Geysers are eruptive hot springs found chiefly in volcanic districts,
but particularly in the Yellowstone Park, Iceland, New Zealand, Tibet
and the Azores. At intervals these fountains of hot water and steam
sometimes rise to a height of two hundred feet. The eruptions occur at
intervals varying from every hour to once a day.
All the geyser waters hold in solution a considerable quantity of
silica. The highly heated water decomposes the felspar and other
volcanic rocks, and becoming slightly alkaline with the soda or potash
these contain, it is enabled to form a silicious solution. The silica
taken up is deposited again round the mouth of the orifice. Minute
plants termed algæ are known to live in the hot water, and to aid in
throwing down the silica from solution to form the sinter deposits.
The cause of the periodical eruptions is probably to be found in the
gradual increase of heat with the depth of the tube. In the middle and
lower parts the temperature is far above the boiling-point (212° F.) at
the ordinary pressure. But at last the lower portion rises to a position
where the temperature is above the boiling-point at the pressure it
there sustains, and then, flashing into steam, it hurls the column above
into the air. After playing for a few minutes the water falls back into
the basin, and remains quiet for a time.