Volcanics Flashcards
what is an ‘isopleth’?
a line of constant maximum clast size
what is an ‘isopach’?
a line of constant deposit thickness
Name the explosive eruption styles in order of increasing explosiveness
Strombolian, Vulcanian, Vesuvian, Plinian, Ultra-plinian
How can pyroclastic flows be formed?
destabilisation of lava flow fronts and lava domes or by collapsing plumes(larger volumes)
What are the 3 main ways of producing melt
- increasing temperature (e.g hotspots)
- decreasing pressure
- lowering the melting temperature (e.g volatiles)
Magma steps to reach the surface. (page 55)
- extraction
- migration
- magma chamber pooling (feed volcanic edifices)
What would cause the opening of a new conduit?
When the critical pressures inside the magma chamber exceed the tensile strength of the overlying rock (maybe due to new magma supply as well as exsolving gas)
When does ‘fragmentation’ occur?
When the pressure inside the bubbles becomes so large that it breaks the magma fills around it.
Controls whether an eruption is explosive or effusive.
(page 57)
eruption regime diagram: what are the 3 regimes from left to right?
Extrusive (meaning effusive??) , Gas-pyroclast dispersion (explosive) , Gas segregation (Strombolian burps)
Eruption precursors:
changes in seismic activity rate, character and location
volcanic tremor
dome-style uplift or other deformation
changes in amount/ composition of released gas
changes in gravitational/ magnetic field over volcano (direct expressions of magma movement)
changes in hydrology
What are ‘tsunami earthquakes’?
earthquakes that produce tsunami that are particularly large relative to the magnitude estimated form earthquake waves (page 62)
produce a relatively large amount of low-frequency waves
What is the energy of a tsunami source equal to?
the potential energy of the uplifted water =mgh
How much of the produced magma beneath a volcano eventually erupts?
20-50%
Give values for the time scales on which magma rises from the source to the subvolcano chamber, and velocities of movement. (2011 5 mark exam question)
velocities: cm’s to 100s m’s per year (from crust to mantle) , km’s a day-km’s per hour in brittle crust (through diking)
*time scales: 1000s of years in deeper crust
10-100s of years in shallow magma chamber
What depth are magma chambers located beneath the surface?
10-20km
When do fire fountains develop?
As the mixture of gas and magma first leaves the vent using its momentum which is slowly exhausted, if insufficient additional buoyancy is gained.
Can reach a max of a few km’s tall
When do clasts fall out of the plume?
when their terminal falling velocity exceeds the velocity with which the plume carries them upwards (page 51)
What effect do volatiles in the magma have on column buoyancy?
plumes with larger volatile content require lower exit velocities to become stable (relationship breaks down when water content is 10 wt % or higher due to additional mass of plume) . (page 51)
For achieving buoyancy what effect does a larger vent radius have on the required exit velocities?
exit velocities must be higher
3 types of calderas
Basaltic type (due to summit collapse of shield volcanoes), crater-lake type (due to collapse of stratovolcanoes) and resurgent (have no single localised vent)
What are the velocities of plate motions?
cm/year
What are the velocities of slow creep landslides?
0.1 m/year
What are the velocities of slow debris flows
10 m/hour
What are the velocities of mudflows?
10 km/hr
What are the velocities of debris avalanches?
100 km/hr
What are the velocities of P-wave?
6 km/s
How long does magma pond at stages 3 & 5 of migration & storage phases?
3 (Pond at levels in crust) - Thousands of years
5 (shallow magma chamber) - 10s - 100s of years
How does an eruption start?
When pressure in magma chamber becomes greater than Pressure for edifice sitting on top + Pressure fue to the tensile strength of the crust
How is the pressure increased in the magma chamber>
Degassing as magma cools
Influx of gas rich magma
How can you estimate the volume fraction of gas?
Using the change in volume of the magma chamber.
Measure this geodetically
Why do basalts hold gas for less long that rhyolite (magmas)?
Lower viscosity
How does bubble growth occur?
- Decompression
- From 200 - 0m depth size increases 4x - Diffusion
- Diffusing gas into bubbles which have already formed increasing by 1000x in top 200m
- More efficient in basalt than rhyolite - Coalescence
- Travel at diff speeds and radii so can coalesce
Why dont basalts erupt more explosively when diffusion aiding bubble growth is more efficient in basalts (than rhyolites)
Bubbles decrease density and drive magma up
Viscosity of basalt lower
Bubbles can escape more easily & stop driving fluid
What causes a volcano to erupt explosively rather than effusively?
Fragmentation.
More bubble than fluid
Pressure of bubbles becomes so large that breaks magma films around them
Causes fragmentation which creates a turbulent mixture of gas and magma pieces which erupt explosively
What are the properties that dictate how the crust & magma interact?
- Presence of fluid (hydrothermal, groundwater)
- Stress conditions
- Temperature
- Thermal Properties
- Density
- Pre-existing weaknesses
What are the characteristics of an effusive erruption?
Basaltic
Small volumes of tephra
lava surface gas exsolution
fire fountains
What are the characteristics of a Strombolian eruption?
- Explosive
- Frequent fountain eruptions of tephra
- No stable volcanic plume production
- Bubbles coalesce at top of magma column
- Continuous explosive activity
- Exit Velocity = 20 - 80 m/s
- Height = 100 - 150 m
What is a caldera?
Large depressions on or in the place of volcanic edifices, formed when magma chamber roof subsides
What are the 3 types of caldera?
Basaltic
Crater-lake
Resurgent
What are the mechanisms that can trigger an eruption?
- Overpressure
- EQs
- Fractures
- Geoengineering
What are the limits on how large an eruption can be?
Need long-term pathways
high enough flux
crustal strength has limit
Parameters that control if convective plume forms (7)
- Momentum at vent
- Exit velocity
- Density of magma at vent
- Gas content of magma at vent
- T of magma at vent
- Vent radius
- Crater height
What are the types of hydrovolcanic eruption styles?
Sutseyan and Phreatoplinian
What are the measures of physical source strength for volcanoes?
Exit velocity
Mass discharge rate
What are the primary eruption hazards?
Tephra - pyroclastic flows and falls
Lava flows
Gas emission
Ground deformation
What are the secondary eruption hazards?
Lahars Debris flows, avalanches Landslides Climate Tsunami EQ/shock waves Weather - acid rain & lightening Floods Ground deformation
What are pyroclastic density currents?
A heavier than air mixture of hot gases and volcanic fragments whose loose packing and internal motions keep the mixture fluidized
How are lahars formed?
During eruption involving water (snow, ice, rivers, lakes, rainfall)
After eruption by lake breakouts, heavy rainfall, debris avalanches mixed with water
Requires tephra or mechanically weak rocks and water (and gravity)
What are debris avalanches?
Large masses of rock and soil that fall, slide or flow very rapidly under force of gravity
What triggers debris avalanches?
Magma intrustion
Explosive eruption
EQ
Saturation by rainfall
What gases are released?
H20
CO2
SO2
HF
How much gass (mass) can an eruption release?
100Mt
Which gas affects climate?
SO2 by forming aerosols in the stratosphere
