Paleontologists and anthropologists spend hours, days, and years scouring the ground for fossils and artifacts, often in inhospitable lands under great hardship. But the excitement of discovering evidence of early man or making other finds is reward enough -- one that would be lost in many cases were it not for volcanic eruptions. Burial of a farm, village, or field by lava flows, pyroclastic flows, and lahars causes great damage and generally destroys everything on the ground, but thick falls of volcanic ash are a nearly perfect preservative. Early human history has been preserved and guarded by ash layers for millions of years, and archaeological excavations of such sites have revealed the history of various peoples and their ways of life.

A constant and rapid fall of volcanic ash from a large eruption is ideal for preserving what exists upon the land at the time of deposition. It can bury them intact without moving them and without completely crushing or burning them. No other natural catastrophe is capable of simultaneous burial and intact preservation. Dust storms that coat surfaces with fine sediment are the nearest analog to fallout ash, but the amount of falling dust delivered by a storm can~- not create a deposit thick enough to bury all objects, as can a voluminous fall of ash.

Water that seeps through the deposited ash reacts with glass shards and dissolves or replaces many of them with other elements to create either void space or derivative minerals, such as clay miner~- als. Excess silica dissolved in the water often replaces soft bone materials molecule by molecule, creating an almost indestructible fossil (like petrified wood) that lasts for millions of years. Leaves commonly are preserved only as imprints. Volcanic-ash deposits not only cause the widespread death, but then also provide the substance that preserves the bones of the buried animals.

Pyroclastic flows from Mount Vesuvius in 79 AD buried Pompeii, Herculaneum, and many villas. Horrendous as it was, it preserved the culture like a snapshot of the distant past, allowing us to learn more of our past history.

Bronze Age Greece, has been preserved by volcanic ash from a large caldera-forming eruption in 1628 BC. At Akrotiri on the island of Thera, excavations revealed the agricultural practices of 3,600 years ago.