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Thar She Blows!     CLICK HERE TO SEE JW's VIDEO    BUT PATIENCE!  It takes a LONG TIME for the video to load.  You may have to leave the page open for maybe ten minutes while Old Faithful gets up steam....
	Resting Up for the Next Show - in About 90 Minutes

There’s a lava dome not far beneath the surface of Yellowstone.   Ground water accumulates over it and boils – or sometimes just tries to.  The weight of the water column between the dome and the surface puts the water deeper down under pressure.   If there’s enough pressure, the water at the bottom, even though close to the hot lava, is prevented from boiling;  instead, it becomes “superheated”.

Eventually the heat in the deeper water will transfer upward and, if there’s an opening to the surface, the unpressured water at the top will start to boil.   Bubbles of steam form and spew out.   Entrained water is ejected.   This reduces the weight on the superheated water below, which suddenly now also becomes able to boil, causing yet more water to be ejected, further reducing the weight in the water column.    The progress of this boiling state accelerates downward rapidly through the entire water column, and the gushing becomes explosive. 

Even as the column of water over the dome empties, groundwater keeps seeping in.   If the water enters the underground space above the dome fast enough to keep it from boiling off before it fills the space, and if the space is not enclosed but is restricted enough to prevent the deeper hot water from circulating up freely, leaving the water at the top cool while the space fills, then the conditions are present to create a geyser.   The space above the lava dome will again fill, the water’s weight again keeping the hot water at the bottom from boiling until the cooler water at the top heats up enough to boil and erupt, thereby reducing the column weight and permitting the superheated water further down to boil and itself erupt.   Thus the cycle endlessly repeats.