West Thumb Geyser Basin

West Thumb was a photographic winter wonderland when I was there. It was a postcard picture at every turn. Rime ice was on every tree and every exposed surface. The West Thumb Geyser Basin including Potts Basin to the north, is the largest geyser basin on the shores of Yellowstone Lake. The heat source of the thermal features in this location is thought to be relatively close to the surface, only 10,000 feet down. West Thumb is about the same size as another famous volcanic caldera, Crater Lake in Oregon, but much smaller than the great Yellowstone Caldera which last erupted about 640,000 years ago. It is interesting to note that West Thumb is a caldera within a caldera.

West Thumb was created approximately 162,000 years ago when a magma chamber bulged up under the surface of the earth and subsequently cracked it along ring fracture zones. This in turn released the enclosed magma as lava and caused the surface above the emptied magma chamber to collapse. Water later filled the collapsed area of the caldera, forming an extension of Yellowstone Lake. This created the source of heat and water that feed the West Thumb Geyser Basin today.

The thermal features at West Thumb are not only found on the lake shore, but extend under the surface of the lake as well. Several underwater hydrothermal features were discovered in the early 1990s and can be seen as slick spots or slight bulges in the summer. During the winter, the underwater thermal features are visible as melt holes in the icy surface of the lake. The surrounding ice can reach three feet (one meter) in thickness.

Perhaps the most famous hydrothermal feature at West Thumb is a geyser on the lake shore known as Fishing Cone. Walter Trumbull of the 1870 Washburn-Langford-Doane Expedition described a unique event while a man was fishing adjacent to the cone: "...in swinging a trout ashore, it accidentally got off the hook and fell into the spring. For a moment it darted about with wonderful rapidity, as if seeking an outlet. Then it came to the top, dead, and literally boiled." Fishing Cone erupted frequently to the height of 40 feet in 1919 and to lesser heights in 1939. One fisherman was badly burned in Fishing Cone in 1921. Fishing at the geyser is now prohibited.

Early visitors would arrive at West Thumb via stagecoach from the Old Faithful area. They had a choice of continuing on the stagecoach or boarding the steamship Zillah to continue the journey by water to Lake Hotel. The boat dock was located near the south end of the geyser basin near Lakeside Spring.(wikipedia)