EART 160: Planetary Science
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Transcript of EART 160: Planetary Science
EART 160: Planetary Science
Last Time
• Paper Discussion– Stevenson (2001)
• Planetary Surfaces
• Impacts– Morphology– Mechanics– Ages of Planetary Surfaces– Frequency and Consequences
Today
• HW 2 due Today– How are people doing?
• Planetary Surfaces– Volcanism– What controls where and when volcanism
happens?
Volcanism
• An important process on most solar system bodies (either now or in the past)
• It gives information on the thermal evolution and interior state of the body
• It transports heat, volatiles and radioactive materials from the interior to the surface
• Volcanic samples can be accurately dated
• Volcanism can influence climate
What is it?
• The eruption of magma from the interior of the planet onto the surface, forming new rock.
Phase Diagrams
Why does it happen?• Material (generally
silicates) raised above the melting temperature (solidus)– Increase in temperature
(plume e.g. Hawaii)
– Decrease in pressure (mid-ocean ridge)
– Decrease in solidus temperature (island arcs)
Temperature
Dep
th
solidus
liquidus
Reduction in pressure
Increase in temperature
No
rmal tem
peratu
re
pro
file
Reduction in solidus
Composition• Mantle material: peridotite• Partial melting of (ultramafic) peridotite mantle produces basalt
(mafic magma).• More felsic magma (e.g. andesite, rhyolite) requires more melting,
fractional crystallization
• Low-temperature minerals (e.g. silica) melt first• Magma becomes more felsic with time• Ultramafic rocks no longer form today
Composition
Mode of occurrence Felsic Intermediate Mafic Ultramafic
Intrusive Granite Diorite Gabbro Peridotite
Extrusive Rhyolite Andesite Basalt Komatiite
Solidus TemperatueSilica content, Viscosity
Eruptions• Magma is often less dense than surrounding rock (why?)• So it ascends (to the level of neutral buoyancy)• For low-viscosity lavas, dissolved volatiles can escape as
they exsolve; this results in gentle (effusive) eruptions• More viscous lavas tend to erupt explosively• We can determine maximum volcano height:
d
h
c
m
mmcdh /)(
What is the depth to the melting zone on Mars?Why might this zone be deeper than on Earth?
Cooling timescale• Conductive cooling timescale
depends on thickness of object and its thermal diffusivity
• Thermal diffusivity is a measure of how conductive a material is, and is measured in m2s-1
• Typical value for rock/ice is 10-6 m2s-1 d
hot
cold
Temp.
• Characteristic cooling timescale t ~ d2/• How long does it take a metre thick lava flow to cool?• How long to boil an egg?• How long does it take the Earth to cool?
Types of Volcanoes
• Large volcano• Shallow slopes• Built up by multiple flows of
low-viscosity magma
• Built up by solid fragments (ash) ejected from volcanic fent
• Steep• Small (< 1 km high)
Shield Volcano Cinder Code
Pancake Domes on Venus
Magellan Radar Images
65 km
High-viscosity, silica-rich magmaHigh atmospheric pressureWhy do they get so big?
Global resurfacing ~750 Mya
Tharsis Rise on Mars
Up to ¼ of the planets surfaceCentered on equator (why?)
Olympus MonsTallest volcano in SS27 km high
Lunar Maria
• Giant impact basins formed during LHB (4.0 Gya)
• Filled with basaltic lava (3-3.5 Gya)
• Near-side ONLY
Rilles
• Lava Channels– often lead back to vent
• Classified by shape– Sinuous– Linear– Arcuate
Prinz Crater – Apollo 15
Mercury
• Smooth Intercrater Plains• Floor of Caloris Basin• Similar to Lunar Maria
Io• Volcanism is basaltic – how
do we know?• Resurfacing very rapid,
~ 1cm per year• What is the eruption
speed?
April 1997 Sept 1997 July 1999
400kmPele
PillanGalileo images of overlapping deposits at Pillan and Pele
Pele
Loki
250km
Io
The lavas of violent Io,Though they may look like pico de gallo
Erupt and then rainOn the sulfurous plain
Looking nothing at all like Ohio.
Tupan Patera -- Galileo
Tvashtar Plume – New Horizons
Eruption Speed
g
vH
2
2
CryovolcanismRock IceMagma Water
Why is this hard?
Schenk et al. Nature 2001
Lobate flow(?)
Caldera rim
This image shows one of the few examples of potential cryovolcanism on Ganymede. The caldera may have been formed by subsidence following eruption of volcanic material, part of which forms the lobate flow (?) within the caldera. The relatively steep sides of the flow suggest a high viscosity substance, possibly an ice-water slurry (?).
Examples
Fountains of Enceladus -- Cassini
Lineaments on EuropaLike Mid-ocean ridges?-- Galileo
Ganesa Macula on Titan-- Cassini
Nitrogen Geysers on Triton-- Voyager 2
Next Time
• Planetary Surfaces– Tectonics