When/why Do Glaciers Move?

Geographically the phenomenon revolving around the movement of glaciers is quite a fascinating one.  In a more basic context, glaciers are able to move when all those materials like rocks, loose ice and snow that are above it apply a magnitude of pressure that is greater than the weight of the ice of the glaciers.  Under the weight of these substances, the glaciers move in a flow like tar, although with an eminently slow pace.  However there is one condition attached to this entire process – in order for the glacier to be exerted pressure on and move in attribution to those pressure forces, it is necessary for the glacier to be quite old in nature.  On an estimate, the distance that glaciers cover in the span of one day is about one fifty feet.  As for their coverage on the earth's surface, glaciers constitute a percentage of 10 percent of the total Earth surface.

"NOVA" says there is a new factor and that is the glaciers which are now on top of rivers of water and thereby moving a lot faster than before. Or the problem is now compounded, for the other answer is correct also. Mine is a Nova up date!
Or the ice is sitting in the water caused by all this melting snow and ice.
AND THAT'S "NOVA"!

Glaciers move when new ice appears and when the ice melts. When  Ice sheet melts and form the glaciers will slowly slide and glide on it. Glaciers moving is not noticable though because they move so slow.