Watching the geyser build up to the spout was very interesting. The vent slowly filled with water from the surrounding bowl. This bowl is fed from a small stream but most of the water in the bowl is collected from the previous spout. The column of water in the vent oscillates up and down, the oscillations getting larger as the vent fills. When there is enough water in the vent the oscillation becomes large enough to throw water out back into the bowl. The loss of water from the top of the column causes a sudden loss of pressure in it allowing the water to boil. This is when the spout happens.