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| The mystery of Siberia's exploding craters |
It appeared suddenly and explosively,
leaving a ragged pockmark on the landscape.
Around the crater’s edge, the earth is a
torn, grey jumble of ice and clods of permafrost. The roots of plants – newly
exposed around the rim – show signs of scorching. It gives some idea of just
how violently this hole in the middle of the Siberian Arctic materialized.
From the air, the freshly exposed dirt
stands out against the green tundra and dark lakes around it. The layers of
earth and rock exposed further inside the cylindrical hole are almost black and
a pool of water is already forming at the bottom by the time scientists reach
it.
Among them is Evgeny Chuvilin, a geologist
at the Skolkovo Institute of Science and Technology, based in Moscow, Russia,
who has flown out to this remote corner of the Yamal Peninsula in north-west
Siberia to take a look. This 164-foot-deep (50m) hole could hold key parts
of a puzzle that has been bothering him for the past six years since the first
of these mysterious holes was discovered elsewhere on the Yamal Peninsula.
That hole, which was around 66ft
(20m) wide and up to 171ft (52m) deep, was discovered by helicopter pilots
passing overhead in 2014, around
The latest crater was spotted in August
this year by a TV crew as they flew past with a team of scientists from the
Russian Academy of Sciences during an expedition with local authorities in
Yamal. It brings the total number of confirmed craters to have been discovered
on Yamal and the neighboring Gydan Peninsula to 17.
But exactly what is causing these enormous holes in
the permafrost to appear and how suddenly they form is still largely a riddle.
There are also unanswered questions about what they mean for the future of the
Arctic, along with the people who live and work there. For many of those who
study the Arctic, they are a disquieting sign that this cold,
largely unpopulated landscape in the north of our planet is undergoing
some radical changes.
Recent research, however, is now starting to provide
some clues about what might be going on. What is clear is that these holes are
not forming due to some gradual subsidence as the permafrost melts and shifts
below the surface. They explode into being.
“As the blast occurs, blocks of soil and ice are
thrown hundreds of meters from the epicenter,” says Chuvilin. “We are faced
here with a colossal force, created by very high pressure. Why it is so high
still remains a mystery.”
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Chuvilin
is one of a group of Russian scientists – collaborating with colleagues from
around the world – who have been visiting these craters to take samples and
measurements in the hope of understanding more about what is going on beneath
the tundra.
Some
scientists have compared the craters to cryovolcanoes –
volcanoes that spew ice instead of lava – thought to exist in some of the
distant parts of our solar system on Pluto, Saturn’s moon
Titan and the dwarf planet Ceres. But as more Arctic
craters have been studied in various stages of their evolution, they have
become known as “gas emission craters”. The name gives some clues as to how they
are thought to form.
“Analysis
based on satellite imagery shows that a blast makes a giant hole in the
place of a pingo, or mound,” says Chuvilin. Pingos are dome-shaped
hills that form when a layer of frozen ground is pushed up by water
that has managed to flow underneath it and started to freeze. As the water
freezes, it expands to create a mound. Also known in Russia by the local Yakut
name “bulgunnyakhs”, they tend to rise and fall with the seasons. Some in
Canada have been found to be up to 1,200 years old. In most parts
of the Arctic, however, these mounds tend to eventually collapse in on
themselves rather than explode.
It is clear that the mounds in northwest
Siberia are behaving differently. They swell “very fast, rising to several
meters” before they blow their top suddenly, explains Chuvilin. And instead of
freezing water, the uplift appears to be caused by a build-up of gas beneath
the ground.
“Pingos take decades to form and last a
long time,” says Sue Natali, an Arctic ecologist who studies permafrost and
director of the Arctic program at the Woodwell Climate Research Center in
Woods Hole, Massachusetts. “These gas-filled mounds form in the order of
years.”
One study of tree rings in willow shrubs
found among the debris thrown out by the explosion that created the first
crater discovered in 2014 suggests the plants had been experiencing
stress since the 1940s. The researchers say this could have been due to the deformation of the ground.
