The jury is out on when the earth’s plates started moving. Speculation in that debate ranges from one billion to four billion years ago. However, the geological evidence that narrows down the range probably lies within the cratons of our planet. Recent research on the Pilbara Craton in Western Australia uncovered some evidence that the actual point in time when plates started moving is closer to four billion years.
The world of ancient rocks usually centers around continental cratons. Craton is the terminology used to designate the geological core of a continent. It constitutes a large, stable segment of the earth’s crust around which a continent forms. The North American Craton underlies much of the USA and Canada. The Kaapvaal craton cores the southern part of Africa, and the Pilbara Craton on the western edge of Australia is one of three Archean age cratons beneath Australia.
The Pilbara Craton is one of the most pristine Archean cratons in the world. It gets this reputation because of the well-preserved granite-greenstone terranes found in the eastern part of the Craton. Because much of the original rock fabric is relatively undeformed, geologists study these rocks for insights into conditions on the early earth. So, these rocks provide a lens for viewing our planet some 3.5 billion years ago.
Magnetism tells a story
Researchers recently drilled into part of the Pilbara Craton and collected magnetic data from a formation named the Honeyeater Basalt. The data showed that when these basalts formed some 3.2 billion years ago, the Craton was in motion. The data indicate a drift rate of 2.5 centimeters per year. Since this rate is similar to continental drift rates today, plate tectonics is a likely explanation for this motion.
Basalts start as magma or lava flows on the surface of the earth. As the basalts cool and solidify, they record the orientation of the earth’s magnetic field. Part of the information recorded is the magnetic inclination. This measurement reveals the angle between the earth’s magnetic field and a horizontal plane. Since the earth is round, the magnetic inclination varies with latitude.
So, differing magnetic inclinations between younger and older basalts indicate a change in latitude. This change is a direct indication that the spatial location of the basalts was moving over time with respect to the poles. Because individual volcanoes are spatially fixed in relation to the land around them, any lateral movement means that the continental crust below them is drifting.
Magnetism from the Honeyeater Basalts points to the Pilbara Craton as an early mover in the plate tectonics dance. It is an indication that continents started drifting very early in the earth’s history. But it is only an indication, not proof. Additional work is needed to build confidence in the “early drift” model.
The Kaapvaal Craton and early life on planet earth (Source: ArcheanWeb) – https://archeanweb.com/2020/04/15/the-kaapvaal-craton-and-early-life-on-planet-earth/ Also:
The Axial Seamount: a very active volcano (Source: ArcheanWeb) – https://archeanweb.com/2020/03/20/the-axial-seamount-a-very-active-volcano/ Also:
Ancient Australian Rocks Suggest Earth’s Continents Were Shifting Earlier Than Thought(By CARLY CASSELLA; Science Alert) – https://www.sciencealert.com/ancient-australian-rocks-suggest-earth-s-continents-have-been-moving-for-over-3-billion-years Also:
Paleomagnetism, Polar Wander, and Plate Tectonics (Source: Penn State College of Earth and Mineral Sciences) – https://www.e-education.psu.edu/earth520/content/l3_p5.html Also:
Feature Image: Folded Jaspilite (Modified) – By James St. John – Folded jaspilite BIF (