Yellowstone National Park's stunning vistas of mountains, valleys, lakes, and wildlife cover a terrible secret. The park's variety of thermal features and igneous rock layers in turn betray it. Roiling beneath Yellowstone is a magma hotspot; an up-swelling of material from deep within the earth that fuels the park's system of geysers and mineral springs, as well as the 1000-3000 earthquakes that happen there per year. Periodically, this hotspot has become so unruly that it vents itself in a pyroclastic fury of unimaginable scale. In its last major explosion 640,000 years ago, 240 cubic miles of ash and debris were thrown into the atmosphere, falling back to earth as far south as the Mexico border. Its caldera measures 34 miles by 44 miles across.
What exactly causes the Yellowstone Hotspot is unknown, but it has lain beneath North America for approximately 16 million years. As the continent moved through plate tectonic action, a succeeding number of volcanic blasts carved out the Snake River Plain that cuts a swath through southern Idaho, terminating at Yellowstone. Moist air from the Pacific channelled up this valley condenses and collapses on Yellowstone, dumping 150 to 300 inches of snow each winter. Some 2.1 million years ago, the hotspot arrived beneath Yellowstone. That was when the first of four eruptions happened that shaped the park as it stands today. A second and smaller explosion happened just outside the park's modern boundaries about 1.3 million years ago. The third happened 640,000 years ago, with the last minor eruption happening about 174,000 years ago. The caldera of this much smaller eruption was filled in by water from Lake Yellowstone, forming the West Thumb. The shore at West Thumb is dotted with thermal springs and minor geysers that have entertained and entranced generations. In the early days, when such activities were permitted, guests could fish in Yellowstone Lake and then swing their line over to a geyser, drop the fish in, and cook it on the spot.
|The "Black Pool."|
|The shoreline at West Thumb Geyser Basin,|
dominated by the "Big Cone" geyser.
|"Fishing Cone," where visitors would boil fish on the line.|
|Springs drain into Yellowstone Lake.|