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A crack in the earth itself

A crack in the earth itself Featured

Standing on a high vantage point at Thingvellir Park, I overlook a vast rocky, snow-covered rift valley. Fissures and cracks scar the landscape, filled with water so clear I can see hundreds of coins winking at me from several metres below, representing the cumulative wishes of visitors over the years.

These cracks in the land were not always there. Forces from below have slowly broken the rocks apart over thousands of years.

The largest "crack" actually marks the gradual separation of two continental plates, a dramatic example of the evolving landscape of Iceland.

It always struck me that rocks are incredibly strong, unbreakable almost. But walking around this park leaves a strong impression of how fragile and changeable our Earth really is.

I ask geologist Freysteinn Sigmundsson from the University of Iceland about this. His answer surprises me.

All the physical movements in nature we see on Earth's surface are extremely feeble compared to what is happening deep underground.

Add some intense heat into the mix and the processes slowly ripping apart the landscape at Thingvellir are the same forces that contribute to the eruption of volcanoes and earthquakes around the country.

These create strange flows of magma that scientists are still trying to understand. At Thingvellir, the ground I stand on is an old lava field that flowed some 9,000 years ago. The whole area was originally flat but over the years, the rift valley below my vantage point has subsided 40 metres. As I walk down a bridge built over a deep fissure, rocks on either side tower above me. It is strange to think that I am inside one of Earth's scars, within two borders that were once joined. That is because this park is at the very site where two continental plates separate North America and Eurasia. They have stretched so much that the resulting empty space has caused the land to subside.   

What is especially remarkable is that I am essentially on a sort of no-man's land, on a ridge between two continental plates.

Think of it this way: the surface of the Earth is made up of a mosaic of continental plates. Some slowly push together – a process that can form mountains – whereas others, like here in Iceland, are slowly tearing the landscape apart.

They do so at a speed of about 2cm per year. That is roughly how fast our own fingernails grow.

These separating borders create fractures and fissures in the rocks. It is these cracks in the land that allow hot magma to surge upwards and fill the gaps.

For the landscape to change so much and so quickly is off immense interest to geologists such as Sigmundsson.

Compared to the age of the Earth, 9,000 years ago is a blink of an eye. Scientists can observe the processes here in order to help them understand the impact of earthquakes and volcanoes elsewhere. It is an impressive sight: two sides of land so visibly ripped apart.

The Mid-Atlantic Ridge that is to blame runs through the whole of Iceland. It crosses the Bardarbunga volcano, which erupted on and off from August 2014 until 28 February 2015. It is the largest volcano observed erupting on Iceland in the past 200 years. It emitted an impressive 1.5 cubic km of lava, which drained from the volcano's roots before travelling along vein-like dykes underground.

Bardarbunga also owes its activity to these shifting plates, as well as being above a hot spot called a mantle plume.

This hot spot brings up heat from great depths inside the Earth, perhaps from the mantle itself. The shifting plates and upwelling heat are the two fundamental ingredients for Iceland's volcanoes. With so much lava coming to the surface, the ground around Bardarbunga also sank considerably. It subsided 60m in the six months that the volcano was actively spewing out lava. This resulted in a caldera collapse. The last one of this magnitude occurred in the 1870s, but that took several decades rather than just six months.

But the strangest thing about the 2014-15 Bardarbunga eruption is that the lava did not blast out at the volcano site. Instead it bubbled up to the surface at an eruption site almost 31 miles (50km) away, at  times spouting out 50m-high lava fountains.

  As soon as Bardarbunga became active, volcanologists began monitoring the extensive lava flow and its impact on the surrounding region.

It is the first eruption that has been studied in so much detail, both before and during the eruption. That this happened at all was quite fortuitous, according to Simon Redfern from the University of Cambridge in the UK.

His team had already been recording tremors in the Earth with seismometers before they knew that Bardarbunga was going to erupt. They monitored thousands of small earthquakes as the lava was travelling along fissures under the glacier covering the volcano.

Redfern says that geologists spend a lot of time looking at historic eruptions and events in the geological record. But Bardarbunga unfolded "right in front of us".

  Although Bardarbunga produced an immense volume of lava, it was actually a fairly tame eruption. Had the lava erupted from under the glacier, instead of travelling so far through dyke tunnels, the impact could have been much more dramatic.

We might have seen an ash cloud similar to the one from the 2010 Eyjafjallajökull eruption, which caused travel chaos across Europe simply because it erupted beneath a glacier.

 

 

 

 

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