Geo 107 Midterm#2

The Volcanic Explosivity Index (VEI) of historical events ranges from _______________.

0 to 8

Over _________ of volcanism is associated with the edges of tectonic plates, and close to about _________ of Earth's magma extruded through volcanism takes place at the oceanic spreading centers.

90%, 80%

The formation of a giant continental caldera includes all but which of the following?

Basaltic eruptions begin forming circular fractures surrounding the bulge.

__________ melting occurs when a rock melts due to a lowering of pressure.

Decompression

At oceanic spreading centers, magmas are rhyolitic in composition.

False

Less than 20 percent of Earth's magma extruded through volcanism occurs at oceanic spreading centers.

False

Mt. Rainier, Washington, is number one on the danger list of many U.S. volcanologists because of its __________.

Great height Extensive glacial cap Frequent earthquakes Active hot-water spring systems, which have weakened the mountain internally (all of these choices are correct)

The most peaceful eruptions are __________ eruptions.

Icelandic type

Crater Lake, Oregon, fills the caldera of ____________ which collapsed about 7,600 years ago.

Mt. Mazama

A genuine success story of advance warning before a large eruption occurred in the Philippines in 1991 before the climactic eruption of _________________.

Mt. Pinatubo

The two most active Cascade Range volcanoes over the past 4,000 years are _______________.

Mt. St. Helens and Mt. Shasta

Among different types of volcanic eruptions, __________ eruptions are the most violent types of explosive eruptions.

Plinian-type

The most famous of all volcanoes probably is Vesuvius, and the most famous of its eruptions are those of 79 ce, which buried the cities of ____________.

Pompeii and Herculaneum

Spreading centers have relatively peaceful eruptions of magma because the __________.

SiO2-poor magma is at high temperatures.

Mt. Kilimanjaro in Tanzania, Mt. Shasta in California, Mt. Rainier in Washington, and Mt. Fuji in Japan, are ______________, which are steep-sided, symmetrical volcanic peaks built of alternating layers of pyroclastic debris capped by high-viscosity andesitic to rhyolitic lava flows that solidify to form protective caps.

Stratovolcanoes

__________ are steep-sided, symmetrical volcanic peaks built of alternating layers of pyroclastic debris capped by high-viscosity andesitic to rhyolitic lava flows that solidify to form protective caps.

Stratovolcanoes

Although there are 92 naturally occurring elements, a mere eight make up more than 98 percent of Earth's crust.

True

Hot spots can be under the oceans, under the continents, in the center of plates, and at spreading centers.

True

Oxygen and silicon are so abundant in Earth's crust that their percentages dwarf those of all other elements.

True

Stratovolcanoes are often associated with subduction zones.

True

The radiocarbon process of documenting major eruption dates for volcanoes in the past 50,000 years or so is the same as that used to work out dates of major prehistoric earthquakes.

True

The three most recent catastrophic Yellowstone eruptions occurred at 2.6 million, 1.29 million, and 640,000 years ago.

True

When it comes to volcanic hazards, the key problem is how easily the dissolved gases can escape from the magma.

True

_________ eruptions are common first phases in the eruptions of volcanoes as they "clear their throats" before emitting larger eruptions.

Vulcanian-type

Slow flowing, more viscous basaltic lava commonly has a rough, blocky texture called ________.

aa

Water circulating at thousands of feet below the Earth's surface needs to be heated to temperatures _________ 100°C (212°F) in order to reach its boiling point because the pressure of the overlying groundwater body is so great.

above

External processes that increase the probability of a slope failure include __________.

adding mass high on a slope, as in sediment deposition steepening the slope, as by fault movements removing support from low on a slope, as by stream or ocean-wave erosion (all of these choices are correct)

Which of the following is a poor choice when trying to reduce the likelihood of a slope failure?

adding water to the slope during a cold snap to freeze it in place

A caldera collapse occurs ________________.

after the magma chamber is mostly empty

Many hill-slope masses are weak due to preexisting geologic conditions such as __________.

ancient slide surfaces rock layering dipping less than topographic slopes structures within the rocks, such as fractures, ancient faults, and thin clay seams (all of these choices are correct)

Pyroclastic debris is __________.

chucks of magma and rocks blown into the air by gas in a volcanic eruption

The process of mineral formation in a cooling magma is usually called ______________.

