Find the Face
Volcanic Eruptions and Hydrovolcanism
Ocean Ridges
Continental Rifts
Subduction Zones:
Continental
Oceanic
Back Arc Basins
Hot Spots Plumes
Cinder Cones
Composite Volcanoes
Shield VolcanoesCinder
Calderas and Domes
Lava has a great resistance to flow- viscosity- it has a high viscosity
water- pours- low viscosity
molasses- strings- moderate viscosity
Lava – high- 100,000 to 200,000 times that of water
High viscosity-Flows Because
Is erupted at a very high temperature
Heavy- 2.5 to 3.5 heavier (denser) than waterthus flows under its own weight
Viscosity decreases with
Increasing Temperature
But increases with increased
Crystal content
Triggering Volcanic Eruptions
What triggers an Eruption?
• Most magma reservoirs underlying worlds
active volcanoes are in a delicate state of
gravitational balance with enclosing rocksneutral buoyancy
• Thus in this delicate state many factors
can lead to an eruption
Triggering an Eruption
• Can divide factors into 3 processes
– Below chamber
– In Chamber
– Above Chamber
• Below-Rise of new magma into chambercan cause over pressurization and failure
of wall rocks
• Within-Differentiation-cooling-volatile
increase; stoping-magma rises-volatile
increase
• Above-Earth Tides-elastic rise and fall of
surface up to 40cm a day –Pinatubo and
pressure changre
What Triggers and Eruption
• Pressure Change (> in magma, < in
lithosphere)
Earthquake
Volatiles
Magma Mixing-New Injections of
magma
Differentiation
• Force of M + V is > then strength of rocks
in conduit, fractures, or volcanic edifice.
Tectonic Events
• Nearly simultaneous
eruption of 2 or more
volcanoes
• Soufriere and Pelee1902-May 7 and 8
• Unzen and Pinatubo1991-June
• Regional stress- earth
tides which fluctuate with
time
• Full Moon-Tidal stresses
Volatiles
• Pressure of load of overlying rocks at the
level of shallow magma chambers range
from 500 bars to 2 kbars
• Add to this the strength of the rocks- 70160 bars
• Magma pressure has to exceed this
• Water rich magmas can do this-volatiles
coming out of solution exert a tremdous
force
Volatiles
• 1 m3 of rhyolite with dissolved volatiles stored in
a shallow magma chamber can expand to 670
m3 of fragmental material and gas upon reaching
atmospheric pressure
• Can expand 1600 fold as it comes out of solution
• Faults-synvolcanic
Intensity and Type of Eruption
• Mass eruption rate-rate of flow out of
vent/fissure- measure of eruption intensityheight and length; column collapse
• Temperature
• Viscosity-Composition
• Volatile Content: Fragmentation and
vesiculation, accelerate magma out of vent
• Vent Geometry
VEI-Volcanic Explosivity Index
• Measures magnitude of volcanic eruptions
• Based on volume of explosive products
plus height of the eruption column,
duration of eruption
• Each successive category equals a 10 fold
increase in explosive power
• 0 to 8
Hydrovolcanism-Trigger for
Explosive Eruptions
• Hydrovolcanic
processes involve the
interaction between
magma and external
water
• This interaction may
be passive or
explosive
• Magmatic processeslittle or no interaction
with external water
Hydrovolcanism
• Thus there is a continuum between magmatic and
hydrovolcanic processes
• And thus between subaerial and subaqueous
volcanism-Hydrovolcanic processes active in both
environments
• Continuum represent by water– Its presence or absence during an eruption
– Amount of water compared to quantity of
magma
Hydrovolcanism-Water and Magma
Weak to extremely powerful
Products of Hydrovolcanism
Passive or thermal shocking
Pillow Lavas
Hyaloclastites
Self Peperites
Lobe Lavas
Explosive- deposits called
Hyalotuffs
Pyroclastic flows and
surges
Ash showers
Lapilli Falls
Debris flows
Hydrovolcanic Explosions
Governed By:
A) Dynamics of the water to magma
interaction:
1) water/magma ratio
2) confing pressure
3) vent geometry
4) state of the water (liquid, vapor, mix)
B) Composition of the magma
1) chemical composition
2) viscosity
3) volatile content
C) Volatile fragmentation depth
D) Water and sediment depth
Volatile Fragmentation Depth
• Water depth at which volatiles will exsolve
from the magma and expand rapidly
enough to cause rapid “bubble burst”• This has the effect of tearing the magma
apart and triggering an explosive eruption
• The VFD is different for magmas of
different composition
VFD
•
•
•
•
•
Tholeiitic basalt: Less than 100 m
Calc alkaline basalt: less than 200 m
Calc alkaline andesite: less than 400 m
Alkalic basalt ?
