Final answer:
Mt. Rainier is predominantly made of igneous rocks such as basalt and granite, comprised mainly of oxygen and silicon which are abundant in the Earth's crust.
Step-by-step explanation:
Mt. Rainier is mostly made of igneous rock, including oceanic basalt and continental granite. These types of rocks originate from a molten state and are common in the Earth's crust, especially in volcanic areas like Mt. Rainier. Considering that Mt. Rainier is a stratovolcano, it is composed of layers of hardened lava, tephra, pumice, and volcanic ash. The Earth's crust, including the area around Mt. Rainier, is abundantly composed of elements such as oxygen and silicon, which are also the primary components of igneous rocks like granite and basalt.