Sept 21 lecture (volcanoes) Flashcards
magma sources tend to be ___- __km deep into crust and upper mantle
50-250km
volcanoes are generated at: (list 3)
- divergent plate boundaries
- convergent plate boundaries
- hot spots
magma compositions vary in SiO2, iron, Mg, and volatile gases.
True/ false
true
mafic magma= low in ___, but high in ___ and ____
SiO2
high in iron and Mg
*most common
basalt
felsic magma= ____ levels of SiO2 ___levels of iron, Mg
- rare
- ___/___ rock
high
low
obsidian/ rhyolite rock
intermediate magma= ___ levels of everything
This is the most __ type
___rock
intermediate
dangerous
andesite
amount of SiO2 determines ____
viscosity
viscosity determines ___ __ ____
style of eruption
variations in lava are due to differences in:
- composition
- gas content
- lava temp
the higher the silica content and lower the lava temp, the ___ viscous the lava is and the ___ is flows
more
slower
the more __ a lava contains= more violent the eruption is
gas
magma produced at a divergent plate boundary is typically melted ____ material, which is rich in _____ (ultramafic)
- a melt from it is mafic
asthenosphere
ferromagnetism
at divergent plate boundaries, ___ is emplaced as new seafloor at the spreading ridge/ rift
basalt
at divergent plate boundaries, rift systems in continental crust may melt granitic crust and produce ___/ ___ lavas
rhyolite or andesite
what characterizes rift volcanoes?
a bimodal suite of extrusive igneous rocks
magma comes up through gap b/w diverging plates
a fissure eruption=
low viscosity basalt, low volatile load
True/ false
magmas associated with a hot spot volcanoes under continental crust generally will produce a basalt lava
FALSE
- produce felsic lava (often explosive)
- magmas associated with a hot spot volcano in a ocean basin will produce basalt lava
magma at convergent boundaries:
very complex but:
- subducted continental crust may melt and produce rhyolite lava
- subducted oceanic crust may melt and produce basalt or andesite lava
- subduction of sediments derived from the top of the subducted slab may produce a variety of water rich and dangerous lavas
composite volcano=
medium viscosity, but extra volatile (steep sided- mechanically unstable)
any volcanic rock that’s ejected into the air is called a
pyroclast
____ are typically characteristic of viscous, siliceous rhyolitic, and andesitic lavas, which typically trap more gas
pyroclasts
pyroclasts eventually fall, often lithified together upon cooling, forming rocks knows as ___. Rocks resulting from smaller fragments= ___ ___ while larger fragments produce ___ ___
tephra
volcanic tufts
volcanic breccias
volcanic bomb=
fragments ejected as blobs of lava that become rounded and cooled in flight
Or
chunks torn loose from previously solidified volcanic rock (very large)
volcanic domes consist of viscous, ___-rich
silica
in volcanic domes,
- viscous lava piles up close to vent –> build compact, high peak
- dome blocks vent, leads to a buildup of gas pressure before much material has emerged
- eg eruption of mount st helens
____ ____ are mixtures of hot gases, ash, and rocks that form superheated, dense currents that can move very fast
pyroclastic flows
aka nuee ardente
pyroclastic flows can be very devastating b/c:
- the solids are buoyed up on a layer of hot gas: little frictional resistance
- generated during some violent eruptions; molten material often cools before falling to earth
combination of older volcanic ash and water can create ____
lahars (mud flow)
where does water come from for lahars? Why do they go so fast?
- precipitation, melt from snow-capped volcanoes
- high, steep slope (unstable) mixed with water = flows fast down steep hill
explain phreatic explosions
- volcanic islands are susceptible to seawater seepage, and when the water meets magma, it’s turned to superheated steam
- result of excessive subsurface pressure due to steam buildup
how can volcanic eruptions induce tsunamis?
often stems from phreatic eruptions underwater, or very large explosions
