Why are Martian volcanoes so lonely?

One of the first Mariner 9 images of the surface of Mars. The poor image quality is due to a planet wide dust storm occuring when the probe arrived. (Image courtesy of NASA/JPL)

Martian volcanoes were confirmed to exist in the early 1970s, from pictures taken by the Mariner 9 orbiter. These first images revealed massive shield volcanoes, like Hawaii on Earth but taller than Mount Everest and wider than Ohio.  One of the first observations was that three of these larger volcanoes, the Tharsis Montes, exist in a straight line, each separated by about 750 km. Rather than a volcanic chain like Hawaii, the line of the Tharsis Montes appears to be related to a rifting feature, but why are they spaced with a certain distance?

Topographic map of the Tharsis region of Mars. (Image courtesy of NASA)

My research focuses on simulating one idea for why the spacing exists. To start I use a computer program to replicate magma flowing up from the hot, deep inside of the planet to the cooler outside layer of the planet. The magma flows upwards because it is less dense than the rock around it, like how an inflatable beach ball raises to the surface of a pool because the air in the ball is less dense than the water. As the magma raises into the cooler layer of Mars it starts to freeze into solid rock. Eventually it freezes too much and no more magma can move up. This point is known as the permeability barrier.

Nature isn't perfect, so the permeability barrier would not be a completely flat layer. There would be points where the barrier may be a little higher than the surroundings. The magma beneath this layer would flow towards these points, like rainwater flowing towards the lowest point on the street. The barrier would be eroded by the heat of the magma at these points, raising it at some rate. All these little instabilities would grow and eat each other until only a few large ones would remain. At the peaks of the large instabilities focused magma may break through making their way to the surface, forming volcanoes.

2-D figure of volcano formation on Mars. Black dashed line represents originally flat permeability layer, solid black line represents later permeability barrier with the volcano wavelength, red lines represent magma flow direction, and green line represent heat flow where longer lines indicate more heat. (Image courtesy of Laurent Montési)

Overall the final system would look like a connected line of upside down funnels. The sides of the funnel are the permeability barrier that magma flows up and the spout is where the magma breaks through to the surface. The series of touching funnels is the wavelength, or spacing, of volcanoes. In summary, Martian volcanoes aren't really lonely, they're holding hands deep underground.

A line of funnels. The sloped funnel is like the permeability altered permeability barrier and the tip of the spout would be the volcano on the surface. (Image courtesy Walmart)