Deck 4: Formation of Magma and Igneous Rocks

Question 1

What process forms rocks with porphyritic texture?

A) quick cooling above ground
B) slow cooling below ground
C) a mixture of quick and slow cooling as rock moves from a warmer environment to a cooler one
D) slow cooling combined with pressure underground

Answer 1

C

Question 2

Why is calcite not common in the igneous rock exposed at Yosemite National park?

A) Calcite isn't found anywhere in California.
B) Calcite is too strong to melt in magma, later becoming igneous rocks.
C) Magma is formed beneath the earth's surface which primarily contains heavier minerals like silicates, not calcite.
D) Calcite can only be in sedimentary rocks.

Answer 2

C

Question 3

A geologist looks at a piece of rock from Yosemite National Park under a microscope and sees tiny crystals of olivine, pyroxene, and plagioclase feldspar. What process might have formed this rock?

A) melting underground, violent volcanic eruption, and solidification above ground
B) melting underground, flowing to Earth's surface, and solidification above ground
C) solidification underground, violent volcanic eruption of ash and solid rock
D) both melting and crystallization underground

Answer 3

B

Question 4

What type of crystal texture is formed when magma cools slowly within warm rock underground?

A) aphanitic
B) porphyritic
C) andesitic
D) phaneritic

Question 5

If volcanic rocks are formed on the earth's surface, what is another word for them?

A) extrusive
B) intrusive
C) plutonic
D) lava

Question 6

Why is silicate the most prevalent mineral in igneous rock?

A) Silicates melt very easily.
B) Silicates make up most of the earth's crust.
C) Silicate is attracted to heat.
D) Silicates are very heavy, so they are perfect for magma.

Question 7

Magma consists of oxides of silicon, aluminum, magnesium, iron, calcium, sodium and potassium. Why is this so?

A) They are all heavy elements located beneath the surface of the earth.
B) They are all attracted to heat.
C) They all have low melting points.
D) They are all oxides.

Question 8

Why does Silica content in most magmas range from 45 to almost 80 percent by weight?

A) Silica is very heavy and therefore throws off the percent weight.
B) Silica is made up of silicon and oxygen, two elements most prevalent in the earth's crust.
C) Silica is a very heavy gas.
D) Silica is extremely light.

Question 9

Why do felsic rocks typically have the lightest color?

A) They contain olivine and pyroxene.
B) They contain pyroxene, amphibole, and plagioclase feldspar.
C) They contain quartz, mica, amphibole, sodium-rich plagioclase feldspar, and potassium feldspar.
D) They contain olivine, pyroxene, and calcium-rich plagioclase feldspar.

Question 10

A volcanologist extracts a piece of rock near a volcano site and observes no visible crystals. After studying the rock for a while, he assumes it is obsidian. Why does he do so?

A) Obsidian is always hot.
B) Obsidian forms underground.
C) Obsidian is green.
D) Obsidian forms in lava flows.

Question 11

Why are igneous rocks that form below earth's surface called plutonic rocks?

A) Pluto was the Greek god of the underworld.
B) Igneous rocks formed underground are the same as those formed on Pluto.
C) Plutonic, in Latin, means intrusive.
D) Plutonic means non-eruptive, and intrusive rocks don't erupt.

Question 12

Why should a student on a field trip in Yosemite National Park assume that a piece of rock, that when under a microscope shows tiny crystals, is basalt?

A) Basalt forms in lava flows.
B) Basalt forms underground.
C) Basalt is green.
D) Basalt is always hot.

Question 13

What is an example of pyroclastic materials?

A) Lava-fountain blobs
B) pumice
C) Lava-fountain blobs, pumice, and ash
D) ash

Question 14

The lava at Kilauea volcano in Hawaii erupts at a temperature of approximately 1000 degrees Celsius. How could geologists overcome this problem when studying the lava flow?

A) Pour water on the lava.
B) Don't bother studying the lava flow.
C) Take pictures from far away and study them in the lab.
D) Wear heat-resistant clothing and use tools that reach the flow from a few meters away.

