face of the volcano
because of weakened rock.
Following the landslide on the north slope of MSH the pyroclastics were erupted in a
horizontal and vertical column.
The “Blast Zone” on the north side of MSH was the result of the slope failure on
the north face and high velocity pyroclastics and gas being ejected in the
horizontal plume.
The most devastating volcanic eruptions are pyroclastic eruptions that are most often
associated with subduction zone volcanism. Dense clouds of super-heated gas and
ash descend down the flanks of the volcano with velocities of 100-160 km/hr.
Pyroclastic eruptions are also known as nuee´ ardentes (glowing avalanche).
Large amounts of volcanic ash (tephra) were erupted into the atmosphere. The ash
plume was blown eastward by the prevailing westerly winds.
The MSH ash plume provided very beautiful sunrises and sunsets on the east side
of the Cascades. Ash plumes can present very dangerous hazards to air traffic and
can destroy machinery. What is the hardness of ash shards?
Very few living organisms survived
the catastrophic effects of the
pyroclastic eruption within the “Blast
Zone” on the north face of MSH.
Most of the 57 fatalities from the MSH eruption occurred in the Blast Zone in
restricted areas.
MSH lahar flows choked streams and inundated low lying topography.
MSH was almost 10,000 feet high prior to the 1980 eruption. Following the eruption
it had an elevation of 8363 feet. It is rebuilding its summit today in the effusive
stage.
Mount St. Helens is rebuilding its dacite lava dome. The lava dome has attained
almost 1400 feet of vertical relief.
Large amounts of volcanic ash (tephra) were erupted into the atmosphere. The ash
plume was blown eastward by the prevailing westerly winds.
Isopach (“Iso” means” the same.” ash layer thickness) map for ash distribution of Mount
Saint Helens (1980) eruption. Why is the ash layer thickest near Ritzville and not
directly proximal to the volcano itself?
Excavating a trench behind the Hyak moraine at Snoqualmie Pass (ca. 1990).
Mt. St. Helens W Tephra
(1500 AD)
Mt. St. Helens Y Tephra
(1900 B.C.)
Mazama O Tephra (4800 B.C.)
How do geologists predict the potential volcanic eruptions? They
determine the past eruptive history of volcanoes using dated ash layers.
How does ash thckness and grain size vary with distance from the
volcano versus magnitude of the eruption?
Tephra distribution of Cascade volcanoes (Mt. Mazama, Mt. St. Helens, and
Glacier Peak).
Caldera-Forming Eruptions
Chemistry:
• Low Fe and Mg content
• High SiO2 content (felsic)
• High volatile content
• Large volumes of magma located in the “magma
chamber.”
Behavior:
• Extremely explosive
• “Cataclysmic” events produce ash and debris
(pyroclastic) deposits 10s to100s of meters thick
near the volcano, and distribute ash (tephra) globally.
• Single eruptions can last for many days.
• Very infrequent (100,000s to millions of years).
• Following caldera formation, more mafic magma
often erupts.
1
4
Crater Lake, Oregon formed from the
collapse of Mt. Mazama following a
cataclysmic eruption 6800 years ago.
Caldera-forming volcanic centers are sometimes referred to as Supervolcanoes.
What is a supervolcano?
‘The term "supervolcano" implies a volcanic center that has had an eruption of
magnitude 8 on the Volcano Explosivity Index (VEI), meaning the measured
deposits for that eruption is greater than 1,000 cubic kilometers (240 cubic miles).’
http://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_sub_page_49.html
For a sense of how large this is, Mt. St. Helens erupted about 0.25 cubic kilometers
of material.
Pyroclastic (Pyro = fiery; clastic = pieces) cones form largely from erupted pyroclasts
(aka, cinders). Pyroclasts solidify in the air and fall to the ground as air-fall.
High gas content is a key component of pyroclastic eruptions. Pyroclastic cones
can be composed of EITHER felsic (pumice) to mafic (scoria) compositions.
Pyroclastic cones may collapse to form maars following an explosive eruptive
phase that empties the underlying magma chamber.
Porous texture
Pyroclasts, such as the pumice lapilli shown above, consist of porous textures
derived from gas bubbles preserved in the rock (see inset image).
Pyroclastic cones attain slope angles ~35°, which is the angle of repose for
unconsolidated lapilli (cinders).
Ancient rocks often tell a story of
previous volcanic activity.
We just need to know what we’re
seeing…
Columnar structures form in basalt flows as a “trade off” due to contraction during
cooling and efficiency in packing. The thicker the lava flow, the thicker the diameter of
the columnar structure (cooling is slower in thick lava flows).
