Volcanoes Flashcards
Where are volcanoes found? percent
90% at plate boundaries
10% in plate interiors/hot spots
What plate boundaries are volcanoes found? percentage of the 90%
convergent (subduction): 7-13%
divergent (mid ocean spreading centers): 80%
Name 2 places under hot spot
hawaii
yellowstone
why does volcanism happen at divergent plate boundaries
separating crust thins, sparking upwelling of mantle and upwarping of asthenosphere due to decrease in pressure, magma rises and creates volcanoes
What is the seafloor made of
volcanic rock-basalt
Basalt
- silica content and other
- silicates
- color
- dominent minerals
percent at surface
low
high in fe, mg, ca
dark silicates
dark
pyroxene and plagioclase fledspar
80
rhyolite
- silica content and other
- silicates
- color
- dominent minerals
percent at surface
high, calcium
light silicates
light
quartz and potassium feldspar
10
Andesite
- silica content
- color
percent at surface
intermediate
10
what are the three compositions of magma
andesitic, basaltic, rhyolitic
how is magma generated?
partial melting of the asthenosphere
Where is basaltic magma coming from?
mid ocean spreading centers
describe the volcanism at mid ocean spreading centers
- volume
- eruption
- hazard
- where
voluminous
gentle
not dangerous
mostly underwater
extensive basaltic flows are products of
hot spot volcanism by mantle plumes
how does basaltic magma propangate andesitic or rhyolitic magma?
basaltic magma rises in continental areas, but ponds at mantle-crust boundary, bc it’s hotter than c.c melt. temp, the continental rocks melt to produce silica rich, less dense, andesitic or rhyloitic magma
where is andesitic or rhylotic magma found
subduction zones
volcanism at subduction zones
- volume
- hazard
- where
- eruption
less volumuous
deadly
continents or islands
explosive
what does basaltic magma produce
lava flows
lava fountains
what does rhyolitic magma produce
ash
what are 2 other words for non explosive eruptions
quiescent
effusive
2 factors that determine how magma erupts
viscosity & volitiles/gas content
Viscosity
what does it mean for something to have high viscocity vs low viscocity
resistance to flow
high: doesn’t flow, thicker; low: flows easily, thin
what are the factors affecting viscosity
temp, silica and oxygen, crystal
Silica and oxygen content for viscosity
why (talk abt structure of atoms)
high SiO: more viscous
low SiO: less viscous
siO have strong close bonds (gas atoms vs solid atoms)
what magma will have the greatest viscocity
why
rhyolitic bc high SiO, low eruptive temp
what magma will be the least viscous
why
basaltic: low in siO, high eruptive temp
relationship btw viscosity and gas content
high viscocity: high gas content
low viscocity: low gas content
what magma will have lots of gas? Least?
rhyolitic
basaltic
volatile
substance thst evaporates easily (turns to gas)
what volatile makes up 90% of magmas? followed by
water vapor
co2
so2
how was our oceans and atmosphere created?
volatiles trapped in earth and brought to surface by volcanism
which is why the volatiles in magma are the sam as the aatmosphere
how do the volatiles (dissolved gases) in the magma stay dissolved?
confining pressure of overlyign rock and cool temp
how do gas bubbles form?
confining pressure reduced or increase in temp causes volatiles to separate from magma
what happens as the gas bubbles rise?
expand bc it’s decreasing in temp, and creates new bubbles until its moslty a gas jet with some globs of magma
lava fountains
- from what magma
- viscocity of that magma
- why it’s able
basaltic
- low viscocity (less thick)
- gases can rise and escape easily
what does it mean for a lava to be fluid?
what is it triggerede by>
low viscocity (can flow)
new molten rock remobilizes semi-liquid lava
inflation of the summit creates fissures for the fluid lava to rise through
why is rhyolitic magma explosive
the gas bubbles remain dissolved bc it can’t escape (very viscous magma)
more volatiles equals
more energetic eruptions
what happens when the gas bubbles in rhyolitic magma rises?
pressure exceeds
expand
the confining pressure of the overlying rock, creating fractures
what happens as it moves through fractures?
