GHC Ch 6: Volcanism Flashcards
We locate volcanoes in the context of what?
plate tectonics
What four factors determine whether eruptions are peaceful or explosive?
chemical composition, viscosity, gas content, and volume
What are the 3 V’s of volcanology
Volatiles, Viscosity, and Volume
90% of volcanism is associated with how many plate boundaries?
Two
80% of volcanoes are at what kind of plate boundary?
spreading centers
About 10% of volcanoes are at what plate boundary? And the rest of the 10%?
at subduction zones. above hot spots
No volcanism associated with which to types of faults?
transform faults and continent-continent collisions
Oceanic volcanoes are peaceful or explosive?
peaceful
Subduction-zone volcanoes are peaceful or explosive?
explosive
Which of the 92 elements are by far most abundant naturally occurring element of Earth’s crust?
Oxygen and silicon
Oxygen and silicon typically join up as what?
SiO4 tetrahedron
Minerals form in magma by what process?
crystallization
Order of crystallization of different minerals in magma is known from what?
observation and experiment
Elements combine to form…?
minerals
Minerals combine to form…?
rocks
Different compositions of magma result in different (…) rocks?
ingeneous
If magma cools slowly and solidifies beneath surface, what type of rock results?
plutonic rocks
If magma erupts and cools quickly at surface, what type of rock results?
volcanic rocks
What is viscosity?
internal resistance to flow
Water is high or low viscosity? Honey?
Low, high
Viscosity determined by what three factors?
temperature, more silicon and oxygen tetrahedra, amount of crystals
More silicon and oxygen tetrahedra results in higher or lower viscosity?
higher
Solubility increases as pressure (…) and temperature (…)
increases, decreases
What is the most abundant dissolved gas in magma?
Water
As magma rises, pressure (…), water becomes (…)
decreases, gas bubbles
Lower water content results in more or less peaceful eruptions.
more peacefull
Rhyolitic magma has higher (…) and (…)
water content and viscosity
In rhyolitic magma, many (…) form and can not escape through thick magma, so explode and result in violent, dangerous eruptions
steam bubbles
At spreading centers, Plates pull apart so asthenosphere (…) and (…) under low pressure.
rises, melts
At spreading centers, eruptions are peaceful or violent?
peaceful
Subduction zones have violent eruptions because…?
Magma is generated by partial melting of mantle above the subducting plate with water in it, and melts overlying crust to produce magmas of variable composition
Three things will cause rock to melt:
Lowering pressure
Raising temperature
Increasing water content
What is the most common way to melt rock?
decompression melting: lowering pressure
Magma at depth is under too much pressure for (…) to form
gas bubbles
Eventually gas bubble volume may overwhelm magma,(…) that explode out as a (…)
fragmenting it into pieces, gas jet
Concentration of water in magma largely determines (…) eruption.
peaceful or explosive
Rhyolitic magma usually erupts violently because of high (…) and high (…).
water content, viscosity
Low-water content, low-viscosity magma leads to…?
lava flows
High-water content, high-viscosity magma leads to…?
pyroclastic debris
What were some non-explosive eruptions?
Pahoehoe, Aa
What are pyroclastic debris?
broken up fragments of magma and rock from violent gaseous explosions, classified by size
Viscosity controls what about magma?
whether magma flows easily or piles up
Volatile abundance controls whether magma…?
may ooze out harmlessly or explode
Greater volume of magma leads to…?
more intense eruption
Three V’s of Shield Volcanoes? What does a shield volcano look like?
Low Viscosity, Low Volatiles, Large Volume. Great width compared to height.
What is the Volcanic Explosivity Index?
Provides a means of evaluating eruptions according to volume of material erupted, height of eruption column and duration of major eruptive blast. Scale from 0 to 8.
Three V’s of Flood Basalts?
Low Viscosity, Low Volatiles, Very Large Volume
Some flood basalts coincide with mass…?
extinctions
Three V’s of Stratovolcanoes? What does a Stratovolcano look like? What are they composed of?
High Viscosity, High Volatiles, Large Volume. Steep-sided, symmetrical volcanic peaks. alternating layers of pyroclastic debris and andesitic to rhyolitic lava flows.
Plinian-type Eruptions? Water content and viscosity?
Gas-powered vertical columns of pyroclastic debris up to 50 km into the atmosphere. High water content and high viscosity.
Vesuvius, 79 CE
Most of 4,000 people who remained in Pompeii killed by thick layers of hot pumice or pyroclastic flows
Three V’s of Calderas? What does a Calderas look like?
High Viscosity, High Volatiles, Very Large Volume. Large volcanic depressions formed by inward roof collapse into partially emptied magma reservoirs
Where do Calderas occur?
Occur where large volumes of basaltic magma intrude to shallow depths and melt surrounding continental rock, to form high-viscosity, high-volatiles magma
Yellowstone National Park has had how many recent catastrophic (ultra-Plinian) eruptions?
3
The 3 mechanisms by which rocks melt? Which is most common?
Heating, Pressure decrease , Addition of water. Pressure decrease
where volcanism is focused?
At the subduction zone
What are the three types of magma?
Basaltic, Andesitic, and Rhyolitic
Describe the anatomy of an eruption.
Magma rises from areas of high pressure to low pressure. Gas comes out of solution. Bubbles form. Bubbles overwhelm magma. Gas jet is created leading to a buoyant plume.
