chapter 6
This image of the Valles Caldera in New Mexico shows a landscape in which there is a large flat meadow surrounded by rocky rims of volcanic rock. The meadow is several miles across. Why is this called a caldera?
A) It is a crater. B) It is several miles across. C) It has volcanic rock. D) All of the above are correct. Answer: D
Which of the following factors help determine whether a volcanic eruption will be violent or relatively quiescent?
A) amount of dissolved gas in the magma B) temperature of the magma C) composition of the magma
Cinder cones ________.
A) have very steep slopes B) are usually less than 300 meters (1000 feet) high C) consist largely of pyroclastics D) All of these are correct statements about cinder cones. Answer: D
Pyroclastic flows move ________.
A) very rapidly, too fast to outrun on foot B) as gravity flows similar to snow avalanches C) as a dense cloud that contains hot magma particles suspended in hot gases and moves rapidly downhill D) all of the above E) none of the above Answer: D
Where can we find the most active volcanoes in the world?
Hawaii
It has been said that living near a Hawaiian volcano puts buildings in danger but not humans. Why is that the case?
Hawaiian volcanoes are not explosive, and the lavas flow at a slow enough speed for humans to get out of the way.
________ tend to increase the explosive potential of a magma body beneath a volcano.
High viscosity and high dissolved gas content
Most of the active volcanoes on land are located in a belt known as the ________.
Ring of Fire
What is the connection between the amount of silica in a melt and how viscous it is?
Silica forms strong bonds, so higher amounts of silica make for more viscous magma.
Which of the following best describes the bedrock in the Sierra Nevada Mountains in California?
The batholiths were washed downhill from distant mountains and deposited here.
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
In what plate tectonic setting are magmas generated predominantly by fluids from ocean waters in trenches "fluxing" the melting process by decreasing melting temperature?
convergent plate margins
Which type of plate margin is characterized by volcanoes that include basalt, andesite, dacite and rhyolite?
convergent plate margins
Magmas are generated at mid-ocean ridges by ________ melting.
decompression
The main mechanism for melting at spreading ridges is ________.
decompression melting of ultramafic mantle rock
Which of the following is a tabular intrusive body with subparallel intrusive contacts?
dike
The Columbia Plateau in the northwestern United States is an excellent example of ________.
flood basalts
Section 6.5 of the chapter notes that most shield volcanoes begin as seamounts and grow from there. What is the driving force behind this action?
fracture zones
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
The composition of a composite cone or stratovolcano makes its viscosity ________.
intermediate, resulting in high volatility
Volcanic activity in Yellowstone National Park is related to ________.
intraplate, hot spot volcanism
A ________ is a near surface, intrusive igneous rock body that results from local inflation of a horizontal sill.
laccolith
Identify what letter A represents in the diagram.
lava flow
Shield volcanoes have extensive deposits of ________.
lava flows
The eruptions of the Hawaiian volcanoes are ________ explosive than the 1980 Mount St. Helens eruption.
less
The image shows the nature of melting at the Mid-Ocean Ridge. The image indicates that melting is ________.
localized below the Mid-Ocean Ridge
Shield volcanoes form from what type of magma?
low viscosity basaltic magmas
identify what letter C represents in the diagram.
magma chamber
Identify what letter D represents in the diagram.
pyroclastic material
Which magma type has the lowest viscosity?
rhyolite
Which one of the following shows the correct order (left to right) of decreasing magma viscosity in order of composition?
rhyolite, andesite, basalt
Which type of basaltic lava flow has a fairly smooth, unfragmented, ropy surface?
scoria
Kilauea is an example of a ________.
shield volcano
A ________ is an intrusive, igneous rock body that is tabular and concordant.
sill
The main destruction from shield volcanoes like Kilauea in Hawaii is ________.
slow burial of man-made features
Volcanoes have ________ impact on weather and climate.
some definite
The largest percentage of total magma production on earth occurs at what plate tectonic setting?
spreading ridges
The most dangerous of the volcano types is the ________.
stratovolcano
Mt. Rainier in Washington State is often thought to be the most hazardous volcano in the lower 48 states of the United States because ________.
the bodies indicate how rapidly the pyroclastic flows came, and this could be an indication of what could happen in the future
Which of the following best describes Shiprock, a famous volcanic feature in New Mexico?
the eroded remains of a volcanic pipe and radiating dikes
The slopes of most cinder cones are usually about 30 degrees because ________
the loose materials ejected from the cone form slopes at that angle of repose
The streamlined ropy flowing lava pictured here has both orange and black parts because ________.
the surface cools faster than the lava below, and is black in appearance
Eruption columns, such as the one pictured above, are unstable and often collapse towards the ground because ________.
they are propelled by the eruption, as in the case of Pompeii
Identify what letter E represents in the diagram.
vent
Which of the following is considered pyroclastic debris?
volcanic ash
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 increase leads to an explosive potential
________ people have been killed by pyroclastic flows erupted from cinder cones
Relatively few
A geologist is working in an area of old volcanic rocks exposed by erosion. He finds a large rock outcrop that contains flatlying 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
Note the image showing a series of batholiths on the landscape. How did those batholiths get exposed at the surface?
