Yellowstone National Park - Fire and Ice

All of Yellowstone's geysers have one thing in common: they result from a link between ancient volcanism beneath the earth and water that falls from above. The same is true of all the water that bubbles, boils, spurts, and hisses from the ground in whatever form throughout the park. It is all surface water, rain and snow, that has trickled down thousands of feet through areas of weakness, or faults, in the earth's crust.

These faults date far back into time, as does the bubble of volcanic fire that heats the water. Some 65 million years ago, when the upthrust of the Rocky Mountains began, this area was a point of great stress. Molten Rock, or magma, was forced upward from deep within the earth and, starting about 50 million years ago, erupted in the form of great volcanoes. The massive eruptions continued off and on for another 10 million years or more, building mountainous cones and spewing ash over vast stretches of the Great Plains.

One of the legacies of this long period of intermittent volcanic activity can be seen on the steep face of Specimen Ridge, in the northern part of the park. Here erosion has exposed entire petrified forests, 27 of them, one atop the other, of standing redwoods, walnuts, sycamores, magnolias, hickories, persimmons, maples, dogwoods, and other warm-climate trees that grew in the broad, gentle valleys of that era.

Each forest grew for several hundred years, spreading at the feet of seemingly dormant volcanoes. Then, when the volcanoes returned to violent life for months, years, even decades at a time, each forest was buried to depths of up to 15 feet by a constant rain of debris, ranging from fine ash to boulder-sized cinders to viscous mud. Leaves, branches, and bark were stripped away; the tops of the trees snapped off or decayed; but beneath the ash many roots were firm and many trunks still stood. Between eruptions, as the devastation was slowly covered by soil and new forests, water seeped down through the ash and leached silica from it; and gradually this silica replaced the living tissue of the trees with quartz, beautifully patterned and bronzed by oxides of iron. The petrified trees had passed a boundary rarely crossed, moving from the organic to the geologic, from the temporal to the eternal.

After dwindling and sputtering for many millions of years, those ancient volcanoes finally died out. Over the eons, most of their cones were eroded away; further destruction took place under the crushing weight of glaciers that pushed down from the Beartooth Mountains to the north some 2½ million years ago.

But the volcanism of ancient times was not dead, merely trapped. Two giant pockets of molten rock and superheated gas were less than a mile below the surface. Between further periods of glaciation, when heavy rivers of ice gouged the land and weighed it down, there were times when the tightly capped pressure had to be released. Then the ground bulged, stretched, and cracked like the unvented crust of a bubbling fruit pie. Finally and suddenly, in giant explosions similar to those that formed Crater Lake, but hundreds of times larger, perhaps the largest explosions the world has ever known, the trapped material burst through the surface. Ash, rocks, and fire spewed across the continent. Three times this happened, 2,000,000 years ago, 1,300,000 years ago, and 600,000 years ago. It was the third explosion that created today's landscape.

Hollowed out, the land collapsed into itself, creating a vast pit, or caldera, the word is Spanish for "cauldron", nearly a mile deep and up to 50 miles across. Still the volcanic fury continued. Over the next 500,000 years, at least 30 lava flows filled the caldera, erasing all but the largest traces of it, such as the fragmented mountains that outline parts of its rim.

These lava flows are the dark gray, pink, and brown rocks seen almost everywhere in Yellowstone today; most of it is rhyolite, a rough-textured rock built up by layers of flowing lava. Earlier volcanism left basalt, the harder, purer rock seen in the columns of the cliffs near Tower Falls. The glassy black face of Obsidian Cliff, in the northwestern part of the park, is a rare result of silica-rich lava's having pushed upward and cooled so quickly that few minerals could crystallize in its substance. Indian peoples made regular journeys to gather the brittle black obsidian, hard enough to scratch glass.

Long after the main caldera collapsed, yet another, smaller explosion and collapse formed the depression that was to become the West Thumb of Yellowstone Lake. Meanwhile, glaciers came and went, the last one melted here 10,000 to 15,000 years ago, and earth-quakes and other violent phenomena cracked the comparatively thin shell of lava-born rock that lies between the surface and the primal heat of the planet.