Types of volcanic constructs:

• shields:
  • formed primarily from flows.
  • steeper upper slopes and shallower lower slopes.
  • characterized by collapse caldera (large than typical on earth).
• tholi;
  • dome shaped.
  • smaller.
  • more viscous magma?
• lowland paterae:
  • even lower angel slopes than shield volcanoes
  • complex summit calderas.
  • Alba Patera is one of the better known ones.
• highland paterae:
  • same as lowland except also characerized by complex radiating channel systems.

Note - what volcanic forms common on earth seem to be missing on Mars?

Are pyroclastic deposits evident on Mars:

• the erodability of many of the volcanic structures indicates so.
• deposits with great lateral mobility (400km for Alba Patera!) also indicates so.
• morphology indicates these were debris flows, not air fall.

Chassigny meteorite:

• fell in France in 1852.
• an olivine cumulate (dunite), Frankel (1996)
• microscopic bubles of gas with content similar that of the atmospher of Mars (Frankel, 1996)
• how did atmospheric gases get trapped in a cumulate?

Nakhlites:

• first example found near Nakhla, Eygpt.
• a pyroxene cumulate.

Shergotites:

• named after Shergotty, India.
• basaltic, volcanic.

SNC - meteorites from Mars. 12 or so 'suspected' Martian meteorites are known.

What are cumulates?

"On Mars, low gravity and low atmospheric pressure at the surface result in a crustal bulk density profile different from other planets (Wilson & Hed, 1994), which means that magma reservoirs are predicted to be deeper than on earth by a factor of four." Head & Coffin, 1997, Large Igneous Provinces: A Planetary Perspective; in Geophysical Monograph 100, American Geophysical Union, p. 420.

How did cumulates get excavated? How did fluid inclusion samples of Martian atmosphere get incorporated?

One distinctive aspect of Martian volcanoes are the large calderas. For Mons Olympus the caldera complex is 80 km across. What are the implications of this large size for plumbing?

One interesting 'problem' is that the flows that should be associated with large scale caldera collapse are not really evident!

Are some Martian flows komatiites?

• ultramafic and very fluid flows.
• require a greater percentage of partial melt (circa 40%).
• since Mars is a smaller body, it overall has less heat content and might be expected to have had smaller percentages of partial melt. However, the lithostatic gradient is also less, because of Mar's smaller mass, and this can increase the ease of partial melting by reducing the inhibiting effect of pressure on partial melting.
• tend to be sulfur rich.

Cooling rates - how does a lava flow cool?

• transfer of heat to atmosphere and or hydrosphere by complex processes.
• radiative blackbody cooling.
• Mars is colder.
• however, with less of an atmosphere cooling rates could be slower..

• earlier ash flows and lava plains
• later bulges and shield volcanoes up until maybe 300 million years ago (see handout).
• may be relationship between changing degree of recycling fluids in crust and diminishment of pyroclastic activity.

What controls how high martian volcanos can grow?

In depth and recent source on Martian volcanism: Peter Cattermole, 1996, Planetary Volcanism, Wiley.

Reading for next time:

A. D. Fortes, Discriminating Between Models for the Formation of the Northern Lowlands of Mars.

Head and Wilson, 2002, Tharsis Radial graben systems as a surface manifestation of plume related dike intrusion complexes ... JGR. Don't read in detail, but focus on abstract and diagrams. Spend an hour and see what you can learn in that time.