Here's why:

* Andesite is an intermediate type of lava, meaning it has a composition between basalt (mafic) and rhyolite (felsic). This composition is common for volcanoes situated along subduction zones, like Mount St. Helens.

* Subduction zones are areas where one tectonic plate slides beneath another. This process melts the crust and mantle, creating magma that is richer in silica and other elements than basalt. This results in the formation of andesite lava.

* Mount St. Helens sits above the Juan de Fuca Plate, which is subducting beneath the North American Plate.

While andesite is the dominant lava type, Mount St. Helens has also erupted dacite in the past, another type of intermediate lava that is slightly richer in silica than andesite.