Yellowstone Natl.Park is home to the worlds most Geyser/Thermal areas. West Thumb is a portion of that around Lake Yellowstone. The large circulur bay off to the left is a excellent example of a volcanic crater/caldera which was cause by a powerful volcanic explosion around 150,000 years ago.The earth's crust collapsed creating the West Thumb Caldera. The depression produced by the volcano later filled with water to become the large bay of Yellowstone Lake.West Thumb caldera itself lies within an even greater caldera-one of the world's largest which encompasses the central and southern portion of Yellowstone Park.So basically Yellowstone sits on top of a collapsed Volcano. The Geyser in the foreground is called Twin Geyser. Twin Geyser has been characterized by short periods of dramatic eruptions and extended periods of dormancy.It was last active in 1975. It can erupt to a height of 70 to 100 feet(21-30 meters)