Welcome to You Ask Andy

Cynthia Rodriquez, age 10, of Rosenberg, Texas, for her question:

What exactly is magma?

Any of the rocky materials of the earth can be made into magma. All we need is heat, tremendous heat and pressure. The earth has pressure cookers of this kind on a grand scale. The ground, of course, gets hotter as we go deeper and every step down adds to the pressure of the weighty rocks above. So the magma making pressure cookers are far below the surface, maybe as deep down as 30 miles or more. They occur where cracks and strains make weak spots in the earth's crust.

These buried hot pots melt the rocks around them to seething liquid called magma. Most of the rocks at this level are heavy basalts, but almost every other type of mineral may be added to the molten mixture. Some heated minerals give up gases and these add bubbly foam to the magma. Ground water also seeps into the seething brew and most pockets of magma contain steam. Volcanos erupt gushers of magma at the surface. The molten mixture then changes its name to lava.

 

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