Isostasy and volcanoes

Question

1° Is there a root beneath volcanoes?

2° Are volcanoes balanced/ in equilibrium ?

Answer 1

Volcanic eruptions often give birth to volcanic chains of mountains. That depends on the nature of the lava flowing(i.e. you could also have a volcanic plateau). http://en.wikipedia.org/wiki/Isostasy#Is...

Answer 2

1.  I'm not sure what you mean by a "root".  There is a magma chamber beneath both active and dormant volcanoes, but I would not call it a root.

2.  Volcanoes are unbalanced and not in equilibrium, that is almost a functional definition of a volcano.  An extinct volcano (as opposed to one which is merely in a dormant period) could be deemed to have reached equilibrium, in that it is no longer growing or likely to do so in the future.

As a general comment on your question - continental crust is always thicker in mountainous areas due to the "plastic" (i.e. semi liquid) nature of the mantle beneath.  As tectonic processes (over thrusting, folding, etc.) cause the mountain chain to rise into the air, it is also becoming thicker and sinking down into the mantle at the same time.  Some laymen use the term "root of the mountain" to generally refer to this fact, but it is only due to the mass of crust bearing down on the softer, pliable materials below (similar in many ways to ice floating in a glass).  More to the point of your first question, as a volcanic cone builds up above the surface of the earth, the crust becomes thicker both above ground level and below.