Collapse Pit - John Seach
Pit Crater

Summit pit craters are found in many types of volcanoes and are generally thought to be the
product of collapse into an underpressured reservoir caused by magma withdrawal.
Three basic collapse mechanisms are dependant on the roof aspect ratio (roof thickness/roof width).
One occurs at low aspect ratios (<1)
An intermediate mechanism occurs at aspect ratios of 1.1-1.2.
A third mechanism occurs at ratios >1.2

Examples of Pit Craters
Kilimanjaro (Tanzania), San Cristobal, Telica and Masaya (Nicaragua), and Ubinas (Peru).
 

Collapse Pit
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