Delving into the Depths of the Earth’s Geologic History

It was only in the 1960s that the scientific theory of plate tectonics transformed how we understand the geology and movements on our planet, Earth.

Earth, July 1969, from Apollo 11

In plate tectonics, Earth’s outermost layer, or lithosphere—made up of the crust and upper mantle—is broken into large rocky plates. These plates lie on top of a partially molten layer of rock called the asthenosphere. Due to the convection of the asthenosphere and lithosphere, the plates move relative to each other at different rates, from two to 15 centimeters (one to six inches) per year. This interaction of tectonic plates is responsible for many different geological formations such as the Himalaya mountain range in Asia, the East African Rift, and the San Andreas Fault in California, United States. National Geographic

A new study  from the University of Copenhagen is suggesting that this is a more recent feature of the Earth’s geologic history.

Zhengbin Deng, former assistant professor at the University of Copenhagen and first author of the new study, explained:

“Our new results suggest that for most of Earth’s history, convection in the mantle was stratified into two distinct layers, namely upper and lower mantle regions that were isolated from each other.”

The study goes on to propose that in  the past, recycling and mixing of subducted plates into the mantle was restricted to the upper mantle, where there was strong convection. This is in compete contrast to how plate tectonics is viewed today where subducting plates sink to lower mantle.

 The scientists took ultra-high precision measurements of the isotopic composition of the element titanium in various rocks.

Isotopes are versions of the same element that have slightly different masses. The isotopic composition of titanium is modified when crust is formed on Earth. This makes titanium isotopes useful to trace how surface material like the crust is recycled in Earth’s mantle through geologic time.

Using this new technique, they determined the composition of mantle rocks that formed as early as 3.8 billion years ago all the way down to modern lavas.

If this study is correct it means that the lower mantle could contain undisturbed primordial material.

The concept of a primordial mantle refers to a reservoir of mantle material that has remained relatively unchanged and preserved since the early stages of the Earth’s formation, about 4.5 billion years ago.

The idea that a primordial reservoir exists in the deep Earth is not new and has been suggested based on the isotopic composition of rare gases trapped in lavas from modern deep-seated volcanoes. However, the interpretation of these data is ambiguous, and some have suggested that this isotope signal comes from Earth’s core as opposed to the deep mantle. Because titanium is not present in Earth’s core, it provides a fresh perspective on this long-standing debate.

Click on this link to access, Earth’s evolving geodynamic regime recorded by titanium isotopes, published in Nature

Sligachan in Skye with the rive in the foreground and the mountains of Skye behind

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