I was browsing through my copy of Volcanoes of the World when I came across an interesting tidbit that led to other interesting tidbits. This book is subtitled "a regional directory, gazetteer and chronology of volcanism during the last 10,000 years." It was last published in 1994, so it's not totally out of date.
The main body is a listing of all the world's volcanoes, each given its own ID code and a single line summary, followed by every known eruption, each in its own line. So the information there is pretty compact, and requires a key to decipher what all the symbols and characters and numbers men. The inside cover contains that key, and there I noticed the first tidbit of interest. This book is based on a work done in 1951: the 22 volume "Catalog of Active Volcanoes of the World", known as CAVW. Since I've not seen that, I have no idea how they filled that many volumes, assuming each volume is not more than a few pages. But in the chart for "Eruptive Characteristics", listed is not only the codes for the book, but those used in CAVW.
Thanks to computers and the limitations of the ASCII character set, most tables these days limit themselves to a small set of characters. That wasn't the case in 1951, and the CAVW used a lot of specialized little icons instead. Some of these have made it into the Unicode character sets, and more on that later. But what caught my eye was that one of the eruptive types listed is "Fumarolic activity", along with the CAVW symbol, a circle with a double headed arrow pointing up. That made me curious, were there symbols for geysers, hot springs and other hydrothermal activity? Let's find out.
Some online searching for a list of CAVW symbols turned up nothing, but I did discover something else. There's a Unicode symbol for "hot springs." Unicode is an attempt to encode all sorts of standard alphabets and symbols, leaving how they are displayed up to the computers involved. But inside the definitions are just about every math or typographic symbol you've ever encountered, and many more: Chess set symbols, arrows, and other decorative items used in printing. Seems that the "hot springs" a standard symbol used on Japanese maps and that's reason enough to add it. And here it is (assuming you are using a Unicode compliant browser):
I also found that the fumarolic activity arrow exists, too:
To use them yourself, on webpages or anyplace where an attempt to display HTML is made, just use "♨" for the hot springs, or "⥉" for the fumaroles. I still need a symbol for geyser, though.