Volcanism in the United States is geographically widespread, representing a diverse range of geological processes that have shaped the continent and its territories. This activity spans from explosive mountain ranges to remote island chains and ancient, massive calderas. The volcanic features across the country vary in age, from deeply eroded ancient structures to currently active sites that are constantly monitored by scientists. The geological settings that produce this volcanism include the convergence of tectonic plates, the movement of a plate over a stationary mantle plume, and unique stretching within the continental crust.
Volcanic Arcs of the Pacific Coast
The Pacific coast of the contiguous United States is defined by the Cascade Volcanic Arc, a chain of high, snow-capped peaks extending from northern California through Oregon and Washington. This arc is a direct result of the Cascadia Subduction Zone, where the Juan de Fuca tectonic plate dives beneath the larger North American plate. This process causes the subducting plate to release water into the overlying mantle, generating magma that rises to the surface.
The volcanoes here are predominantly stratovolcanoes, also known as composite volcanoes, characterized by their steep, conical shape. They are built up from alternating layers of hardened lava, ash, and pumice. The magma feeding these volcanoes is typically viscous, meaning it is thick and sticky, which traps gases and leads to highly explosive eruptions.
High-profile examples include Mount St. Helens in Washington, famous for its catastrophic lateral blast in 1980, and Mount Rainier, the highest peak in the arc at 14,411 feet. Rainier is considered hazardous due to its massive ice cover. Oregon is home to Mount Hood, which last erupted in the mid-19th century, and the remnants of Mount Mazama. Mazama collapsed to form Crater Lake following a massive eruption approximately 7,700 years ago. These mountains remain geologically active and are monitored for signs of renewed unrest.
Hotspots and Remote Volcanic Fields
Volcanism in the remote regions of the US is governed by two different mechanisms: a deep mantle plume and an oceanic subduction zone. The Hawaiian Islands are the result of a stationary hotspot deep within the Earth’s mantle, over which the Pacific Plate slowly moves. This mechanism has created the Hawaiian-Emperor seamount chain, a 3,900-mile-long trail of volcanoes, with the youngest located over the plume.
The volcanoes of the Big Island of Hawai‘i, such as Kīlauea and Mauna Loa, are classic shield volcanoes. They are characterized by broad, gently sloping profiles, resembling a warrior’s shield. This shape is due to the eruption of fluid, basaltic lava that flows easily and spreads out over great distances before cooling. Mauna Loa, the largest active volcano on Earth, has been erupting for at least 700,000 years and frequently erupts alongside Kīlauea.
Alaska contains the Aleutian Volcanic Arc, a 1,600-mile-long chain of volcanoes that forms the boundary between the Pacific and North American plates. This arc is a subduction zone system, similar to the Cascades, where the Pacific plate is diving beneath the North American plate. The arc includes approximately 80 volcanic mountains, with about 41 having been active in historical times. The Aleutian volcanoes are primarily composite cones, featuring explosive eruptions that often produce ash clouds impacting air traffic.
Interior and Ancient Volcanic Features
Away from the major plate boundaries, the interior of the US contains unique volcanic features, most notably the colossal Yellowstone Caldera. Located primarily in Wyoming but extending into Montana and Idaho, this feature is the surface expression of the Yellowstone Hotspot, a mantle plume beneath the North American continent. The present caldera, measuring about 30 by 45 miles, was formed by the third of three massive, caldera-forming eruptions, the most recent of which occurred 640,000 years ago.
The Yellowstone system is often called a “supervolcano” due to the immense volume of magma ejected during its caldera-forming events. While no magmatic eruption has occurred in about 70,000 years, the system remains active, evidenced by its hydrothermal features, including geysers and hot springs. The caldera floor itself undergoes cycles of uplift and subsidence, driven by the movement of magma and hydrothermal fluids beneath the surface.
Other interior fields represent significant ancient or sporadic volcanism across the Southwest. The San Francisco Volcanic Field in northern Arizona, near Flagstaff, covers about 1,800 square miles and contains over 600 volcanoes. Most of these features are small, monogenetic basaltic cinder cones that erupted only once. Sunset Crater, the youngest feature in the field, last erupted between 1080 and 1150 CE. This area demonstrates that volcanism can occur far from plate edges, often due to localized stretching or heating of the continental crust.