That technique has provided a timetable for periods of normal and reversed polarity, showing 171 reversals in the earth's magnetic field in the past 76 million years.
Paleomagnetic studies of the ocean floor have been of decisive importance in establishing the modern theories of continental drift and seafloor spreading .
Geochronology is the science of dating and determining the time sequence of events in the history of the Earth.
This web page provides an overview of selected geochronology methods used by USGS scientists.
The reversely magnetized lavas (the Siemens Creek formation of Powder Mill group, the lowermost part of North Shore volcanics, Osler volcanics, and the lower part of Mamainse Point formation) are found in many locations around Lake Superior (see figure from ) during the normal polarity interval.
During this interval, a sequence of Portage Lake lava flows erupted within a two to three million year interval around 1095 million years ago.
As a member, you'll also get unlimited access to over 55,000 lessons in math, English, science, history, and more.
Plus, get practice tests, quizzes, and personalized coaching to help you succeed.
This record is preserved by many rocks from the time of their formation.
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.
Archaeomagnetic dating is a method of dating iron-bearing sediments that have been superheated—for example, the clay lining of an ancient hearth. When these clays are heated to high temperatures, the iron in them aligns with the earth’s magnetic field at that moment.
Archaeomagnetic dating works because the earth’s magnetic field "wanders," continually changing its position in response to changes in the flow of liquid iron in the planet's core. As the clay cools, the alignment of the iron “fixes,” preserving a record of the magnetic field at a specific time in the past.