Jason Hill, age 14, of Winston Salem, N.C., for his question:

HOW WERE ROCKS FORMED?

The bulk of the earth's mass is made up of rock. Scientists have estimated that it weighs approximately 6 sextillion tons  that's the number 6 followed by 21 zeros.

Rocks are made up of minerals. They contain metal ores and other useful substances that man has learned to utilize.

There are three kinds of rocks: igneous, sedimentary and metamorphic. Igneous rocks were made from what scientists call magma, or liquid rock. If the molten rock cooled slowly, minerals within it had time to form crystals. The more slowly it cooled, the larger the crystals.

Granite is a form of igneous rock. It always contains the mineral quartz and usually either mica or hornblende. It was cooled slowly within the earth.

Felsite and basalt cooled more rapidly from molten rock either within the earth or as lava that flowed from volcanos. volcanic lava cooled so fast that it became either glassy or, like pumice, filled with trapped gas bubbles.

Sedimentary rocks were made of weathered fragments or sediments of igneous rocks. They run the range from loose sedimentary rocks, such as clay, sand and gravel, to shale, limestone, sandstone and compacted sand and gravel.

Crushed shells of sea animals cemented together form a type of sedimentary rock. Many in this classification can be broken very easily and usually feel gritty.

The third classification of rocks, metamorphic, are those that have been altered by tremendous pressures and heat. They differ from igneous rocks, whose crystals are mixed, by having the crystals of each mineral more or less lined up in bands. They have that layered appearance found in sedimentary rocks but are much harder and crystalline. They are not gritty.

Among the metamorphic rocks is gneiss, which resembles granite but has a banded appearance, slate and marble. Slate resembles shale but is harder and will not crumble or break. Marble is a recrystallized limestone.

Rocks break down and are exposed to the weather and atmosphere, so there is a constant change in their makeup. Boulders can become pebbles when other rocks rub against them or they are crushed. Water can dissolve some rocks. it is

also possible that heat and pressure within the earth's crust may be great enough to remelt the deeply buried rocks and to form a new magma. And a new cycle could start over again.