geology

Geologists Zero In On Plate Tectonics Start Date

According to new geochemical evidence, the process of Earths grinding plates dipping beneath each other, known as plate tectonics, started about 3 billion years ago.

Scientists analyzed trace element ratios that relate to magnesium content in ancient Earth’s crust to come up with their results. The findings were published in the journal Science.

plate-tectonics

At left depicts what Earth might have looked like more than 3 billion years ago in the early Archean period. The ocean appears green because of a high amount of iron ions in the water at the time. The timeline traces the transition from magnesium rich (mafic) crust to a magnesium poor (felsic) crust. Image credit: Ming Tang/ UMD

“By linking crustal composition and plate tectonics, we have provided first-order geochemical evidence for the onset of plate tectonics, which is a fundamental Earth science question,” said Ming Tang, a graduate student at the University of Maryland (UMD) and lead author of the study. “Because plate tectonics is necessary for the building of continents, this work also represents a further step in understanding when and how Earth’s continents formed.”

Before this, some scientists thought plate tectonics started as early as 4.5 billion years ago, while others suggested a more recent start some 800 million years ago.

A team of graduate students from the China University of Geosciences, on a one and a half year research visit at UMD, discovered a correlation in the ratio of elements among ancient rocks. The team was looking to identify magnesium amounts – the key to understanding when plate tectonics began, because Earths ancient crust transitions from high magnesium, to low magnesium during the formation of plate tectonics.

However, magnesium is notoriously difficult to find because it’s easily washed away into the ocean once rocks are exposed to the surface.

The students got around this problem by analyzing water soluble elements (nickel, cobalt, chromium and zinc) that are still left behind long after the magnesium is gone. The researchers found that the ratios of these elements hold the signature of magnesium: higher ratios of nickel to cobalt and chromium to zinc both correlate to higher magnesium content in the original rock.

“To our knowledge, we are the first to discover this correlation and use this approach,” Tang said. “Because the ratios of these trace elements correlate to magnesium, they serve as a very reliable ‘fingerprint’ of past magnesium content.”

The team studied a variety of rocks between 4 and 2.5 billion years old, and compiled a database of trace elements. Using the correlation, they were able to determine the magnesium content in the rocks when they first formed.

The results suggest a transition period took place about 3 billion years ago, where Earth’s crust went from 11% magnesium to about 4% magnesium within the span of about 500 million years. This may have been the onset of plate tectonics because the water that the grinding plates brought deep into the Earth made magnesium levels plunge.

“Because the evolution of continental crust is linked to many major geological processes on Earth, this work may provide a basis for a variety of future studies of Earth history,” Tang said. “For example, weathering of this magnesium-rich crust may have affected the chemistry of the ancient ocean, where life on Earth evolved. As for the onset of plate tectonics, I don’t think this study will close the argument, but it certainly adds a compelling new dimension to the discussion.”

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