Martian Plate Tectonics - or not

[MigL]

Well while we've got you here Ophiolite...

 

I was aware that Mars does have a molten core because it shows evidence of volcanic activity.

However, Earthly volcanos show a linear motion through time, such that the volcano creates a 'crest'.

On Mars they don't, a volcano will happily sit in one location, indicating that there is no plate movement.

 

If I'm mistaken in my assessment, please correct me and elaborate.

If I'm correct, why is there no plate motion over the molten core ?

 

And if this causes a thread derailment, please split off.

.

[pavelcherepan]

That's mostly because Mars doesn't seem to have or ever had plate tectonics. On Earth either plate moves over I "hot spot" like in case of Hawaiian volcanoes and this creates a linear stretch of volcanoes, or in subduction zones due to change of subduction regime over time (for example the angle of subduction) the area where molten material encounters the surface shifts over time and then you have a series of volcanic ranges, like in case with Kamchatka peninsula in Russia.

[MigL]

OK, but why no plate tectonics on Mars ?

So, elaborate.

( or are you gonna make a lazy guy do some searching ? )

[pavelcherepan]

It's a grey area. For starters there's still no universally accepted theory as to how plate tectonics started here on Earth. I'll post some links to papers later when I get to a computer.

[Ophiolite]

I was aware that Mars does have a molten core because it shows evidence of volcanic activity.

However, Earthly volcanos show a linear motion through time, such that the volcano creates a 'crest'.

 

Plate tectonics on Earth does not take place over a molten core. It takes place over a solid, though mobile and partially molten asthenosphere, part of the upper mantle. The high temperature core is probably implicated, ultimately, in the process, but the precise connection has not yet been established.

 

It is true that some"Earthly volcanoes show a linear motion through time", but not the majority of terrestrial volcanoes. You are likely thinking of examples like the Emperor Seamount/Hawaiian Islands chain, or Yellowstone calderas.

 

In contrast the volcanoes running the western length of the Americas remain in place over the subduction zone that created them, while the upwelling magma is responsible for the tens of thousands of miles of mid-ocean ridge.

 

Pavelcherepan said:

That's mostly because Mars doesn't seem to have or ever had plate tectonics.

 

Not necessarily true. This is an extract from some notes I made that relate to the topic:

 

Possible Plate Tectonics on Mars

 

Southern Hemisphere Plate Tectonics

 

During the aero-braking maneuver to place it in orbit, Mars Global Surveyor detected linear magnetic anomalies in the southern hemisphere of Mars (i). Researchers, using the data from two full Mars years of polar orbital data and with an improved technique to eliminate the effects of external fields, constructed a global map of remnant crustal magnetization (ii) (I note in passing that the strength of this magnetism is more than an order of magnitude greater than comparable magnetism on the Earth.)

 

The similarity between these parallel patterns of magnetic reversal on Mars and those flanking mid-ocean ridges on Earth, immediately suggested a Martian equivalent to sea floor spreading. The patterns are absent from large impact basins, such as Hellas and Argyre, and from the volcanic terrain of the Tharsis bulge. This is consistent with the loss of the Martian dynamo early in history, before the final Heavy Bombardment Phase and Tharsis volcanism, but after crustal formation.

 

The researchers provisionally identified two major faults from offsets in the magnetic patterns. The character of these faults shows them to be transform faults, rather than simple strike slip faults. “The great faults in Meridiani are consistent with the properties of transform faults and define an axis of rotation (23°S and 80.5°E) describing the relative motion of two plates, north and south of the equator. The separation of the faults (1,200km) and offset of the putative ridge axis (+/-240km) in Meridiani are comparable with what is observed along ocean ridges on Earth.”

 

The Sound of One Hand Clapping

 

Zhong’s explanation (iii) for the crustal dichotomy (Thick crusted southern upland, thin crusted northern plains) is that uneven convection in a young Mars thinned the northern crust. In the south, hot mantle material rose, cooled, and then sank in the north.

 

This announcement does not seem to offer anything especially new.

