Minor Forms of Extrusive Activity Geysir Stokkur, Iceland Geysers are hot springs that erupt periodically. Water in the lower crust is heated by rocks and turns into steam; pressure increases and the steam and water explode onto the surface. The ingredients needed for geyser activity are: heat, water, and underground rock hard enough to withstand intense pressures. Other features A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are hot springs all over the earth, on every continent and even under the oceans and seas. Boiling mud: hot water mixes with mud and surface deposits. Lassen Volcanic Park mudpot A fumarole is an opening in Earth's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The name solfatara is given to fumaroles that emit sulfurous gases. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic flows. A fumarole field is an area of thermal springs and gas vents where magma or hot igneous rocks at shallow depth are releasing gases or interacting with groundwater. From the perspective of groundwater, fumaroles could be described as a hot spring that boils off all its water before the water reaches the surface. Hverarönd sulphuric mud pools (solfataras). The ground is unstable there and you cannot walk everywhere. Blue-gray mud is boiling and produces small or big (depending on water content) bubbles that burst. The ground is yellow and reddish and the noise from an abandoned hot water well is penetrating. The smell of sulphuric dioxide is everywhere. Close-up view of the solfatara at Ijen, Indonesia with fumarole temperature of more than 220°C.