CH. 4: Magma & Igneous Rocks

Magmas are classified based on their SiO2 content. Match the following composition terms with the amount of SiO2 contained in a rock of the composition. 1. felsic 2. intermediate 3. mafic 4. ultramafic

1. 66-76% 2. 52-66% 3. 45-52% 4. 38-45%

For each of the tectonic settings listed below, choose the most likely form of melting you would expect to see: 1. mid ocean ridge 2. felsic magma in a continental rift 3. oceanic hot spot 4. subduction zone 5. continent continent collision

1. decompression melting 2. heat transfer melting 3. decompression melting 4. volatile melting 5. no melting

Match the following magma producing processes with their definitions: 1. the melting temperature of hot material is lowered as the surrounding pressure decreases 2. water is added to a region with rocks at a temperature where they will melt if water is added 3. hot material moves into a region with higher temperatures. than its melting temperature 4. hot material moves into a region with water or carbon dioxide, which lowers its melting temperature

1. decompression melting 2. volatile melting 3. heat transfer melting 4. not a form of melting

Based on what you learned about minerals and igneous rocks, and based on the following mineral compositions, are following mafic or felsic minerals? 1. olivine - Mg2SiO4 2. quartz - SiO2 3. biotite mica - KMg3AlSi3O10(OH)2 4. feldspar - KAlSi3O8

1. mafic 2. felsic 3. mafic 4. felsic

Cooled magma chambers from _____ whereas cooled volcanic feeders form ____, which are both ____ igneous features.

1. plutons 2. dikes 3. intrusive

What is the general temperature range that most silicate magmas exist?

650-1100 degrees Celsius

What is required to generate an igneous rock with a fine-grained texture?

A fine-grained texture is the product of rapid cooling and crystallization of lava.

Which of the following pairs of igneous rocks exhibit aphanitic texture?

Andesite; rhyolite

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

Because the extrusive magma cools quickly and mineral grains do not have time to grow

In general, why does Earth's crust sit on top of the mantle?

Earth's crust is less dense than the mantle.

You have found a mafic rock with a phaneritic texture. What is a potential rock name?

Gabbro

What do pumice and obsidian have in common?

Glassy texture

Which rock would be most likely to contain visible quartz and potassium feldspar crystals?

Granite

How are felsic magmas formed?

Heat from the mantle melts part of the lower crust.

Intrusive and extrusive igneous rocks are classified based on what criteria?

How quickly the molten rock cools and crystallizes, and where this occurs with respect to Earth's surface

How are igneous rocks formed?

Igneous rocks are formed by the cooling and crystallization of molten rock.

Why does magma have a tendency to rise after its formation?

It is less dense than the surrounding rock.

What is one way a glassy texture forms?

Lava cools rapidly.

Which of the following is true about mafic igneous rocks?

Mafic igneous rocks are rich in Fe, Mg, and Ca

Based on the diagram shown in the video, name two minerals that are unlikely to coexist in the same igneous rock.

Muscovite and pyroxene

What type of igneous rock consists of very coarse crystals?

Pegmatite

Which of the following textures describes an igneous rock with anomalously large crystals?

Pegmatitic

Which term describes a texture of igneous rocks that crystallized over a single, long period?

Phaneritic

Which texture listed below would be most unlikely to occur in an extrusive igneous rock?

Phaneritic

Which of the following textures indicates two stages of cooling and crystallization?

Porphyritic

How do mafic volcanic rocks get to Earth's surface?

Pressure from overlying crust forces mantle magmas through cracks to Earth's surface. These magmas erupt out of volcanoes, cool, and solidify into rock.

Based on what you learned in this video, what minerals make up rhyolite?

Quartz, K-feldspar, and plagioclase feldspars, with relatively fewer biotite and amphiboles

Which of the following statements about rock melting is most accurate?

Rock can remain solid, become partially melted, or melt completely at the same depth within Earth, depending on the temperature.

Why is decompression melting common at mid-ocean ridges but not at subduction zones?

Tectonic plates are moving apart at mid-ocean ridges, resulting in a lowering of pressure beneath the ridge. Tectonic plates are moving together at subduction zones, resulting in an increase of pressure under new mountains.

Why is water a necessary component of the melting process in subduction zones?

The addition of water lowers the melting temperature of rock.

What does it mean if an igneous rock has an aphanitic texture?

The igneous rock exhibits mineral crystals too small to see with the naked eye.

What does an igneous rock with a pyroclastic texture tell a geologist?

The rock was likely formed by a violent volcanic eruption.

