Two fantastic, factual looks at Earth; The Great Extinction: The Solution to One of the Great Mysteries of Science: The Disappearance of the Dinosaurs, by Michael Allaby and James Lovelock. New York: Doubleday & Co. 175 pp. $13.95.; Volcano Weather: A Case Study: 1819, the Year Without a Summer, by Henry Stommel and Elizabeth Stommel. Newport, R.I.: Seven Seas Press (524 Thames Street, 02840 ) 160 pp. $15.

These two fascinating books about seemingly fantastic catastrophes outdo the best of science fiction because they deal soberly and factually with what has actually happened to our own planet Earth.

The first describes the impact of a massive asteroid that devastated vast areas and wiped out many species living some 65 million years ago. The second chronicles the climatic impact of a major volcanic eruption that, although half a world away, turned a New England summer into winter in the early 19th century.

Michael Allaby, a science writer, has teamed up with James Lovelock, a world-renouned atmospheric chemist, to tell the remarkable story of how, during the past decade, a number of scientists have put together what may well be the true story of the dinosaurs' demise. Readers should be cautioned, as the authors are at pains to point out, that the story still is scientifically controversial. But it has enough evidence behind it and enough support from a variety of leading scientists to be taken quite seriously.

Briefly, the theory holds that a major asteroid, or planetismal, struck Earth. The enormous energy of its impact could have boiled the oceans, thrown sun-darkening dust into the air, and as a result of these and other effects, killed off many plant and animal species. There are many signs in the geological record which scientists holding this theory now interpret as indicative of such an event. They include traces of elements, such as a form of iridium, which indicate an extraterrestrial origin in clays laid down at the time.

The authors lay all this out for the reader, presenting each factor in the context of arguments for and against the theory. In the end, they come down rather strongly in its favor.

Where they go from there is as controversial as the scientific debate. As have other experts, they note that such a collision is expectable over geological time and could happen again. They suggest, as have others, that a portion of the arms race effort could go into a cooperative East-West program to establish a watch for any asteroid on a collision course and to set up a missile defense against it.

So far, no political leader has ever indicated he or she took the suggestion seriously. But it is scientifically credible. And, unlike the proposal for an antimissile laser system, an antiasteroid defense is known to be feasible. It could be established with present-day technology.

''Volcano Weather'' deals with a less-apocalyptic, but nonetheless spectacular, subject - what happens to a region when summer fails to come and many crops freeze. Oceanographer Henry Stommel, another world-class scientist, and his wife, Elizabeth, have traced what this meant to New England. The many anecdotes they have found speak eloquently of the hardships of those affected and of their capacity to survive. Not all crops failed. Wheat did much better than corn, for example. So the region pulled through.

The larger significance of the story is the light it sheds on the possible role of volcanic eruptions in climatic change. Here again, there is scientific controversy over the chain of cause and effect. However, there is fairly wide agreement that dust from the eruption of Mount Tambora in 1815 enveloped Earth and dimmed the sun. There is considerable evidence that the following year without a summer in New England was linked to that event.

As with the asteroid story, the moral is clear. A natural catastrophe occurred that could happen again. Its impact on a region, indeed a world, more densely populated now than it was at the time could be much more serious today. It would be prudent for planners to remember this. At the very least, food reserves could be maintained to allow for massive crop failures. And contingency plans could be part of the community disaster preparedness that now is common in many places.

Thus these books are not ''scare stories'' meant to titillate or merely entertain, although they are fascinating reading. They are welcome efforts by leading scientists and their writing colleagues to share insights into rare, but important, natural events of which we all should be aware.

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