Depth of oceans 4. Paleomagnetic patterns on ocean floor and ocean crust age. The Earth has a magnetic field, caused by the circulation of liquid iron in Earth's core. Today, because of the magnetic field, compasses point to the north. However, in the past, the magnetic field has reversed so that a compass would have pointed to the south.
Basalts act as paleo-compasses.
Iron-rich components within the basalt align with the magnetic field. Pacific and North American plates. The Pacific Plate is moving northward relative to the North American Plate. The San Andreas Fault marks the junction between these plates. View a quicktime movie of continental drift through geological time.