On November 1st, 1755, the Portuguese capital Lisbon experienced a very nasty earthquake. Up to 100,000 people died and much of the city was destroyed.
A Portuguese scientist working in Australia now says the earthquake was caused by an 'embryonic subduction zone' that may split the Eurasian tectonic plate and cause the Atlantic ocean to close.
Before we go on, a quick and very simple tectonics refresher. Earth's crust is comprised of “plates”, colossal slabs of rock that move above the rather less solid stuff beneath.
When plates collide, the denser plate slips beneath the other and heads for the earth's mantle. As this happens, the top plate gets pushed upwards and hot rocks from the mantle get a chance to rise. The result is more volcanic activity, new volcanoes and mountain formation.
This all happens at a rate of centimetres a year, except when big slips occur. When that happens, earthquakes are the result as the plates slip, wobble about and settle.
Which explains Lisbon 1755 … except that no-one had spotted any subduction zones near the city that would explain the quake.
Enter Dr João Duarte who, as part of his PhD studies, conducted an extensive survey of the ocean floor off Portugal, especially a feature called Gorringe Bank. As Dr Duarte explained to The Reg , and reveals in a newly-released paper “Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin” from today's issue of Geology, the underwater mountain that is Gorringe Bank's main feature looks an awful lot like a nascent subduction zone, at least based on newly-observed fault lines.
Dr Duarte, a research fellow at Australia's Monash University, says his surveys found “the very beginnings of an active margin - it's like an embryonic subduction zone.”
The term “margin” is important because the Eurasian plate extends all the way to the mid-Atlantic ridge. Dr Duarte thinks the nascent subduction zone will split that plate in two, one continental and one oceanic.
Intriguingly, Dr Duarte hypothesises the new subduction zone is forming because the collision of African and Eurasia has just about run its course, having already spawned the band of mountains from Gibraltar to the Himalayas. When such events end, new subduction zones start to appear. Dr Duarte says we know this because, as the Pacific plate settles down, two newish zones have already been spotted in the East Atlantic. One is in the North Atlantic, near the Caribbean Sea, and another lies between South America and Antarctica, beneath the 55th parallel. The features observed at Gorringe Bank resemble those new zones.
The Gorringe Bank's sea mount, which rises nearly 5000 metres from from the sea floor to just 30 metres below sea level, is therefore a sign the plate is already starting to subduct and cause new mountains to form. And, as Lisbon knows only too well, making earthquakes.
Newly-appearing subduction zones are, Dr Duarte added, to be expected as part of Earth's “super-continent cycle” that sees much of earth's land come together in very large continents. That cycle takes 300m to 500m years, and Dr Duarte said he thinks the appearance of the new subduction zone off Portugal shows that the process continues and that the Atlantic will one day become part of a new super-continent.
That's bad news for Portugal and even for Britain, which Dr Duarte thinks will eventually become part of the subduction zone. The good news is that the zone isn't expected to become fully active for another 20 million or so years. And it'll be ten or eleven times that long before the Atlantic's gone. ®