The toxic soup of volcanic gases (carbon monoxide, carbon dioxide, sulfur dioxide, hydrogen sulfide) is familiar to most everyone. Today I learned about a new one being produced on La Palma in the Canaries:
One river of lava reached the ocean near Playa de Los Guirres on Sept. 28. It poured off a 300-foot-tall cliff into the seawater below, prompting authorities to urge residents to remain indoors with their windows closed to limit the entry of outside air. When lava enters the ocean, it heats up seawater extremely rapidly, splitting water molecules into hydrogen and oxygen ions. Some of the hydrogen combines with chlorine ions in the seawater to form hydrochloric acid and produce a gas that is toxic when inhaled.
You learn something every day (though hydrochloric acid shouldn't exist as a gas - perhaps they mean hydrogen chloride, or else the HCl is aerosolized as inhalable droplets).
Image cropped for size from the original.