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