David Walmer, age ll, of Mount Vernon, Wash., for his question:

What causes those rocks called thunder eggs?

Andy is just as curious about things as his young readers and this question mystified him. At first suspect    ed that thunder eggs were the fossilized eggs of the dinosaur thunder lizard  but he was wrong. The correct answer was found after lots of research into volcanic minerals.

Rock collectors of the Northwest hunt for stones they call thunder eggs. Most of them measure only a few inches and on the outside they look very dull and uninteresting. They seem to be made of rough, dark lava material, often pitted and scarred. But many thunder eggs have caches of secret treasure hidden inside. Their shabby coats may conceal stores of rainbow colored crystals and other lovely minerals.

A rock collector uses a special saw to slice through a thunder egg. The stuffing inside the stone is almost always a pleasant surprise. The smooth cut surface may suggest a miniature picture in delicate colors. There may be fronds and foliage or tropical scenes with waving palms. Sometimes the colored minerals are arranged like colored ribbons in rings or rows or even bows.

A thunder egg is often born in the thundering upheaval of a volcanic eruption. Flames and smoke and fuming gases spurt into the clouds. Bombs of blazing minerals, ashes, dust and debris are tossed into the air. Rivers of red hot lava pour over the ground. The lava is often a foamy mixture of molten minerals and gases. As it cools, the liquid minerals harden around bubbles and pockets of the trapped gases.

The hardened mineral forms a layer of bed rock riddled with hidden holes. The pockets fill up with ground water and perhaps steam from the volcanic eruption. The steam and water are loaded with chemicals dissolved from magma and volcanic lava. Many of these chemicals are silicates that tend to arrange themselves in crystals. And silicates can form an endless array of tinted crystals and rainbow colored stones. The pockets in the drab bed rock become stuffed with rainbows of semi precious gems. They are much harder than the bed rock and when it crumbles away they are left behind. The pockets of treasure are found on the ground, each coated with a layer of crusty lava. They may be called geodes or :Mexican agates and some collectors call them thunder eggs.

Quartz is the most common silicate and the quartz minerals form an endless array of colored crystals and semi precious stones. They may form rainbow ribbons of agate or dark and glossy moon glowing chalcedony and sunshiny carnelian, leaf green chrysoprase and midnight jasper. They form glassy crystals of rose and violet, smoke and lemon. Traces of these and many other glamorous minerals may stuff the pockets and paint the pictures inside a drab looking thunder egg stone.