So the Earth is, or was, a kind of giant balloon. We know seafloor spreading is still ongoing, and can affect global sea levels on historical timescales.
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Ancient fragments of Earth’s crust acted as ‘seeds’ for new crust to grow from, says LiveScience.

Around 3 billion years ago, Earth’s crust ballooned during a massive growth spurt, geoscientists have found.

At that time, just 1.5 billion years after Earth formed, the mantle — the layer of silicate rock between the crust and the outer core that was more active in the past — heated up, causing magma from that layer to ooze into fragments of older crust above it.

Those fragments acted as “seeds” for the growth of modern-day continents.

The researchers found evidence for this growth spurt hiding in ancient zircon crystals in stream sediments in Greenland. These extremely durable crystals — made up of zirconium silicate — formed during the growth spurt around 3 billion years ago.

“There have probably been multiple crust-forming events in the Earth’s history,” lead researcher Chris Kirkland, a professor of geoscience at Curtin University in Australia, told Live Science. “But this global injection event 3 billion years ago is definitely one of the biggest.”

Continental seeds

Before this massive growth period, Earth’s ancient crust was much thinner and weaker than it is today. Eventually, it broke apart into crustal fragments that served as floating “life rafts” from which new crust could grow.

“We think of crust as this buoyant material that sits on top of the mantle,” Kirkland said. “This means it constantly gets an injection of new material coming from below. The longer it sits on top, the more new material is injected into it and the larger it becomes.”

Mantle temperatures peaked at the time, due to the radioactive decay of elements like uranium and potassium in Earth’s core, as well as from residual heat left the planet’s formation.

Since that globe-wide peak in temperatures spurred the process, the crust continued to balloon on a massive scale for a period of roughly 200 million years, according to the researchers.

At the end of this period, the first continents began to take shape, eventually allowing for the development of complex life on land around 400 million years ago.

Continued here.

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January 23, 2021 at 06:03AM