St. Vincent volcano: massive eruption sends ash to 20,000 ft
A strong explosion occurred this morning at the volcano at 08:40 am local time. The eruption produced an ash column that rose several km above the active lava dome.
The Belmont volcano observatory indicated the height of the ash plume as high as 8 km (24,000 ft), while the Volcanic Ash Advisory Center of Washington issued alerts of ash to 20,000 ft. The plume drifted east from the island.
Ash fall occurred at the Argyle International Airport and all persons in the red hazard zone, which comprises the northern third of the island, were asked to evacuate immediately. In Georgetown alone, 1,700 people needed to leave their homes.
Change in eruptive behavior
This marks a change in the eruptive style of the young lava dome, from growing rather calmly by effusion of viscous lava towards a much more dangerous mix of effusive and explosive eruptions.
Explosive activity so far had only been minor only, even though videos and images taken the day before (attached) show an increase in degassing, moderate ash venting and glowing rockfalls from the dome, suggesting that something was changing. In any case, the interesting question is what caused this apparent relatively sudden (if not brutal) change. It is not easy to answer this without closer investigation of the eruption products and other parameters only scientists on location have access to, but could help reveal the cause of today's explosion.
One obvious possible reason could be the arrival of more gas-rich magma, which should reflect in a different composition of the eruptive products. Or was it simply that gas-rich parts of the dome's interior, or even trapped pockets of gas had slowly grown beyond a threshold, literally blowing off the cork today?
Certainly, the fact that volume of the accumulated lava in the dome has continued to grow facilitates the process of trapping pockets of gas or gas-charged hot rocks inside. In fact, both explanations can be at work, but we can so far only speculate on this.
It also follows a pattern frequently observed at dome-building volcanic eruptions, such as at Sinabung since 2013, or as a geographically very close example, Soufrière Hills on Montserrat during 1995-2000, but also many others. In the first stage, the eruption is mainly effusive and often brusquely turns to a mix of effusive and explosive activity.