Ch 4 HW

How do the crystallization and settling of the earliest formed minerals affect the composition of the remaining magma?

After the earliest formed minerals have settled, the resulting magma will be more felsic than the original magma.

How does magmatic differentiation occur?

As the mafic magma cools, firstly such minerals as pyroxene and olivine crystallize, not quartz and feldspar. Therefore, the remaining melt becomes richer in silica and closer to the felsic magma.

How is basaltic magma transformed into felsic magma?

Basaltic magma partially melts the continental crust, which is more felsic in composition.

How do batholiths, stocks, and laccoliths differ?

Batholiths are the largest type of igneous bodies and occur in a linear fashion with a distance of 100km or more; stocks are smaller than batholiths; laccoliths bend the sedimentary layers above them, whereas the sedimentary layers below remain relatively undeformed.

Can you determine what types of igneous rock share the same mineral composition, but are different in texture, and are therefore different rocks?

Coarse Grained Granite Diorite Gabbro Peridotite Fine Grained Rhyolite Andesite Basalt Komatiite Rhyolite has same composition as Granite but finer texture. Diorite has same composition as Andesite but coarser texture. Gabbro has same composition as Basalt but coarser texture. Peridotite has same composition as Komatiite but coarser texture.

Bowen's reaction series contains a discontinuous series and a continuous series. The discontinuous series is shown in the table, from highest crystallization temperature to lowest. The continuous series contains plagioclase feldspar, which can have any number of chemical formulas, ranging from calcium-rich plagioclase (CaAl2Si2O8CaAl2Si2O8) to sodium-rich plagioclase (NaAlSi3O8NaAlSi3O8) as the melt gets cooler. The following minerals form after the discontinuous and continuous series: Mineral/Molecular formula potassium feldspar/KAlSi3O8KAlSi3O8 muscovite/KAl2(AlSi3O10)(OH)2KAl2(AlSi3O10)(OH)2 quartz/SiO2 Consider a melt that has cooled to a temperature such that olivine, pyroxene, amphibole, and calcium-rich plagioclase feldspar have crystallized. Which elements will be enriched in the melt, and which will be depleted in the melt? Drag the appropriate items to their respective bins. Each item may be used only once.

DEPLETED magnesium (MgMg), iron (FeFe), calcium (CaCa) ENRICHED sodium (NaNa), silicon (SiSi), potassium (K)

How do light-colored igneous rocks differ from dark-colored rocks?

Dark-colored igneous rocks have a higher iron and magnesium content than light-colored rocks.

What causes an elevated geothermal gradient in a divergent plate setting?

Decompression melting of the mantle

Which of the following choices classifies the three listed rocks by composition and texture?

Diorite is intermediate with a phaneritic texture; rhyolite is felsic with an aphanitic texture; basalt porphyry is mafic with a porphyritic texture.

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

Earth's crust is less dense than the mantle.

What is the difference between extrusive igneous rocks and intrusive igneous rocks?

Extrusive igneous rocks cool and solidify on the Earth's surface, whereas intrusive igneous rocks cool and solidify beneath the Earth's surface.

Once a source rock partially melts, what does it produce?

Felsic magma and mafic residue

How are felsic magmas formed?

Heat from the mantle melts part of the lower crust.

Can you determine what type of igneous rock would result from various cooling histories and locations?

Intrusive igneous rocks Diorite and Granite Extrusive igneous rocks Rhyolite Porphyritic igneous rocks Andesite porphyry

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

It is less dense than the surrounding rock.

How does magma differ from lava?

Magma exists strictly below Earth's surface, whereas lava is found on Earth's surface.

As shown in the figure below, ultramafic rocks (peridotite/komatite) are composed of minerals with high crystallization temperatures. In contrast, felsic rocks (granite/rhyolite) are composed of minerals with low crystallization temperatures. Therefore, ultramafic rocks are typically the first rocks to crystallize from magma and felsic rocks are typically the last. Arrange the points A, B, and C in the following diagram from most mafic to most felsic. Notice that points A and C reside in the magma melt, while point B resides in formed igneous rocks.

Most mafic -> Most felsic Rock B -> Melt A -> Melt C

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.

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 a porphyritic texture indicate about the cooling history of an igneous rock?

The rock began cooling under the Earth's surface, but then was moved to above the Earth's surface where it finished cooling.

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.

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.

How does decompression melting occur?

When hot, solid mantle rock ascends, it moves to areas of lower pressure. This reduction in pressure allows the hot material to melt into a less compressed liquid phase.

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

almost always

What are the six major igneous rock textures?

aphanitic, phaneritic, porphyritic, vesicular, glassy, and pyroclastic

Pyroclastic debris from a volcanic eruption can include

ash and cinders

How are igneous rocks classified?

by their textures and compositions

Which of the following could cause rock to melt?

change in composition increase in temperature decrease in pressure

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

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

Fill in the blanks in this sentence using the terms below: Granite and rhyolite are both ____, but they differ in ____.

felsic; crystal size

According to Bowen's Reaction Series, __________ is one of the first minerals to melt, but last to crystallize.

quartz

Which group of minerals makes up most igneous rocks?

silicates

What experiments did Tuttle and Bowen perform?

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

What are the three components of magma?

the liquid portion (mobile ions of the eight most common minerals found in the Earth's crust) called melt, the solid portion which consists of silicate mineral crystals, and the gaseous portion called volatiles

As a result of partial melting, magma originates in

the mantle and crust

In a subduction zone, water driven from subducted oceanic crust causes __________.

the melting temperature of mantle rocks to decrease

What factors influences the texture of an igneous rock?

the rate at which the rock cools, the amount of silica that is present in the magma from which it forms, and the amount of dissolved gases present in said magma

What is peridotite?

the rock making up the mantle

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

water

When will magma rise to Earth's surface?

when the magma is less dense than adjacent rock