geology chapter 6
In the movie "Volcano" basaltic lava flows are shown running down flat streets at speeds faster than humans could run. From your knowledge of magma viscosity, is this realistic?
no the viscosity of basaltic magma is much higher than water, and thus, would flow much slower than water
the famous destruction of the Roman city of Pompeii was produced by ___
a pyroclastic eruption from a composite volcano that buried the city in ash and smothered the people living there
Mount Pelée on the island of Martinique killed thousands of people in
a pyroclastic flow, also called a nuee ardante Nuée Ardante
laccolith
a tabular intrusive body with subparallel intrusive contacts
How does the viscosity of the magma fed to volcanoes affect nearby life and property?
a volcano that expels highly viscous magma is greater threat to life and property because it is more explosive, and thus, is more difficult or impossible to predict
which type of basaltic lava flow has its surface covered with sharp-edged, angular blocks and rubble
aa
how are volcanic eruptions determined as violent or relatively quiescent
amount of dissolved gas in the magma, temperature of the magma, and the composition of the magma
sill
an intrusive, igneous rock body that is tabular and concordant
most lava flows erupted by Mt St Helens are ___
andesite
what type of magma is considered to be intermediate in composition
andesitic
volcanic bombs originate
as erupted magma blobs that partly congeal before falling to the ground
pyroclastic flows move ____
as gravity flows similar to snow avalanches; very rapidly, too fast to outrun on foot; as a dense cloud that contains hot magma particles suspended in hot gases and moves rapidly downhill
most granitic batholiths are generated ____
as intrusions beneath composite volcanoes along convergent plate margins
the dominant rock type erupted by submarine volcanoes at spreading ridges is
basalt
cinder cones
consist largely of pyroclastic, have very steep slopes, and are usually less than 300 meters high
Mt. St. Helens is located along what type of plate tectonic boundary
convergent
in what plat tectonic setting are magmas generate predominantly by fluids from ocean waters in trenches "fluxing" the melting process by decreasing melting temperature
convergent plate margins
the main mechanism for melting at spreading ridges is ____
decompression melting of ultramafic mantle rock
tabular, intrusive bodies
dikes and sills
dikes and sills
dikes are discordant igneous intrusions, and sills are concordant igneous intrusions; both are tabular
which type of plate boundary generates the greatest quantity of magma
divergent
When you open a bottle of soda slowly, it tends not to spill. Whereas if you open it too fast, you may get a wet hand. What parallel is there with effusive vs. explosive eruptions
effusive eruptions = slowly. the air is released, but cannot build up in the magma, thus keeping the eruption non-violent
If you are in the path of a pyroclastic flow or a lahar, how much time would you have to escape
enough time in a car, but not enough on foot
t/f: a basaltic volcano is likely to produce an explosive column that would endanger flying aircraft
false
t/f: during the eruptive period of Mt. St. Helens, the mountain peak was substantially built up by new lava flows and pyroclastic debris
false
t/f: powerful, explosive, volcanic eruptions can result in climatic cooling for millions of years following the eruption
false
Kilauea is an example of a ________.
shield volcano
crater
funnel-shaped depression around the vent
what does a fumarole emit
gases only
over what time span do most typical cinder cones form
generally the product of a single eruption and thus are constructed in less than a year, maybe less than a month
the bedrock in the Sierra Nevada Mountains in the California are best described as
granite and batholiths
________ tend to increase the explosive potential of a magma body beneath a volcano.
high viscosity and high dissolved gas content
the steep, upper slopes of composite volcanoes are generally produced by __
high-viscosity, low-volume silicic magmas that are erupted from the summit
the higher the viscosity of a magma, the ____ the potential for explosive eruptions
higher
the viscosities of magmas _____ with increasing percentages of silica
increase
volcanic activity in Yellowstone National Park is related to ___
intraplate, hot spot volcanism
pluton
intrusive body
a ____ is a near surface, intrusive igneous rock body that results from local inflation of a horizontal sill
laccolith
why is Mt. Rainier considered the most hazardous volcano in the lower 48 US states
large amounts of glacial ice on the mountain could be melted by even a small eruption, potentially generating dangerous lahars that would travel rapidly down river valleys to heavily populated areas
cinder cone volcanoes
short lived volcanoes that last a few days to months and then shut down
the big hawaiian volcanoes are best described how?
situated in the interior of a large, Pacific plate above a hot spot deep in the mantle
stocks
smaller than batholiths
volcanoes have ____ impact on weather and climate
some definite
which of the following was a precursor to the 1980 eruptions of Mt St Helens
steam and ash eruption, bulge beneath the volcano, small earthquakes
the most dangerous of the volcano types is
stratovolcano
rocks melt because of the introduction of water mainly at ___
subduction zones
the eruptions of the Hawaiian volcanoes are ____ explosive than the 1980 Mount St. Helens eruption
less
shield volcanoes form from what type of magma?
low viscosity basalt magmas
what is the source of magma for most intraplate volcanism
mantle plumes
what primary factors determine the nature of volcanic eruptions
temperature, composition, dissolved gases
lahar
mudflow of volcanic debris saturated with water
What role do water and other volatiles play in the formation of magma?
the addition of water and other volatiles allows the rock to melt at lower temperatures than if they had not been added
Note in the above image there are bodies buried by the pyroclastic flows from Mt. Vesuvius in 79 AD. These are not the actual bodies, as those were vaporized by the heat, but Plaster of Paris casts of individuals affected by the reaction, with casts made in the areas where the bodies were. How could this discovery help local residents to plan for the future?
