Of Yellowstone Park’s 3475 square miles, about 1000 square miles are composed of the Yellowstone Caldera. (The exact area of the caldera is uncertain because of erosion). “Caldera” is Spanish for “cauldron” which is a good description of this landform. A caldera is formed when a volcano explodes, forming a mountain, which collapses from the weight of the lava.
The caldera is bounded by Mount Washburn in the north and Mount Sheridan in the south. It includes the Midway Geyser and Upper Geyser Basins and much of Yellowstone Lake.The Yellowstone caldera was formed by a volcanic explosion, which took place 650,000 years ago. It overlies older calderas and lies northeast of the smaller Island Park caldera. A hotspot, or pocket of molten rock over 400 miles tall lies 30 miles beneath the earth’s surface under Yellowstone. Hotspots occur all over the globe and their cause in uncertain, but it is known that over the past 16.5 million years the hotspot under Yellowstone has produced over 140 volcanic explosions. The earliest known caldera is located at the Idaho-Nevada-Oregon border, and a pathway of calderas is seen in a southwest-northwest direction because the Earth’s crust has been migrating southwest over the hotspot since it was formed.
Mount Washburn is the highest peak in the Washburn Range, reaching 10,223 feet at its summit. It was named for explorer Henry D. Washburn, leader of the Washburn expedition in 1870.
Mount Sheridan’s summit is 10,308 feet high and is named for General Philip Sheridan, one of the park’s first superintendents. It was named by a member of the Hayden Expedition in 1871.
Midway Geyser Basin, along Firehole River, includes Excelsior Geyser, which throws over 4000 gallons of water into the river per minute, and the Grand Prismatic Spring, the largest hot spring in Yellowstone. The spring is 370 feet wide and 121 feet deep.
Upper Geyser Basin contains the famous Old Faithful Geyser, Castle Geyser, Grand Geyser, Daisy Geyser and Riverside Geyser. All erupt at regular intervals. Castle Geyser, which appeared much like a castle back in 1870, has seen some erosion since then, but its basic shape still gives some idea of how it got its name. Its large eruptions occur about every 13 hours and its smaller ones erupt at irregular intervals. Grand Geyser, as its name suggests, is the tallest of the predictable geyser, with fountains rising as much as 200 feet for 9 to 12 minutes. Each eruption consists of from 1 to 4 or 5 bursts. Daisy Geyser’s eruptions, 75 feet tall, erupt every 110 to 240 minutes for 3 to 5 minutes. The 2002 Denali earthquake in Alaska temporarily affected Daisy Geyser. For a few weeks after the earthquake the geyser erupted more frequently, but then returned to its usual activity. Riverside geyser erupts 75 feet over the Firehole River, sometimes making rainbows.
Biscuit Basin and Black Sand Basin are also part of the Upper Geyser basin. The hills defining the basin were formed from lava flows much later than the eruption that formed the caldera.
The latest lava flow from the caldera, about 70,000 years ago, formed the West Thumb of Yellowstone Lake. The water is so hot in West Thumb Geyser Bay that fishing is off limits. The Bay is a caldera within a caldera. Other thermal features have been observed underwater in the Thumb, and melted ice can be seen over them in the winter.
A cluster of earthquakes struck around the caldera last year. Geologists constantly monitor the caldera and the entire Yellowstone region in an attempt to predict the next caldera-producing volcanic eruption.
Image Citation: “Windows into the Earth, The Geologic Story of Yellowstone and Grand Teton National Park”, Robert B. Smith and Lee J. Siegel, Oxford University Press, 2000.