The Shattered Summit.

On May 18, 1980, the north face of Mount St. Helens collapsed in the largest terrestrial landslide in recorded history, triggering a cataclysmic lateral blast.

Unlike the gentle flows of Hawaii, Mount St. Helens is a **Stratovolcano** (or composite volcano). These are fueled by viscous, silica-rich magma that traps gas under extreme pressure. When the pressure exceeds the strength of the rock, the result is a **Plinian eruption**—a violent explosion that can propel ash columns 20 miles into the stratosphere.

The Anatomy of a Collapse

In the months leading up to the eruption, magma rising within the volcano created a massive "bulge" on its northern flank. When a magnitude 5.1 earthquake struck, the bulge failed. The sudden removal of the overlying rock's weight caused the superheated water and gas within the magma to flash into steam, exploding sideways at speeds exceeding 600 miles per hour.

Lahar and Pyroclastic Flows

The eruption didn't just produce ash. It created **Pyroclastic flows**—seething clouds of hot gas and volcanic matter that behave like a fluid, incinerating everything in their path. Simultaneously, the heat melted glaciers on the summit, triggering **Lahars** (volcanic mudflows) that choked the surrounding river systems with concrete-like debris.