Geyser calcAQ Application

Erupting Geyser. Reykjavik, Iceland.

Geysers are very sensitive to environmental changes around them. Natural shifts in the geologic formations and human events can disrupt the fragile systems. Earthquakes can create cracks in the plumbing systems or close them off completely. Man's use of water or construction patterns can lower the water table or disrupt the movement of water underground enough to cause the geyser to lose its steam - so to speak.

Geysers require four components to exist. These are

  • Heat source
  • Water supply
  • Plumbing
  • Special chemistry

The heat source and water supply are obvious requirements for a geyser. The heat is needed to superheat the water and generate the pressure that will force the water and steam up the geyser column. Most geysers are found near some area of volcanic activity, which provides the heat source near enough to the surface to interact with the normal water table.

Not just any area with volcanic activity close to the surface, however, will produce geysers. It is the third, and closely related fourth, component of a geyser that make them unique. For a geyser to function in the spectacular manner exhibited by Old Faithful or Geysir, the geyser column must provide a tight seal or else the pressure will not build up enough for the geyser to erupt. Porous formations or formations that crack and shift easily do not make for good geyser plumbing. Most geologic formations will crack and are porous to at least some extent. This is where the chemistry component is important. The silica from the surrounding rocks at depth is dissolved at the high temperature and pressure and transported up the column where it precipitates out as the temperature and pressure decrease forming a pressure tight seal. This occurs over an extended period of time but effectively closes up all of the small cracks and pores that would otherwise dissipate the built up pressure.

Depth Water Temperature Pressure
ft °C atm

0

90 1.00
20 120 2.65
40 132 4.12
60 154 6.68
80 166 8.72
100 175 11.0
120 178 12.2

Temperture and Pressure versus Depth in a Geyer Column

This calcAQ application uses temperature and pressure measurements of a geyser in Iceland to profile the solubility of SiO2 over the length of the geyser column. The calculated values show that the solubility at depth is high enough to dissolve appreciable amounts of silica and that the change in solubility along the column is significant enough to cause an appreciable amount to precipitate out and form the required seal. The application also demonstrates a unique feature of calcAQ itself. That is that both pressure and temperature can be varied as part of the survey in an independent manner. In fact, any input value or parameter can be varied independently for a survey. The benefit is that the calculation results are easily plotted versus a distinct process variable, in this case depth along the geyser column. For an engineering process, the variable could distance along a reactor tube, time, or some other operational parameter particular to the piece of equipment.

The worksheet is laid out using the row format where each row represents a temperature - pressure measurement at a particular depth. The isothermal array function is used to calculate the equilibrium composition and properties at each depth. Reporting the composition of dissolved silica in mg and the aqueous volume in L, then dividing one by the other gives the solubility concentration of SiO2 in water. The solubility is easily plotted against depth using an x-y scatter plot, which is tweaked to show the depth on the y axis in inverse order than normal to illustrate the solubility relative to ground level (top of plot) and the bottom of the column (bottom of the plot). It is seen that the solubility of silica monotonically decreases as the water moves up the column. However, the solubility curve is not completely smooth because the change in temperature and pressure with respect to depth is not constant.


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