Geology 101 Quiz 5 Flashcards
in both volcanoes and carbonated beverages
gas is dissolved under pressure; when the pressure is released, gases expand and carry part of the liquid out the top with the escaping gas
in a volcano, rising magma does what to overlying rock?
stresses, which eventually creates fractures that reach the surface
as gases expand, what gets carried up through the fractures along with the expanding gases?
magma
Mt. St. Helens in Washington had a ? eruption in what year?
explosive; 1980 - the entire side of the volcano was blasted away
are all volcanic eruptions explosive?
no
the explosiveness of a volcano depends on two criteria
gas content & magma composition
most of the gas in volcanoes is
simple water and carbon dioxide
some of the gases come from ?
subducted plates that carry seawater along with the descending crust
if the gas content of the magma is (low/high), the potential for an explosive eruption is much higher
high
the composition of magma is important because it affects ?
viscosity
viscosity
the resistance to flow
something with a (low/high) viscosity does not flow easily (like molasses)
high
something with a (low/high) viscosity flows easily (like water)
low
if a magma has a low viscosity,
gases coming out of the magma can easily rise to the surface and vent
if a magma has a high viscosity,
gases will tend to remain trapped within the magma; magma cannot get out of the way easily when gases are rapidly expanding so lots of it will get carried out with the escaping gases
(felsic/mafic) magmas have a higher silica content and thus are (less/more) viscous
felsic; more
(felsic/mafic) magmas tend to make more explosive eruptions
felsic
Mt. St. Helens if fromed from andesitic lavas/tephra (?)
felsic
Kilauea is formed from basaltic lavas (?)
mafic
felsic (rhyolitic)
higher silica content, more viscous, lower melting temperature
mafic (basaltic)
lower silica content, less viscous, higher melting temperature
a lower melting temperature means that
felsic magmas can exist when it is not hot enough for mafic rocks to melt
Felsic magma will still be more viscous than mafic magma, however, even if ?
the gas content is high
very high explosiveness
felsic + high gas content + high viscosity
low explosiveness
felsic + low gas content + very high viscosity
intermediate explosiveness
mafic + high gas content + very low viscosity
very low explosiveness
mafic + low gas content + low viscosity
eruptions will not be explosive without
dissolved gas
80% of volcanoes on Earth are
basaltic (most oceanic volcanoes)
10% of volcanoes on Earth are ? and the other 10% are ?
andesitic & rhyolitic (most continental volcanoes)
extrusive igneous rocks are divided into two categories
lava and pyroclastic material (tephra)
pyroclastic material
if magma and solidified fragments spend part of their journey from the volcano hurled through the air
pyroclastic material is divided into three categories based on size
ash, laphilli, bombs
ash
dust size tephra up through the size of a BB (< 2 mm)
laphilli
BB to baseball size (2-64 mm)
bombs
larger than a baseball (>64 mm)
lava
magma that flows over the surface of the ground
pahoehoe
smooth, ropey lava formed from high gas content, low viscosity lava
aa
rough, jagged lava formed from low gas content, high viscosity lava
lava tubes
tunnels that lava travels through; form when the upper surface of a lava flow hardens, but the lava beneath continues to flow
three different types of volcanoes that result from various combinations of lava and pyroclastic material
shield volcano, stratovolcano, cinder cone (or tephra cone)
shield volcano
formed from lava flows; successive flows builds up a wide, gently sloping volcano with a shield shape
examples of shield volcanoes
Mauna Loa and Mauna Kea, Hawaii
stratovolcano
formed from lava and pyroclastic material; forms a more traditionally recognized, steeply sided volcano
example of stratovolcano
Mt. Fuji, Japan
cinder cone (or tephra cone)
formed from pyroclastic material; similar shape as stratovolcanoes, but tends to be smaller
example of cinder cone (tephra cone) volcano
Cerro Negro, Nicaragua
caldera
large crater formed when a volcano collapses into a massive, partially emptied underlying magma chamber
example of a caldera
Crater Lake, Oregon
when a large crater is left behind after an eruption,
the volcano has collapsed into the magma chamber (not blasted away)
volcanic hazards
lava flows, explosive eruptions, lahars, tsunami, dangerous gas release
lava flows
mostly a hazard to property since most flows are slow enough, or channeled enough to give time to get out of the way
explosive eruptions
the primary danger in an explosive eruption is the formation of a pyroclastic flow
pyroclastic flow
a dense cloud of super-heated tephra and air that flows at high speed down the side of a volcano; these events are also referred to as incandescent avalanches
two stories of historic pyroclastic flows
Mt. Pelee in Martinique (1902) and Mt. Vesuvius in Italy (79 AD)
Martinique is one of the islands in a volcanic arc formed by
the convergence of two oceanic plates
Mt. Vesuviius is associated with a
subduction zone
iahars
mudflow of tephra and water; similar to flowing, wet cement; they flow with great force down slopes and in stream channels, knocking over structures in their path and burying lowlands in thick deposits of debris
tsunami
a huge wave caused by a sudden displacement of water in a large body of water (used to be called a “tidal wave”)
If a volcano erupts violently under water in the ocean or other large water body, it will displace a large volume of water above it that will trigger
a tsunami
dangerous gas release
volcanic emissions do include noxious sulfur gases, but the most deadly release in recorded history was simply carbon dioxide
If a volcano tends to erupt every so many years, this ? may be used to estimate the next time the volcano will erupt, though the estimate is rarely precise
recurrence interval
early warning signs
seismic monitoring, ground deformation (tilt & expansion), water or ground temperature changes, changes in gas emissions (such as SO2 or HCl)
seismic monitoring
watching for increased number or intensity of earthquakes
ground deformation, tilt & expansion
as magma rises, the volcano expands and the slope angle increases
as signs of volcanic activity increased, the ? got smaller and smaller until it was down to a few days
prediction window
