What's Up With the Quakes Before a Volcano Erupts?

EARTHQUAKES AND VOLCANOES are intimately linked. They are both products of the plate tectonic process that is constantly reshaping the surface of the Earth. But even beyond that, earthquakes are some of the best clues we have that a volcano might be heading towards an eruption. So, why are there earthquakes in the days-months-years before a volcano erupts and why do some earthquakes lead to an eruption and some don’t? Well, it is a little more complicated than it seems.
The "Whaleback" at Mount St. Helens seen in February 2005. The movement and eruption of this slug of magma generated earthquakes under St. Helens

Erik Klemetti on this issue on Wired >>