Christchurch earthquake breaches Castle Rock

The earthquake that tore into Christchurch also assaulted the Citadel of Kahukura - Te Tihi o Kahukura, or Castle Rock. Its upper ramparts were sent shaken down its Port Hills' slopes, tumbling and sliding some hundreds of metres.

Astoundingly, some ground to a halt just short of the Bridle Path and the Heathcote portal of the road tunnel. One boulder the size of a beer keg found its way under the Heathcote underpass on the motorway, coming to rest on the road just short of the Gondola entranceway!

As the photograph shows, some of the rocks are the size of a large truck, and no amount of tunnel-control persuasion would have prevented them from entering the tunnel if they had been of a mind. The main flow of debris from the fall has forever altered the silhouette of the Valley's central geological feature.

Castle Rock is an exposed 'dyke', or volcanic dome, with well-developed columnar jointing. Dykes are ridges formed of molten rock which, under enormous pressures, surge their way up through fractures and rents in the volcano's wall.

The greater hardness of the dyke structure, most resistant to erosion, has meant that the surrounding softer rocks have eroded far more quickly over the years, leaving the dykes exposed as spectacular outcrops. In this case, it appears that the central portion of the dyke has contained a large fracture or fractures and these have been given impetus by the jolt of the quake.

In one of the more pleasant ironies, amidst the local chaos, while the quake has managed to topple the most prominent part of the local geological landscape, that manmade blot on the landscape that is the Maltworks' silo has stubbornly refused to budge!