Mount St. Helens at al

VOLCANOES W. H. Freeman, 1981. 244 pp. Illustrated $17.95, hardbound; $8.95, paper VOLCANO WATCHING Hawaii Natural History Association, 1980 80 pp. $6.95 by Professor Robert and Barbara Decker

The parlor-game, question is: How many active volcanoes are there in the world today - 5, 50, or 500? The correct number depends on the definition of "active." The authors of these excellent books consider a volcano active if it has erupted in historic times. Five years ago, perhaps only old Vesuvius or Mount Fuji would have peaked above our level of indifference toward volcanoes, but now that we have an unconscious yet steady-state watch on Mount St. Helens, the Deckers' books are especially appropriate reading.

Volcanoes is both comprehensive and illuminating, in no way prosaic. It presents a likely volume of information for the generalist reader, and it does so vividly.

Robert W. Decker, adjunct professor of Earth Sciences at Dartmouth, and his wife Barbara deftly concentrate on their subject, but it is intriguing how far, and in how many directions, that takes the reader. Their book has appeal for the amateur geographer, geologist, meteorologist, botanist, space buff, and, of course, environmentalist. But the appeal doesn't stop there. The lucid writing stirs musings about the "Big Bang" and a number of other far-reaching wonderments. But these thoughts must be inferred. The authors present colorful information, not philosophy.

For the person who has never absorbed much more than headlines and TV pictures of earth eruptions, Volcanoes is a revelation, for each chapter comprises a handful of egregious nuggets: Were it riot for volcanoes' production of water and atmosphere, our blue planet might consist of rock and metal; if volcanoes weren't generous in distributing iron pyrites around the earth, we might be awash in sulphuric acid as our twin, Venus, seems to be. At times the reader gets the feeling that volcanoes can be geological heroes.

The overall quality of this book by the Deckers, members of the U.S. Geological Survey's Hawaiian Volcano Observatory, presents the reviewer with the dilemma of writing a seemingly uncritical rave. But there were some momentary misgivings. Chapter One, on "Seams of the Earth," could make the reader wonder if the rest of the book might be "technical." It is not. No scientific background is required to make this handsome book a pleasure to read. Another factor in the authors' style that might create mild apprehension is the inclusion of terms that are exotic, even arcane. But then the following passages occur. In Hawaii, the kind of lava flow just described is called an aa flow, pronounced ah ah. Its major characteristic is the rubble of lava blocks on its surface. Adventurers with thick boots have walked on these rubble surfaces while the flows are still slowly moving, but aa flows are hard enough on boots after they have stopped and cooled . . . Another kind of Hawaiian lava flow is called pahoehoe, pronounced pa hoy hoy. ..." The misgivings dissolve.

"Volcanoes' many subtopics fascinate: We may have skimmed the earth of its diamonds, but there may be many tons of them way down; Mount St. Helens' recent eruption was a considerable blast, but it didn't chill the world for a few years the way Krakatau did in the last century. Of topical interest is the forecasting of eruptions. The Deckers include a firm appraisal of the progress made so far.

Volcano Watching is a paperback with lively writing and dramatic illustrations that reflect the incandescence of molten lava. Its 80 pages put the reader on some truly wonderful scenes, mostly in Hawaii but also around the world. There is

even a fey spirit to it as it creates the image of volcano watchers trying to cook a meal on flowing lava and seeing their frying pans drift out of reach. Like Volcanoes, this book has many exciting elements - lost Atlantis and Mexico's almost-brand-new volcano, Paricutin, come into view.

Take Volcanoes and Volcano Watching with you on your trips abroad. Included in them is a section called "The World's 101 Most Notorious Volcanoes." These are in addition to the 400 other active volcanoes scattered around the earth.

A free-lance writer in the Hanover area, TedThorne was many years a member of the NewYork Times editorial staff and a science writerwith primary interests in space and aviation.