“However, there is evidence that the life
cycle of gas emission craters can be very short, ranging from 3-5 years,” says
Alexander Kizyakov, a cryolithologist at Lomonosov Moscow State University in
Russia. One crater that formed in the early summer of 2017, known as SeYkhGEC,
was found in satellite images to have first begun deforming the ground
in 2015.
Similar scars and mounds related to gas pocket
emissions have been found on the floor of the Kara Sea, just off the Yamal
Peninsula, and others have been found in the Barents Sea. But so far, says
Natali, nothing similar has been found on land elsewhere in the Arctic.
Something about the permafrost in Yamal and Gydan
makes them prone to these exploding mounds. “There are some characteristic
features of the landscape there,” she says. “It is an area where there is a
very thick layer of ice, called tabular ice, which forms a cap across the
permafrost. It is also an area where there are a lot of features known as
cryopeg, which are areas of unfrozen ground surrounded by permafrost – a kind
of permafrost sandwich. The third feature is very deep deposits of gas and
oil.”
One crater recently examined by Chuvilin – a
66ft-wide (20m) hole known as the Erkuta crater after the river whose
flood plain it appeared on – appears to have formed on the spot of a
dried-up oxbow lake. When the lake vanished, it left behind an unfrozen
patch of soil beneath it known as a talk, where gas then built up. But
Chauvilin says the exact source is still largely unclear. “The key issue in
crater research is identifying the source of gas that builds up under the
permafrost surface,” says Chuvilin. “Once the crater is there, the gas is
already gone.”
Retracing
the evolution of these mounds and how the gas gets there is now an intense
source of study. “It is intriguing that there could be a new or previously
unknown geochemical process happening that we would never have imagined,” says
Natali.
Researchers
brave enough to abseil down into the craters have found elevated levels
of methane in the water pooling at the bottom, suggesting the gas may
be bubbling up from below. One leading theory is that these deep deposits of
methane gas under the permafrost find their way up to the unfrozen pocket of the ground beneath the icy cap. Another idea is that high levels of carbon
dioxide dissolved in the water in these unfrozen pockets begin to
bubble out as the water starts to freeze, and the remaining water cannot hold
onto the dissolved gas.
An
alternative source of both methane and carbon dioxide could be microorganisms
thriving in the unfrozen pocket of ground-breaking down organic
material and releasing the gases, says Chuvilin. Isotopic analysis of
methane at one particularly dramatic crater appeared to confirm this,
but the activity of methane-producing microbes, however, has been found to
be particularly low in the lakes at the bottom of recently formed
craters – even for the cold conditions where they are found.
But methane could also be leaking out from
the ice itself. Gases can become trapped inside the water crystals in
permafrost to form a strange frozen material known as a gas hydrate.
As it melts, the gas is liberated.
“It is thought that there may be different
formation mechanisms which can hardly be described by a single model,” says
Chuvilin. “Much depends on the environment and landscape.” At least one crater
has been found in a riverbed, he points out.
Regardless of the source, it is thought
that the gas builds up in the unfrozen pocket of the ground, pushing the
solid tabular ice cap upwards by 16-19ft (5-6m) until it ruptures like
a boil. (While graphic, the furuncle analogy is not a bad one – much like
internet users are fascinated by videos of pimples popping, some scientists
find themselves drawn to the Yamal craters. “It was the combination of the
unknown and risk related to these craters that attracted me,” admits Natali.)
When they finally burst, they certainly
appear to be spectacular. Mud and ice above the gas-filled pocket, along with
much of the material in the unfrozen section itself, is flung outwards
up to 980ft (300m) away. The force is so great that blocks of earth
up to 3ft (1m) across are thrown outwards, leaving a crater with a raised
parapet, a wide mouth, and a narrower cylindrical hole –
thought to be the unfrozen pocket – left behind. Local reindeer herders
reported seeing flames and smoke after one crater explosion in June 2017 along
the banks of the Myudriyakha River. Villagers in nearby Seyakha – a settlement
about 20.5 miles (33km) south of the crater – claimed the gas kept burning
for about 90 minutes and the flames reached 13-16ft (4-5m) high.
In this sparsely populated region of the
world, for one to occur so close to a settlement has led to concern.
