A caldera is a large circular hollow, that can have a diameter ranging from one kilometer to over 25 kilometers, and are usually formed by the collapse of surficial rocks into an emptied or unsupported magma chamber. A result of these collapsing rocks are a series of concentric normal faults, also known as ring faults. The faults do not form a continuous ring around the caldera, but displacement from one fault overlaps with the next, forming a circular pattern (Fig. 1). The faults are terminated by the chamber boundary, therefore displacement only occurs in the rock layers above the chamber. Calderas can become highly fragmented if collapse occurs in successive volcanic events. Ring faults are not just associated with calderas, but can occur whenever there is a pocket or hole below the surface where rocks can cave into it. An example of a caldera is Crater Lake on Mount Mazama in Oregon, whose diameter is 9 km.
Another structural boundary of calderas are indicated by ring dikes. Ring dikes are subsurface concentric dikes, where magma usually fills existing fractures. Ring dikes are a subsurface expression of the circular pattern occurring at the surface. They tend to dip vertically or outwards at high angles. In some cases, the fractures due to faulting act as conduits for the magma, not only for dikes, but for a future volcanic event.
Melissa Nihsen, 2/21/97Return to Structural Geology Notebook index page.>