The United States is home to a vast and diverse collection of volcanic structures, ranging from towering peaks in the Pacific Northwest to immense shield volcanoes in the Pacific Ocean. The question of which volcano is the largest is not easily answered with a single name, as size is determined by various geological metrics. Different forms of measurement lead to different volcanoes holding the title of “largest.” Understanding their scale requires moving beyond a simple measurement of height.
Understanding How Volcanic Size is Measured
Geologists use three primary criteria to assess the size of a volcano: elevation, area, and volume. Elevation is the most straightforward measurement, defined as the height of the summit above sea level, or subaerial height. This metric determines the tallest peaks that are visible and most often associated with mountain-like volcanoes.
The area, or footprint, measures the total surface extent of the volcano at its base, providing an indication of its overall breadth. This measurement is particularly relevant for shield volcanoes that spread out over a wide region.
Volume is often considered the best measure of a volcano’s true magnitude, as it quantifies the total mass of rock that makes up the entire edifice. Calculating volume requires taking into account the entire structure, from the base on the ocean floor or continental crust up to the summit. This complex calculation can result in a volcano that is not the tallest still being the most massive.
The Largest Volcano in the United States
When size is measured by volume and mass, which accounts for the entire structure, the largest volcano in the United States is Mauna Loa. Located on the Island of Hawaiʻi, Mauna Loa is an immense shield volcano built up by countless eruptions of fluid, low-viscosity lava over hundreds of thousands of years. This steady accumulation of material has created a mountain with broad, gentle slopes.
Mauna Loa constitutes over half of the area of the entire Island of Hawaiʻi. Its estimated volume is about 18,000 cubic miles (75,000 cubic kilometers), establishing it as the largest active volcano on Earth by mass and volume. This sheer bulk is what sets it apart from all other US volcanoes. Its most recent eruption occurred in late 2022.
The volcano rises approximately 13,679 feet (4,169 meters) above sea level. However, the bulk of the mountain lies beneath the ocean surface, extending downward about three miles to the seafloor. Measured from its true base on the ocean floor to its summit, Mauna Loa stands roughly 30,085 feet (9,170 meters) tall, making it significantly taller than Mount Everest measured from sea level.
Furthermore, the enormous weight of Mauna Loa has caused the Earth’s crust beneath it to depress by an additional 5 miles (8 kilometers) in a process called isostatic adjustment. The total vertical extent of the volcano, from the deepest point of the crustal depression to the summit, is approximately 56,000 feet (17,000 meters).
Notable US Volcanoes and Calderas
Mauna Kea, Mauna Loa’s neighbor on the Island of Hawaiʻi, is technically the tallest volcano in the US when considering only the portion above sea level. Mauna Kea’s summit reaches an elevation of 13,803 feet, slightly higher than Mauna Loa’s.
In the contiguous United States, the largest volcanic features are defined by different metrics entirely. The Yellowstone Caldera system, spanning parts of Wyoming, Montana, and Idaho, is not a single conical mountain but a massive, ancient collapse structure. The caldera itself is an immense basin, approximately 30 by 45 miles across, formed by colossal past eruptions.
Yellowstone is considered one of the world’s largest explosive volcanic systems. Its size is measured by the vast volume of material erupted in past events, such as the 240 cubic miles (1,000 cubic kilometers) of material released 640,000 years ago. This makes it the largest system by scale in the continental US, defined by its enormous subterranean magma reservoir and resulting collapse structure.
Among the more traditionally shaped, conical volcanoes in the contiguous United States, Mount Rainier in Washington State is one of the most massive. Its summit stands at 14,411 feet above sea level, and it holds more glacial ice than any other peak in the contiguous US. Mount Shasta in northern California is another enormous stratovolcano, rising to 14,179 feet and representing a significant bulk of material in the Cascade Range.