Geology exam 2

what is a rock?

an aggregate of minerals

what does the term composition mean when applied to igneous rocks?

what kind of volcanic magma they came from

in what three ways does rock melt?

heat, reduced pressure, and adding water

in what environment does reduced pressure melt rocks?

divergent plate boundaries

in what environment does heat melt rocks?

hot spots

in what environment does adding water melt rocks?

convergent plate boundaries

what are the two kinds of igneous rocks?

plutonic and volcanic

what feature distinguish plutonic from volcanic igneous rocks?

texture

what is meant by the texture of an igneous rock?

size/arrangement of minerals in the rock

why does magma cool slowly?

it is insulated by surrounding rock underground

why does lava cool rapidly?

the air/temp around it cools it faster

what two features of an igneous rock must be determined before it can be accurately name?

texture and composition

what single factor determines whether a rock will acquire a course texture or a fine texture?

if the magma cools underground or above ground

what is a phenocryst?

large crystals in porphyritic rocks

how does an igneous rock acquire a porphyritic texture?

by cooling partially underground then completing crystallization above ground

what is the volcanic equivalent of gabbro?

basalt

what is the volcanic equivalent of diorite?

andesite

what is the volcanic equivalent of granite?

rhyolite

why are certain groups of minerals found together in a rock?

they crystallize at similar temperatures and lock together to form the rock

what is the molten rule?

when a rock partially melts, it produces magma with a slightly higher percentage of silica

how are the different compositions of magma created by the molten rule?

by melting the rock higher than the other (melt magic rock to get intermediate magma)

at what three specific plate types of plate boundaries do igneous rocks form

divergent plate boundaries, oceanic-oceanic convergent plate boundaries, and oceanic-continental convergent plate boundaries

what is a pluton?

intrusive igneous rock bodies

what are baked zones?

a rim of altered host rock that surrounds the pluton

how are baked zones formed?

hot magma cooks/bakes the surrounding rock created a baked zone

what is a xenolith?

a rock fragment which falls into magma and doesn't melt so it envelopes in a larger rock during the crystallization and entombs it creating a

what is the largest of all plutons?

batholiths

what two states have the largest batholiths?

California and Idaho

what one rock makes up most batholiths and stocks?

granite

what are the similarities and differences between dikes and sills?

they both are cracks in host rock and are basically the same shape; but dikes cut across host rock and sills form parallel to the layers of host rock

how does a volcanic neck form?

as magma solidifies in the conduit of the volcano

how does a laccolith form?

magma rises vertically, then spreads laterally, and finally inflates (creating a lava blister)

what are the four main parts of a volcano?

magma chamber, volcanic throat/conduit, vent, and edifice (cone)

what causes magma to rise against gravity while moving up the throat of a volcano?

gases dissolved in the magma (gas pressure)

what is the ultimate \#1 factor that controls the style of a volcanic eruption?

the composition because it determines viscosity (then gas buildup, etc)

how does viscosity affect the gas content of the magma?

low viscosity won't trap gas and has passive eruptions; high viscosity traps gas and has violent eruptions

what one substance turns magma violent?

steam

what is lava?

rivers of molten rock flowing on earth's surface

what are pyroclastics?

bits of magma blown apart by steam

what is pahoehoe?

hot, runny, thin lava that travels far and cools wrinkled/contorted

what is aa?

erupt partially solicified, thicker, doesn't travel far, and cools to a rough/jagged surface

what mafic volcano erupts from a number of long cracks in the ground?

flood basalt volcano

why is a mountain-like structure not built during the eruption of a flood basalt volcano?

because the lava is runny and doesn't pile up

which mafic volcano forms from thousands of eruptions of fluid lava from a central vent?

shield volcano

how does a lava tube form?

pahoehoe lava erupts, cools on top and bottom from cold air and ground, lava in middle remains insulated, and may leak out draining the interior leaving a hollow tunnel behind \= lava tube

which mafic volcano forms when lava blobs are sprayed into the air and solidify before hitting the ground?

cinder cone

what composition of magma erupts from a composite cone?

intermediate magma

what are the two volcanic materials erupted from composite cones?

pyroclastics and lava flows

what rocks form when a composite cone erupts?

varieties of andesite

what is a volcanic arc?

a chain of volcanoes parallel to the trench built by subduction (o-o is in ocean, o-c is on land)

what are the dominant volcanoes within volcanic arcs?

composite cones

along what specific type of convergent plate boundary is a continental volcanic arc generated?

oceanic-continental convergent plate boundary

what is the name and location of the continental volcanic arc in the USA?

the cascade range in california

along what specific type of convergent plate boundary is a volcanic island arc generated?

oceanic-oceanic convergent plate boundary

what and where is the only volcanic island arc in the USA?

the aleutian islands in alaska

what are the five phases of a composite cone eruption?

1) major vertical blast 2) fire hose effect 3) mushroom cloud 4) pyroclastics flows 5)ash fallout

what happens during the major vertical blast phase of a composite cone eruption?

loud cannon-like shot of steam

what happens during the fire hose effect phase of a composite cone eruption?

rush of ash/gas into air out of the vent

what happens during the mushroom cloud phase of a composite cone eruption?

ash column spreads out to create what looks like a mushroom

what happens during the pyroclastics flows phase of a composite cone eruption?

the column of ash collapses and hot clouds race and flow out away from cone

what happens during the ash fallout phase of a composite cone eruption?

downwind ash falls like snow up to hundreds of miles away from eruption

what is the temperature range of pyroclastic flows?

1000-1500°F

what is the speed of pyroclastic flows?

60-80 miles per hour

what is the distance travelled of pyroclastic flows?

5-20 miles (20 being a lot)

what rocks can form from a pyroclastic flow?

varieties of tuff and pumice

why do felsic composition volcanoes almost always erupt violently?

because felsic magma has high viscosity that traps gas and creates a violent eruption

why do lava domes initially form by an eruption that acts like toothpaste oozing out of a tube?

they initially erupt de-gassed lava meaning no explosion

what interesting and cool looking rock is associated with lava domes?

obsidian

how does obsidian form?

it is dry felsic (most viscous) magma that is so thick the atoms cannot move to form crystals creating glass

why does a caldera form?

because eruption is so large, it empties magma chamber and collapses above rock

how could a large caldera eruption affect the climate on a global scale?

by blasting so much ash into the atmosphere that it blocks out the sun

what type of rock makes up a flood basalt volcano?

basalt

what type of rock makes up a shield volcano?

basalt

what type of rock makes up a cinder cone volcano?

scoria

what type of rock makes up a composite cone volcano?

varieties of andesite

what type of rock makes up a lava dome volcano?

rhyolite and obsidian

what type of rock makes up a caldera volcano?

rhyolite-tuff and pumice

what type of volcano exists at keweenaw peninsula in michigan?

flood basalt volcano

what type of volcano exists at mt. st. helens in washington?

composite cone

what type of volcano exists at yellowstone national park in wyoming?

caldera volcano

what type of volcano exists in the hawaiian islands?

shield volcanoes