Volcano

how deep is a normal magma chamber

4-5 km

 typical melt fraction in magma and the other remaining materials

15%

volatiles, unmelted minerals, glasses

volcanic glass is made by

quenching

what is the role of water at convergent boundaries

water lowers the solids and thus lowers the Geotherm intersects the water-saturated solidus and things can melt

melting divergent plate boundaries

Adiabatic rise of mantle with no conductive heat loss lowers the pressure but maintains the temperature. this leads to Decompression partial melting of at least 30%.

two processes that create melt in subduction and mid ocean ridges

• Flux melting (subduction zones)

• Decompression melting (mid ocean ridges)

• Two types of magmas in the mantle 

• Tholeiitic and alkaline basalts-both generated beneath the ocean

•  minerals that have a water percentage sufficient to be called a volatile and subduct in subduction zones to create melt

Serpentine, amphibole and chlorite

fractional crystallization process

• Magma starts with basalt then andesite then rhyolitic

• Increasing Si Na and K concentrations 

• Decreasing Mg Fe and Ca concentrations

• mafic to felsic

magmatic systems inn the crust (draw these)

anatomy of a silicic volcano (draw this image)

significance of rheological lock up

• Melts can only erupt when they are hotter than the point of rheological lockup

  • Crystals in this melt will record temps in the magma chamber when erupted

• tells us when crystal volume reaches roughly 40% to 60% in a melt, so it cannot erupt/flow

structural relaxation time (and how it relates to glass)

• process by which a substance tries to reequilibriate its structure in order to account for a change in pressure, temp, ect.

• When the temperature of a substance drops, the volume will have a smaller sudden drop, but then a over a longer period of time the volume will drop slower until it reaches an equilibrium

  • A glass does not not relax, and therefore is quenched 

what is crustal residence time and how can we obtain these times

crystal residence time is the amount of time that a crystal spends suspended in a melt

we can get these time values through “crystal clocks;” this is where changes in temperature, pressure, water content, or fresh magma injections leave distinct chemical or textural "growth rings".

scientists use diffusion chronometry

glass transition temp and what it is for three types of magma

• The temp range at which a substance changes from a glassy to a rubbery state

• Above this temp, the material is flexible and can change in energy

• Transition temp for a basalt in 900 C

• For an andesite is 800 C

• For a rhyolite is 700 C

what's magma?

Molten rock, plus solids and dissolved gases or gas bubbles

magma is propelled by __ in an eruption

volatiles

three types of volcanic gasses

volcanic

magmatic

hydrothermal

solubility of gasses ___ when pressure drops

decreases

passive vs. active degassing

passive-quiet, slow degassing through fumaroles, vents, ect. without explosions

active-explosive outburst of volatiles due to gas pressure build up

composition of volcanic gasses

95% h2O

1.6% CO2

1.3%SO2

.4% H2S

.7% HCl

magma volume ___ from rhyolite to andesite to basalt

decreases

more SO2 emissions means ___ lava erupted

more

degassing volcano (draw)

2 key ratios in volcanic degassing and what they tell us

CO/CO2-as this increases, Fumarole outlet temperature increases

H2/H2O-same as above

also tell us oxygen fugacity rates which can predict eruption potential and magma ascent dynamics

CO2/SO2 ratio graph (draw)

excess sulfur problem and why it matters

volcanoes emit far more sulfur during eruptions than can be accounted for by measuring the sulfur dissolved in erupted magma

This matters because relying strictly on traditional petrologic tools can underestimate the true volatile and explosive potential of a volcanic system.

CO2/SO2 ratio

CO2/SO2- ratio increases with more pressure and more depth within the earth

• If the CO2/SO2 ratio decreases, this means that magma is moving toward the surface

• In order to raise the ratio, new magma must enter the system which has not yet degassed and lost its CO2

CO2 is ___soluble than SO2 in a melt

less

• CO2 degasses ___ H2O when magma is rising

before

CO2 vs. H2O degassing paths

CO:Low solubility (exsolves early/deeply)

H2O: High solubility (exsolves late/shallowly)

factors that affect flow regimes

• volatile/gas content

• Magma bulk viscosity

• Conduit geometry

• Rise velocity/ascent rates 

• Water degassing ___ the viscosity of the melt, inducing crystallization by ___the liquidus temperature 

• increases

• raising

• At any pressure, rhyolite has ___ water content than basalt 

more

• Higher confining pressure=magma can dissolve ___ volatiles 

more

• Eruption style is determined by…

gas content and how the gas separates from magma 

how gas separates from magma (4 ways)

• “bubbly”=passive

  • Gas bubbles carried by magma

• “slug”=explosive

  • Gas pockets that look like slugs 

• “Annular”=fountaining 

  • Looks continuous 

• “Dispersed”

  • Magma fragments carried by gas

  • Looks like inverse bubbly 

High silica content=___ explosive eruptions

more

fragmentation

• -break apart of vesicle rich magma into solid, gas, liquid 

two types of fragmentation

• Rapid acceleration

  • vesicle form as a result of magma moving upward and depressurizing

  • as the magma continues to move up, the vesicles expand until the point that the magma cannot hold them anymore, and fragments, leading t explosive eruptions

  • Hawaiian and plinian style eruption 

• Rapid Decompression 

  • Top down process

  • Plinian style eruptions

  • sudden decompression from dome collapse leads to fragmentation of magma beside the pressurized gas inside the magma cannot reequilibrriate fast enough

high viscosity magma=more___explosions

ashy

eruption syle diagram

eruptive styles in order of least to most eruptive (4)

hawaiin

Strombolian

vulcanian

plinian

plinian eruption

• Very explosive

• Intermediate to felsic eruption

• Rhyolite or dacite magmas 

• More viscous

• Associated with Calderas

Strombolian eruptions

• Mafic

• Cinder cones

• explosive

Hawaiin eruptions

• Lava flows

• Mafic melts 

• Basalts

• Fissure eruptions-magma that bubbles up from a joint 

• Effusive

vulcanian eruptions

• Glass in an eruption can create plugs

• Plugs can create domes

  • The domes build up pressure and when erupted they are strong and fast 

  • These eruptions are plinian style (usually)

run up vs. repose time and what they are affected by

run up=time of precursory unrest before an eruption

repose=time between eruptions

affected by viscosity, silica content, magma recharge rate

match each flow regime to its eruption

bubbly-passive degassing

annular-fountaining

slug-strombolian

dispered-plinian

volatile content of a magma is influenced by…(two things)

determined by sources + pressure (i.e., conduits and reservoir conditions)

as pressure decreases, does gas volume in a magma go up or down

answer: gas volume goes up when pressure decreases because the volatiles exsolve and form bubbles