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Discovery Magazine 7/1/2006

Liquid Rock

by  Brad Harrub, Ph.D.

Most people enjoy roasted marshmallows. There’s not much better than a sweet, warm, gooey marshmallow that is pulled out of a fire on a stick. But many marshmallows have met an early demise because someone accidentally got them too close to the fire. They can catch on fire and immediately turn black on the outside. Imagine trying to roast marshmallows over hot, molten lava. Lava is what erupts out of a volcano—an incredibly hot liquid rock mixture that ranges in temperature from 700 to 1,200 degrees Celsius. Not only would the marshmallow burn up, but so would the stick, and maybe even your arm! Just imagine how hot a rock would have to be in order to melt. Those marshmallows would not have a chance at such extremely high temperatures.

Magma is the term used to describe lava when it is still underground. It is only when it reaches the ground that it is called lava. However, these two terms identify the exact same material. After reaching the surface of the ground, lava begins to cool quickly and harden. As it cools, it forms different shapes and textures. For instance, pahoehoe (puh-HOY-hoy) lava normally appears smooth and dense and is easy to walk across. Whereas `a`a lava forms individual rocks that are porous and very jagged—making it extremely difficult to walk on.

Since scientists who study volcanoes know when certain volcanoes erupt, it allows them to know something about the age of these newly formed rocks. In addition, it provides them with the ability to check dating methods against rocks of a known age. For instance, since we know the precise date when the hardened rock formed, we can measure it using radiometric dating techniques to see if those dates are accurate. This has been done with rocks from Mount St. Helens that were known to be less than 10 years old, and yet laboratories gave results that suggested these rocks were between 350,000 and 2.8 million years old! Unfortunately, many people put a great deal of faith in these dating methods—arguing that the dates they give are “proof” of an ancient, “evolutionary” Earth. In truth, lava provides good evidence that these dating methods are incorrect.

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