Rare Earth:possible evolution of plate tectonics (Introduction)

by David Turell , Tuesday, February 28, 2017, 16:30 (2826 days ago) @ David Turell

The Earth may have started with a solid shell with plates appearing later. Plate tectonic activity is vital to the development of life:

https://www.sciencedaily.com/releases/2017/02/170227120347.htm

"Plate tectonics -- a defining feature of modern Earth and the driving force behind earthquakes, volcanoes and mid-ocean spreading ridges -- did not start until later in Earth's history, new research suggests. The work is the latest salvo in a long-standing geological debate: did plate tectonics start right away, or did Earth begin with a solid shell covering the entire planet? The new results suggest the latter.

"Today's Earth is a dynamic planet with an outer layer composed of giant plates that grind together, sliding past or dipping beneath one another, giving rise to earthquakes and volcanoes. Others separate at undersea mountain ridges, where molten rock spreads out from the centers of major ocean basins.

"But new research suggests that this was not always the case. Instead, shortly after Earth formed and began to cool, the planet's first outer layer was a single, solid but deformable shell. Later, this shell began to fold and crack more widely, giving rise to modern plate tectonics.

"Models for how the first continental crust formed generally fall into two groups: those that invoke modern-style plate tectonics and those that do not," said Michael Brown, a professor of geology at the University of Maryland and a co-author of the study. "Our research supports the latter -- a 'stagnant lid' forming the planet's outer shell early in Earth's history."

"To reach these conclusions, Brown and his colleagues from Curtin University and the Geological Survey of Western Australia studied rocks collected from the East Pilbara Terrane, a large area of ancient granitic crust located in the state of Western Australia. Rocks here are among the oldest known, ranging from 3.5 to about 2.5 billion years of age. (Earth is roughly 4.5 billion years old.) The researchers specifically selected granites with a chemical composition usually associated with volcanic arcs -- a telltale sign of plate tectonic activity.

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"Using the Coucal basalts and Pilbara granites as a starting point, Brown and his colleagues constructed a series of modeling experiments to reflect what might have transpired in an ancient Earth without plate tectonics. Their results suggest that, indeed, the Pilbara granites could have formed from the Coucal basalts.

"More to the point, this transformation could have occurred in a pressure and temperature scenario consistent with a "stagnant lid," or a single shell covering the entire planet.

"Plate tectonics substantially affects the temperature and pressure of rocks within Earth's interior. When a slab of rock subducts under the Earth's surface, the rock starts off relatively cool and takes time to gain heat. By the time it reaches a higher temperature, the rock has also reached a significant depth, which corresponds to high pressure -- in the same way a diver experiences higher pressure at greater water depth.

"In contrast, a "stagnant lid" regime would be very hot at relatively shallow depths and low pressures. Geologists refer to this as a "high thermal gradient."

"Our results suggest the Pilbara granites were produced by melting of the Coucal basalts or similar materials in a high thermal gradient environment," Brown said. "Additionally, the composition of the Coucal basalts indicates that they, too, came from an earlier generation of source rocks. We conclude that a multi-stage process produced Earth's first continents in a 'stagnant lid' scenario before plate tectonics began.'"

Comment: As with other aspects of development, it appears God uses an evolutionary process to create an Earth suitable for life.