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Earth
 Isostacy

Isostacy
Reading Level
     edHelper's suggested reading level:   high interest, readability grades 5 to 6
     Flesch-Kincaid grade level:   5.69

Vocabulary
     challenging words:    asthenosphere, downward, mathematician, theory, continental, material, europe, mantle, fluid, therefore, mass, response, roots, upward, liquid, decrease
     content words:    Remember Archimedes

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Isostacy
By Patti Hutchison 		  
1     As he was taking a bath one day, a mathematician named Archimedes discovered something. He noticed that if you place an object in water, the water level rises. This led to Archimedes' Principle. This principle says that an object applies a force that pushes downward on the liquid. Another force pushes upward on the object that is placed in the fluid. If the object is denser than the fluid, it will sink. If the object is less dense than the fluid, it will float. If an object floats, it will, however, sink lower in the water as more mass is added to it.
 
2     Think of it this way. An iceberg floats on water. The ice is less dense than the water it is floating on. But part of the iceberg is still under water. Now, imagine that a polar bear climbs onto the iceberg. The water level will rise around the iceberg and there will be more of the iceberg under the water. Now, imagine that the polar bear gets off and swims away. The iceberg has less mass, so it will now float higher in the water. Less of its mass will be under water.
 
3     So, what does all this have to do with earth science? It helps explain earth's isostacy. Do you remember the theory of plate tectonics? This theory basically says that the crust of the earth is broken into giant pieces. These pieces float around on the partly liquid mantle (asthenosphere) below it. The crust floats because it is less dense than the mantle.

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