Continental drift: plate tectonics from 3.2 bya (Introduction)

by David Turell , Thursday, March 25, 2021, 19:39 (1129 days ago) @ David Turell

The earliest evidence of plate tectonics found in studies of western Australian rock:

https://www.quantamagazine.org/ancient-rocks-reveal-when-earths-plate-tectonics-began-2...

"Tungsten-182 should be relatively abundant in rocks from early in Earth’s history. Once plate tectonics started, however, the convective churning of the mantle would have mixed up tungsten-182 with the other four isotopes of tungsten, yielding rocks with uniformly low tungsten-182 values.

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"The tungsten-182 concentrations started out high in rocks formed before 3.3 billion years ago, showing that the mantle wasn’t mixing yet. Then the values declined over 200 million years until they reached modern levels by 3.1 billion years ago. That decline reflects the dilution of the ancient tungsten-182 signal as the mantle beneath Pilbara began to mix. That mixing shows plate tectonics had begun.

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"The tungsten-182 concentrations started out high in rocks formed before 3.3 billion years ago, showing that the mantle wasn’t mixing yet. Then the values declined over 200 million years until they reached modern levels by 3.1 billion years ago. That decline reflects the dilution of the ancient tungsten-182 signal as the mantle beneath Pilbara began to mix. That mixing shows plate tectonics had begun.

"Earth would quickly transform from a water world studded with Iceland-like volcanic islands into a world of continents with mountains, rivers and floodplains, lakes, and shallow seas.

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"Life started beforehand, more than 3.9 billion years ago, and was making hummocky little stacks in sediments at Pilbara called stromatolites by 3.48 billion years ago. This shows that plate tectonics isn’t a prerequisite for life at its most basic level. Yet it’s probably no coincidence that life diversified just as plate tectonics got underway. (my bold)

"With plate tectonics came shallow sunlit seas and lakes fertilized with nutrients weathered from continental rock. Bacteria evolved in these environments to harvest sunlight through photosynthesis, generating oxygen.

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"Gradually, though, plate tectonics provided the land and sediments in which to bury more and more of the carbon (while also providing plenty of phosphorus to stimulate photosynthetic bacteria). The atmosphere eventually oxygenated 2.4 billion years ago.

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"Progress toward complexity stalled during the “boring billion” era, the roughly billion-year reign of the supercontinent Nuna-Rodinia. With the continents stuck in a jam, Ming Tang of Peking University and colleagues argue, mountains eroded completely, reducing the flow of nutrients into the ocean and lowering oxygen levels.

"Eventually the supercontinent broke apart, and new mountains grew and exported nutrients again. Only then — around 600 million years ago — did complex organisms diversify and get bigger, riding Earth’s second rise in oxygen.

"Complex animal life exploded in the oceans 540 million years ago, and on land soon after. Dry land was now habitable because oxygen in the stratosphere formed ozone that protected land life from ultraviolet radiation."

Comment: All the chemical cycles from the effects of plate tectonics, that provide for life, have been described, but now we know when it began. It now appears life to start did not need plate tectonics, but to advance and complexify, continuing tectonics are vital.