Wade Martinez, age 17, of E1 Paso, Tex., for his question:

JUST WHAT ARE METAMORPHIC ROCKS?

There are three main kinds of rocks: igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks formed when the earth’s liquid magma cooled and solidified. Sedimentary rocks consist of materials that once were part of older rocks or of plants and animals. And metamorphic rocks are those that have changed in appearance and sometimes mineral composition.

Metamorphic rocks are changed by heat, pressure and chemical solutions at great depths below the earth’s surface.

Most metamorphic rocks are very old. They are seen only because they have been heaved up to the surface during changes in the earth’s crust.

Many metamorphic rocks contain layers and streaks of different mineral composition and texture. Geologists call these gneiss.

Some metamorphic rocks are uniform in composition and contain minerals that appear in parallel plates. Geologists call these schist. Schist generally splits into thin plates. Metamorphosed sandstone is called quartzite. Metamorphism changes limestone into marble.