Geo CH 4 & 5
Intrusive Igneous Rocks
Granite, Diorite
What type of volcano is Mount St. Helens?
composite
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
Which of these most directly triggered the main eruption?
A massive landslide
What is an accessory mineral?
A mineral that makes up a relatively small portion of the total rock composition
Porphyritic Igneous Rocks
Andesite Porphyry
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 calderas form?
Calderas form when the summit of a volcano collapses.
What causes an elevated geothermal gradient in a divergent plate setting?
Decompression melting of the mantle
In general, why does Earth's crust sit on top of the mantle?
Earth's crust is less dense than the mantle.
Once a source rock partially melts, what does it produce?
Felsic magma and mafic residue
You have found a mafic rock with a phaneritic texture. What is a potential rock name?
Gabbro
Which of the following best describes the difference between granite and rhyolite?
Granite is phaneritic and rhyolite is aphanitic.
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
If previous eruptions had left Mount St. Helens with a different shape, what differences might have influenced the timing of the eruption?
If the northern slopes had been less steep initially (same height), the eruption would likely have happened later. If the volcano had been significantly shorter initially (same steepness), the eruption would likely have happened sooner.
How are igneous rocks formed?
Igneous rocks are formed by the cooling and crystallization of molten 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
Asthenosphere
Magma with the HIGHEST temp
There had been many earthquakes in the area prior to this eruption. Why did this last earthquake trigger a massive landslide?
Moving magma had over-steepened the mountain slopes.
Based on the diagram shown in the video, name two minerals that are unlikely to coexist in the same igneous rock.
Muscovite and pyroxene
Which of the following textures describes an igneous rock with anomalously large crystals?
Pegmatitic
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.
According to Bowen's Reaction Series, __________ is one of the first minerals to melt, but last to crystallize.
Quartz
What minerals make up rhyolite?
Quartz, K-feldspar, and plagioclase feldspars, with relatively fewer biotite and amphiboles
Extrusive Igneous Rocks
Rhyolite
In addition to rock deposits, what evidence do we have that the eruption that preceded the formation of Crater Lake was very large?
The eruption was so large that the magma chamber under Crater Lake partially emptied, causing the volcano to become unstable and collapse.
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.
Why does Crater Lake have an island in it?
The volcano is still active, and a new volcanic peak has formed.
What do pumice and scoria have in common?
They both exhibit a vesicular texture.
Which of the following describes a texture containing bubble-shaped cavities?
Vesicular
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.
What geologic process is related to caldera formation?
Volcanism
Explosive eruption
Volcano fed by silica-rich magma
Upper Lithosphere / Crust
a. Magma with GREATEST silica content b. Magma with density EQUAL TO or LESS THAN the crust c. Magma undergoing degassing during an eruption
Lower Lithosphere
a. Magma with density LESS THAN the LOWER lithosphere but HIGHER THAN the crust b. Magma undergoing PARTIAL crystallization of HIGH-DENSITY minerals.
Nonexplosive eruption
a. Volcano fed by mafic (basaltic) magma b. Volcano fed by magma with low amounts of dissolved gases c. Volcano fed by high-teperatures (1000 C) magma
Not related to eruption explosiveness
a. Volcano that has not erupted for a long time b. Volcano at high elevation (>10,00 ft above sea level)
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 drives melting at divergent boundaries?
decompression melting
Igneous rocks of felsic composition have __________ and __________ relative to mafic igneous rocks.
greater silica; lower melting temperature
A phaneritic texture is characteristic of a(n) _______ igneous rock that cooled _______.
intrusive; below the surface
What lies underneath volcanic features at Earth's surface?
magma chambers and volcanic conduits
At what tectonic setting is Hawaii located?
oceanic hot spot
Ultramafic rocks contain __________ and are commonly found in __________.
olivine; mantle
In a subduction zone, water driven from subducted oceanic crust causes _______.
the melting temperature of mantle rocks to decrease
Which of the following tectonic settings does NOT produce volcanism?
transform boundary
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
