According the USGS, there are 169 volcanoes in the U.S. that are considered active. Please note that active is not with synonymous with erupting. Most are located in Alaska. There are eleven other states with active volcanoes: Arizona, California, Colorado, Hawaii, Idaho, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming. Aside from steam eruptions, and excluding the volcanoes of Hawaii and Alaska, three volcanoes have erupted since the U.S. was founded: Mount St. Helens, Washington ; Lassen Peak, California ; Mount Hood, Oregon.
Q. Where is there evidence of volcano calderas in Missouri? A. "The St. Francois Mountains is a region of formerly extentive [sic] volcanic terrane comprising several calderas, cauldron subsidence structures, ring intrusions, and resurgent cauldrons. Three calderas are recognized: the Taum Sauk, the Butler Hill, and the Hawn Peak [sic] [i.e., Hawn Park]" -- https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=MOpCAv%3B0
Q. Are any of Missouri calderas still capable of producing an eruption? A. No.
Q. Where is there evidence of volcanic activity in Arkansas? A. ""Crater of Diamonds offers park visitors a one-of-a-kind experience—the adventure of hunting for real diamonds. You’ll search over a 37 ½-acre plowed field that is the eroded surface of the world's eighth largest diamond-bearing volcanic crater. If you find a diamond, it is yours to keep." -- http://www.craterofdiamondsstatepark.com/
What type of eruption is likely if Yellowstone erupts again? According to the USGS, "The most likely explosive event to occur at Yellowstone is actually a hydrothermal eruption—a rock-hurling geyser eruption—or a lava flow. Though the worst-case scenario for a giant Yellowstone eruption is indeed bad, and could have global implications, most past eruptions at Yellowstone were not highly explosive. Of the past 50 or so eruptions, almost all were simple lava flows. If they occurred tomorrow, or next year, they would have minimal direct effect outside Yellowstone National Park." https://www.usgs.gov/faqs/what-type-eruption-will-occur-if-yellowstone-erupts-again?qt-news_science_products=0#qt-news_science_products
That La Garita experienced the largest eruption in what is now the U.S. is in dispute. Wah Wah Springs, Colorado may have been even larger.
"80 percent of the United States active volcanoes lie on Alaskan soil. Alaska boasts the planet's largest eruption in the last century: the 1912 eruption of Novarupta, 450 miles southwest of Anchorage in what is now Katmai National Park." -- Alaska Public Lands Information Centers, -- https://www.alaskacenters.gov/volcanoes.cfm
According to the USGS, Mauna Loa is the world's largest active volcano World's Largest Active volcano
Includes information on the volcanoes of California and the Cascades, excluding Mt. St. Helens. Mt. St. Helens information is located on a separate page.