3. Calderas Dan Barker March 2009 Ngorongoro, Tanzania

Calderas are volcanic collapse depressions greater than 1 km in diameter. The biggest caldera so far recognized on Earth is Lake Toba in Indonesia, 35 by 100 km. Some calderas form by collapse along arcuate faults, others by sagging without faulting, and others by piecemeal collapse of sections between intersecting short faults. According to an old idea, a caldera is formed by catastrophic emptying of an underlying magma chamber by eruption of a large-volume pyroclastic flow; for each caldera there was supposed to be a single corresponding pyroclastic flow deposit. We now know that some calderas have produced more than one pyroclastic flow, and others have produced none, because magma drained laterally through dikes low on the flanks of the volcano.

The potentially short lifetime of a caldera is illustrated by Kilauea's (foreground), with a volume of 1.4 km 3. If all the magma were to erupt in the caldera, not on the flanks, at the average rate of 3.4 to 3.5 m 3 /sec, the caldera would be filled in a few tens of years.

Crater Lake in Oregon is a beautiful example of a caldera that formed on a stratovolcano.

The cliffs surrounding Darwin Bay in the Galapagos Islands are the walls of a caldera that formed on a now -submerged basaltic shield volcano.

Rana Kau on Rapa Nui is another caldera on a basaltic shield volcano.

Kona caldera, Gariboldi Pass, Ethiopia, is the collapsed top of a trachyte shield volcano.

Famed Ngorongoro Crater in Tanzania is another caldera on a basaltic shield volcano.

Here's more of Ngorongoro.

Las Canadas caldera on Tenerife, Canary Islands, formed on a large trachyte/phonolite shield volcano, and then large stratovolcanoes started to fill in the caldera.

Furthermore, the floor of Las Canadas caldera is being covered by lava domes and scoria cones.

A wall and floor of Fogo caldera, Cape Verde Islands. The surviving members of Magellan's crew first realized that they had circled the globe when they saw Fogo and said "Hey, we saw this before!" but in 16th century Spanish.

The eastern part of Fogo caldera (right) fell into the sea in a major flank collapse.Scoria cones cover part of the floor.

After the eastern flank of Fogo collapsed,a stratovolcano grew and was persistently active for centuries. The 1951 vent is in the foreground in this 1988 photo, but apparently was destroyed in 1995.

Some large calderas are not on top of stratovolcanoes or shield volcanoes, but are at the centers of broad depressions and are surrounded by thin aprons of pyroclastic debris. The eruptions were so energetic that debris could not accumulate near the vents. Taupo on the North Island, New Zealand, and Campi Flegrei in Italy, are examples. They are not very photogenic. This is the Taupo caldera.

Commonly, when a caldera floor collapses, parts of the walls break off and become parts of the caldera fill.This is Tortilla caldera, AZ.

The reddish blocks fell from a wall of Caliente caldera, NV, forming megabreccia.

These blocks slid across the Caliente caldera floor, deforming lake sediments.

The Green Tuff (topmost unit) filled its caldera and then slopped over the caldera wall at Scauri, Pantelleria.

Here is another view of the Green Tuff, thick inside the caldera (right) and thin where it spilled over the rim in the center of the photo.

Some calderas, especially large ones, have uplifted portions of floor or caldera fill, called resurgent domes. This is Sour Creek dome at Yellowstone caldera.

The largest well preserved caldera close to Texas is the Valles caldera, north- central New Mexico.

This is the "moat" in the southeastern part of the Valles caldera.

A resurgent dome fills part of the Valles Caldera. For more on this caldera, see the session on Lava Domes.

These walled depressions in agricultural land on Sao Nicolau Island, Cape Verdes, are called caldeiras. They are supposed to retain moisture around plants. The sandstorm isn't helping any.