Here's why:

* Dacite is an extrusive igneous rock with a silica content ranging from 63% to 68%. This high silica content makes it relatively viscous, meaning it flows slowly and tends to form steep, dome-shaped structures.

* Low-silica lava, such as basalt, is much more fluid and flows quickly, creating flatter features like shield volcanoes.

Mount Hood's history of eruptions, characterized by explosive events and the formation of lava domes, points to a high silica content. The high silica content in dacite contributes to its explosiveness, as dissolved gases trapped within the viscous magma build up pressure until they release in a forceful eruption.