Mount Rainier, the highest peak in the Cascade Range and the most glaciated peak in the contiguous United States, formed over millions of years through a process called volcanic activity. Here's a breakdown of how it came to be:

1. Subduction Zone: The Cascade Range, including Mount Rainier, is situated above a subduction zone, where the oceanic Juan de Fuca Plate is diving beneath the North American Plate.

2. Magma Generation: As the oceanic plate descends, it melts due to the intense heat and pressure. This molten rock, known as magma, rises towards the surface.

3. Volcanic Eruptions: The magma eventually erupts through the Earth's crust, forming volcanoes. Mount Rainier is a stratovolcano, a type of volcano characterized by steep slopes and layers of lava flows and ash.

4. Eruptions and Growth: Over time, numerous volcanic eruptions, some explosive, some effusive, built up the cone-shaped mountain we see today. These eruptions deposited alternating layers of lava, ash, and volcanic debris.

5. Glacial Shaping: In addition to volcanic activity, glaciers have played a significant role in shaping Mount Rainier's distinctive features. During the Ice Age, massive glaciers carved out valleys and sculpted the mountain's flanks, creating its characteristic glacial features like cirques, moraines, and valleys.

Mount Rainier is still an active volcano, although its last major eruption was approximately 100 years ago. It continues to pose a threat of future eruptions and is closely monitored by scientists.