When I climbed the top of Indonesia's highest volcano, Gunung Kerinci (3805m) in West Sumatra, I was first gasping for oxygen. Then as I neared the top's crater, the smell of rotten eggs hit my nose. Who was the culprit of this terrible odour? It came from a column of steam, constantly rising from the crater and wafting over the edge towards us hikers. The mucus membranes of our eyes, mouth and ears were simply itching.
What's That Smell?
In chemical terms the cause of this smell and itch is hydrogen sulphide (H2S). Minor quantities are simply irritating, but - and this may come as a surprise - in large volumes, H2S becomes odourless as it numbs the receptors in our noses and becomes lethal within a matter of seconds.
Gases play a major role in the eruptions of volcanoes, and sulphur dioxide (SO2) is an important one. If enough gases accumulate in the magma chamber of a volcano, the material in the vent blocking the gas may then be belched out, taking at times the whole top off a volcano like the proverbial cork popping out of a champagne bottle.
The biggest eruption in recorded history was right her in Indonesia's Gunung Tambora. It happened in 1815, and this volcano on the island of Sumbawa was estimated to be about 4200m before the eruption. It now stands at 2850m. This was followed by the Krakatau eruption in 1883 in the Sunda Strait, which was estimated to be about 2000m high before the eruption - resultingin a 300-metre deep underwater crater. Between the two, Gunung Tambora's eruption catapulted about 150km³ of volcanic material into the air, about eight times more than the Krakatau.