Gravitational invluence of glaciers

[Aschere]

I remember listening to the Naked Scientist radio show a while back on the topic of climate change, and one way, they said, melting glaciers could pose a threat was that, due to their humongous mass, could relieve pressure on the crust when melted and open up the possibility for easier plate movement, e.g. more earthquakes, more volcanic activity and so on.

I was thinking, wouldn't it increase the pressure on the plates, given that a glacier's weight, in water form as opposed to ice, would weigh more? Or perhaps I'm thinking of density...

 

[billiards]

Well the glaciers are in some places above sea level, a thick chunk of ice will weigh more than a thin/no layer of water.

You might expect an increase in seismicity as the lithospere readjusts, I don't see how this would affect volcanism.