A geyser is a periodically erupting  pressure cooker. It consists  of long,narrow,vertical hole into which underground streams seep. The column of water is heated by volcanic heat below  to temperature exceeding  100°C. This can happen because the relatively deep vertical column of water exerts pressure on the deeper water,thereby increasing the boiling point. The narrowness of the shaft shuts off convection currents, which allows the deeper portions to become considerably hotter than the water surface. Water at the surface is less than 100°C.but  the water temperature  below, where it is being heated, is  more than 100°C,high enough to permit boiling before water at the  top reaches the boiling  point. Boiling  therefore begins near the bottom, where the rising bubbles push out column of water above, and the eruption starts. As  the water gushes  out, the pressure on the remaining water reduced. It then boils rapidly and erupts with great force.
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