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(Lesson #9)

Hot Spots-Hawaii and Yellowstone

Geysers and Hot Springs


Old Faithful geyser in Yellowstone Nation Park is a famous tourist attraction. Every hour or so it sends a stream of scalding hot water from 135-200 feet in the air. Why does this occur? The same scientific principle that makes a volcano erupt turns a hole in the ground into a spectacular fountain.
Hot springs and geysers form over magma chambers in very similar ways. Geysers though, are more complex in how they form and much more spectacular in the display that they put on. Here is how the Earth works like a giant hot water heater and boiler.  
Rain water seeps into the ground and slowly percolates down through cracks in the layers of the upper crust. Here it collects in porous rock that holds the water like a sponge.  
The huge magma chamber that sits under the park is the heating source. This magma chamber is located over two miles below the porous rock layer that holds the water. The rock below radiates the heat up to the water by a method called conduction. You have felt conduction when you have picked up a glass handled dish of hot water from the microwave oven. The glass handle is hot because the heat from the water radiates through the bowl to the handle.  
The water in the layer of porous rock is heated but will not boil because it is under extreme pressure from the overlying rock. The water is superheated like in a steam boiler. The temperatures may reach over 500 degrees Fahrenheit! At the same time that the water is heating, more water from the surface keeps coming into the rock layer. This cooler water sinks to the bottom causing the hot water to rise. When the water rises the pressure from the surrounding rock layers drops. The result is the hot water will continue to rise untill it reaches the surface of the Earth. Some of these hot springs become filled with mud and form hot mud pots. People throughout the world come to hot springs and mud pots for enjoyment and some even believe they have medicinal or magical healing powers.
Geysers, the giant boilers of the Earth, are produced in a slightly more complex way. As the heated groundwater rises it collects in rock pockets that are under extreme pressure. Because of the high pressure the water is not able to boil. The temperature continues to rise until some of the water boils. The steam then rises very fast and takes some of the non-boiling water with it. This reduces the pressure in the rock pocket. Thr superheated groundwater then heats to steam quickly because of the drop in pressure around it. When this happens the rest of the water in the rock pocket explodes out through the fissure and will continue to erupt until the steamy groundwater is gone.  
Sometimes these eruptions will last for over an hour. When the eruption is over the rock pocket will fill with groundwater and start the cycle again.  
Old faithful in Yelowstone Nation Park goes through this cycle every 65 minutes or so.  


1. What is the difference between a hot spring and a geyser?

2. Why does the release of pressure cause the geyser to erupt so explosively?