• In 2001 Zhong reported that conventional models of mantle convection could not account for an internal explanation of the dichotomy. However, a thicker and weaker asthenosphere would allow a single convection cell, which in turn might allow the apparent thinning of the northern crust by mantle convection currents, or build up the crust in the southern hemisphere.
• In 2004, working with Roberts (iv) he is struggling to generate the required degree-1 convection cell within a stagnant lid setting. They broach the possibility that “The mobile lid regime may be appropriate if the crust and mantle are sufficiently warm as to decouple the crust and the mantle, as one may expect for the early Mars”.
• In 2006, again with Roberts (v), Zhong reached the conclusion, based on further FEA modeling, that “degree-1 mantle convection induced by a layered viscosity structure may be responsible for the formation of the crustal dichotomy.”
• Also in 2006, in another paper, they offered a description of the process “Degree-1 mantle convection develops within the first few hundred Ma. The one-plume structure drives a TPW event that places the plume near the equator. Melt associated with the plume is erupted onto the surface above it, thickening the crust in that hemisphere. This melt cools in the ancient global magnetic field and produces remnant magnetism, consistent with suggested paleopole positions near the present-day equator. As the planet cools the lithosphere thickens, reducing the dynamic topography. When T_e exceeds about 30km, the geoid above the plume becomes negative and the plume rotates the planet such that it is near the south pole.”
• Roberts and Zhong (vi)continue this line in 2007, declaring “that the crustal thickness variations associated with the dichotomy may have driven true polar wander, establishing the north south orientation of the dichotomy very early in Martian history.” In other words, the entire crust was moving relative to the mantle.
• Also in 2007 Zhong (vii) argues the case for degree-1 convection, over the alternate endogenic hypotheses (magma ocean(viii); ‘conventional’ plate tectonics )(ix).

 

(Note: I have referenced only about half of the papers Roberts and Zhong wrote on the subject over this period. A search of the ADS data base will reveal the rest.)

 

Conclusion

These words almost sum up my thoughts: “If Mars ever had plate tectonics, it operated only briefly early in Martian history and was a very feeble mechanism quite dissimilar to Earth’s plate tectonics.” (x) I say almost because if Zhong is correct and the entire crust of Mars moved as one to achieve the current ‘balanced’ orientation – in a manner reminiscent of Hapgood’s hypothesis for the Earth (xi)– then the mechanism is anything but feeble.

 

 

 

Connerney, J.E.P., et al. (1999) Global distribution of crustal magnetization discovered by the Mars Global Surveyor MAG/ER experiment. Science 284, 794–798.

[ii] Connerney, J.E.P., et al (2005) Tectonic implications of Mars crustal magnetism. P.N.A.S. , 102, 14970–14975

[iii] Zhong, S. et al (2000) Degree-1 mantle convection and the crustal dichotomy on Mars. Earth and Planetary Science Letters, Volume 189, Issue 1-2, p. 75-84.

[iv] Roberts, J.H. & Zhong, S. (2004) Degree-1 mantle convection as a process for generating Martian hemispheric dichotomy. Workshop on Martian Hemispheres

[v] Roberts, J.H. & Zhong, S. (2006) Degree-1 convection in the Martian mantle and the origin of the hemispheric dichotomy. Journal of Geophysical Research, Volume 111, Issue E6,

[vi] Roberts, J.H. & Zhong, S. (2007) The cause for the north south orientation of the crustal dichotomy and the equatorial location of Tharsis on Mars. Icarus, Volume 190, Issue 1, p. 24-31.

[vii] Zhong, S. (2007) Understanding the Early Evolution of Mars and the Formation of Crustal Dichotomy. Lunar and Planetary Science XXXVIII.

[viii] Elkins-Tanton et al., (2005) Earth and Planetary Science Letters, 236, 1 –12.

[ix] Sleep, N.H. (1994) Martian plate tectonics. Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. E3, p. 5,639-5,655

[x] Kargel, J.S. (2004) Mars – A Warmer, Wetter Planet Springer-Verlag (Praxis) p.59

[xi] Hapgood, C.H. (1956) The Earth’s Shifting Crust. Museum Press

 

Edited December 15, 2015 by Ophiolite

[MigL]

Very thorough, Ophiolite.

Thank you.