Water: Above and Below Ground - Geology - page 186

Heating up underground: Geysers
A geyser is like an artesian spring: Groundwater flows up to the surface, against the pull
of gravity. But a geyser is not forced upward due to pressure and the impermeability of
surrounding rock layers like an artesian spring. A geyser flows upward because the
groundwater has become very hot from heat deep in the earth's crust.
Where the permeability of rock and sediments allows groundwater to flow deep into the
earth's crust, the water is heated by the mantle. (I explain the layers of the earth in
Chapter 4.) After it's heated, the water rises back toward the surface of the earth as a
geyser. (I explain why heated materials rise upward in Chapter 10.) Figure 12-9 illustrates
how groundwater is heated by exposure to magma : melted rock deep in the earth's
crust, near the mantle.
The most famous example of a geyser is Old Faithful located in Yellowstone National
Park, Wyoming. It shoots heated water up into the air on a fairly predictable schedule,
which makes it an ideal attraction for people curious about geysers but not willing to
wait hours (or even days) in the hope of seeing an eruption.
That sinking feeling: Karst, caves, and sinkholes
When rocks and sediment made of certain minerals (see Chapter 6) contact water, the
minerals may dissolve and be carried away in the water flow, leaving holes in the rock.
Such rocks are said to be soluble. When groundwater flows through soluble rock (usually
carbonate rocks like limestone), it creates small spaces where the soluble minerals used
to be. Continuous groundwater flow enlarges the spaces over time. An area of the
earth's crust that has been eroded by groundwater in this way is called karst.
Figure 12-9:
Groundwater
heated by magma
rises to the sur-
face as a geyser.