[karajorma]

Originally posted by Grey Wolf 2009
That's what I get for a quick web search. You're right, Lightspeed, it is tidal stresses from Jupiter that cause Io's volcanic activity.

However, IIRC, heat from decaying radioactive isotopes play a role in heating the cores of planet....

Yep. They play a major role. Io is in fact something of an oddity because it's vulcanism has nothing to do with radioactivity.

[Lightspeed]

The cores usually are not *that* radioactive. The heat primarily comes from the time they were formed, and they contiunously lose warmth to the surrounding space.

Now, volcanic activity can happen due to a lot of causes. It can start through tectonic activity, gravitational influence, meteor impact, planet climate conditions, etc.

Now for mars it (probably) has the following reasons:

* Less own gravity than earth - lava can more easily be transported to the planet's surface

* Mars volcanoes are fix spots since the surface doesn't move (no tectonic activity)

* The volcanic activity comes from Mars' rotational axis wobbling around

* The crust in the southern hemisphere is thicker - more rock melts on the north side and the landscape sinks till an isostatic balance is reached. This causes gravitational anomalies and thus - volcanism.