chapter 4
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
Where do ultramafic rocks form?
At the upper mantle
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. Rank from most mafic to most felsic.
B A C
What is peridotite?
the rock making up the mantle
Which of the following could cause rock to melt?
- Change in composition - Decrease in pressure - Increase in temperature
oldest events to youngest events
-Magma is injected far below Earth's surface, cutting through sedimentary country rocks. -Magma crystallizes and a dike is formed. -Sedimentary country rocks are eroded away. -Humans build a tunnel and road through the resistant igneous rocks.
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
Which of the following substances is not a volatile found in magma?
02
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 can a magma body change its composition? CHOOSE ALL THAT APPLY.
Assimilation Magma mixing Crystal settling Magmatic differentiation
Why does partial melting result in a magma having a composition different from the rock from which it was derived?
Because different minerals melt at different temperatures, minerals that melt at lower temperatures will melt from the rock first, creating a magma formed from the minerals that melted first.
What is the name of the preexisting rock through which intrusive igneous bodies intrude?
Country rock
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?
Depleted: Ca, Fe, MgEnriched: K, Na, Si
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.
What was the rate of cooling for an extrusive igneous rock and why?
Fast because it was erupted from a volcano
How are felsic magmas formed?
Heat from the mantle melts part of the lower crust.
Will density increase or decrease with the crystallization of a magma?
Increase
The partial melting of mafic rocks would produce what composition of magma?
Intermediate
Can you determine what type of igneous rock would result from various cooling histories and locations? Review the following rock photographs and determine their cooling histories. Place each image in its appropriate bin.
Intrusive Igneous Rock- DIORITE & GRANITEExtrusive Igneous Rock- RHYOLITEPorphyritic Igneous Rock- ANDESITE PORPHYRY
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.
Match the rock with the correct igneous composition.
Rhyolite −− Felsic Rhyolite −Felsic Pumice −− Felsic Pumice −Felsic Gabbro −− Mafic Gabbro −Mafic Diorite −− Intermediate Diorite −Intermediate Andesite −− Intermediate
Which mineral group makes up most igneous rocks?
Silicates
What is the geologic definition of texture?
Size, shape, and arrangement of mineral grains in the sample
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 is the best interpretation for how the porphyritic texture of these rocks formed?
The magma cooled slowly at first, forming large crystals, and then quickly, forming small crystals from the remaining magma.
Which of the following is true based off your observation of intrusive igneous rocks on Earth's surface?
These rocks have been exposed by uplift and erosion.
Peridotite is the main constituent of which part of the Earth?
Upper mantle
What substance makes ion migration easier in the later stages of magma crystallization, which allows abnormally large mineral crystals to form and create pegmatites?
Water
Where does the water involved in melting at subduction zones come from?
Water contained within minerals in the subducting plate is released during metamorphism.
What role does water play in generating magma?
Water lowers the melting temperature of the rock, allowing it to melt.
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
Which two processes commonly generate magma?
decompression melting and wet melting (the addition of volatiles)
The granitic igneous rocks that make up the Sierra Nevada Mountains in California and Nevada are ________.
intrusive
The photos below show both intrusions and the layered sandstones that they intrude. Igneous rocks are outlined in red, while the orientation of sandstone layers is traced with smaller yellow lines. Drag the appropriate items into their respective bins
on left: Injected parallel to sedimentary layers, Sill, Nearly horizontal orientation on right: Dike, Nearly vertical orientation, Cuts across sedimentary layers
The partial melting of ____ will yield mafic (basaltic) magma.
peridotite
An igneous rock with a ______ texture solidified deep within Earth.
phaneritic
Which group of minerals makes up most igneous rocks?
silicates
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
Imagine you are out in the field and have to sketch and label the view below. Based on observations from the two previous photos, which of the following sketches is the most accurate
the one with two dikes
When will magma rise to Earth's surface?
when the magma is less dense than adjacent rock
