Phlegethon Catena

This VIS image is located on the eastern flank of Alba Mons. Linear faults and graben surround the volcano, intersecting and deflected around the summit. The large graben is called Phlegethon Catena. The term catena means a string of craters or circular depressions. The circular depressions in this image were likely formed by the collapse of the preexisting surface into a subsurface void. Graben are formed by extension of the crust and faulting. When large amounts of pressure or tension are applied to rocks on timescales that are fast enough that the rock cannot respond by deforming, the rock breaks along faults. In the case of a graben, two parallel faults are formed by extension of the crust and the rock in between the faults drops downward into the space created by the extension. Several sets of graben are visible in this THEMIS image, trending from north-northeast to south-southwest. Because the faults defining the graben are formed perpendicular to the direction of the applied stress, we know that extensional forces were pulling the crust apart in the west-northwest/east-southeast direction. Phlegethon Catena is 400km long (248 miles).