Structural Collapse at Poás Volcano Triggers Major Lake Surge and New Gas Outlets

Costa Rican volcanologists report a structural collapse at Poás Volcano, causing a sudden ash plume, a three-meter rise in lake levels, and new gas vents.

By: AXL Media

Published: Apr 17, 2026, 9:52 AM EDT

Source: The Tico Times

A Shift from Explosive to Structural Activity

Volcanologists from the Volcanological and Seismological Observatory of Costa Rica (OVSICORI) have identified a significant change in the behavioral patterns of Poás Volcano. Unlike the traditional magmatic eruptions observed in 2025, the April 10 event was driven by a structural failure in the western sector of the crater lake. Part of the crater wall, weakened by ongoing hydrothermal activity, slumped into the lake. This mass of material, estimated at 20 to 30 meters deep, triggered a rapid decompression of underground gases, resulting in a substantial ash plume that reached residents in the surrounding Sarchí region.

The Physics of the Three-Meter Lake Surge

One of the most striking outcomes of the collapse is the sudden rise of the crater lake’s water level. During field inspections this week, scientists confirmed the water had surged upward by approximately three meters. According to OVSICORI volcanologist Geoffroy Avard, this increase cannot be attributed to recent rainfall. Instead, the volume of material falling from the crater wall effectively changed the shape of the lake bed, displacing the acidic water upward. This physical displacement underscores the massive scale of the structural slump and its immediate impact on the crater's internal geography.

Emergence of New Fumarole Fields

The internal reorganization of the volcano has also altered its gas emission points. The inspection revealed a brand-new field of fumaroles in a sector where gas output was previously negligible. Rather than a singular new vent, scientists observed a cluster of several gas outlets appearing simultaneously. Conversely, a sulfur-rich zone on the eastern side of the lake that had been a focal point of activity for months has shown signs of weakening. This shift suggests that the internal plumbing of the hydrothermal system has migrated from the eastern to the western side of the crater following the collapse.