Beacon Rock was formed by lava flows, not a single volcanic eruption, and was subsequently eroded by the Columbia River.

Here's a breakdown:

* Formation: Beacon Rock is a volcanic plug, meaning it was formed by the slow cooling and hardening of lava within the vent of a volcano. This lava was part of a series of flows from the Cascade Range volcanoes millions of years ago.

* Erosion: Over time, the Columbia River carved its path through the surrounding landscape, eroding the softer rock around Beacon Rock and leaving the hardened lava plug standing tall.

So, while Beacon Rock was formed by volcanic activity, the Columbia River is the main force responsible for its distinctive shape and prominence.