Intense heat from lava entering seawater initiates a chemical reaction between salt ions dissolved in seawater and water which results in hydrochloric acid (HCl). Geologists love to carry small bottles filled with this substance because it makes a great tool for testing carbonate rocks. Actually, we all carry it along because this is the acid that fills our stomachs. It is diluted, though, but so are acid in geologist’s bottles and also the cloud you see on the picture below.
These “smokes” are mostly composed of water vapor but they also contain a significant amount of hydrochloric acid. They rise from the place where lava flows onto the sea bottom from a lava tunnel. These white acidic plumes are also known as laze (lava + haze). Picture taken in Hawai’i.
Lava itself was not visible but you can see the same flow about 6 kilometers “upstream” in this post: Flowing lava of Pu’u O’o.
This place is surrounded by kilometers of absolutely fantastic and otherworldly landscape.
Be careful when you venture near “smokes” like that because they can be hazardous. It is not recommended to expose your lungs, eyes, etc. to it. People have lost their lives because of acidic laze clouds.