Stories of the Lithosphere

How Does it Work--Geysers

Earth Science Essentials

by Russ Colson

 

Introduction

 Geysers are rare.   We might ask why.  

 

Geysers require that we have lots of heat, generally meaning a volcanically-active region.   In addition, geysers need lots of water.   Volcanically active regions without lots of water do not have the ground-water circulation needed to get geysers.   Finally, geysers need pressure-tight plumbing.   This places constraints on the kind of rock that is present.   Generally, we need rock that contains lots of silicate that will dissolve in water and then re-precipitate to line the plumbing system to make a water-tight seal and create constrictions in the piping. Thus, geysers or more likely in regions with silica-rich volcanics like rhyolite.

 

The main material for sealing the piping is silica-rich material like geyserite.

The silica-rich geyserite often precipitates at the surface also, where it makes a variety of interesting nozzles and cones from which the geysers emerge.

 

White Cone Geyser, Yellowstone NP, 1994

 

All these constraints don't happen in the same place very often.   They are all present on the high Yellowstone plateau, which receives large amounts of winter snow—providing lots of water.   The plateau is underlain by a hot spot in the mantle, and a cooling magma chamber in the crust—providing lots of heat.   The kind of rock at Yellowstone includes not only basalt, but also rhyolite, providing silica to seal up the piping.

 

About half of the world's geysers are found in Yellowstone National Park.

  

We might ask why geysers erupt suddenly and spectacularly instead of quietly boiling like some hot springs do.

 

One key idea is that plumbing in a geyser system has constrictions that prevents water circulation up and down.   Hot water moves up from depth in a hot spring and is replaced by cooler water moving down.   This keeps the water at depth in a hot spring from getting to the boiling temperature.

 

As the heat builds up in a geyser, the water at depth will eventually reach the boiling temperature—which is much higher than 100 ° C, since the weight of all the overlying water makes the pressure higher. Then water vapor bubbles up through the plumbing system and throws a bit of water out of the geyser.   Once the gases throw out some water, the pressure at depth drops because there is less water above it, which causes a drop in boiling temperature at depth.

 

Suddenly, the water at depth is above its boiling temperature.   It flashes to steam, which throws out more water, which decreases the pressure and boiling temperature even more, so there is more boiling, and more throwing out of water.   This continues until the geyser either runs out of heat to boil the water or out of water to boil.

 

Then, the plumbing system refills with cool water which has to heat back up again, until we get another eruption.   This can result in a certain predictability to eruptions—like the famous predictability of old faithful.   The timing of eruptions depends on how long it takes the system to fill back up with water, and to heat up to the boiling temperature.

 

Most geysers are connected to other geysers and hot springs in a complex network of plumbing pipes, which makes them less regular or predictable.

 

The key to understanding how a geyser works is found in understanding the process of water starting to boil, pressure dropping, more boiling, and so on.   Understanding this process is best achieved by considering the phase diagram for the boiling of water.   As discussed in the lecture, water does not always boil at 100°C!

  

Understanding the Phase Diagram

 The puzzles below reinforce understanding of the phase diagram important in understanding how a geyser works.   They presume that you have watched, understood, and taken notes on the lecture.   You may need to review your notes and/or review the lecture again.

 

The dotted lines above show conditions of temperature and pressure 1) at depth just before an eruption, 2) at depth just after an eruption, 3) in your syringe experiment and 4) in a pot of water boiling on the stove at sea level.

  

 

 

 

Suppose that we changed the axes of the graph.   Which of the following is the correct phase diagram?

 

 

Modeling the Geyser Eruption

Which of the arrows in the phase diagram below shows how conditions change in water that is rising up toward the surface in an eruption?

 

 

Which of the arrows below shows how conditions change at depth in the geyser as the plumbing refills with cool water after an eruption.

 

  

 

Often hot springs are boiling at the surface.   However, the superheating that occurs at depth with geysers doesn't happen.   Suppose that water circulates in the hot spring plumbing, rising from depth, and begins to boil as it nears the surface.   Which of the arrows shown below might illustrate the conditions that lead to this non-geyser boiling.

 

 

 Last updated Oct 13, 2015.   All text and pictures are the property of Russ Colson.

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