Lassen Volcanic National Park, California

Lassen Volcanic National Park offers the only place in the lower 48 states where all four types of volcanoes—shield, cinder cone, stratovolcano, and plug dome—converge in one landscape, a geological feat unmatched by any other park on this list. Lassen Peak, the park's centerpiece, erupted violently in 1915, leaving behind steaming fumaroles and boiling mud pots that rival Yellowstone's hydrothermal features in raw drama but draw fewer than 500,000 annual visitors—over 3.5 million less than #1. This 30% lower visitation than the average national park allows for profound solitude amid volcanic wonders, making it a standout for geology enthusiasts seeking uncrowded exploration.