On April 20, 2010, a bubble of methane raced up the drill column of the Deepwater Horizon oil rig, bursting through the seals and barriers in its way. By the time it exploded on the platform’s surface, it had grown to 164 times its original size. The rig, severed by the explosion, caught fire and sank two days later, allowing oil and gas to spew into the Gulf of Mexico for 83 long days.
This chaotic methane bubble was just a vanguard. With the well unsealed, substantial amounts of the gas were released into the gulf. This plume of dissolved methane should have lurked in the water for years, hanging around like a massive planetary fart. But by August, it had disappeared. On three separate trips through the gulf, John Kessler from Texas A&M University couldn’t find any traces of the gas above background levels. He thinks he knows why – the methane was eaten by bacteria.
Small earthquakes in unexpected locations are often a cause for concern. The worry is that these rumbles are harbingers of bigger quakes to come. But not always – a new study suggests that many of these tremors aren’t warnings, but aftershocks. In particular, those that happen in the middle of continents, far away from the major fault-lines that separate tectonic plates, probably reflect past quakes rather than future ones.
Earthquakes are a common occurrence on the boundaries between tectonic plates, and they occur at predictable spots. But they can often strike areas that are far away from such boundaries and where old fault-lines have seen little seismic activity over the past hundred years. The central United States, for example, experiences many such unexpected tremors.
But Seth Stein from Northwestern University and Mian Liu from the University of Missouri think that many of these small quakes are aftershocks of two bigger magnitude-7 tremors that shook the Midwest around 200 years ago.
The first hit a town called New Madrid in 1811 and triggered three shocks of similar magnitude that, together, reactivated an ancient set of faults in the continent’s interior. The second big one hit Charleston, South Carolina in 1886. Low-level seismic activity in both areas, New Madrid and Charleston, is often interpreted as a sign that they will once again be hit by large earthquakes in the future, painting two imaginary bull’s-eyes of risk in middle America.
New Madrid afer the 1811 quake
Large earthquakes are often followed by aftershocks, the result of changes in the surrounding crust brought about by the initial shock. Aftershocks are most common immediately after the main quake. As time passes and the fault recovers, they become increasingly rare. This pattern of decay in seismic activity is described by Omori’s Law but Stein and Liu found that the pace of the decay is a matter of location.
At the boundaries between tectonic plates, any changes wreaked by a big quake are completely overwhelmed by the movements of the plates themselves. At around a centimetre per year, they are regular geological Ferraris. They soon “reload” the fault, dampen the aftershocks, and return the status quo within 10 years. In the middle of continents, faults move at less than a millimetre every year. In this slow lane, things can take a century or more to return to normal after a big quake, and aftershocks stick around for that duration.
Stein and Liu’s study could help scientists to more accurately predict the risk of future earthquakes, especially in unexpected areas. If they’re right, then it would be positively misleading to base such assessments on small quakes that could sometimes be aftershocks of historical events. In the longer term, Stein and Liu predict that such approaches will “overestimate the hazard in some places and lead to surprises elsewhere”. The disastrous earthquake that hit China’s Sichuan province in May 2008 highlights the catastrophic impact that unexpected mid-continent quakes can have.
To begin with, we need to better understand the network of faults that criss-crosses continents. Fortunately, such work is already underway. Palaeoseismology – a field of research that reads traces left by prehistoric earthquakes – is providing a much longer history of tremors than our pitifully short records do. Meanwhile, GPS mapping can reveal places where plates are being deformed. These are the sorts of data that will allow us to separate the aftershocks of earthquakes past from indicators of future quakes.
Again, New Madrid proves the principle – a cluster of large earthquakes hit the area in the past thousand years, but the crust shows no sign of recent deformation according to two decades of GPS measurements. It seems that recent activity really is the legacy of centuries-old quakes, a threat that has since shut down.
Reference: Nature doi:10.1038/nature08502
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science. The blog is on holiday until the start of October, when I’ll return with fresh material.
You don’t normally hear continents described as speedy, but it’s now clear that some are much faster than others. India, in particular, is the Ferrari of continents and now, scientists have discovered why.
Rewind 150 million years and the Earth looked very different. Most of the land in today’s southern hemisphere were united in a single super-continent called Gondwana, including Africa, Australia, South America, Antarctica, India and Arabia.
The Earth’s crust is not a stationary shell but an ever-shifting mosaic of tectonic plates that constantly (albeit slowly) reshape the face of the planet. Underneath the crust lies the much hotter mantle, and plumes of super-heated rock occasionally erupt out of this layer, causing hotspots of volcanic activity.
Geologists believe that a particularly large ‘mantle plume’ kick-started the break-up of Gondwana. Now, Prakash Kumar and colleagues from the National Geophysical Research Institute in India have found that the plume also gave India a turbo boost.
Hundreds of thousands of years ago, one of the largest floods in Earth’s history turned us into an island and changed the course of our history. Britain was not always isolated from our continental neighbours. In the Pleistocene era, we were linked to France by a land ridge called the Weald-Artois anticline that extended from Dover, across what are now the Dover Straits.