Question 15

A student notes that a rock contains mica and plagioclase feldspar and concludes that the rock is an intermediate rock. What piece of further information could best determine if the student's conclusion was the correct one?

A) the grain size of the crystals
B) the color of the rock
C) the amphibole content of the rock
D) the silicate content of the rock

Question 16

The composition of the magma determines the minerals that form in an igneous rock.

Question 17

All igneous rocks exhibit identical grain size.

Question 18

The lava at Kilauea volcano in Hawaii moves down slope at speeds of 1-10 meters per second; fast for a lava flow. What could account for this?

A) The lava is spewing out of the cone at fast speeds.
B) The lava is more thin.
C) The lava is very thick.
D) The slope is steep.

Question 19

Why is plutonic a synonym of intrusive?

A) Pluto was the Greek god of the sea; intrusive rock is formed in the sea.
B) They mean the same thing in everyday talk.
C) Pluto was the Greek god of the underworld; intrusive rock is formed underground.
D) Pluto was the Greek god of fire; intrusive rock is formed during volcanic eruption.

Question 20

How can you tell apart volcanic and plutonic rocks?

A) Look at their fracturing properties.
B) Volcanic rocks are fine grained; plutonic rocks are coarse grained.
C) Volcanic rocks are colder than plutonic rocks.
D) Volcanic rocks are older than plutonic rocks.

Question 21

Why are dissolved gases present in magma, but not lava?

A) The gases condense to form liquids once magma hits the earth's surface.
B) The gases escape once magma hits the earth's surface.
C) The gases combust once magma hits the earth's surface.
D) The gases crystallize once magma hits the earth's surface.

Question 22

Why is crystal size an important component in igneous rock texture?

A) It shows if the rock formed underwater or underground.
B) It shows if the rock underwent metamorphism or lithification.
C) It shows what color the rock is.
D) It shows if the rock cooled underground or above ground.

Question 23

A sample contains 60 % Plagioclase Feldspar and a medium amount of silica. What would you call it?

A) an intermediate igneous rock
B) a mafic igneous rock
C) an ultramafic igneous rock
D) a felsic igneous rock

Question 24

Why is methane gas the least abundant gas in magmas?

A) Methane is only found deep within the earth.
B) Methane is repelled by heat.
C) Water vapor, carbon dioxide, and odorous sulfur compounds are the primary gases.
D) Methane turns to liquid in the presence of magma.

Question 25

Igneous rocks are overwhelmingly composed of silicate minerals. Why is this so?

A) Silicates are very heavy.
B) Silicates compose a huge amount of the earth's crust.
C) Silicates are only found in igneous rocks.
D) Silicates are very rare.

Question 26

Why are igneous rocks that consist of very small crystals called aphanitic?

A) Aphanitic comes from the Greek word phaneros, meaning visible.
B) Aphanitic comes from the Greek word aphaneros, meaning invisible.
C) They form deep within the earth's crust.
D) They form deep within the earth's ocean.

Question 27

A scientist identifies a rock sample as andesite. Why did he do so?

A) He observed that the rock has a lot of silica, making it mafic.
B) He observed that the rock has a lot of silica, making it ultra-mafic.
C) He observed that the rock has a decent amount of silica, making it intermediate.
D) He observed that the sample has little silica, making it felsic.

Question 28

Why are rocks rich in magnesium and iron called mafic?

A) Mafic is the opposite of felsic.
B) Ferric is the same as mafic, which means rich in magnesium and iron.
C) Mafic comes from magnesium and ferric.
D) Mafic is the Latin word that means rich is magnesium and iron.

Question 29

Why is dacite considered a fine-grained rock felsic rock?

A) It contains very little silica.
B) It contains a lot of silica and feldspar.
C) It contains a lot of iron and magnesium.
D) It contains an intermediate amount of silica.

Question 30

A scientist identifies a rock sample as very rare. Why did he do so?