(rhyolitic)
more bubbles are created bc of drop in confining pressure (collapse of flank reduces pressure)
what happens when rhyolitic magam reaches surface
are these eruptions a one time thing
describe lava flow
blown into pumice and ash creating an eruption column (cloud of gas and ash)
series
viscous so it piles up and its slow
what propels lava fountains
what comes first lava flow or lava fountain
escaping gas pushes lava up
lava fountain
pahoehoe
basaltic lava flow
ropy
fluid, more volatiles
aa
aa and pahoehoe relationship
basaltic lava flow
blocky and rough texture
less fluid, less volatile
pahoehoe>aa but not vice versa f/ temp decrease and gas bubble formation
lavatube
crust of solidified lava surrounding lava flow so it can travel farther
pillow basalts
toothpaste, lava solidifies but interior lava breaks through and repeats steps
what is pyroclastic material
aka
pyro-fire
clastic-fragments
fragments of magma from explosive eruptions
tephra
4 pyroclastic materials in order of small to large particles
ash
cinders
blocks and bombs
what’s the difference btw blocks and bombs
what do they hold in common
blocks: solid when airborne
bombs: liquid when airborne
size and close to vent
scoria vs pumice magma
scoria: basaltic
pumice: rhyolitic
Describe the cycle of pyroclastic material
magma rises and gases expand which creates a frothy melt, explosively ejected as tiny glass (ash) in a pyroclastic plume, which rains down ash and cinders which falls and creates tuff
airfall depostis
layered and sorted by grain size
pyroclastic flows
aka
eruption column collapses and ssssends cloud down volcano
not sorted by grain size and not layered, thick
ash flow deposits
welded tuff vs regular tuff
welded: magma hot enough for ash to fuse
harder, denser
shield volcanoes are made of
- shape
- how does viscosity affect shape
- other characteristics
- end of growth
ex
fluid basaltic lava
- broad with gentle slopes
- basaltic lava is less viscous, so it easily flows fast & far
- lava tubes
- calderas (depressions in summit)
- eject pyroclastic material which is more visocus so it will be short and thick, cinder cones
mauna loa
what are cinder cones?
- rock made of
- what can it produce
- shape
- viscocity and volatiles
- life span
piles of basaltic pyroclastic debris around central vent
- scora
- lava flows
- deep craters, con shape ,s teep slope,
- medium viscosity and medium volatile
- erupt once and done, produced in one eruptive event
shield volcanoes in height
tallest from seafloor up
example of cinder cones
paricutin
Composite volcanoes aka
stratovolcanoes
where are most stratovolccanoes/composite volcanoes
thee are associated with what type of plate boundary
ring of fire continental volcanoes
subduction (convergent)
describe the things composite volcanoes are built on
alternating layers of pyroclastic debris and andesitic to ryholitic lava flows (cake)
how do composite cones get their shape from viscosity
medoum to high viscosity (andesitic to rhyolitic)
silica rich equals magma doesn’t move as far before hardeneing=steep slopes and
summit steep bc ocarse material accumulaate near sourse but finer sediment flatten the flank of cone
shorter flows armour summit area
how do cinder cones get theri shape
low wvisscosity, when cinder cone volcanoes erupt, the lava spews high into the air and then falls back right on the volcano.
are composite volcanoes explosive of effusive?
eexplosive (pyroclastic)
stratovolcano eexample
mt fuji
mt vesivous
what type of material covered the cities close to vesuvoius
lava flow comp
pumice
Rhyolotic
what is the vei scale
relative measure of the explosiveness of volcanic eruptions.
things that are basaltic
scoria, lava flow, tube, fountain, pahoehoe, aa, lava bombs, pillow basalts
things that are rhylolitic
tuff
welded tuff
oumice
pyroclastic material
obsidian
What kind of magma produces shield? Cone? Composite?
Basaltic
Andesitic
