A caldera is a large depression formed when a volcano erupts and collapses. During a volcanic eruption, magma present in the magma chamber underneath the volcano is expelled, often forcefully. When the magma chamber empties, the support that the magma had provided inside the chamber disappears. As a result, the sides and top of the volcano collapse inward. Calderas vary in size from one to 100 kilometers (0.62 to 62 miles) in diameter.

Some calderas form a lake as the bowl-shaped depression fills with water. A famous example is Crater Lake, in Oregon. This caldera formed about 7,000 years ago when a stratovolcano, Mt. Mazama, violently erupted. For several thousand years after this eruption, smaller volcanic eruptions continued inside the caldera. One of these eruptions was so large it formed an island in Crater Lake named Wizard Island.

Calderas such as Crater Lake and those in Yellowstone National Park result from dramatic eruptions, but slower eruptions can also create calderas. This often occurs with shield volcanoes, which are typically flatter and more gradually sloped. Lava flows from shield volcanoes more slowly and often at regular intervals. Over time, this creates a series of nested depressions. The Kilauea caldera on Kilauea, one of the volcanoes that make up Hawai’i, is one example.

Another type of caldera is a resurgent caldera. These broad, vast calderas result when very large magma chambers empty quite forcefully, causing a series of pyroclastic flows. Over time, the refilling of the magma chamber pushes up the caldera floor. This upward movement is why the caldera is called resurgent, which means “risen again.”