A) He observed that the rock is fine grained and has a lot of silica, making it mafic.
B) He observed that the sample is fine grained and has very little silica, making it ultra-mafic.
C) He observed that the rock is coarse grained and has a lot of silica, making it ultra-mafic.
D) He observed that the rock is fine grained and has very little silica, making it felsic.

Question 31

A scientist identifies a rock sample as basalt. Why did he do so?

A) He observed that the rock has a lot of silica, making it mafic.
B) He observed that the rock has a lot of silica, making it ultra-mafic.
C) He observed that the sample has little silica, making it felsic.
D) He observed that the sample has little silica, making it mafic.

Question 32

Why is tonalite considered a coarse-grained felsic rock?

A) It contains very little silica.
B) It contains a lot of silica and feldspar.
C) It contains a lot of silica and magnesium.
D) It contains a lot of iron and magnesium.

Question 33

A scientist identifies a rock sample as rhyolite. Why did he do so?

A) He observed that the rock has a lot of silica, making it mafic.
B) He observed that the rock has a lot of silica, making it ultra-mafic.
C) He observed that the sample has little silica, making it felsic.
D) he observed that the sample has a lot of silica, making it felsic.

Question 34

Which minerals usually give mafic and ultramafic rocks their dark colors?

A) plagioclase feldspar and pyroxene
B) amphibole and pyroxene
C) olivine and pyroxene
D) plagioclase feldspar and olivine

Question 35

Why is diorite considered a coarse-grained intermediate rock?

A) It contains a medium amount of silica.
B) It contains a lot of silica.
C) It contains very little silica.
D) It is made up of the elements in the middle of the periodic table.

Question 36

Why is granite considered a coarse-grained felsic rock?

A) It contains a lot of feldspar and silica.
B) It contains little feldspar and silica.
C) It contains a lot of magnesium and iron.
D) It contains a lot of feldspar and a little silica.

Question 37

Why is gabbro considered a coarse-grained mafic rock?

A) It contains a lot of silica.
B) It is extremely rare.
C) It contains a lot of feldspar.
D) It contains a lot of magnesium and iron.

Question 38

Why are rocks rich in feldspar and silica called felsic?

A) Felsic is the Latin word that means rich in feldspar and silica.
B) Felsic is the opposite of mafic.
C) Felsic comes from feldspar and silica.
D) Felsic is the opposite of ultramafic.

Question 39

Why is peridote considered a coarse-grained ultramafic rock?

A) It contains a lot of silica.
B) It contains a lot of feldspar.
C) It contains a medium amount of silica.
D) It contains very little silica.

Question 40

Which igneous rocks are typically the lightest in color?

A) ultramafic
B) felsic
C) intermediate
D) mafic

Question 41

Why do igneous rocks have so many uses (anything from building stone to abrasive in toothpaste)?

A) They are harmful to the environment, so we use them to prevent harm done.
B) They are very easy to attain.
C) They are strong because their finely intergrown mineral crystals are difficult to fracture along grain boundaries.
D) They are cheap.

Question 42

Names given to the different types of igneous intrusions are attributed to their

A) shape and size.
B) composition and crystal size.
C) location and depth.
D) process of formation.

Question 43

Name the type of volcano described here: "Thick, steep, lava-flow domes form these young volcanoes made of dacitic and rhyolitic rock."

A) shield volcano
B) dome complex volcano
C) cinder cone volcano
D) composite volcano

Question 44

Which of the following is not a property of a cinder cone?

A) usually less than 600 meters tall
B) typically produced by one, prolonged eruption
C) can be up to 10000 meters tall
D) composed of basaltic to andesitic cinder

Question 45

A scientist at a canyon notices observes a dike cuts across the sedimentary layers. How did it get there?

A) It was there before the layers; the layers built upon above and beneath it over millions of years.
B) There was an earthquake  a layer of igneous rock was thrust into the sedimentary layers..
C) Magma rose from within the earth and intruded the sedimentary layers.
D) A piece of a volcano broke off beneath the earth..

