The record of the strength and direction of Earth’s magnetic field (paleomagnetism, or fossil magnetism) is an important source of our knowledge about the Earth’s evolution throughout the entire geological history.This record is preserved by many rocks from the time of their formation.In dendrochronology, the age of wood can be determined through the counting of the number of annual rings in its cross section.
The paleomagnetism of rocks of the ~1.1 billion year old North American Midcontinent Rift have been intensively studied since early 1960s (for example, see a review in Halls and Pesonen, 1982).
The rifting began during an interval of reversed polarity of geomagnetic field.
These rocks represent the main stage of the rift-related magmatism.
All younger sedimentary and igneous suites exposed on the Keweenaw peninsula (the Copper Harbor conglomerate, LST, etc) have normal polarity magnetization.
Those laid down during the fall and winter have a dark color because of the presence of dead vegetation; those deposited during the rest of the year have a light color.