3. Conservative (transverse or transform) plate boundaries – fault lines
A conservative plate boundary is where two plates slide past each other. Land is neither created nor destroyed at these boundaries. These boundaries are marked by fault lines; they are thousands of kilometres long and up to eight km deep. They are associated with shallow earthquakes. Most occur under the sea. A few conservative boundaries occur on land. The best known is the San Andreas
Fault in California. This fault is 1,300 km long and tens of kilometres wide in places. At this fault the Pacific plate and the North American plate are both moving in a
northwesterly direction. The Pacific plate, however, is moving faster at a rate of about five cm per year. The slippage is not smooth and the plates may stick for many decades. This leads to the build-up of enormous pressure between the two plates until they suddenly lurch past each other causing an earthquake. In the 1906 San Francisco earthquake the Pacific plate moved over six metres in one minute. Los Angeles will eventually arrive beside San Francisco if the Pacific plate keeps
moving northwards, but it will take 16 million years! The Haiti 2010 earthquake also occurred on a transform boundary between the Caribbean and North American plates.
Fig. 21 Exam Diagram: A conservative plate boundary
Fig. 22 Aerial photograph of the San Andreas Fault