Question 46

The texture of an igneous rock consisting of coarse-grained crystals surrounded by very small crystals is called

A) aphanitic.
B) phaneritic.
C) porphyritic.
D) glassy.

Question 47

Highly vesicular, intermediate-sized pyroclastic fragments of dacite or rhyolite are called

A) cinder.
B) pumice.
C) scoria.
D) lapilli.

Question 48

Which process forms pahoehoe lava formations?

A) Very thick lava squeezes out in large mounds.
B) Thin lava runs quickly over a large area.
C) Lava stops flowing while it is still hot and forms ropes.
D) Lava continues flowing after a crust forms.

Question 49

Despite their large size, batholiths are emplaced over a relatively short period of time.

Question 50

Why is the texture of the igneous rock obsidian glassy?

A) Obsidian commonly forms at the surface of lava flows.
B) Obsidian always forms beneath the earth's surface.
C) Obsidian is aphanitic.
D) Obsidian is phaneric.

Question 51

Volcanic is a product of combustion.

Question 52

Batholiths are larger than stocks.

Question 53

A large intrusion of magma that forms over a long period of time deep under the Earth has 128 square kilometers exposed to the surface by erosion. Analysis proves that the rock contains large crystals of gabbro. This formation is called

A) a stock.
B) a batholith.
C) a dike.
D) a sill.

Question 54

Which of the following is a difference between a shield volcano and a composite volcano?

A) Shield volcanoes' lava flows are thin and long whereas composite volcanoes' lava flows are thicker and shorter.
B) Shield volcanoes have a much steeper slope than composite volcanoes.
C) Shield volcanoes are much hotter than composite volcanoes.
D) Shield volcanoes maximum height is much smaller than composite volcanoes.

Question 55

Volcanic bombs do not explode.

Question 56

Highly vesicular, intermediate-sized pyroclastic fragments of basalt or andesite are called

A) pumice.
B) lapilli.
C) ash.
D) cinder.

Question 57

Most volcanoes are found at or near divergent and convergent plate boundaries.

Question 58

Which process forms a'a lava formations?

A) Very thick lava squeezes out in large mounds.
B) Thin lava runs quickly over a large area.
C) Lava stops flowing while it is still hot and forms ropes.
D) Lava continues flowing after a crust forms.

Question 59

The correct size order of pyroclastic material (from smallest to largest) is

A) bomb; lapilli, ash.
B) lapilli; ash; bomb.
C) bomb; ash; lapilli.
D) ash; lapilli; bomb.

Question 60

Why are igneous rocks that consist of coarse-grained crystals called phaneritic?

A) Phaneritic comes from the Greek word aphaneros, meaning invisible.
B) They form deep within the ocean.
C) Phaneritic comes from the Greek word phaneros, meaning visible.
D) They are porphyritic rocks.

Question 61

Volcanoes are present at convergent plate boundaries because

A) the plates rub together, causing friction and heat.
B) water is carried down with the subducting plate and causes the mantle to melt.
C) heat from the inner mantle travels up along the subducting plate.
D) decompression melting due to decreased pressure at the boundary causes the mantle to melt and rise into volcano formations.

Question 62

Why are lahars so dangerous?

A) They are the hottest of all lava flows.
B) They contain deadly chemicals.
C) Hot pyroclastic fragments melt ice and snow; this mixture flows extremely fast.
D) They are the most explosive of all volcanic eruptions.

Question 63

Many economically important metallic elements like gold and silver are disseminated throughout magma. Why are we able to mine these metals?

A) They are easy to pick out of lava flows with tools that reach a few meters away.
B) They don't melt; they float in lava flows and are then trapped when the lava melts; humans go in and mine the lava flows.
C) They become concentrated by fractional crystallization into the last bit of the magma that remains after most of the silicate minerals have formed.
D) They float up to the surface of the lava and harden on top of the flow.

Question 64

What is a benefit of ash and lapilli ejections?

