Ch. 6 - HW (Volcanic Eruptions)

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 pumice, contains so many holes it can float on water.

true

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

true

While there are many hundreds of different minerals, the overwhelming majority of Earth's crust is composed of just eight common rock-forming minerals.

true

No magmas contain dissolved gases.

false

Crater Lake in Oregon is which type of caldera?

formed from the summit of a stratovolcano

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

If two magmas have the same composition, the hotter of the two magmas will have a lower viscosity.

true

The largest number of hot spots lies beneath Africa.

true

The most peaceful eruptions are __________ eruptions.

Icelandic type

____________ in the Aegean Sea underwent an explosive series of eruptions around 1628 BCE that buried the Bronze Age city of Akrotiri on Thera to depths of 70 meters.

Santorini

Mineral growth in magmas at the surface with temperatures around 1,000 to 1,200°C occurs in the following way

Both of these are correct.

__________ 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

What erupts in a geyser?

superheated water and steam

What is the cause of volcanism at Italy's Vesuvius, Stromboli, Vulcano, and Etna?

the subduction of Mediterranean seafloor beneath Europe

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

true

At a depth of 32 km, basaltic rock melts at around 1,400°C but this same rock will melt at only 1,250°C at Earth's surface.

true

At the end of the Triassic (201 million years ago), flood basalts occurred during the opening of the Atlantic Ocean.

true

The three 'Vs' of volcanology are __________________.

viscosity, volatiles, and volume

In magma, __________ is the most abundant dissolved gas.

water vapor (H 2O)

A shield volcano has a great ___________.

width compared to its height

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

Basaltic eruptions begin forming circular fractures surrounding the bulge.

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

SiO2-poor magma is at high temperatures.

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

Mt. Mazama

On 20 February 1943, a new volcano named __________, a scoria cone, was born when an eruption rose up through a farm field near a village in Mexico.

Paricutin

Airborne pyroclasts have their finest grains settle down from the atmosphere first, closest to the volcano, followed by progressively coarser material at greater distances away.

false

At oceanic spreading centers, magmas are rhyolitic in composition.

false

Because volcanologists are aware of the potential hazards, no volcanologist has ever been killed studying a volcano.

false

Eruptions in Hawaii are always peaceful.

false

Gas bubbles decrease in number and volume as magma keeps rising upward to lower pressures, helping to propel magma upward through fractures or pipes toward an eruption.

false

Hawaiian volcanoes produce andesitic lava.

false

Hot spots occur only under oceanic plates.

false

Most basaltic rocks are high in Potassium-rich feldspar and muscovite mica.

false

Decreasing the pressure on hot rock generates most magma.

true

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

true

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

true

If lava reaches the sea or a lake it cools rapidly into ellipsoidal masses called pillow lava.

true

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

true

Lava domes form when high-viscosity magma with low-volatile content cools quickly, forming a hardened dome a few meters to a kilometer or so in height.

true

Magma at great depth does not contain gas bubbles because the high pressure at depth keeps gas dissolved in solution.

true

Magma rising in a subduction zone tends to get more "contaminated" with high-silica minerals than does magma rising in a spreading ridge.

true

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

true

The basaltic magmas that erupt on the ocean floor have a low SiO2 content, high temperature, and allow gas to escape easily, all resulting in non-explosive eruptions.

true

The lowest temperatures and highest SiO 2 contents occur in rhyolitic magma, material so viscous that it commonly does not flow.

true

The most abundant dissolved gas in magmas is H20.

true

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 lowers the temperature required for the adjacent overlying mantle lithospheric rock to melt.

true

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

true

As minerals form in magma kept at a constant temperature, what happens to the viscosity of that magma?

It increases.

This volcano erupted in 1883 in the Sunda Strait between Sumatra and Java, exploding with a loudness heard 3,000 miles away, then collapsing into its magma chamber, making a caldera and setting off a tsunami that killed at least 36,000 people.

Krakatoa

__________ 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

_________ 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

A caldera collapse occurs ________________.

after the magma chamber is mostly empty

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

all of these choices are correct

Volcanic __________ are big fragments that are liquid when airborne.

bombs

Pyroclastic debris is __________.

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

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

rhyolite

Silicon and oxygen link up to form the silicon-oxygen ______________.

tetrahedron

A well-formed conical volcano located above an active subduction zone that has not erupted in 12,000 years is __________.

dormant, but will likely erupt again at some point

Antarctica does not lie above any hot spots.

false

Most rifting occurs at spreading centers __________.

located below sea level

The presence of water ________ the melting point of rock.

lowers

Which of following correctly lists the order in which the listed minerals form in a cooling magma from the first to the last to form?

olivine, pyroxene, amphibole, biotite mica, and quartz

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

pillow

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

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?

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

Rock may melt by _______________.

-increasing its water content -raising its temperature -lowering the pressure on it *All of these choices are correct.

The viscosity of magma is lowered by __________.

-increasing temperature -decreasing SiO 2 content -decreasing crystal content *All of these choices are correct.

Spreading centers are an ideal location for volcanism because ______________.

-they sit above the high-temperature asthenosphere -the oceanic plates pull apart causing hot asthenosphere rock to rise and undergo decompression melting to form magma that continues to rise -the asthenosphere rock has low percentages of SiO 2 *all of these choices are correct

Analysis of global sites of magma extrusion found that hot spots, such as Hawaii, represent __________ of the total magma extrusion.

12%

Over _________ of Earth's magma extruded through volcanism takes place at the oceanic spreading centers

20%

About _________ of volcanism is associated with the edges of tectonic plates.

90%

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

Decompression

Hawaiian volcanoes unlikely to erupt include _________________.

Diamond Head, Oahu

The volcanic explosivity index (VEI) measures size of volcanic eruptions on a scale of 0 to 8. Between 1500 and 1980, one VEI 7 eruption occurred. This was ______________.

Tambora in 1815

If the magma cools and solidifies below the surface, it crystallizes as volcanic rocks.

false

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

false

Most volcanism is associated with hot spots.

false

Scoria cones form from repeated eruptions over the course of many decades.

false

The Volcanic Explosivity Index (VEI) ranges from 1 to 100.

false

The higher the viscosity of magma, the more fluid is its behavior.

false

The highest temperatures and highest SiO2 contents are in basaltic magma, giving it the lowest viscosity and easiest fluid flow.

false

Transform faults have lots of associated volcanism because the strike-slip motions keep a "lid" on the hot asthenosphere below, allowing it to melt.

false

The number of active "hot spots" on Earth over the last 10 million years active is __________.

more than 100

About 80 percent of the magma reaching Earth's surface is basaltic, with only about 10 percent andesitic and 10 percent rhyolitic.

true

Basaltic magma has the lowest viscosity, so more of it reaches the surface; the more viscous rhyolitic magmas are so sluggish that they tend to be trapped deep below the surface where they cool, solidify, and grow into the larger mineral crystals of plutonic rocks.

true

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

true

The three most recent catastrophic Yellowstone eruptions occurred at 2.06 million, 1.29 million, and 640,000 years ago.

true

The variations in the composition of magmas govern whether eruptions are peaceful or explosive.

true

Water circulating at thousands of feet below the surface can be heated to temperatures far above 100°C (212°F) without boiling because the pressure of the overlying groundwater body is so great.

true