Pillow basalts on
ocean floor.
palagonite
Pillow basalts form when basalt is erupted into water, such as along a seafloor
spreading margin or into a lake. Palagonite is reddish-orange clay mineral that
forms from rapidly weathered basaltic glass. It often surrounds pillow structures.
palagonite
Pillow basalts form when basalt is erupted into water, such as along a seafloor
spreading margin or into a lake. . Palagonite is reddish-orange clay mineral that
forms from rapidly weathered basaltic glass. It often surrounds pillow structures.
Plutons consist of intrusive igneous rock that slowly crystallized from magma
underground.
Plutons are classified based on their size and orientation relative to the
layered sedimentary rock in which they are intruded.
How do plutons become exposed at the surface of the earth?
Uplift and erosion of overlying country rock. The plutons that you observe
near Mt. Rainier and Mt. Index today formed under the Cascade volcanic arc
14 to 20 million years ago and were exposed due to continued uplift and
erosion of the overlying rock.
Gabbro
Diorite
Granite
Plutons are composed of coarse-grained igneous rock of all
compositions.
Large batholiths can form along
collisional and convergent tectonic
boundaries. Large batholiths underlie
or are exposed within the Cascades,
Rockies, Sierra Nevada, Appalachian
and Himilayan Mountain Ranges.
Roof pendant (dark metamorphosed sedimentary rock) comprised of remnant “country
rock” is draped over the younger intruded Sierra Nevada batholith (light colored
granodiorite). Why is the roof pendant metamorphosed?
The Sierra Nevada batholith was intruded into the country rock over 90 million
years ago. Batholiths are irregular in form and have a surface area greater
than 100 km2.
Dark xenoliths (mafic inclusions) are preserved in the plutonic rock and represent piece
of the magma chamber that did not completely melt and become assimilated into
magma body. Check out the stairs adjacent to Suzzalo library, can you see xenoliths
preserved in the granodiorite rock matrix?
Plutonic bodies, such as the stocks shown above, are more resistant to erosion and
stand in relief as high topography. Stocks are irregularly shaped plutons which are
less than 100 km2 in surface area. Note that a stock may be an extension of a larger
batholith not completely exposed by erosive processes.
Stocks are irregular-shaped plutons that have areas <100 km2. The stock shown
above has deformed the layered country rock.
The Henry Mountains, Utah, is a classic example of an exhumed laccolith.
Dikes are tabular intrusive bodies that are oriented prependicular or oblique to layered
country rock.
Sills are tabular plutonic bodies that are oriented parallel to pre-existing sedimentary
layers.
Volcanic necks form when
resistant magma is intruded
into the vent and radial
fractures of a volcano.
Why do the radial fractures
form as the magma rises?
Over time, the less resistant
rock (i.e., pyroclasts and
less consolidated lava flows)
comprising the flank of the
volcano is eroded away
leaving the resistant rock
exposed in relief.
Shiprock, New Mexico is a classic example of a volcanic neck. Note radial dikes
projecting outward from beneath the vent.
Footprints of ealry hominids
are preserved in volcanic ash
in East Africa.
Volcanic hazards have posed a
problem for humans since their
earliest origins in the East African rift
zone.
The St. Pierre pyroclastic eruption in 1902
killed over 30,000 people on the island of
Martinique in the Carribbean. There were
only two survivors from this devastating
eruption of Mt. Pelee.
Mt. Vesuvius (seen in the background) had a cataclysmic
pyroclastic eruption in 79 AD which destroyed the Roman cities of
Pompeii (in foreground) and Herculaneum.
The pyroclastic flows buried the city of Pompeii, killing its inhabitants, including Pliny
the Elder. The modern city of Naples (seen in the background) has been constructed
on top of the pyroclastic and mudflows (seen in the background above the preserved
ruins. Note that the ancient Roman architecture was well preserved when entombed
within the silicate ash flows.
Roman columns of Pompeii’s buildings were preserved under thick ash flows. Important
archaeological finds relevant to Roman culture and every day life were unearthed
following their discovery in the 18th century.
Casts of Roman citizens who perished in the eruption were well-preserved in the
Mount Vesuvius ash. You can still see wrinkles in the cloth of the Roman toga.
How do volcanologists monitor
volcanoes?
http://volcanoes.usgs.gov/observatories/cvo/
monitoring_videos.html
• Deformation (tilt meters, GPS)
• Gas emissions (sampling at the vent,
airplanes, satellite remote sensing)
• Earthquakes (seismometers deployed
around the volcano)