Chapter 6

The volcanic explosivity index has___ levels, each of which is a tenfold increase in the volume of material erupted.

8 or eight

Which of these best describes a caldera?

A large crater formed from the collapse of a volcano

What is viscosity?

A liquid's resistance to flow

What happens to the viscosity of a liquid as temperature increases?

Viscosity decreases.

What is the relationship between silica content and magma viscosity?

Viscosity increases with more silica.

What is tephra?

Volcanic particles of any size

Minerals that are rich in silica have ______ minerals that are silica-poor.

a lower melting temperature than

Mountains can be found in areas of thin continental crust as _______ causes the rocks in the area to expand.

heat

The oceanic ridge system is actually an underwater mountain range caused by the addition of___ from the upwelling magma

heat

A magma with a high viscosity likely has a ______.

high silica content

The melting temperature of silica-poor minerals is ______ that for silica-rich minerals.

higher than

Volcanoes in the interior of plates are located at newly formed divergent boundaries and above_________ spots.

hot

The viscosity of a liquid will decrease as the temperature

increases

The eruption of a volcano in a sideways or horizontal direction is called a ____ blast.

lateral

Different plate boundaries produce different types of___ because each setting melts a different source rock.

magma

The varying compositions of magma found at different plate boundaries result from ______.

melting different source rocks

The gases within a low-viscosity magma will escape ______ compared to those within a high-viscosity magma.

more easily

Mount St. Helens was visibly ______ after it erupted in 1980. Multiple choice question.

shorter

This is the name given to volcanic particles of any size that are blasted out during an eruption.

tephra

The volcanic explosivity index, or VEI, is based on ______.

the volume of material erupted the atmospheric height of the erupted ash the length of the eruption event

Thick continental crust with mountain belts is typically found near convergent plate boundaries; this is where ______.

thrust faults push pieces of crust on top of one another

The word we use to describe a liquid's resistance to flow is

viscosity

Where are most of the world's active volcanoes (above sea level) located?

Along convergent plate boundaries

What was the cause of the May 1980 eruption of Mount St. Helens?

An earthquake caused a landslide that allowed the lateral eruption of the volcano.

After the Himalayas, the tallest mountains in the world are found in the ______.

Andes

What is the term used to describe the balance between Earth's surface topography and the thickness of the underlying rocks?

Isostasy

How does the continental crust vary in thickness?

It is thick below mountain belts and thin at the margins.

The 12-million-year-old ash bed in Nebraska that preserves hundreds of animal skeletons came from the eruption of a supervolcano in present-day Idaho. Where is this supervolcano now?

It is under Yellowstone National Park.

What type of volcano is typically very large, composed of low-viscosity lava, and has gently sloping sides?

Shield

What is partial melting?

The incomplete melting of a source rock to create magma of a different composition

What is isostasy?

The relationship between Earth's topography and the thickness of the underlying crust

What is a lateral blast?

The sideways eruption of a volcano

The volcanic explosivity index has ___levels, each of which is a tenfold increase in the volume of material erupted.

8

Approximately how many active volcanoes, those that might erupt, currently exist?

1,500

The eruption of Mount St. Helens in ______ was one of the United States' most destructive natural disasters.

1980

The hot spot that currently is underneath Yellowstone National Park is responsible for the three ash deposits shown in this map. These most recent eruptions of the supervolcano resulted in ash deposits as wide as ______ kilometers. This much ash in the atmosphere would have resulted in years of global cooling.

2,200

How many of the approximately 1,500 active volcanoes in the world have a complete network of instruments for monitoring seismic and volcanic activity?

20

The low-viscosity lava flows from Hawaiian volcanoes and Nyiragongo in Congo are good examples of how far lava can flow away from a volcano; in these cases, the lava has moved up to ______ kilometers.

50

What is a volcanic bomb?

A type of tephra that is formed from an airborne blob of lava

An impending volcanic eruption commonly is preceded by changes in the volcano's shape, an increase in volcanic_____ emissions, and an increase in____ frequency.

Blank 1: gas Blank 2: earthquake

This two-word term is used to describe the creation of magma from a source rock that does not completely melt.

Blank 1: partial Blank 2: melting

What type of plate boundaries create mountains, which in turn are the locations of the thickest continental crust?

Convergent

What term is defined as mass per unit volume?

Density

Choose the three most common indicators of an impending volcanic eruption.

Earthquakes occur more frequently. More volcanic gases are emitted. The volcano changes shape.

How fast does lava flow over open ground? Assume we are discussing low-viscosity lava.

Fairly slowly—1 kilometer per hour

Which of the following describe a typical shield volcano?

Gently sloping Large Formed from low-silica lava

Within your lifetime, how much mountain erosion would you notice?

Hardly any, since it might equal 10 millimeters

How many of the approximately 1,500 active volcanoes in the world are monitored with instruments?

Less than 200

Do gases escape more easily from high-viscosity or low-viscosity magma?

Low-viscosity

What volcano in the Cascade range erupted in May 1980, causing approximately $1 billion in damage and 57 deaths?

Mount St. Helens

What were the two primary physical changes to Mount St. Helens and its surroundings immediately following its eruption in May 1980?

The volcano was about 400 meters (1,300 feet) shorter. All the trees within 12 kilometers (7 miles) of the crater were destroyed by the lateral blast and mudflows.

The Columbia River plateau shown here is made of many eruptions of basaltic lava. How common are such features?

They are found on all the continents and ocean basins.

True or false: Mountain ranges affect climate.

True

True or false: The density of a substance is its mass compared to its volume.

True

Where is the continental crust the thickest?

Under mountain ranges

This one-word term is used to describe a volcano that might erupt. Fifteen hundred of these currently exist.

active

Volcanoes found in the interior of plates are located ______.

along divergent boundaries above hot spots

The Himalayan Mountains were formed about 50 million years ago in a continental collision. These are the tallest mountains on Earth, indicating that mountains ______. Multiple choice question.

are long-lived features

Lava plateaus are found across the globe and represent enormous eruptions of _______ lava.

basaltic

Volcanoes are most common along___________ plate boundaries.

convergent