Re: Why do volcanos erupt only once in a while?

First, you must remember that Earth isn't just a solid ball where
everything sits still.  It is constantly in motion, with the plates
of the crust sliding against one another, being driven upward, being
driven downward.  Pressure deep in the earth is generating heat,
so there is always magma being formed as rock is melted.

Now, as for why volcanoes do not erupt all the time, it is because
a volcano can be thought of as the planet's "relief valve."  Magma
presses its way up against a weak spot in the crust, and when it 
finally breaks through, it spews out for a while until the local
pressure decreases.  Once the force of the eruption slows, the
cooling magma solidifies into rock, and the whole thing dies down...

...until later, when the pressure builds up again, and the magma starts
looking for a way out once more.

Think of it as blowing up a balloon.  You can only puff so much air out
into it before you just don't have any pressure left inside.