A caldera is a large, often circular or oval-shaped geological depression. Unlike typical volcanic craters, which form from explosive ejection, calderas result from a volcano collapsing into itself. The term “caldera” originates from the Spanish word for “kettle,” reflecting their bowl-shaped nature.
How Calderas Form
Caldera formation begins with a shallow magma chamber beneath a volcano. When a substantial volume of magma is rapidly ejected during a powerful eruption, the underlying chamber becomes partially or largely emptied. This removal of molten rock compromises the structural integrity of the overlying ground, including the volcano’s edifice. The weight of the unsupported rock above the emptied chamber becomes too great for the remaining structure to bear.
This process involves the development of a roughly circular fracture system, known as ring faults, around the edges of the magma chamber. These ring faults act as conduits for escaping magma during the eruption. Once the magma chamber is sufficiently depleted, the ground surface within the ring faults begins to subside and collapse inward, creating the characteristic large depression. This collapse can occur as a single, catastrophic event or in stages following a series of eruptions.
Distinctive Features of Calderas
Calderas are expansive, basin-like depressions, often measuring over a kilometer in diameter and sometimes extending for dozens of kilometers. They are rimmed by steep, inward-facing cliffs or scarps, remnants of the collapsed volcanic cone. These features frequently fill with water, forming scenic caldera lakes, such as Crater Lake in Oregon.
Renewed volcanic activity within some calderas can lead to new cones or domes on the caldera floor after the initial collapse. A resurgent dome, an uplifted area within the center of the caldera, is found in some of the largest calderas. This dome forms as new magma pushes the collapsed floor upwards, indicating continued magma movement in the underlying reservoir.
Notable Calderas Around the World
The Yellowstone Caldera, located primarily in Wyoming, is one of the world’s largest calderas. It is an immense volcanic complex, approximately 50 by 70 kilometers (30 by 45 miles) in size, formed by multiple massive eruptions over millions of years. The geothermal features of Yellowstone National Park, including its geysers and hot springs, are direct manifestations of the active magmatic system beneath this caldera.
Crater Lake in Oregon formed about 7,700 years ago from the collapse of Mount Mazama. This caldera is notable for its deep blue water, which fills the 8 to 10-kilometer (5 to 6-mile) wide depression. Within Crater Lake, Wizard Island stands as a post-caldera volcanic cone that emerged after the main collapse.
The Santorini caldera in Greece is a largely submerged caldera, forming the iconic crescent-shaped island group in the Aegean Sea. This caldera, approximately 12 by 7 kilometers (7.5 by 4.3 miles) with steep cliffs rising 300 meters (980 feet) above the sea, formed from a massive eruption around 3,600 years ago. Its ongoing volcanic activity makes it a geologically active site.