When magma (molten rock) comes close to the surface, it sometimes induces geothermal activity such as gas exhalations, hot water springs, bubbling mud pools and even geysirs.
The heat from the magma can be transported by three materials:
1. Rock. This is very slow, but there are places in the world where the ground gets so hot you can't touch it.
Example: In Timanfaya National Park on Lanzarote (Canary Islands) the heat in pits only 1-2m deep is so intense it lights dry wood. To boil an egg, you bury it in the hot sand, just 20cm deep. A bucket of water, filled into a iron pipe rammed 5m vertically into the ground, reappears after 5 seconds as a small geysir. The restaurant "El Diablo" had to be built entirely from non inflamable materials. It has a large barbeque which is heated by hot air rising from a well about 5m deep.
2. Water. Ground water sometimes gets in contact with warm or hot rocks. It is heated and rises to the surface to form geothermal springs. When the heat is great enough, the water is heated above boiling point. In then appears as steam or as a geysir.
3. Gas. Magma often contains huge amounts of gas, mostly sulfur oxides and carbon dioxide. When the magma cools down, the gas is released and rises to the surface. In conjunction with water bubbling pools are created. Note that the bubbles not neccessarily indicate the water is boiling.
The following text gives some details concerning geothermal activities.
Symbol: Steaming water
When ground water gets heated, it gets lighter than the surrounding cold water and rises to the surface. There, it may form hot springs and hot water pools. The temperature ranges from luke warm to boiling hot. So be carefull!
Often the water contains sulfur compounds which smell like foul eggs. Take off your silver jewelry when you're bathing in a hot pool: It will get black because the silver reacts with the sulfur. But no fear, some polishing will fix that. I learned this the hard way when I bathed at Ketetahi Hot Springs, Tongariro National Park.
Symbol: Boiling mud
The geothermally heated water often is very acidic and turns the stone it passes through into fine greyish mud and clay. When the water is accompanied by gas, the mud seems to boil. Depending on the amount of water in the mud, it bubbles like boiling water or splutters like a thick soup. If it gets too dry, the mud forms cones which look like small volcanoes. They easily get eroded by the next rain.
Don't go too close as the mud often is quite hot. Apart from this, mudpools sometimes spit at you (see the Waimangu Valley section to read how I got to know this).
Symbol: Erupting, steaming fountain
A geysir (the islandic word for "bubbling") is a special form of hot water spring. Several meters under the surface the pressure of the water column prevents the water from boiling even if it is hotter than 100°C. Eventually a temperature is reached when the superheated water begins to boil. Bubbles rise to the surface and reduce the weight and pressure of the water column. More water begins to boil, and steam and hot water are erupted into the air.
After an eruption, it takes some time to fill the vent with water again. Then the process repeats. The intervall varies widely with the geysir. Some geysirs erupt every couple of minutes, some only a few times per day.
Symbol: Steam and sulphur deposits.
Fumaroles are exhalations of volcanic gases released from the magma.
They can be very hot. Some exceed 1000°C and heat surrounding rocks
until they are glowing red.