Mount Rainier is a stratovolcano, which means it's formed by layers of ash and lava flows. The lava that forms Mount Rainier is andesitic. This type of lava is:

* Intermediate in composition between mafic (basaltic) and felsic (rhyolitic) lavas.

* Viscous, meaning it flows slowly and tends to solidify quickly.

* Often explosive, due to its high gas content.

The andesitic lava from Mount Rainier has created the characteristic steep slopes and cone-shaped form of the volcano. The slow-moving lava has also built up layers of ash and other volcanic materials, contributing to its impressive height.