crystallization

The primary reason magma forms at subduction zones is that the subducting plate carries a cover of sediments, water, and hydrated minerals down with it, which usually ____________ the temperature required for the adjacent overlying mantle lithospheric rock to melt.

decreases

Why does the magma from some volcanoes flow smoothly and relatively peacefully, while the magma from other volcanoes blasts forth violently and deals death over wide areas?

differences in the chemical and mineral makeup of magmas variations in the temperature, water and gas content, and viscosity of magmas different geographic positions with respect to hot spots and edges of tectonic plates (all of these choices are correct)

When it comes to volcanic hazards, the key problem is how easily the _______ can escape from the magma.

dissolved gases

Several geologic phenomena are being studied as signs of an impending volcanic eruption. These include seismic waves, _____________, and the release of gases.

ground deformation

Magma at great depths usually does not contain gas bubbles because ___________________

high pressure at depth keeps gas dissolved in solution

Beneath continents, rising basaltic magmas are contaminated by sediments and continental-crust rocks, altering magma compositions; the resultant andesitic-to-rhyolitic magmas have _______ contents of SiO2, relatively low temperatures, and _________ viscosity.

high, high

As minerals continue to form in the magma kept at a constant temperature, the viscosity of that magma will ___________.

increase

The viscosity of magma is lowered by __________.

increasing temperature decreasing crystal content decreasing SiO2 content (all of these choices are correct)

Below the ocean basins, the basaltic magmas have _____ contents of SiO2, high temperature, ________viscosity, and allow easy escape of gases, producing peaceful eruptions.

low, low

If two magmas have the same composition but at different temperature, the hotter of the two magmas will have a ________ viscosity.

lower

Rock may melt by _______________.

lowering the pressure on it raising its temperature increasing its water content (all of these choices are correct)

The presence of water usually ________ the melting point of rock.

lowers

Spreading centers are an ideal location for volcanism because all but which of the following? ____

magma formation due to the melting caused by incorporation of water in asthenosphere minerals

Highly fluid basaltic lava may cool with a smooth, ropy surface called _______, whereas lower temperature, slower flowing, more viscous basaltic lava commonly has a rough, blocky texture called _________.

pahoehoe, aa

The typical trend in a rising plume of subduction-zone magma is to increase the ____________.

percentage of SiO2 viscosity explosive potential of the magma by holding in the gases more tightly (all of these choices are correct)

If basaltic lava reaches the sea or a lake, it cools rapidly into ____________ lava.

pillow

When gas escapes quickly and violently from lava it may produce a frothy glass full of holes left by former gas bubbles; this porous material, known as ____________, contains so many holes it can float on water.

pumice

Violent causes of death from volcanic eruptions include _______________.

pyroclastic flows tsunami lahars poison gases (all of these choices are correct)

If all of the following rocks were to completely melt and reach the same final temperature, which would produce magma with highest viscosity?

rhyolite

Of the following types of mass movements, which can move fastest?

rock fall

A mantle hot spot has generated a long-lived plume beneath Yellowstone National Park, and the North American continent is moving __________ above it about 2 to 4 cm/yr.

southwestward

Some of Earth's most beautiful mountains are ________, including Mt. Kilimanjaro in Tanzania, Mt. Shasta in California, Mt. Rainier in Washington, and Mt. Fuji in Japan.

stratovolcanoes

Magma rising in a __________ tends to get more "contaminated" with _______ than does magma rising in a spreading ridge.

subduction zone, high silica minerals

In magma, silicon and oxygen often link up to form the silicon-oxygen ______________.

tetrahedron

Active volcanoes today in Oregon and Washington, including Mt. St. Helens, result from _________________.

the subduction of the Juan de Fuca Plate beneath North America

The immediate causes of slope failures, such as earthquakes or heavy rainfall, are called __________.

triggers

The three 'Vs' of volcanology are __________________.

viscosity, volatiles, and volume

If the magma reaches the surface, it forms _________, named for Vulcan, the Roman god of fire.

volcanic rocks

In magma, __________ is the most abundant dissolved gas.

water vapor (H2O)

A shield volcano has a great ___________.

width compared to its height

The Houston-Galveston, Texas, area has been sinking due to the __________.

withdrawal of groundwater by pumping