The ground we walk on may seem unchanging, but the rocks that make up the Earth’s crust are actually subject to a cycle of birth and death that has altered the planet’s surface over billions of years.

The tectonic plates that make up the Earth’s crust are constantly jostling against each other, brushing past one another in some places, moving apart in other areas, and butting head-on in others.

Now Scottish university scientists believe volcanoes on Greenland, believed to be some of the oldest on earth, could provide new insights into how and why this phenomenon happens.

A study by scientists at the University of St Andrews, published today in Nature Communications, reveals the fate of the Earth’s ancient crust, the outer layer that is made up of different types of rock.

And it tells of how that could help solve the mystery of how the Earth’s surface and mantle, which is below the crust, are connected.

The Earth’s tectonic plates move around and collide at regions called subduction zones.

Where head-on collisions occur, denser oceanic crust is shoved beneath lighter continental crust, causing it to melt in the ferocious temperatures and pressures of the Earth’s mantle. 

In areas of collision, crustal materials get transported into the deep mantle which, because of its high temperature and pressure, slowly flows and fuels the world’s volcanoes.

And one of the grand challenges in Earth Sciences is to understand what happens to this crust.

At a few volcanoes on Earth geologists can find traces of these ancient crustal materials in the erupted lava. To date most of this work has focused on oceanic islands like Hawaii or the Canaries.

However, oceanic islands are only present at the surface of the Earth for a few million years before they themselves subside and are subducted back into the mantle.  So it can only provide a tiny snapshot of crustal recycling over the four billion years of Earth history.

The St Andrews team investigated a suite of alkaline magmas erupted in continental rifts similar to the modern day East African rift.

The Scottish scientists say that although these magmas have very unusual chemistries, they show many similarities with those oceanic lavas and, crucially, are found throughout Earth’s geological record. 

The team focused on an alkaline province in south-west Greenland using cutting-edge isotope techniques to chemically fingerprint ancient crustal material in the source of these magmas.

Through a combination of remote field work – by boat and helicopter – and careful chemical analysis the team was able to show that these magmas were tapping into ancient crust subducted into the mantle 500 million years before the volcanoes started erupting.

Once the team understood these processes in Greenland they compiled a global data on alkaline magma chemistry and were surprised to find that the vast majority contained a recycled crustal component in their magma source.

Lead author Dr Will Hutchison, from the School of Earth and Environmental Sciences at the University, said: “Our key result is that the isotopes of alkaline magmas are highly variable and this suggests that their recycled crustal sources have changed through geological time.

“The beauty of our global dataset is that it extends back over two billion years and so these unique alkaline rocks represent an extremely powerful record for understanding crustal recycling over Earth history.

“By carefully bringing together the igneous and sedimentary isotope records, this might allow us to understand how changing crustal input is tied to volcanic output, and ultimately build a much better understanding of what happens to tectonic plates once they get transported into the deep Earth.”

It is not the first time that Greenland has been picked to try and uncover some of the Earth’s mysteries. Some scientists have previously believed Greenland may have been home to a mud volcano where life on Earth may actually began.
Researchers discovered the chemical elements crucial to the formation of life in mud volcanoes at Isua in south-west Greenland.

It was believed life began in underwater geysers, but research conducted eight years ago argued that these could not have sustained life, and that the volcanoes offered the oldest environment where life could begin.

The ices of Greenland and Antarctica bear the fingerprints of a monster - a gigantic volcanic eruption in 539 or 540 AD that killed tens of thousands and helped trigger one of the worst periods of global cooling in the last 2,000 years. 

And in August, it emerged, that after years of searching, a team of scientists had finally tracked down the source of the eruption.
The team’s work laid out  evidence that ties the natural disaster to Ilopango, a now-dormant volcano in El Salvador. Researchers estimated that in its sixth-century eruption, Ilopango expelled the equivalent of 10.5 cubic miles of dense rock, making it one of the biggest volcanic events on Earth in the last 7,000 years. The blast was more than a hundred times bigger than the 1980 Mount St. Helens eruption.