Most igneous rocks never reach the surface. However, igneous rocks other than those formed in volcanoes are found exposed on many parts of Earth. How do you account for this phenomenon?

They formed at depth and have been exposed by uplift and erosion.

Why is Tuttle and Bowen's experiment useful to geologists?

This experiment allows geologists to interpret where granitic rock solidifies beneath Earth's surface.

Which of the following describes a texture containing bubble-shaped cavities?

Vesicular

Where does the water involved in melting at subduction zones come from?

Water contained within minerals in the subducting plate is released during metamorphism.

How does water content of granitic magmas change as the magmas move towards Earth's surface?

Water content decreases.

How does water content of melt change with increasing pressure?

Water content increases with increasing pressure.

In general, when will melted rock in the mantle rise?

almost always

Based on what we learned about igneous composition and texture, which of the following statements is TRUE?

any composition of magma/lava can cool to form any of the discussed textures, because the two properties are independent from one another

Pyroclastic debris from a volcanic eruption can include _____.

ash and cinders

Which term describes the changing of the composition of magma by incorporating surrounding host rock?

assimilation

What is the dominant lava erupted from volcanoes on Hawaii and Iceland?

basalt

The three most common volatiles in magma are:

carbon dioxide, sulfur dioxide, and water vapor

Which of the following could cause rock to melt?

change in composition, decrease in pressure, increase in temperature

Which of the following is a factor that does not affect the generation of magma?

crystal size

Which two processes commonly generate magma?

decompression melting and wet melting (the addition of volatiles)

When lava erupts at Earth's surface, what type of rock is produced?

extrusive

The last minerals to crystallize on Bowen's Reaction Series result in igneous rocks with a __________ composition.

felsic

Which of the choices below lists the four basic compositional groups of igneous rocks in order from highest silica content to lowest silica content?

felsic, intermediate, mafic, ultramafic

What type of texture occurs in igneous rocks that cool so rapidly that unordered ions are "frozen" in place?

glassy

determine the composition of the granite and basalt below

granite = felsic basalt = mafic

Igneous rocks of felsic composition have __________ and __________ relative to mafic igneous rocks.

greater silica; lower melting temperature

Why does a higher temperature cause the viscosity of a magma to decrease?

heat breaks bonds between atoms and allows them to move easily past one another

If a body of magma becomes more felsic (richer in Si), its viscosity will:

increase

Which of the following could cause rock to melt?

increase in temperature, change in composition, decrease in pressure

Considering a felsic magma, what was the likely composition of the source of that magma, knowing that most magma forms by partial melting of a source rock?

intermediate

A phaneritic texture is characteristic of a(n) __________ igneous rock that cooled __________.

intrusive; below the surface

What does grain size generally indicate about the cooling of a melt?

larger crystals indicate that the melt cooled very slowly at depth

Magmas are composed of which of the following components:

liquid melt, solid minerals, gasses (either dissolved in the melt or as gas bubbles)

This is a glassy textured igneous rock that is black in color. What rock is this?

obsidian

Which of the following minerals crystallize early in Bowen's reaction series?

olivine

Which volcanic rock is extremely vesicular and glassy?

pumice

Which of the following is a chemical equivalent of granite?

rhyolite

Most igneous rocks are composed of ____ minerals.

silicate

What experiments did Tuttle and Bowen perform?

tests of the temperature and pressure conditions required to melt granite and form magma

What term describes the sizes, shapes, and arrangements of mineral grains in an igneous rock?

texture

Why does water induce melting in a subduction zone?

the addition of water lowers the melting of rock

What is the geothermal gradient?

the increases in temperature of the Earth with increased depth

The hotter the magma...

the less viscous it becomes

If you had the ability to travel anywhere on Earth, inside or out, where would be the best place to go find peridotite?

the mantle

As a result of partial melting, magma originates in __________.

the mantle and crust

What does pyroclastic texture in an igneous rock suggest about its formation?

the rock likely formed by a violent volcanic eruption

What is peridotite?

the rock making up the mantle

Which of the following are used for studying rocks with a polarizing or petrographic microscope?

thin sections

A __________ is an open cavity in a volcanic rock that was filled by a gas bubble when the lava was still mainly liquid.

vesicle

What material was added to powdered rock during Tuttle and Bowen's experiments?

water

Water induced melting at subduction zones comes from where?

water contained within minerals in the subducting plate is released during metamorphism

Which of the following igneous rocks has a pyroclastic texture, meaning it was formed from a glowing avalanche?

welded tuff

When will magma rise to Earth's surface?

when the magma is less dense than adjacent rock