A new idea about how Earth’s outer shell was first broken into tectonic plates

Solid Earth activity, for example, volcanoes in Java, earthquakes in Japan, etc., is well understood within the context of ~ 50 year plate tectonics theory. This theory postulates that Earth’s outer layer (Earth’s “lithosphere”) is subdivided into plates that move relative to each other, concentrating the most activity along the boundaries between the plates. It may be surprising, then, that the scientific community does not have a firm concept of how plate tectonics started. This month, Dr. Alexander Webb of the Division of Earth and Planetary Sciences and Space Research Laboratory of the University of Hong Kong has presented a new answer, in collaboration with an international team in an article published in Nature’s Communications. Webb serves as the corresponding author on the new work.

Dr. Webb and his team proposed that the shell of early Earth be heated, causing an expansion that created cracks. These cracks grew and merged into a global network, subdividing the shell of the early Earth into plates. They illustrated this idea through a series of numerical simulations, using a fracture mechanics code developed by the paper’s first author, Professor Chunan Tang of Dalian University of Technology. Each simulation tracks the stress and strain experienced by a thermally expanding shell. Deposits can generally withstand approximately 1 km of thermal expansion (Earth’s radius is ~ 6371 km), but further expansion leads to the onset of fracture and rapid establishment of the global fracture network (Figure 1).

Although this new model is simple enough (Earth’s first shell warmed, expanded, and cracked) superficially, this model resembles long-discredited ideas and contrasts with the basic physical precepts of Earth science. Before the plate tectonic revolution of the 1960s, Earth’s activities and the distribution of oceans and continents were explained by a variety of hypotheses, including the so-called expanding Earth hypothesis. Luminaries like Charles Darwin postulated that great earthquakes, mountain building, and the distribution of land masses were believed to be the result of the Earth’s expansion. However, because Earth’s main internal heat source is radioactivity, and the continuous decomposition of radioactive elements means that less heat is available as time progresses, thermal expansion could be considered much less likely than its opposite: thermal contraction. Why, then, does Dr. Webb and his colleagues think that the early Earth’s lithosphere experienced thermal expansion?

“The answer lies in considering the main mechanisms of heat loss that could have occurred during the early periods of Earth,” said Dr. Webb. “If volcanic advection, which carried hot material from deep to the surface, was the primary mode of early heat loss, that changes everything.” Mastering volcanism would have an unexpectedly chilling effect on Earth’s outer layer, as documented in the earlier work of Dr. Webb and co-author Dr. William Moore (published in Nature in 2013).

This is because the new hot volcanic material taken from the depths of the Earth would have been deposited as cold material on the surface; heat would be lost in space. Deep evacuation and accumulation on the surface would eventually require the surface material to sink, bringing the cold material down. This continuous downward movement of the cold surface material would have had a cooling effect on the early lithosphere. Because the Earth was generally cooling, heat production and corresponding volcanism would have slowed down. Consequently, the downward movement of the lithosphere would have slowed down over time, and therefore, even as the planet generally cooled, the cooled lithosphere would have become increasingly heated through conduction from the hot material. and deep underneath. This heating would have been the source of the thermal expansion invoked in the new model. The new model illustrates that if Earth’s solid lithosphere expands thermally enough, it would fracture, and the rapid growth of a network of fractures would divide Earth’s lithosphere into plates.

Dr. Webb and his colleagues continue to explore the early development of our planet, and of the other planets and moons in the solar system, through integrated field, analytical, and theoretical studies. Their field-based explorations take them to remote sites in Australia, Greenland, and South Africa; his analytical research investigates the chemistry of ancient rocks and their mineral components; and his theoretical studies simulate various proposed geodynamic processes. Together, these studies destroy one of the biggest remaining mysteries of planetary science: how and why did Earth go from a molten ball to plate tectonics?