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Basic Information

A Geyser is a hotspring "dialed up to 11". It is a rare geologic terrain feature where underground water is heated by magma deep beneath the earth. As it heats up, it expands and generates pressure, which is then released when the water is projected into the air via a regular or semi-regular hydrothermal explosion. Extremely hot water, mist, or steam (potentially enough to boil the flesh off your bones) sprays up into the air, often with little or no warning. Geysers are dangerous. Thankfully, they only occur in a few places on earth, all of which feature other volcanic activity:

Real-World Geyser Hotspots:

Typically, the "plumbing" that contributes to a geyser runs about 2,000 feet deep (~600 meters). A mineral called Geyserite, which is mostly silicon dioxide, dissolves in the hot water and accentuates the pressure build up that leads to eruptions. The cycle of explosions might be a quick blast every minute or two, or it could be 12 hours or more between eruptions which in that case might last for 15 minutes straight and unleash a huge amount of steam.

It is also entirely possible for a geyser to eject toxic - or at least suffocating - gases as well as water … the "rotten egg" smell often associated with geothermal hot water, for example, is usually due to the presence of hydrogen sulphide.

Related features and phenomenon for comparison and contrast:



Game and Story Use

  • A geyser field is essentially a naturally-occurring trap, almost akin to a minefield.
    • Even when not in the moment of eruption, a geyser can still be dangerous. Many geysers are surrounded by pools of boiling water, or have a cave-like opening at their center that someone might fall into and become trapped within. There are some real-world horror stories of people attempting to test if a hotspring was at a safe temperature for bathing, only to die when they either fell in, or got scalded by a sudden cyclical blast of deadly vapor at the water's edge.
    • If the local geyser(s) have a regular steady pattern or recharge cycle, it's something of a puzzle or riddle to be solved as well as a trap.
    • A creature that is resistant to fire, steam or heat (such as a dragon) might make it's lair in or near a geyser. Now you're combining that trap or puzzle with a linked combat encounter. Reconnaissance scouting to determine the timing of the blasts could be critical to surviving the adventure.
    • In theory, humans or other less-resistant creatures might be able to incorporate a geyser into their defenses. It would be dangerous (don't try this at home), but in a fantasy setting, someone might build a castle or fortress in such a way that a geyser is on or just off the side of the causeway or road leading up to the gate house. Locals would know not to travel that route at the specific times of day when a blast was likely. An invading army wouldn't know when it was or wasn't safe, and would have to work to time their assaults for the windows of opportunity when their column of soldiers won't get burnt and blasted.
  • A geyser could get plugged (by a rockslide, or by some big creature or group of people intentionally filling it with rocks or debris). This might neutralize it permanently, or it might build up pressure behind the blockage until it eventually exploded with much greater force and disruption than normal.
  • A geyser can sometimes die off or go inactive if the local water table is disrupted, or a local river diverted.
  • Presumably, diverting water into a geothermally active area might generate geysers…
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