| MadSci Network: Astronomy |
That's a good question, one that has been pondered by planetary geologists quite a bit. The final answer is still out, but it seems unlikely that the hot spot idea works.
One of the reasons that the hot spot model appears unlikely is that it requires plate tectonics so that a part of the Martian surface can slide over the plume of hot material rising up through the mantle. But Mars lacks spreading and subduction zones, which are required for plate tectonics. (Mars does have tectonics, however. The cracking of the crust that created Valles Marineris is an example. In this case, the cracking is probably due to the large mass of the Tharsis bulge nearby.) In fact, to date Earth is only planet known to have plate tectonics.
It is possible that the entire crust of Mars has slid relative to the mantle, but that also seems a bit unlikely. It is difficult to do this and there would probably be a lot of other features lined up the same way, in that case.
So why are the Tharsis volcanoes lined up so neatly? No one seems to know. It could just be coincidence: the human eye is drawn to such patterns, so spotting such an alignment isn't always significant. Or it could be that the physics of mantle plumes allow for linear features. (There is some evidence that they can cause the differences in the hemispheres of the Moon and of Mars. The physics of mantle convection is still being researched quite actively, so stay tuned.)
I'd like to thank Hanna Sizemore and James Roberts for sharing some of their Mars-knowledge with me for this answer.
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