Here's why:

* Andesite is a type of volcanic rock that is intermediate in composition between basalt (mafic) and rhyolite (felsic).

* Mount Fuji is a stratovolcano, which are known for their explosive eruptions, and andesite is a common magma type found in these volcanoes.

* The magma produced by Mount Fuji is slightly viscous and contains a moderate amount of silica, leading to eruptions that are often explosive but also produce lava flows.

While it's primarily andesitic, there can be some variation in the composition of Mount Fuji's magma depending on the specific eruption and location within the volcano. But, andesite is the dominant type.