How do geysers work? I'm wondering how geysers do a spontaneous explosion of water and heated vapor instead of a more continuous, homogeneous in time phenomena, such as constant boiling. I don't understand the mechanism that makes geysers explode a certain amount of time the last time they erupted. Please explain me.
I understand that the energy of the eruption comes from heat, which ultimately comes from magma deep in the earth, or from heated water in contact with that magma. the thing I don't understand is this: let's suppose the geyser chamber starts to fill with cold water, and starts to heat. I don't see why it doesn't boil like in a pot, that is gradually and slowly.
I've seen this video, and I don't understand why suddenly when the bubble pushes out water out of the chamber, that makes a pressure drop inducing vaporization in all the chamber. 
Another doubt about the video I linked is why, after mass vaporization, there is a eruption. I think that changing from liquid to gas shouldn't suddenly increase the pressure so much, why it should?
 A: Suppose the chamber is a vertical cylinder of area $A$. The pressure at the bottom is $W/A$ where $W$ is the weight of the water. If some water spills the pressure at the bottom is less.
Boiling doesn't increase the pressure. There is already high pressure at depth. Removing (some of)  the weight of water allows the steam to expand. The pressure at the bottom is actually decreasing.
A: the water in the geyser's "plumbing" is distributed as a vertical column of water many tens of feet in depth. at the bottom of this (deep) water column, the ambient pressure is greater than atmospheric and the boiling temperature of the water at that depth is thereby increased, as it is in a pressure cooker on your kitchen stove. So the deep water in the column is prevented from boiling by the hydrostatic pressure and it becomes superheated. 
At that moment when the deep water finally gets hot enough to boil at that pressure, the expansion of the resulting vapor bubbles begins to push water out of the geyser's surface opening, which reduces the hydrostatic pressure exerted on the superheated water down below, and that water then explodes into vapor, violently ejecting all the rest of the water in the column.  
