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 Depth solidus liquidus Reduction in pressure Increase in temperature Normal temperature profile 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 occurrenceFelsicIntermediateMaficUltramafic IntrusiveGraniteDioriteGabbroPeridotite ExtrusiveRhyoliteAndesiteBasaltKomatiite Solidus Temperatue Silica 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c mm 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 m 2 s -1 Typical value for rock/ice is m 2 s -1 d hot cold Temp. Characteristic cooling timescale t ~ d 2 /  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 VolcanoCinder Code

Pancake Domes on Venus Magellan Radar Images 65 km High-viscosity, silica-rich magma High atmospheric pressure Why do they get so big? Global resurfacing ~750 Mya

Tharsis Rise on Mars Up to ¼ of the planets surface Centered on equator (why?) Olympus Mons Tallest volcano in SS 27 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 km Pele Pillan Galileo 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 rain On the sulfurous plain Looking nothing at all like Ohio. Tupan Patera -- Galileo Tvashtar Plume – New Horizons

Eruption Speed

Cryovolcanism Rock  Ice Magma  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 Europa Like Mid-ocean ridges? -- Galileo Ganesa Macula on Titan -- Cassini Nitrogen Geysers on Triton -- Voyager 2

Next Time Planetary Surfaces –Tectonics