Here's why:

* Volcanic Explosivity Index (VEI): Mount Shasta has a history of eruptions that are classified as moderate to strong (VEI 3-4), which is a characteristic of andesite eruptions.

* Composition: Andesite is an intermediate volcanic rock with a silica content between 52% and 63%. This composition leads to eruptions with moderate viscosity and explosive potential, aligning with Mount Shasta's history.

* Physical Properties: Andesite lava flows are typically viscous, meaning they flow slowly and form steep-sided cones like Mount Shasta. The magma also has a higher gas content, contributing to the explosive nature of eruptions.

It's important to note that while andesite is the dominant magma type, there is evidence suggesting that Mount Shasta has also erupted with dacite magma, which is even more silica-rich.