A) They produce highly fertile soil in tropical regions.
B) They contain extremely valuable minerals.
C) The chemicals cause extremely important rainfall in tropical regions.
D) They often rid farms of pests.

Question 65

Why do pyroclastic flows always have higher casualty tolls than lava flows?

A) Pyroclasic flows are much hotter.
B) Lava flows are better predicted.
C) Pyroclastic flows occur more often.
D) Pyclastic flows move at velocities exceeding 100 km/hr.

Question 66

Volcanoes composed of multiple, overlapping volcanic domes are called a

A) dome complex.
B) shield volcano.
C) composite volcano.
D) cinder cone.

Question 67

Figure 4.22
According to Figure 4.22 which types of volcanic rock can be found in composite volcanoes?

A) Basaltic, Andesitic, and Dacitic
B) Basaltic, Andesitic, Dacitic and Rhyolitic
C) Basaltic and Andesitic
D) Andesitic

Question 68

In which ways does pressure influence different rock compositions' melting points?

A) the more pressure, the higher the melting point
B) the more pressure, the lower the melting point
C) pressure changes the composition of the rock, raising the melting point
D) pressure changes the composition of the rock, lowering the melting point

Question 69

Which is one of four processes that produces different varieties of igneous rocks?

A) Single source rocks melt to form different types of magma.
B) Magma in a volcano changes mineral composition as soon as it is ejected.
C) Magma stays the same after assimilating nearby rocks.
D) Magma can change its composition by assimilating nearby rocks.

Question 70

Which of the following is considered an indirect volcanic hazard?

A) lava flows
B) ash fall
C) lahars
D) tsunamis

Question 71

Why is pegmatite an economically important igneous rock?

A) Pegmatite's crystals are huge, so they are very valuable.
B) Pegmatite is very rare, so it is valuable.
C) Pegmatite's crystallization process causes the enrichment of rare ore minerals.
D) Pegmatite is perfect for the production of certain weaponry.

Question 72

Figure 4.22
According to Figure 4.22 which types of magma can erupt as lava flows?

A) Basaltic, Andesitic, and Dacitic
B) Basaltic, Andesitic, Dacitic and Rhyolitic
C) Basaltic and Andesitic
D) Basaltic

Question 73

Briefly describe how igneous rocks are classified. (4.2)

Question 74

How do igneous products form and why do geologists study these processes? (4.1)

Question 75

Why does Basaltic magma have the lowest viscosity?

A) Basaltic lava has more silicate.
B) Basaltic lava has less silicate.
C) Basaltic lava is hotter than other flows.
D) Basaltic lava is less hot than other flows.

Question 76

Which of the following causes the formation of large crystals such as pegmatite?

A) high fluid content of magma
B) high heavy metal content of magma
C) high silica content of magma
D) high solidification factor of magma

Question 77

Why would decompression of the mantle cause the production of mafic magma?

A) Mafic magma is produced at high pressures.
B) With less pressure comes higher melting points.
C) Mafic magma is produced at low pressure.
D) With less pressure comes lower melting points.

Question 78

Which is not a danger associated with volcanic ash?

A) Ash gets caught in jet engines causing them to fail.
B) Ash can set fire to everything it falls on.
C) Ash can cause roofs to collapse.
D) Ash, if exposed to enough heat, can turn back to lava.

Question 79

Figure 4.22
According to Figure 4.22 which types of magma form cinder cones?

A) Basaltic, Andesitic, and Dacitic
B) Basaltic, Andesitic, Dacitic and Rhyolitic
C) Basaltic and Andesitic
D) Basaltic

Question 80

What is an effect of a volcano with Rhyolitc magma?

A) Rhyolitic magma has low gas content; volcanoes with this type of magma erupt gently.
B) Rhylotic magma has low viscosity; volcanoes with this type of magma erupt more often.
C) Rhyolitic magma has high gas content; volcanoes with this type of magma erupt violently.
D) Rhyolitic magma has low gas content; volcanoes with this type of magma erupt less often.