Alec Brenner on When Plates First Moved
In the podcast, Alec Brenner describes how he used paleomagnetism to determine the latitude of rocks in the East Pilbara region of Western Australia. He found that the rocks changed their latitude from about 53°N to about 77°N over a period of several million years 3.5 billion years ago. This rapid motion in the Pilbara contrasts with the apparent lack of motion during the same period of the Kaapvaal craton. It tells us that plates were moving independently, fully a billion years earlier than any previous such detection. The results do not mean modern-style plate tectonics was already occurring, but it does rule out a stagnant lid, such as what we see on Mars or Venus. Brenner also explains that his results have a bearing on the Earth’s core and the atmosphere during the Paleoarchean. The image shows him with 3.47 billion-year-old pillow basalts in the East Pilbara.
Podcast Illustrations
Images courtesy of Alec Brenner unless otherwise noted.
Brenner’s Results
The paleomagnetic studies showed that between 3,481 million years ago (Ma) and ~3,475-3,451 Ma, the East Pilbara craton moved from a latitude of 53°N to about 77°N at a rate of 47 cm/yr, with an error of +70 cm/yr to -36 cm/yr. This is comparable to the fastest plate motions observed today. Over the same period, other workers have shown that the Kaapvaal craton in South Africa did not move in latitude. This is evidence that the two crustal blocks were moving independently of each other, with some kind of a boundary between them.
Field Work—Obtaining the Rock Samples
Brenner and his colleagues preparing to obtain samples in East Pilbara in Western Australia.
Roger Fu, Brenner’s PhD supervisor at Harvard University, drilling out one of the core samples used for Brenner’s study.
Sarah Steele and Öykü Mete orienting 3,481 million-year old core samples. A crucial part of the paleomagnetic method is to record the in-situ orientation of the rock samples that are taken to the lab where their magnetic field direction is measured.
Core samples ready for measurement.
Magnetization of the Sampled Seafloor Lavas
In the podcast, Brenner explains that the magnetic signal he measured in his samples was created by hydrothermal events occurring shortly (a few million years) after the lavas were extruded. The timing of these hydrothermal events can be bracketed quite accurately owing to the presence of layers of zircon-rich sedimentary rocks on either side of the lava flows.
Testing the Authenticity of the Magnetic Signal
A crucial part of paleomagnetic studies is to establish that the magnetic signals being measured date back to the original events being studied, rather than to some intervening event, such as metamorphism or hydrothermal flows, that could erase or overprint the original magnetization. In the podcast, Brenner describes several tests that are performed to this end. The figure illustrates two of them: the fold test and a cross-cutting intrusion test. In the fold test, the magnetic signal is original only if the field aligns when the effect of folding is removed (green layer). In the cross-cutting intrusion test, the field is original if it deviates from the measured direction only within (grey) or adjacent to (red) the cross-cutting intrusion, reverting to the measured direction further away from the intrusion (green).
Models Consistent With the Observed Plate Motion
Diagrammatic cross section of the Archean Earth showing local subduction of the lithosphere without a global system of plates.
Computer simulation of global episodic plate motion.
Courtesy of Diogo Lourenço