Why do volcanoes near subduction zones often erupt more explosively than hot spot volcanoes?

At subduction zones, one tectonic plate sinks beneath another and carries water-bearing minerals into the mantle. That water lowers the melting point and helps generate magma that is often richer in silica and dissolved gases. Because the magma is thicker, gas gets trapped and pressure can rise sharply before eruption. This is why many Cascade volcanoes, such as Mount St. Helens and Mount Rainier, are capable of explosive eruptions.