However once sea floor spreading established itself with the creation of the Southern Ocean, the southern marging of Australia changed to a passive margin, located far from the mid ocean ridge plate boundary. Some of the detailed information which enables us to understand these processes is summarised in the following illustrations, shown in the lectures. Note that the mid-ocean ridges are not continuous, but characeristically cut but curved faults. These are called transform faults, forming sets of parallel faults, defining circles around a point of rotation. Indeed the seven plates of the lithosphere can only move by developing these transform faults.
Transform faults have an important effect on plate boundary activity.
Consider the case of New Zealand. Further south, plate movement is accomodated by transform fault rather than subduction. Such large scale movement deforms rocks to produce characteristic fabrics which will be described next week. The second example to illustrate the relationship between sea floor spreading and transform faults is along the west coast of America. Further south, plate movement is accomodated by a major transform fault and there is no active volcanism.