What was the biggest volcanic eruption in the United States ?

It occurred about 15 million years ago in what is now Washington state. The explosion spewed out 600 cubic miles of material. The largest this century happened in 1912 in Alaska where a volcano sent 15 cubic miles of ash, rock, and magma into the air. In comparison, Mount St. Helens only exploded 1/2 cubic mile of material.

I suspect that this one was bigger.

Long before any recorded human history in Yellowstone, a massive volcanic eruption spewed an immense volume of ash that covered all of the western U.S., much of the Midwest, northern Mexico and some areas of the eastern Pacific Coast. The eruption dwarfed that of Mount St. Helens in 1980 and left a huge caldera 43 miles by 18 miles (70 km by 30 km) sitting over a huge magma chamber (see Geology section and Yellowstone Caldera)

It wasn't until the 1960s and 70s that it was realized that Yellowstone is one huge bowl that is a volcanic crater. The good news is that the huge eruption was long before recorded history. The bad news is that the ground in many areas of Yellowstone is bulging, with some parts 100' higher than it used to be. Norris Geyser in Yellowstone is spewing hotter water than it used to. Any guess as to what is causing all this?

The Yellowstone Caldera, sometimes known as the Yellowstone supervolcano, is a volcanically active region in Yellowstone National Park. It measures 55 kilometers (34 mi) by 72 kilometers (44 mi). The caldera was discovered based on geological field work conducted by Bob Christiansen of the United States Geological Survey in the 1960s and 1970s.
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Supervolcano Questions

What is a supervolcano?

The term "supervolcano" implies an eruption of magnitude 8 on the Volcano Explosivity Index, meaning that more than 1,000 cubic kilometers (240 cubic miles) of magma (partially molten rock) are erupted. The most recent such event on Earth occurred 74,000 years ago at the Toba Caldera in Sumatra, Indonesia.



What are some other examples of supervolcanoes?

Volcanoes that produced exceedingly voluminous pyroclastic eruptions and formed large calderas in the past 2 million years would include Yellowstone, Long Valley in eastern California, Toba in Indonesia, and Taupo in New Zealand. Other "supervolcanoes" would likely include the large caldera volcanoes of Japan, Indonesia, and South America.

Yellowstone Volcano Observatory