Chapter 4: Igneous Rocks, Intrusive Activity
Which of the following could cause rock to melt?
-Change in composition -Increase in temperature -Decrease in pressure
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.
-Sill -Injected parallel to sedimentary layers -Nearly horizontal orientation -Dike -Cuts across sedimentary layers Nearly vertical orientation
Which of the following is evidence of glaciation in Yosemite National Park?
-Striations in granite -U-shaped valleys
What is the difference between a dike and a sill?
A dike intrudes across sedimentary layers, and a sill intrudes between sedimentary layers.
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.
What is an accessory mineral?
A mineral that makes up a relatively small portion of the total rock composition
What is the rock name of an intermediate rock with two distinct grain sizes?
Andesite porphyry
How is basaltic magma transformed into felsic magma?
Basaltic magma partially melts the continental crust, which is more felsic in composition.
How do sheet joints and exfoliation domes form in granite plutons?
Confining pressure on the plutons is removed through erosion of surrounding rock. Plutons then expand.
What causes an elevated geothermal gradient in a divergent plate setting?
Decompression melting of the mantle
Why does magma composition change during fractional crystallization?
Different elements in the magma form crystals at different rates, leaving behind more of the unused elements.
How do black smokers get their name?
Dissolved minerals are ejected from chimneys and rapidly precipitate out of solution, creating a dark color.
In general, why does Earth's crust sit on top of the mantle?
Earth's crust is less dense than the mantle.
Why is the monolith El Capitan in Yosemite National Park more resistant to erosion than other igneous plutons in the park?
El Capitan is unfractured.
Once a source rock partially melts, what does it produce?
Felsic magma and mafic residue
Which of the following best describes the difference between granite and rhyolite?
Granite is phaneritic and rhyolite is aphanitic.
Can you determine what types of igneous rock share the same mineral composition, but are different in texture, and are therefore different rocks?
Granitic = Rhyolite Andesitic = Diorite Basaltic = Gabbro Ultramafic = Peridotite
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
__________ naturally concentrate(s) the less common elements of Earth's crust.
Hydrothermal solutions
How are igneous rocks formed?
Igneous rocks are formed by the cooling and crystallization of molten rock.
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 Rocks: Diorite, Granite Extrusive Igneous rocks: Rhyolite Porphorytic Igneous Rocks: Andesite porphyry
A phaneritic texture is characteristic of a(n) __________ igneous rock that cooled __________.
Intrusive; below the surface
For this question, think about what you have learned regarding the formation of dikes and how these features are exposed at the surface today. Rank the following events in order from oldest to youngest in age.
Oldest -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.
Ultramafic rocks contain __________ and are commonly found in __________.
Olivine; mantle
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.
According to Bowen's Reaction Series, __________ is one of the first minerals to melt, but last to crystallize.
Quartz
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?
Sketch #2
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.
Which of the following statements best represents the relationship between magma and crystals formed from that magma?
The crystals are denser than the magma.
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 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.
What do pumice and scoria have in common?
They both exhibit a vesicular texture.
Why are volcanic rocks often exposed at Earth's surface as hills, ridges, and mountains surrounded by areas of lower elevation?
Volcanic rocks are often more resistant to erosion than surrounding rock.
Which of the following best describes the water transport path before exiting through a black smoker at the seafloor?
Water circulates through the ocean crust, causing it to heat up and become saturated with dissolved minerals.
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 type of magma would you expect to see after crystallization of minerals containing significant amounts of iron?
a magma depleted in iron
In general, when will melted rock in the mantle rise?
almost always
What is a magma chamber called after the conduit has solidified and been exposed by erosion?
batholith
What large, overall igneous feature makes up the Sierra Nevada Mountains in California?
batholith
What is fractional crystallization?
crystallization of part of a magma, leaving behind melted rock with a different composition from the original magma
Which two processes commonly generate magma?
decompression melting and wet melting (the addition of volatiles)
Pegmatites are igneous rocks with __________ crystals, formed from a __________.
large; wet magma
What lies underneath volcanic features at Earth's surface?
magma chambers and volcanic conduits
What is a roof pendant?
older rock capping the batholith. This rock would have been the roof of the magma chamber.
What are the large crystals visible in this figure, and what do they indicate about the rock they formed in?
phenocrysts; precipitated out as magma cooled
In a subduction zone, water driven from subducted oceanic crust causes __________.
the melting temperature of mantle rocks to decrease
What is peridotite?
the rock making up the mantle
When minerals precipitate in a fracture, they are referred to as __________.
vein deposits
What is a volcanic conduit called after the conduit has solidified and been exposed by erosion?
volcanic neck
When will magma rise to Earth's surface?
when the magma is less dense than adjacent rock