the bodies indicate how rapidly the pyroclastic flow came, and this could be an indication of what could happen in the future
conduit
the circular path through which magma ascends to the vent
country rock
the crustal rocks into which igneous bodies intrude
Shiprock, New Mexico
the eroded remains of a volcanic pipe and radiating dikes
what is the geothermal gradient, and how does it relate to the melting curve of peridotite
the geothermal gradient is the increase in temperature with depth under the Earth's surface. the melting curve of peridotite is consistently higher than the geothermal gradient
batholiths
the largest type of igneous bodies and occur in a linear fashion with a distance of 100km or more
vent
the surface opening of the conduit
Mount Pelee on the island of Martinique erupted in the early 20th century. why is this eruption famous
the volcano produced a pyroclastic flow that swept down the mountain, killing thousands of people
t/f: cinder cones are composed of generally basaltic pyroclastic material and scoria
true
pyroclastic debris
volcanic ash
how do volcanic bombs differ from blocks of pyroclastic debris
volcanic blocks are made of hardened lava, whereas volcanic bombs are made incandescent lava
________ is a major dissolved volatile constituent in both magmas and volcanic gases.
water
________ are usually the most abundant gases emitted during basaltic volcanism.
water and carbon dioxide
As magma approaches the surface, water contributes to the explosive potential of the eruption because
water dissolved in magma flashes to steam as pressure decreases, and the volume increases leads to an explosive potential
how do Hawaiian-type calderas form
when magma gradually drains away from the magma chamber, causing the summit to collapse
Mt St Helens has formed as a result of subduction of the _____ lithospheric plate beneath the ____ lithospheric plate
Pacific Oceanic, North American oceanic
caldera classifications
several miles across, volcanic rock, crater
what are the two types of lava flows
Aa and Pahoehoe
Mt St Helens is located in the ____ Range, which is classified as a ____
Cascade Range, continental margin volcanic arc
where can we find the most active volcanoes in the world
Hawaii
it has been said that living hear a Hawaiian volcano puts buildings in danger but not humans. why?
Hawaiian volcanoes are not explosive, and the lavas flow at a slow enough speed for humans to get out of the way
in 1980, _____ became the first Cascade Range volcano to erupt since Mt. Lassen in 1915-16
Mt. St. Helens
the location of a currently active shield volcano is
Muana Loa in Hawaii
if you had to live on a volcano, which of the following would you choose if safety was your only concern
Muana Loa in Hawaii, on the Big Island of Hawaii near Kilauea
most of the active volcanoes on land are located in a belt known as the
Ring of Fire
what region of the world has the greatest concentration of composite volcanoes
Ring of Fire
what type of structure is Shiprock, NM and how did it form
Shiprock is a volcanic neck that formed when the volcano itself eroded away, leaving the crystallized igneous rock piercing through the Mancos shale
in addition to composite volcanoes, what other type of volcanic landform can generate pyroclastic flow
a lava dome
a volcanic deposit comprised of red to black lapilil-sized pyroclastic material associated with pahoehoe laval flows was probably deposited on ___
a cinder cone
dissolved gases may comprise up to ____ of a magma
a few percent of
pyroclastic flow
a hot mixture of gas, ash, and pumice that races down the flanks of volcanoes at speeds of up to 125mph
A geologist is working in an area of old volcanic rocks exposed by erosion. He finds a large rock outcrop that contains flat lying gravels in the base of the outcrop. These gravels are overlain by basalt that forms a series of layers representing flows. The uppermost basalt lava flows have a dip of about 10 degrees to the west. What does this outcrop record?
basalt flows formed after gravels were deposited, covered the local terrain, and built a shield volcano with dips of about 10 degrees
___ magma is the most abundant type of magma erupted on Earth
basaltic
pillow lavas
basaltic lava tubes erupted on the seafloor in volcanic piles (think of toothpaste being pushed out of a tube)
how do batholiths get exposed at the surface
batholiths crystallize below the surface but were later exposed by way of erosion of overlying material
laccoliths
bend the sedimentary layers above them, and the ones below remain undeformed
volcanic blocks and bombs
bombs are ejected as magma limps; blocks are ejected as solid fragments
what volcanic events formed Crater Lake, Oregon? when?
caldera collapse followed major ash and pyroclastic-flow eruptions 6000 years ago
paricutin volcano in Mexico is an example of
cinder cone
cinder cones
cinders are most commonly basaltic, they are built mostly or entirely during one eruptive cycle, and they are small volcanoes with fairly steep sides
Mount Pelee on Martinique is an example of a __
composite volcano
what type of volcano would be most likely to generate large landslides
composite volcano
Mt. St. Helens in the U.S. and Mt. Fujiyama in Japan are examples of
composite volcanoes
When contrasting lava from composite volcanoes with lava from shield volcanoes, _________________.
composite volcanoes most often produce silica-rich lavas with high viscosities, whereas shield volcanoes produce basaltic lavas with low viscosities
where do most shield volcanoes form
on the ocean floor
Which type of basaltic lava flow has a fairly smooth, unfragmented, ropy surface?
pahoehoe
what type of volcanic material is indicative of eruption under water
pillow lavas
the location of most volcanoes closely correlates with
plate boundaries
pumice often floats, yet the density of the rock is greater than water. why
pumice is a pyroclastic rock comprised largely of open voids formed by gases released when the magma solidified as froth-like material
_____ people have been killed by pyroclastic flows erupted from cinder cones
relatively few
order of decreasing magma viscosity in order of composition
rhyolite, andesite, basalt
correct order of magmas from the most silica rich to the least
rhyolitic, andesitic, and basaltic
What is scoria, and how does it differ from pumice?
scoria is a vesicular ejecta produced from mafic magma. pumice, however is felsic magma
