The lower the silica content, the more fluid the lava. It moves along the ground in long streams, such as this Hawaiian flow. Mount St. Helens has had eruptions in the past like this, such as the one that formed Ape Cave. It often forms a mound of lava called a lava dome. A volcano's shape is usually determined by the type of lava it erupts most often.
Basalt lava builds long, broad volcanoes called shield volcanoes.
The stiffer lavas, dacite, andesite, and rhyolite, form steep-sided volcanoes called composite volcanoes. Mount St. Helens is a composite volcano. Magma can produce a very explosive eruption if it has a high silica content and a high concentration of gas. This explosion breaks the magma into pumice and tiny ash particles. This happened during the May 18th eruption of Mount St. Helens.