A caldera is a large, basin-shaped volcanic depression that forms as a result of a massive collapse. This geological structure represents some of the most significant events in Earth’s history. The term “caldera” comes from the Spanish word for “kettle” or “cauldron,” aptly describing the immense, sunken shape of the feature. Unlike many other volcanic landforms, a caldera is created by the inward subsidence of the ground, not the explosive outward ejection of material.
Defining the Geological Structure
A caldera is defined by its substantial size, typically measuring one mile (about 1.6 kilometers) or more in diameter. These features are characterized by steep, inward-facing scarps or walls, which mark the boundary where the overlying rock dropped down. The floor of the caldera is often relatively flat, and the structure can range in shape from nearly circular to elliptical.
The distinction between a caldera and a volcanic crater lies in their formation process and scale. A crater is a smaller, bowl-shaped depression formed by the explosive removal of material from the central vent. Conversely, a caldera forms from the subsidence or collapse of the ground into an evacuated space beneath the surface. Craters span up to a couple of kilometers across, whereas calderas commonly span many tens of kilometers in width.
The Catastrophic Process of Formation
The creation of a caldera is a geological event resulting from the sudden removal of underground support, most often following a massive eruption. This process begins when a shallow magma chamber beneath a volcano empties rapidly, either through a colossal explosive eruption or through the extensive withdrawal of molten rock. The volume of magma ejected during these events can be immense, removing support for the rock mass above the chamber.
As the magma chamber is drained, the unsupported roof of the volcano fractures and collapses inward under its own weight. This collapse occurs along a roughly circular or ring-shaped pattern of faults, known as ring fractures. The overlying rock mass then subsides like a piston into the voided reservoir, forming the steep-walled depression that defines the caldera. The total area that collapses can encompass hundreds of square kilometers.
Distinguishing Features and Variations
Following the initial collapse, calderas can evolve in different ways. One significant variation is the formation of a “resurgent caldera,” which is marked by a broad, elevated central mass called a resurgent dome. This dome forms when the magma chamber beneath the collapsed floor is partially refilled, and the pressure from the new magma pushes the caldera floor upward.
Resurgent calderas are very large, often exceeding 8 miles (about 13 kilometers) in diameter, and are associated with viscous, silica-rich magmas. Post-caldera activity includes the formation of lakes, known as crater lakes, when the depression fills with water. Smaller volcanic cones or lava domes may also grow on the caldera floor, and extensive hydrothermal features, such as geysers and hot springs, are common due to the underlying heat source.
Famous Caldera Locations
Several famous examples illustrate the scale of caldera formation. The Yellowstone Caldera in Wyoming, United States, is a prime example of a massive resurgent caldera. This structure, measuring approximately 30 by 45 miles (50 by 70 kilometers), was formed by three immense eruptions over the last 2.1 million years. The area remains active today, featuring numerous hydrothermal features driven by the underlying magma system.
Another well-known example is the caldera formed by the 1883 eruption of Krakatoa in Indonesia, which involved a dramatic, visible collapse. Crater Lake in Oregon, United States, is a classic example, formed by the collapse of Mount Mazama about 7,700 years ago. The depression subsequently filled with water to become the deepest lake in the country.