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GEOLOGY

Few landmarks are recognizable from outer space - among them:
         The Grand Canyon
         The Great Wall of China
         and The Valles Caldera. 

This caldera, one of the largest in the world, measures about 16 miles from rim to rim. On satellite images it appears clearly as a round “bullseye.” The Caldera is a complex, long-lived volcanic field active for the last 13 million years.  Its center is seen today as a circular grass-and-elk-filled depression near the center of the Jemez Mountains of New Mexico—approximately seven miles east of Cathedrals Canyon. Indeed, most of the caldera is encompassed in the new Valles Caldera National Preserve. But the privately held 45 acre Cathedrals Canyon Reserve is outside the preserve…a part of the massive caldera wall. 

Geologic History

About1.2 million years ago (very recent in geologic history as earth itself is 4.6 billion years old), a body of molten rock formed about two miles beneath the earth’s surface. This body--some ten miles in diameter--was charged with water vapor, gases, and heat energy trying to escape. The pressure built until there was an eruption in magnitude thousands of times larger than Mount St. Helens in 1980. In both of the separate major eruptions which probably occurred some 400,000 thousand years apart, about two hundred cubic miles of incandescent glassy ash and pumice fountained many thousands of feet into the air and pyroclastic flows cascaded along the ground at up to100 miles per hour. Some flows traveled a distance of 20 miles; ash made its way into what is now Texas and Oklahoma. As hot deposits up to 600 feet deep formed all around the Jemez Mountains, the caldera collapsed under its own weight forming a large basin. The huge bowl in turn collected ash flow more than 5000 feet deep.

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When the flowing, glowing ash stopped, its retained heat caused the material to re-fuse. Tensile stress from later cooling and contracting created deep polygonal fractures. In the subsequent years, water draining from mesa top to valley caused erosion in these fractures. And thus Cathedrals Canyon with its spectacular, sheer-rock spires came into being! Geologists refer to compressed volcanic ash flows as “ignimbrites” and to the specific ignimbritic flows of Cathedrals Canyon as “Bandelier Tuff.”

On local geologic survey maps, such eroded outcrops are termed “tentrocks.” Of these, the tentrocks of Cathedrals Canyon are the most spectacular and complete and have been the subject of intensive study.

Volcanologist Professor Stephen Self writes that the geological history revealed in Cathedrals Canyon is “exceptional—perhaps the single most complete record extant of the great Valles Eruptions.” In 1989, Self led a multinational team of volcanologists through the Jemez Mountains. While at the canyon, Self and forty ignimbrite specialists from thirty-one countries were able to examine evidence that led to a revised theory of the formation of Valles Caldera on the basis of all this Canyon reveals.