Yellowstone's volcanic and hydrothermal history suggests the potential for various kinds of eruptions in the future. The likelihood of a certain type of eruption occurring in the future can be judged by how often eruptions have occurred in the past.
The most likely type of eruption would not be volcanic but, rather, hydrothermal. This type of small, but still explosive eruption can occur from shallow reservoirs of steam or hot water rather than molten rock. These reservoirs are the sources of Yellowstone's famous geysers, hot springs, and fumaroles. Such explosions could blast out shallow craters more than a kilometer wide; as has occurred in the northern Yellowstone Lake Basin, including Mary Bay and nearby Turbid Lake and Indian Pond, and in western Yellowstone National Park north of Old Faithful.
A recent and notable hydrothermal explosion occurred in 1989 at Porkchop Geyser in Norris Geyser Basin. The remains of this explosion are still clearly visible today as an apron of rock debris 5 m (15 ft) across surrounding Porkchop’s central spring. In the 1880s and early 1890s, a series of powerful hydrothermal explosions and geyser eruptions occurred at Excelsior Geyser in the Midway Geyser Basin. Some of the explosions hurled large rocks as far as 15 m (50 ft).
The largest hydrothermal-explosion crater documented in the world is along the north edge of Yellowstone Lake in an embayment known as Mary Bay. This 1.5-mile (2.6 km)-diameter crater formed about 13,800 years ago and may have had several separate explosions in a short time interval. What specifically triggered these very large events is not firmly established, but earthquakes or a pressure release caused by melting glaciers or rapid changes in lake level may have been a significant factor.
These very large and violent hydrothermal explosions are independent of associated volcanism. None of the large hydrothermal events of the past 16,000 years has been followed by an eruption of magma. The deeper magma system appears to be unaffected even by spectacular steam explosions and crater excavations within the overlying hydrothermal system.