Sector collapse

A sector collapse, also called a lateral collapse, is when a very large part of a volcano falls apart and slides down. The collapsed piece must be at least 1 km³ (0.24 cu mi) in size. Unlike smaller flank collapses, a sector collapse can also involve the central volcanic pipe. Some writers once used this term only for such events in arc stratovolcanoes, but today it is used for large events on any volcano.

Sector collapses are among the most dangerous volcanic events. They can cause lateral blasts, landslides, and changes in how a volcano erupts.

A sector collapse can be caused by earthquakes, volcanic eruptions, slow volcanic deformation, and other processes. These events can happen on volcanoes at convergent and divergent plate boundaries.

Sector collapses usually happen very suddenly. Some scientists have tried to predict when they will happen.

Causes

Sector collapse can happen because of processes inside a volcano. An eruption can damage magma chambers that were once stable, causing part of the volcano to fall apart. It can also happen without any eruption. Magmatic movement can lead to collapse too. Dikes crack and bend rock, which leaves the volcano weaker. Hydrothermal activity, likely caused by reactions between acid-sulfates, can weaken volcanic rock. Collapse can also happen when a steep slope reaches the critical angle of repose. The slope angle is a major factor in these events.

Outside forces can also cause collapse. Seismic activity is a major cause, since earthquakes can weaken a volcano. This can lead to a sudden collapse, or to one that happens later. Harsh weather and heavy rainfall can cause erosion, which makes collapse more likely. Glacial melting is another cause. Most glacial melt collapses happened during the Pleistocene. Melting ice makes the slope steeper and lowers pore pressure, which can lead to collapse. Sea level change has also been linked to sector collapse.

Sector collapse events happen suddenly and over short periods of time, so they are hard to predict. Instead, scientists usually judge how much risk a volcano has of collapsing. A volcano becomes structurally unstable when it is at risk of collapse. Scientists can judge this by its slope angle, its composition, its deformation, and other factors.

Consequences

Some volcanoes do not change much after a sector collapse. Others have different eruption patterns and types of magma. A collapse is often followed by a phreatic eruption, because the pressure in the magma chamber drops. The damage can also form a new volcanic plumbing system, which may change how often the volcano erupts. Sector collapse often leads to eruptions of more mafic magma. This is because the collapse removes some of the weight above, so the denser mafic magma can rise more easily.

Sector collapses and the landslides they cause have harmed people and damaged property. One especially dangerous effect is a tsunami. The Oshima-Oshima collapse caused a tsunami that hurt many people. These events can also force people to leave their homes, which leads to homelessness.

Prehistoric sector collapses are kept in the geological record as debris avalanche deposits and collapse scars. Debris avalanche deposits are usually found up to 20 km (12 mi) from the site of the collapse. In the case of the Hawaiian islands, they are found 200 km (120 mi) away. Studying these deposits helps us learn when the collapse happened and which volcano it came from. Collapse scars are also clues. They are often described as "amphitheatre" or "horseshoe" shaped. One example is the Sciara del Fuoco, which formed on the Stromboli volcano because of a sector collapse.

Examples

Prehistoric

• Antuco
• Chimborazo
• Popocatépetl
• Mount Rainier
• Mount Shasta
• Shiveluch
• Stromboli

Historic

• Anak Krakatoa, (/wiki/2018_Sunda_Strait_tsunami)
• Mount Bandai, (/wiki/1888_eruption_of_Mount_Bandai)
• Bezymianny, 1956
• Oshima-Oshima, (/wiki/1741_eruption_of_Oshima–Ōshima_and_the_Kampo_tsunami)
• Ritter Island, (/wiki/1888_Ritter_Island_eruption_and_tsunami): largest historical collapse
• Mount St. Helens, (/wiki/1980_eruption_of_Mount_St._Helens)