Calc alkaline-tholeiitic rhyolite: less than
400 to 1000 m?
• VFD’s less than these –instantaneous
expansion, not just exsolution
• Gas bubbles compose > 50-70% of
magma prior to explosive eruption
• To get this need shallow water depths
• VFD’s less than these –instantaneous
expansion, not just exsolution
• Gas bubbles compose > 50-70% of
magma prior to explosive eruption
• To get this need shallow water depths
Water-Magma Interactions
• Heat-thermal energy of the magma is
rapidly transported into the fluid
encountered
• Kinetic energy of these kind of explosions
very high compared to dry (magmatic
volitiles only) eruptions.
• Important boundary condition:
Water-Magma Interactions
• Depth at which magma encounters water
• when water heated to 100c at a pressure
of 1 kb (3km) volume increases only by a
factor of 6.
• At earth’s surface steam has a volume that
is 2000 times larger than water
Water-Magma Interactions
• Because the volume ratio of steam to
water increases with decreasing pressure
and therefore depth, explosions are most
effective in the upper 300m, especially in
the upper 100m.
Water-Magma Interactions
• When lava flows into water explosive
reactions are rare
• Its only when water and magma come
together in an enclosed space, where
magma and water vapor cannot escape
• Can generate enormous excess pressure
with high explosive potential
• Hence need closed or partially closed
system.
• Pressure cooker valve
• At a temperature of 100c and pressure of
1 atmos the ratio of thermal expansion is
15bars/1c;
• Means when water becomes heated and
its volume remains constant the pressure
rises by15 bars for each 1 degree of
heating
Water and Sediment Depth
• Magma approaches the surface in dykes which
advance by crack propagation/faults
• Thick piles of unconsolidated sediment is not
rigid enough to crack
• Rising magma may stall out in the sediments
• Heat of magma boils pore water- shallow get
explosive eruption
• Deep water boils and expansion pushes grains
apart-viscous fluid- allows magma to spread
laterally- peperites-hypabyssal complexes
Hydrovolcanic Processes Produce Deposits
and Volcanic Landforms Governed by
•
•
•
•
•
Water Depth
Water/Magma ratio
State of water
Volatile Content of Magma
Chemical Composition of magma
Reactions that occur when water and
magma interact are divided into 2 major
processes
• Fuel-Coolant (FCI)-ContactSurface Steam Explosivity
• Bulk Interactive explosivity
FCI
• Fuel is the magma and it is at a
temperature greater than the boiling point
of the coolant
• Coolant is the external water
• Interaction vaporizes the coolant and chills
or quenches the fuel
FCI
• Vaporization commonly occurs at
explosive rates (flashing of water to
steam) and rapidly converts thermal to
mechanical energy
• Pulsating Eruptions or episodic
• Abundant water-milder more continuous
eruptions
BSE
Water is trapped close to the magma
Water, along with unconsolidated sediment
or pyroclastic material, is engulfed in the
magma
Heat of the magma converts water to steam
and resultant expansion tears magma apart
Any cooled magma will be torn apart by
the associated shock waves
Non-Explosive Fragmentation
• Cooling contraction granulation- higher
water/magma ratios
• Occurs at any water depth
• Hyaloclatites
• Planar surfaces, right angle corners, 0 to
20% vesicles
• Smooth surface flows generate less
hyaloclatite than rough surface flows
Volcanic Eruptions and Deposits Can, in a
general way, be classified on the bases of
• Water/magma ratio
• Rate of interaction
• Volatile content of the magma
Volcanic Eruptions and Deposits Can, in a
general way, be classified on the bases of
• Water/magma ratio
• Rate of interaction
• Volatile content of the magma