Types and Processes Gallery

Craters
Craters differ from calderas both in size and origin. Craters are much smaller features than calderas and are typically defined as being less than 1 km in diameter. Although both craters and calderas are most often associated with explosive eruptions, craters are typically formed by the explosive ejection of material in and surrounding the upper part of the conduit, rather than by collapse. Steep-walled pit craters, in contrast, often found on shield volcanoes, are more passive features formed when magma drains from a fissure, leaving overlying lava flows unsupported. Multiple explosive eruptions can form overlapping or nested craters, and adjacent craters may reflect localized areas of eruption along fissures, as seen in craters rows in Iceland that extend for tens of kilometers. Maars and tuff rings are broad, low-rimmed craters formed during powerful explosive eruptions involving magma-water interaction. The accumulation of material ejected from craters contributes to the formation of their surrounding rims.

Chichinautzin
Xico is a low circular tuff ring within Mexico City that is part of the Chichinautzin volcanic field. It is seen here in an aerial view from the south. This volcano was formed by phreatomagmatic eruptions through the middle of Lake Chalco, whose remains can be seen surrounding the tuff ring. Chalco and Lakes Texcoco and Xochimilco were formed when lavas from the Chichinautzin volcanic field blocked river drainages to the south. The lakes were largely drained during the 16th and 17th centuries following the Spanish conquest.

Photo by Hugo Delgado, 1994 (Universidad Nacional Autónoma de México).


Cotopaxi
The glacier-capped summit of Ecuador's Cotopaxi volcano is truncated by two nested craters. The outer crater, seen here from the SE, is 800 x 550 m wide. A cone that grew inside this crater is cut by a smaller crater that is 250 m wide and 120 m deep. Frequent explosive eruptions during historical time have modified the shape of the summit crater. In 1903, prior to growth of a broad central cone, it was 450 m deep.

Photo by Tom Simkin (Smithsonian Institution).


Erebus
The summit of Antarctica's Mount Erebus, the world's southernmost historically active volcano, contains an elliptical, 500 x 600 m wide crater whose NE side is cut by the 250-m-wide Inner Crater. The flat, snow-covered floor of the Main Crater lies about 100 m below its rim. Steam rises from the steep-walled, 100-m-deep Inner Crater, which has a phonolitic lava lake that has been active since 1972.

Photo by Bill Rose, 1983 (Michigan Technological University).


Kelimutu
A double crater lake of Kelimutu volcano on Indonesia's Flores Island is seen in this aerial view from the SW. Tiwu Nua Muri Kooh Tai (Lake of Young Men and Maiden) on the left and Tiwu Ata Polo (Bewitched, or Enchanted Lake) are separated by a narrow septum about 35 m high. Phreatic eruptions have occurred from Tiwu Nua Muri Kooh Tai in the 19th and 20th centuries, and continuous upwelling occurs at both lakes.

Photo by Tom Casadevall, 1986 (U.S. Geological Survey).


Lengai, Ol Doinyo
The northern crater of Ol Doinyo Lengai is seen here in February 1980 from the summit of the volcano. The inner crater is a steep-walled pit that remained after the powerful explosive eruptions of 1966 and 1967. Three years after this photo was taken, another eruption began. Slow lava effusion completely filled in the inner crater. By December 1988, lava had overflowed the near southern crater rim, at the lower center of the photo.

Photo by Peterson, 1980 (courtesy of Celia Nyamweru, Kenyatta University).


Maly Semyachik
Troitsky crater, the youngest of six craters at the summit of Kamchatka's Maly Semiachik volcano, was formed during a major explosive eruption about 400 years ago. The crater, seen here from the west with the Pacific Ocean in the background, truncates the snow-covered slopes of Ceno-Semiachik, the youngest of the four overlapping stratovolcanoes that comprise the Maly Semiachik massif. The crater, now filled by a hot, acidic lake, has been the source of all historical eruptions from the volcano.

Photo by Dan Miller, 1990 (U.S. Geological Survey).


Pilas, Las
Lake-filled Laguna de Asososca maar, in the foreground, and the conical Cerro Asososca at the upper right were formed by eruptions at the southern end of a N-S fissure system of Las Pilas volcanic complex in Nicaragua. The ages of these vents are not known. This view looks from the NE across the broad plain at the foot of the Marrabios Range to the Pacific Ocean in the distance.

Photo by Jaime Incer, 1981.


Telica
This aerial view on July 1987 looks almost directly down into the double summit crater of Telica volcano in Nicaragua. An older shallower crater is located on the SW side (top). Steam rises from fumaroles in the NE crater (bottom), the source of recent eruptions. The steep-walled crater is 120 m deep. Ash from frequent historical eruptions keeps the outer slopes of the cone unvegetated.

Photo by William Melson, 1987 (Smithsonian Institution).