A new idea on how Earth's outer shell first broke into tectonic plates


Date:
July 20, 2020
Source:
The University of Hong Kong
Summary:
Plate tectonics theory posits that Earth's outer shell is subdivided
into plates that move relative to each other, concentrating most
activity along the boundaries between plates, yet the scientific
community has no firm concept on how plate tectonics got started. A
new answer has now been put forward.



FULL STORY ==========================================================================
The activity of the solid Earth -- for example, volcanoes in Java,
earthquakes in Japan, etc -- is well understood within the context of the ~50-year-old theory of plate tectonics. This theory posits that Earth's
outer shell (Earth's "lithosphere") is subdivided into plates that move relative to each other, concentrating most activity along the boundaries between plates. It may be surprising, then, that the scientific community
has no firm concept on how plate tectonics got started. This month, a
new answer has been put forward by Dr. Alexander Webb of the Division
of Earth and Planetary Science & Laboratory for Space Research at the University of Hong Kong, in collaboration with an international team in
a paper published in Nature Communications. Webb serves as corresponding
author on the new work.


==========================================================================
Dr. Webb and his team proposed that early Earth's shell heated up, which
caused expansion that generated cracks. These cracks grew and coalesced
into a global network, subdividing early Earth's shell into plates. They illustrated this idea via a series of numerical simulations, using a
fracture mechanics code developed by the paper's first author, Professor
Chunan Tang of the Dalian University of Technology. Each simulation
tracks the stress and deformation experienced by a thermally-expanding
shell. The shells can generally withstand about 1 km of thermal expansion (Earth's radius is ~6371 km), but additional expansion leads to fracture initiation and the rapid establishment of the global fracture network.

Although this new model is simple enough -- Earth's early shell
warmed up, expanded, and cracked -- superficially this model resembles long-discredited ideas and contrasts with basic physical precepts of
Earth science. Before the plate tectonic revolution of the 1960's,
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 such as Charles Darwin posited that major earthquakes, mountain-building, and the distribution of land-masses
were thought to result from the expansion of the Earth. However, because Earth's major internal heat source is radioactivity, and the continuous
decay of radioactive elements means that there is less available heat
as time moves forward, thermal expansion might be considered far less
likely than its opposite: thermal contraction. Why, then, do Dr. Webb
and his colleagues think that early Earth's lithosphere experienced
thermal expansion? "The answer lies in consideration of major heat-loss mechanisms that could have occurred during Earth's early periods,"
said Dr. Webb. "If volcanic advection, carrying hot material from depth
to the surface, was the major mode of early heat-loss, that changes everything." Dominance of volcanism would have an unexpectedly chilling
effect on the Earth's outer shell, as documented in Dr.

Webb and co-author Dr. William Moore's earlier work (published in Nature
in 2013). This is because new hot volcanic material taken from Earth's
depths would have been deposited as cold material at the surface --
the heat would be lost to space. The evacuation at depth and piling up
at the surface would have eventually required that the surface material
sank, bringing cold material downwards. This continual downward motion
of cold surface material would have had a chilling effect on the early lithosphere. Because Earth was cooling overall, the heat production
and corresponding volcanism would have slowed down. Correspondingly,
the downwards motion of lithosphere would have slowed with time, and
thus even as the overall planet cooled, the chilled lithosphere would
have been increasingly warmed via conduction from hot deep material
below. This warming would have been the source of the thermal expansion
invoked in the new model. The new modeling illustrates that if Earth's
solid lithosphere is sufficiently thermally expanded, it would fracture,
and the rapid growth of a fracture network would divide the 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, via integrated field-based, analytical, and theoretical
studies. Their field-based explorations bring them to far-flung sites
in Australia, Greenland, and South Africa; their analytical research
probes the chemistry of ancient rocks and their mineral components;
and their theoretical studies simulate various proposed geodynamic
processes. Together, these studies chip away at one of Earth and planetary science's greatest remaining mysteries: how and why did Earth go from
a molten ball to our plate tectonic planet?

========================================================================== Story Source: Materials provided by The_University_of_Hong_Kong. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. C. A. Tang, A. A. G. Webb, W. B. Moore, Y. Y. Wang, T. H. Ma,
T. T. Chen.

Breaking Earth's shell into a global plate network. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-17480-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200720102101.htm

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