Geology chapter 4

What mafic volcanic rock is formed at seafloor spreading ridges?

Basalt

Magmas that contain 45 to 50% silica are called

mafic

Landslides may lead to volcanic eruptions when ______.

the removal of material allows gases to escape

Which of the following describe mafic magma?

Contains 45 to 50% silica Has a relatively high amount of iron and magnesium

What will a decrease in pressure do to a rock that is already nearly hot enough to melt?

It will cause the rock to start melting.

Which of the following describe stratovolcanoes?

They are also known as composite volcanoes. They are known to erupt explosively. They are made up of both pyroclastic and lava flow layers

Which of the following describe the volcanoes of the Cascade Range?

They are composite volcanoes. They are created through the process of subduction.

Which of the following describe lahars?

They are made of ash and water. They move quickly.

How did the residents of Heimaey in Iceland deter erupting lava from closing off their harbor?

They pumped seawater on it to cool and harden it.

Melting of the asthenosphere can occur due to a(n)

drop, decrease, lowering, or reduction

The ______ associated with active volcanic activity are often inconvenient for the nearby residents and business owners, as it can be difficult to predict how long the activity will last.

evacuation

The viscosity of magma is its resistance to flow and is controlled by the silica content. Which is more viscous: mafic or felsic magma?

felsic

Eruptions of lava from cracks in the earth's surface, as opposed to from single vents, are called

fissure

Carbon monoxide, carbon dioxide, fluorine, sulfur compounds, and water vapor are all

gases

While lava usually is not life threatening, it is

hot

scientists are still uncertain the reason for

hot spots

How will a large amount of volcanic ash and dust from a single large eruption affect global climate?

it will cause regional or global cooling for months up to a few years.

A rare success story in diverting a lava flow occurred in the early 1970s, when residents on the island of Heimaey in Iceland pumped ocean water to cool the lava in order to ______.

keep their fishing harbor open

A fast-moving volcanic mudflow created through the combination of ash and water is called a

lahar

The asthenosphere is described as being ultramafic, meaning that it is very rich in ferromagnesian minerals. When it melts, it usually forms ______ magma.

mafic

The partial melting of the ultramafic mantle creates magma that is

mafic

Explanations for hot spots include that they lie above regions in the mantle or in the outer core that are hotter due to a higher percentage of ______.

radioactive materials

Magma that is rich in silica is called ______.

felsic

How many of the world's 300 to 500 active volcanoes are actively monitored?

some of tthem

The volcanoes of the Aleutian Islands are ______ volcanoes that are ______ in composition.

strato-; andesitic

What type of volcano is composed of both pyroclastic and lava flow layers?

stratovolcano

Most magma originates in the upper mantle because the ______ is high enough and the ______ low enough to at least partially melt the rock there.

temperature; pressure

earthquakes, harmonic tremors, and changes in topography and gas emission

activity

Most of the world's active volcanoes are located ______.

along plate boundaries

What type of pyroclastic material causes the following problems: it can bury cities, it is slippery, it can cause roofs to collapse, it is bad for people's lungs, and it is damaging to engines and other mechanical equipment?

ash

Formed by either the collapse of a volcano into an emptied magma chamber or the enlargement of an existing crater, a ______ represents a very large volcanic eruption.

caldera

A very large volcanic crater is known as a(n

caldera

Magma composition is controlled by the type of

rock

Most magma originates in the ______.

upper mantle

Evacuations necessitated by impending volcanic eruptions have saved significant numbers of lives, yet they can be seen as inconvenient and disruptive because ______.

volcanoes don't always erupt in the time and fashion they are expected to

Choose all of the statements that correctly describe the relationship between plate setting, magma behavior, and hazards.

Each plate setting creates magma of a certain composition. The composition of the magma controls how the magma erupts. The way a magma erupts determines the type of hazards associated with it.

How is magma behavior related to plate setting and volcanic hazards?

Each tectonic setting produces a distinct type of magma which erupts in a way directed by its composition.

How is volcanic activity associated with landslides

Earthquakes associated with moving magma may cause landslides. Steep and/or coastal volcanoes may have materials that are weakened and fail suddenly. Landslides may allow volcanic gases to escape.

How does volcanic ash and dust cause cooling of the climate?

It blocks sunlight from reaching earth.

Choose all of the hazards associated with volcanic ash.

It causes mechanical failures. It can bury entire cities. It is dangerous to breathe since it is composed of microscopic glass.

Which of the following best describes the eruptive history of the Yellowstone caldera?

It has had three major explosive eruptions in recent geologic history.

Which of the following describe a pyroclastic flow?

It is a very dangerous volcanic hazard. It is also known as a nuée ardente.

What is the composition of the magma and rocks formed at seafloor spreading ridges?

Mafic

Many of the world's active volcanoes are located along the rim of the ______.

Pacific Ocean

What volcanic hazard is associated with the eruption of Krakatoa in 1883? This event is famous for blowing the island apart, creating a tsunami, and allegedly creating the loudest noise ever heard.

Phreatic explosion

Where does the majority of volcanic activity occur?

Seafloor spreading ridges

Which of the following commonly precede a volcanic eruption?

Seismic activity Changes in the volcano's shape Changes in gas emissions

what are the two primary controls on magma composition?

The extent of melting The parent material

Which of these volcanoes in the Cascades is the most recently active?

St. Helen

Of all the gases listed below that are emitted by volcanoes, which is the primary?

Water vapor

Fissure eruptions occur ______ and can result in massive outpourings of basalt.

on continents or on the seafloor

Water that is heated because it is near magma may cause a volcanic eruption called a(n

phreatic

explosion of Krakatoa in 1883 was so forceful that it blew the island apart.

phreatic

The largest on earth, __________ volcanoes are created from hundreds to thousands of runny basalt flows.

shield

What type of volcano is produced from fluid basalt flows that create a large, broad, gently sloping structure?

shield

Seafloor spreading ridges are the most volcanically active areas on earth's surface because ______.

they are so widespread

Lava is typically ______ or hotter.

850

What is the primary hazard of concern associated with the stratovolcanoes in the Aleutians?

Ash causing aviation hazards

When a volcano erupts due to the creation of large amount of steam, it is called a ______.

phreatic explosion

The entire town of St. Pierre, save for one person, died in a(n)

pyroclastic

What volcanic hazard killed nearly every person in the town of St. Pierre on the island of Martinique when Mont Pelée erupted in 1902?

pyroclastic