Collapses form when a large magma chamber is emptied by a volcanic eruption or by an underground magmatic movement. The unsupported rock that forms the roof of the magma chamber then collapses, forming a large crater. It is believed that the crater lake and many other calderas formed through this process. A caldera is a great depression that forms when a volcano erupts and collapses. Calderas in spaceThe Earth is not the only planet to have calderas. Other planets have them too, including Venus and Mars. The moon also has calderas. Non-explosive calderas form at the top of shield volcanoes during particularly large eruptions of lava flows, either at the top or along the crack areas of the flank. A caldera (/kɔːlˈdɛrə, kæl-/[1] kawl-DERR-ə, kal-) is a large cauldron-shaped hollow that forms shortly after emptying a magma chamber during a volcanic eruption. If large amounts of magma are excavated in a short period of time, the structural support of the rock above the magma chamber is lost. The surface of the ground then collapses in the emptied or partially emptied magma chamber, leaving a great depression on the surface (from one to tens of kilometers in diameter).

[2] Although sometimes described as a crater, the feature is actually a kind of sinking and collapse, as it is formed by subsidence and collapse rather than an explosion or impact. Compared to the thousands of volcanic eruptions that occur every century, the formation of a caldera is a rare event that occurs only a few times a century. [3] It is known that only seven caldera-forming collapses occurred between 1911 and 2016. [3] More recently, a caldera collapse occurred in Kīlauea, Hawaii, in 2018. [4] Ash and pumice eruptions: The catastrophic eruption began in a vent on the northeast side of the volcano as an imposing ash column, with pyroclastic currents spreading northeastward. Caldera collapse: As magma entered, cracks opened around the summit which began to collapse. Fountains of pumice and ash surrounded the collapsing summit, and pyroclastic currents descended on all sides of the volcano. Steam explosions: When the dust settled, the new caldera had a diameter of 5 miles (8 km) and a depth of 1.6 km (1 mile). Groundwater interacted with hot deposits, causing explosions of steam and ash. Today: In the first hundreds of years after the catastrophic eruption, new eruptions built Wizard Island, Merriam Cone and the central platform. The water filled the new caldera and formed the deepest lake in the United States.

Illustration modified from diagrams on the back of the 1988 USGS map «Crater Lake National Park and Vicinity, Oregon». Illustration and legend of the United States Geological Survey. When an erupting volcano empties a shallow magma chamber, the volcano building can collapse into the empty reservoir, forming a steep bowl-shaped depression called a caldera (Spanish for cauldron or cauldron). These characteristics are very variable in size and range from 1 to 100 km in diameter. In most cases, they are easily distinguished from summit craters, which are usually much smaller and formed by the explosive erosion of the central vent. The rocky calderas are surrounded by thick rock covers of pumice stone from the eruption of voluminous pyroclastic streams of leaves. Explosive caldera eruptions are produced by a magma chamber whose magma is rich in silica. Silica-rich magma has a high viscosity and therefore does not flow easily like basalt.

[10]: 23–26 Magma generally also contains a large amount of dissolved gas, up to 7% by weight in silicon dioxide-rich magmas. [16] As magma approaches the Earth`s surface, the drop in enclosure pressure causes trapped gases to quickly gush out of the magma and fragment the magma to create a mixture of volcanic ash and other tephras with the very hot gases. [17] The summit regions of many active shield volcanoes are characterized by calderas. Hawaiian examples are the Mokuaweoweo caldera on Mauna Loa and the Kilauea caldera on kilauea. Others are the Erta Al Caldera in Ethiopia, the summit caldera of Piton del la Fournaise on Reunion Island and the spectacular basaltic caldera on the shield volcanoes of the Galapagos Islands. Most basalt shield volcanic calderas on Earth have a diameter of 1 to 5 km. However, those observed on Mars are extraordinarily large, the largest being the Caldera Olympus Mons with a diameter of more than 60 km! For their 1968 work,[6] which first introduced the concept of a resurgent caldera into geology,[5] R.L. Smith and R.A. Bailey chose the Valles caldera as a model. Although the Valles caldera is not exceptionally large, it is relatively young (1.25 million years old) and exceptionally well preserved[30] and remains one of the best-studied examples of a resurgent caldera.

[5] The ash-flowing tuffs of the Valles caldera, such as the bandelier tuff, were among the first to be carefully characterized. [31] Explosive calderas form during silica-containing magma eruptions that emit high-volume ash stream tuffs and form ultra-plinian eruptive columns. Caldera Demonstration: This video shows a teaching activity that clearly shows how a caldera is formed. It can be difficult to explain or draw how a caldera forms. This table model is a great demonstration. Teachers can do this activity with their students or simply show the video in class with a computer projection. Dina Venezky and Stephen Wessells, 2010, Caldera Demonstration Model: U.S. Geological Survey Open-File Report 2010-1173.

Explosive calderas form when very large magma chambers filled with silica-rich cast iron and abundant gas move upwards from the depths. Silica-rich magmas have a very high viscosity, which allows them to retain gas bubbles under very high pressures. As they rise to the surface, the reduction in pressure causes the gases to expand. When a breakthrough occurs, the result can be a huge explosion that carries away large amounts of rock to form the caldera. Some of these explosions emit many cubic kilometers of magma and rock. Aniakchak Caldera in Alaska: The Aniakchak Caldera in the Aleutian Islands of Alaska formed during a huge explosive eruption that emitted more than 50 km3 of magma about 3,450 years ago. The caldera has a diameter of 10 kilometers and is 500 to 1,000 meters deep. Subsequent eruptions formed domes, ash cones, and explosion pits on the caldera floor. Enlarge the image.

The mixture of volcanic ash and gas initially rises into the atmosphere in the form of an eruption column. However, as the volume of erupting matter increases, the eruption column is unable to absorb enough air to remain floating, and the eruption column collapses into a tephra well that falls to the surface to form pyroclastic currents. [18] Eruptions of this type can spread ash over large areas, so the ash flow heels placed by caldera eruptions containing silica are the only volcanic product whose quantities compete with those of flood basalts. [10]: 77 For example, when the Yellowstone caldera last erupted about 650,000 years ago, it released about 1,000 km3 of matter (measured in dense rock equivalent (DRE)) that covered a significant part of North America with up to two meters of debris. [19] Mauna Loa Volcano: Snow-capped Moku`aweoweo Caldera on Mauna Loa Shield Volcano (Mauna Kea in the background) on the island of Hawaii. The caldera is 3 x 5 km wide, 183 m deep and would have collapsed 600 to 750 years ago. Several pit craters along the upper southwest zone of the Mauna Loa Rift (bottom right) were also formed by ground collapse. Image and legend of the USGS. Enlarge the image.

Note: A crater is formed when a volcano explodes outward, but a caldera forms when a volcano collapses inward. A caldera is a depression that forms after a volcano partially collapses after releasing most of its magma chamber during an explosive eruption. Explosive calderas result from violent eruptions of large amounts of magma containing silica. These eruptions produce massive eruption columns that extend into the stratosphere and voluminous pyroclastic currents. Eruptions that produce explosive calderas typically range from 6 (colossal) on the Volcanic Explosive Index (VEI) to 8 supereruptions (apocalyptic). The different types of explosive calderas vary in size, eruption strength, diameter, age, and whether they formed on top of a pre-existing volcano.