Here's why:

* Mount Rainier is an andesitic volcano. This means its magma (and therefore lava) is rich in silica, making it thicker and stickier.

* High viscosity lava flows slowly. It tends to build up steep, cone-shaped volcanoes like Mount Rainier. This is in contrast to low viscosity lava, which flows more easily and creates flatter, shield volcanoes.

While Mount Rainier's lava is high in viscosity, it's important to remember that volcanic eruptions are complex events. The actual viscosity of a lava flow can vary based on factors like:

* Temperature: Hotter lava is less viscous.

* Gas content: More gas makes the lava more fluid.

* Composition: The specific mineral content can impact viscosity.

Therefore, the viscosity of Mount Rainier's lava can vary from eruption to eruption, but overall, it is considered high viscosity.