The research team led by Dr. Hanika Rizo now of the Université de Montréal was surprised to find the material dating back to the days of Earth’s formation into an iron-rich core, a mantle of silicate and an outer rocky crust. 4.5 billion years ago, the Earth’s crust was still being created, from the upward rise of melted mantle followed by the downward sinking of its denser elements back towards the Earth’s core.
These denser elements of crust formation were previously thought to have been thoroughly re-mixed back into the mantle, but by narrowing their search to an isotope of tungsten -one of the materials thought to have been fully re-mixed – within lava rocks on both Baffin Island and the Ontong-Java Plateau in the Pacific Ocean north of the Solomon Islands, researchers hit upon their unique discovery: a higher concentration of tungsten-182 than previously known, indicating that this rock was present all those years ago when the Earth’s crust was still under construction.
“This demonstrates that some remnants of the early Earth’s interior, the composition of which was determined by the planet’s formation processes, still exist today,” says Dr. Rizo.
The finding sheds new light on this early period in Earth’s history as well as providing greater insight on what’s actually to be found deep within the Earth’s mantle. “The survival of this material would not be expected given the degree to which plate tectonics has mixed and homogenized the planet’s interior over the past 4.5 billion years, so these findings are a wonderful surprise,” says Dr. Richard Carlson, Director of the Carnegie Institution for Science’s Department of Terrestrial Magnetism and co-author of the study.
The dynamic movement of materials on the Earth’s surface, known as plate tectonics, is actually a rarity among planets in our solar system. In fact, Earth is the only planet on which the surface is in such continual change, with plates colliding and sliding into one another, forever forming mountain ranges and splitting apart continents.
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For years, scientists were puzzled as to why this is so – and, in particular, how the process started on Earth, as it has been determined that during the first one or even two billion years of Earth’s history, the dynamism of plate tectonics did not occur. Now, researchers at the GFZ German Research Centre for Geosciences have come up with an explanation.
For the surface to move, rise up and sink down into the Earth’s mantle, three conditions need to be met. First, the upward flow of hot liquid rock (known as a mantle plume) has to be large and hot enough to produce a lot of melt in the Earth’s crust, effectively weakening it. Second, the crust has to be made of material thick and heavy enough to sink back into the mantle. And third, there has to be liquid water in abundance to “lubricate” the movement of materials of the tectonic plates.
All three factors, of course, exist on Earth, whereas on our sister planet, Venus, while mantle plumes are in fact common, the crust is too light and there is no liquid on the surface.
