Paula Phillips, Age 11, of Dallas, Tax. For her question:

What are igneous rocks?

Igneous rock may be pale pumice, like frozen foam, or a dark column of dense basalt. It may be gritty granite or glossy obsidian, smooth as glass. It may be an ore, bearing gold, or pegmatite studded with gems. All these and more are classed together as igneous rocks, because they share a past history of fire and fury.

Rocks are formed from minerals in the earth's crust, and igneous rocks are formed by fire. Their story begins deep underground when volcanic activity generates enough heat to melt rock. This forms buried pools of magma, molten mixtures of minerals and seething gases. Igneous rocks form when molten magmas cool and become solid. The cooling process may be fast or slow.

When magmas cool slowly, their minerals have time to sift and sort themselves into sizable grains and crystals. They form gritty granites and crystal studded pegmatites. Fast cooling magmas form few or no grains. They form hyalites and smooth obsidians. Some magma erupts and spills rivers of lava over the ground. They form extrusive rocks that cool fast on the surface. Other magmas Seep among the underground rocks and cool slowly. They form intrusive rocks below the surface.

All igneous rocks are rich in silicate minerals of silicon and oxygen. Some are light colored, abounding in quartz. Others are dark, containing compounds with iron and magnesium. Intrusive rocks may be light, such as granite, or dark, such as gabbros. Light and dark rocks form from different minerals in the magmas, yet the same magma may form many different rocks.

A molten mixture of magma may form coarse grained granite, cooling slowly underground. It may cool a little faster just under the surface and form fine grained hyalite. It may reach the surface and cool fast in the chilly air, forming glassy obsidian or frothy pumice, which becomes solid while it is still seething with bubbling gases. Granite and hyalite, obsidian and pumice may have the same mineral content of feldspar, quartz and mica. They differ so greatly because of differences in the cooling process.

Intrusive rocks form massive underground layers and the cores of many mountains. After ages of weathering, some are exposed to the surface. The vast Columbia Plateau and the Palisades along the Hudson River are intrusive igneous rocks. Extrusive rocks form rough or smooth surface layers. The ropy lavas and cindery caves of Hawaii are extrusive igneous rocks.