Geology Exam 1.1

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What factors affect the melting of rocks?

1. Pressure 2. Volatiles 3. Heat

Which rock needs a microscope to see individual mineral grains?

Aphanitic

________ magma is the most abundant type of erupted at oceanic spreading centers.

Basaltic

the volcanoes of southwestern alaska and the aleutian islands

Composite/Stratovolcano

Which kind of eruptive activity is most likely to be highly explosive?

Eruptions of big, continental margin, composite cones or stratovolcanoes.

Bowen's reaction series predicts the sizes of the different mineral grains that grow from crystallizing magmas.

False

The last minerals to crystallize on Bowen's Reaction Series result in

Felsic

________ tend to increase the explosive potential of a magma body beneath a volcano.

High viscosity and dissolved gas

Why do magmas rise toward Earth's surface?

Magmas are mainly liquid and contain dissolved fluids such as water; most are less dense than the adjacent solid rock.

In a porphyritic rock, which mineral grains are the last to crystallize?

Matrix or groundmass

In 1980, ________ was the first Cascade Range volcano to erupt since Mt. Lassen, California, in 1915-16.

Mt. St. Helens

what mineral crystallizes first?

Olivine

The ________ ocean basin is rimmed by the most subduction zones.

Pacific

Which type of basaltic lava flow has a fairly smooth, unfragmented, ropy surface?

Pahoehoe

What is characterized by very coarse mineral grains?

Pegmatite

What are mineral grans that are of equal size and coarse enough to be seen w/out a microscope?

Phaneritic

What texture represents a single, long period of cooling and crsytallization?

Phaneritic

What rock has two or more, distinctly different sized populations of mineral grains?

Porphyritic

What volcanic rock is extremely vesicular?

Pumice

Why would a plume of solid silicate rock rising slowly from deep in the mantle begin melting as it neared the base of the lithosphere?

Temperatures remain high as lowered pressures decrease melting temperatures.

Why are lava flows typically finer grained than intrusive igneous rocks?

The extrusive magma cools quickly so the mineral grains do not have time to grow

What is a good description of an aphanitic texture?

The rock is crystalline; mineral grains are too small to be visible without a magnifying lens or microscope

The Icelandic volcanoes are related to plate tectonics because ________.

They lie on a spreading center where two plates are moving apart

Basalt is the aphanitic or fine-grained equivalent of gabbro.

True

Glassy igneous rocks form when magma cools too fast for mineral grains to grow.

True

In an igneous rock with a phaneritic texture, the mineral grains are visible to the unaided eye

True

Olivine is an important mineral in peridotites and other ultramafic rocks

True

The viscosities of magmas increase with increasing percentages of silica.

True

________ is a major dissolved volatile constituent in both magmas and volcanic gases?

Water

Which type of basaltic lava flow has its surface covered with sharp-edged, angular blocks and rubble?

aa

Mount St. Helens is ________.

an explosive stratovolcano

Which of the following statements best describes the big Hawaiian volcanoes?

are situated in the interior of a large, pacific plate above a hot spot deep in. the mantle

Volcanic bombs originate ________.

as erupted magma blobs that partly congeal before falling to the ground

A ________ is the largest, perpendicular body of intrusive, igneous rock.

batholith

small basaltic cones built during one, short, eruptive episode

cinder cones

large, fairly steep-sided cones composed of lavas and pyroclastic layers

composite/stratovolcano

Basaltic lavas are generally hotter and more viscous than andesite lavas

false

In general, cinder cones are much larger than shield volcanoes.

false

The eruptions of the Hawaiian volcanoes may be described as explosive in comparision to the 1980 Mount St. Helens eruption

false

The recent (geologically) volcanic activity in Yellowstone National Park is ________.

related to intraplate, hot spot volcanism

Which one of the following best describes volcanism in the Cascade Range, northwestern United States?

related to plate subduction

Which one of the following shows the correct order (left to right) of decreasing magma viscosity?

rhyolite, andesite, basalt

A ________ volcano is a very large, gently sloping mound composed mainly of basaltic lava flows.

shield

massive, gently sloping volcanoes built of successive, basaltic lava flows

shield volcanoes

the big volcanoes of Hawaii

shield volcanoes

Which region has the greatest concentration of currently active volcanoes?

the circum-pacific area

Which of the following best describes Shiprock, a famous volcanic feature in New Mexico?

the eroded remains of a volcanic neck and radiating dikes

Which of the following is associated with deep mantle hot spots?

the volcanoes of Hawaii and the Quaternary activity in Yellowstone National Park

Basalt is the most common magma erupted along oceanic rift systems.

true

A ________ is a near surface, intrusive, igneous rock body that results from melt pooling between horizontal layers.

volcanic neck


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