This ridge of chalk separated the North Sea on one side from the English Channel on the other. For Britain to become an island, something had to have breached the ridge.
Now, Sanjeev Gupta and colleagues from Imperial College London have found firm evidence that a huge ‘megaflood‘ was responsible. They analysed a hidden series of massive valleys on the floor of the English Channel – vast gouges of bedrock 50 metres deep and tens of kilometres wide.
These valleys were first noticed by geologists in the 1970s but until now, no one really knew what caused them. Gupta decided to find out with the help of some modern technology. He used high-resolution sonar to create a contour map of the Channel floor, and found that this hidden world was remarkably well preserved.
He saw a clear picture of the huge, linear valleys, branching out in a westerly direction. In and among the valleys lay long ridges and grooves running parallel to the channel, V-shaped scours that taper upstream, and streamlined underwater islands up to 10km long.
In 2005, corals in the large reef off the coast of Florida were saved by four hurricanes. Tropical storms seem to be unlikely heroes for any living thing. Indeed, coral reefs directly in the way of a hurricane, or even up to 90km from its centre, suffer serious physical damage. But Derek Manzello from the National Oceanic and Atmospheric Administation has found that corals just outside the storm’s path reap an unexpected benefit.
Hurricanes can significantly cool large stretches of ocean as they pass overhead, by drawing up cooler water from the sea floor. And this cooling effect, sometimes as much as 5°C, provides corals with valuable respite from the effects of climate change.
As the globe warms, the temperature of its oceans rises and that causes serious problems for corals. Their wellbeing depends on a group of algae called zooxanthellae that live among their limestone homes and provide them with energy from photosynthesis. At high temperatures, the corals eject the majority of these algae, leaving them colourless and starving.
These ‘bleached’ corals are living on borrowed time. If conditions don’t improve, they fail to recover their algae and eventually die. But if the water starts to cool again, they bounce back, and Manzello found that hurricanes can help them to do this.
Antarctica normally conjures images of white and blue, but the frozen continent can sometimes bear more unexpected colours. Take the Taylor Glacier – when geologist Griffith Taylor first explored it a century ago, he found a bizarre reddish stain that seemed to spill waterfall-like from the glacier’s snout. The area became evocatively known as Blood Falls.
The source of the blood-red colour is an underground saltwater lake that was trapped by the encroaching glacier at least 1.5 million years ago. The temperature of the water is -5 Celsius, but it’s so salty that it doesn’t freeze. It’s also rich in iron salts, which are slowly leaching the ice – these are the source of the distinctive red hue. Blood Falls is a rust glacier.
But it also houses another secret, which scientists from Harvard University have started to uncover – it’s home to an entire ecosystem of bacteria, trapped for millennia in conditions that could hardly be more inhospitable to life.
Neither water from the surface nor light from the sun penetrates the thick ice of Taylor Glacier to the lake lying 400 metres beneath. As the glacier slides overhead, trace amounts of gases might seep through, but nothing substantial. There’s hardly any oxygen dissolved in the water, and radioactive-dating of the little carbon suggests that it is incredibly old. But despite the extremely salty water and the lack of light, oxygen and carbon, the microbes have lived there for millions of years, using sulphate ions as their only source of energy.
An international group of scientists have recruited a team of unlikely research assistants to help them study the Southern Ocean that surrounds Antarctica – elephant seals. Boldly going where current buoys, satellites and ships cannot, the intrepid fieldworkers will help to fill blind spots in our knowledge of this most inaccessible of oceans.
Our knowledge of the effects of climate change at the planet’s poles is heavily skewed towards the Arctic. There, it’s clear that the sea-ice cover is gradually shrinking. But at the opposite end of the world, in Antarctica, data is harder to come by. The Southern Ocean plays a vital role in exchanging heat between the atmosphere and surrounding oceans, and it’s too important an area to know so little about.
Satellites and floats provide some readings, but there is a massive zone directly beneath the frozen continent is almost completely unobserved. At 19 million km2, these hidden waters span an area greater than Russia. Satellites can’t penetrate the thick cover of ice, Argo floats can’t transmit through it and research voyages are difficult, slow and costly. Nor do these techniques tell us much about how thick the ice itself is and how quickly it’s forming – both crucial measurements for understanding the effects of climate change.
While many professors might consider pushing their graduate students overboard and making them swim about with sensors, that might be a touch unethical. Instead, a team of scientists led by Jean-Benoit Charrassin decided to enlist the help of the southern elephant seal – an animal that is well at home in these ice-cold waters.
The next time you watch a snowfall, just think that among the falling flakes are some that house bacteria at their core.
It’s a well known fact that water freezes at 0°C, but it only does so without assistance at -40°C or colder. At higher temperatures, it needs help and relies on microscopic particles to provide a core around which water molecules can clump and crystallise. These particles act as seeds for condensation and they are rather dramatically known as “ice nucleators”.
Dust and soot are reasonable ice nucleators but they are completely surpassed by bacteria, which can kick-start the freezing process at higher temperatures of around -2°C.