Batholiths crystallize below the surface but were later exposed by way of erosion of overlying material.
What volcanic events formed Crater Lake, Oregon? When did they take place?
Caldera collapse followed major ash and pyroclastic-flow eruptions 6000 years ago.
Which statement about the May, 1980, eruption of Mount St. Helens is false?
During the eruptive period, the mountain peak was substantially built up by new lava flows and pyroclastic debris.
When you open a bottle of soda slowly it tends not to spill, whereas if you open it too rapidly, you may get a wet hand. What parallel is there with effusive versus explosive eruptions?
Effusive eruptions are like the slowly opened bottle. The air in such eruptions is released so it 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.
A basaltic volcano is likely to produce an explosive column that would endanger flying aircraft.
FALSE
Powerful, explosive, volcanic eruptions can result in climatic cooling for millions of years following the eruption.
FALSE
When magma rises toward the surface, pressure decreases can lead to release of dissolved gases in a magma with a volume increase that can produce an explosion.
FALSE
Considering your knowledge of volcano types in different tectonic settings, if you had to live on a volcano, which of the following would you choose if safety was your only concern?
Mauna Loa in Hawaii, on the Big Island of Hawaii near Kilauea
In 1980, ________ became the first Cascade Range volcano to erupt since Mt. Lassen in 1915-16.
Mt. St. Helens
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.
hen Mt. St. Helens erupted in May of 1980, ash blew as far away as ________.
Oklahoma
What type of volcanic material is indicative of eruption under water?
Pillow lavas
The rock pumice often floats yet the density of the rock is greater than water. Why does it float?
Pumice is a pyroclastic rock comprised largely of open voids formed by gases released when the magma solidified as froth-like material.
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 flows came, and this could be an indication of what could happen in the future.
Which one of the following statements concerning cinder cones is false?
The cinders and other pyroclastic particles are normally consolidated into welded tuff.
Which of the following is not an accurate statement about the melting process in the earth's interior?
The earth's mantle is entirely molten.
Observe the photograph of an entire forest of trees knocked down on Mt. St. Helens. By contrast this is not the case on Kilauea. Why were so many trees knocked down during the Mt. St. Helens eruption while that was not true of the Kilauea volcano?
The force of the blast from Mt. St. Helens caused the trees to be knocked over, while the Kilauea volcano was much less violent.
Observe the image of the individual walking through the lava tube. Why is it hollow there and not solid rock?
The inside of the lava tube was granite, but granite is not resistant to weathering, so has long since eroded away.
Which one of the following statements concerning volcanic blocks and bombs is true?
The surface cools faster than the lava below, and is black in appearance.
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.
Which of the following types of magma is normally at the lowest temperature during an eruption?
They are propelled by the eruption, as in the case of Pompeii.
Which of the following statements best describes the big Hawaiian volcanoes?
They are situated in the interior of a large, Pacific plate above a hot spot deep in the mantle.
Volcano A erupts rocks with SiO2 content ranging from 49% to 52% whereas volcano B, 100km away, erupts materials with SiO2 content of 62-68%. Which statement best characterizes these two volcanoes?
Volcano A is probably a shield volcano with typically quiescent eruptions, whereas Volcano B is likely a stratovolcano characterized by violent, explosive eruptions.
________ is the 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
Which of the following best describes melting of the mantle at subduction zones that leads to island arc volcanoes?
Water released from the subducting plate decreases the melting temperature in the mantle above the downgoing plate to generate magmas from the overlying mantle wedge.
A volcanic deposit comprised of red to black lapilli-sized pyroclastic material associated with pahoehoe lava flows was probably deposited on ________.
a cinder cone
Dissolved gases may comprise up to ________ of a magma.
a few percent of
Which type of basaltic lava flow has its surface covered with sharp-edged, angular blocks and rubble?
aa
The average composition of rocks comprising a large composite cone or stratovolcano is generally ________.
andesitic
Volcanic bombs originate ________.
as erupted magma blobs that partly congeal before falling to the ground
Most granitic batholiths are generated ________.
as intrusions beneath composite volcanoes along convergent plate margins
Magmas are the product of melting in the deep part of the earth that forms a layer of partially molten material called the ________.
asthenosphere
The dominant rock type erupted by submarine volcanoes at spreading ridges is ________.
basalt
Pillow lavas are ________.
basaltic lava tubes erupted on the seafloor in volcanic piles analogous to toothpaste extruded from a tube
Pillow lavas are associated with ____________.
basaltic magmas extruded as piles of bombs that accumulate in piles of rocks that look like a pile of pillows
A ________ is the largest discordant body of intrusive, igneous rock.
batholith
Which of the following is emplaced over millions of years to form a composite intrusive complex?
batholith
The greatest danger from pyroclastic flows is ________.
being incinerated by the hot gas and entrained magma
Which of the following is not a significant volcanic hazard?
being overrun by a basaltic lava flow
identify what letter B represents in the diagram.
bomb
Paricutin volcano in Mexico is an example of a ________.
cinder cone
Which of the following volcano types are short lived volcanoes that last a few days to months and then shut down?
cinder cone
The largest types of volcanoes are the ________.
cinder cones
Mount Pelée 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