Pakistan earthquake: How did it create a 'sudden island'?

It's not uncommon for the Earth's crust to shoot up out of the sea when an earthquake occurs. Many, but not all, of these instant islands are soon swallowed up again, particularly in Pakistan at monsoon time.

Not uncommon for Earth's crust to shoot up, but not all instant islands stick around

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Cartographers might have to put another island on the map after Tuesday's massive earthquake created a new island five kilometres off the southern coast of Pakistan, in the Arabian Sea near Gwadar area.

"It's not a common way for islands to be created," says Andrew Miall, a geology professor at the University of Toronto. "But vertical movement of the crust is really common, and it just so happens that, in this case, the crust was very near the surface of the water."

Sudden islands like this can be swallowed back up by the seas, as has happened in Pakistan in the past. But there have been cases, such as following the Great Alaskan Earthquake almost 50 years ago, when new landforms like these can stay around.

In this case, local residents have been visiting the island and picking up rocks as souvenirs. But Pakistani authorities have been warning them to stay away as methane gas is leaking out of the new structure and can be set off by sparks from a cigarette.

Muhammed Arshad , a hydrographer with the Pakistan navy, says the sea is only about six to seven metres deep in the area where the new land mass appeared.

And rare as it is, this sudden island is not Pakistan's first.  Arshad said that similar islands appeared off a different part of the coast of Pakistan after quakes in 1999 and 2010.

Those islands were swallowed back into the sea during the monsoon season, a period of heavy rain and wind that sweeps Pakistan every summer.

After looking at photos of the new island in Gwadar, John Dixon, a geology professor at Queen’s University, said the new land mass is likely a “mud volcano” made mainly of soft mud, not hard rock.

“Because of this, it is likely that the island will be very temporary — it will probably be washed back into the sea by erosion due to wave action fairly quickly. In fact, one can see from the photo that this is already starting,” he says.

The island is likely composed of sediment from the sea floor that was pushed up above sea level while trying to escape either water or gas that was previously trapped between grains of sediment, he says.

When the earthquake hit, the sediment was reorganized and settled into a more compact arrangement causing the grains to be pushed out and upwards to try to escape the pressure.

This recent earthquake occurred near the bend in the Earth’s plate boundary in southeast Pakistan where there is a combination of a dip and sideways slip between the Arabian and Eurasian plates.

In that region, earthquakes are typically shallow and lead to more sideways slip, so not much crustal uplift is expected, Dixon says.

Longer timeframe

Islands are most commonly formed over decades or even hundreds of years through coastal erosion that causes remnants of land to be left behind.

Another common way for islands to be formed is when volcanoes on the sea floor eventually appear above sea level. Yet some islands, such as the one in Pakistan are formed almost immediately.

Miall explains that it's common for earthquakes to cause movement along the Earth's faults or breaks, and that blocks of crust get pushed either upwards or downwards, causing them to rise or sink.

"In this case, it looks like a large piece of the crust is pushed upwards above sea level," he says.

The newly formed island stands 18 metres high, 30 metres long and 76 metres wide, Pakistani navy geologist Mohammed Danish told Pakistan Geo Television.

Scientists in Pakistan are unsure if the island will be permanent and are trying to get samples of its material. The island's composition will help them determine if it may eventually subside back into the ocean.

"Usually these movements are permanent, but of course the waves could gradually erode it away so it disappears again," Miall says.

Similar land formations happened in March of 1964 when the Great Alaskan Earthquake devastated the coast of southern Alaska and caused it to be suddenly uplifted.

Miall says that the earthquake, which was followed by a tsunami, created new land and lifted up Middleton Island in the Gulf of Alaska by almost four metres. That elevated land, he says, is still there today. 

With files from Associated Press


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