The region is also splattered with pipelines for the oil and gas infrastructure
trying to get at the fossil fuel deposits buried beneath the permafrost.
“We don’t yet know if these are something
that could be a risk to people in the Arctic,” says Natali. She and her
colleagues have been trying to answer this particular question by searching for
signs of other craters in high-resolution satellite images.
“Once we find something that looks like a
crater, we are then using time series very high-resolution imagery [satellite
pictures of the same location taken at different times] to try to work out when
they formed,” she says. Their work seems to be suggesting that there are more
craters out there than was previously believed. “We have so far confirmed and
validated two new crater locations. Considering that back in 2013 we knew
nothing about them, it seems very likely that there are more out there.”
Natali's team went on to discover
a third new crater, in results released in February 2021. They had
identified a further 17 possible craters, but analysis of high-resolution
images led them to conclude they may not have formed from explosive gas
emissions. "It's hard to fully validate until we can be on the
ground," adds Natali. Their research has identified a number of other
abrupt landscape changes in the region that had not been detected before
related to the thawing of the permafrost. In total, they spotted a 5% change in
the landscape between 1984 and 2007.
Eventually, Natali and her team hope to
gather enough data to be able to automate the crater search process. Their aim
is to create an algorithm that can predict craters before they form by looking
out for likely gas emission mounds in satellite images.
“We hope to get to a point where we can
see these before they form,” says Natali. “That is the sort of information you
particularly want to know when these are happening in an area where there are
people living, there are pipelines, and other gas and oil infrastructure.”
Unraveling exactly how common these
craters are is currently a slow process. After their violent birth, most seem
to disappear into the landscape almost as quickly – the void left by the
explosion near Seyakha – which measured 70m (230ft) wide in places and more
than 50m (164ft) deep – flooded with water in just four days due
to its proximity to the river. This transition from hole to lake
seems to be a rather innocuous end to a dramatic event.
Other craters take longer to flood, but
over a year or two the edges of the dark, angry wound erode and they fill with
water to become almost indistinguishable from the thousands of other
small round lakes – known as thermokarst lakes – that dot the
landscape. Exactly how many of these lakes are the scars of gas emission
craters is still unclear.
“It is likely that some of the lakes in
the permafrost are flooded gas emission craters,” says Kizyakov. “It is too
early to say how common this is as a mechanism of lake formation.”
Some researchers have tried to identify
former gas emission craters by measuring the chemicals dissolved in
characteristic lakes, but have been unable to identify any patterns.
Finding out just how common these events
are is driven by more than simple curiosity. There are growing concerns that
the appearance of the craters in northwest Siberia might be related to wider
changes taking place in the Arctic due to climate change.
Surface air temperatures in the Arctic
are warming at twice the rate of the global average, which is
increasing the amount of permafrost thaw during the summer months.
This in itself is transforming the Arctic
landscape, leading to subsidence and landslides known as thaw
slumps. Siberia boasts perhaps the largest thaw slump on the planet – the
Batagaika mega slump, which has grown from being just a gully in the 1960s
to being nearly 3,000ft (900m) wide. (Learn more about the mega slump that locals call “the gate to hell”.)
“There is nowhere else on the planet I
know of that climate change is causing the physical structure of the ground to
change,” says Natali.
Trapped inside the Arctic permafrost are
huge amounts of carbon – about twice as much as the amount currently in
the atmosphere. It is mostly in the form of the frozen remains of plants
and other organic material, along with methane that has become trapped inside
ice crystals – the gas hydrates that Chuvilin mentions earlier. As the ground
thaws, it allows microorganisms to break down the organic matter, releasing
methane and carbon dioxide as byproducts, while the methane trapped in the ice
also breaks free.
As a potent greenhouse gas, this methane
leaking out of permafrost has the potential to accelerate global
warming and drive even more melting.
But in Yamal, the craters have raised the
prospect of another process that is adding even more uncertainty to the complex
feedback loop between rising temperatures, permafrost thaw, and the release of
greenhouse gases. If it turns out that methane deposits trapped deep
underground by the permafrost are starting to seep upwards through
the normally impenetrable permafrost layers, it could be a sign that the
frozen ice cap over the tundra is becoming more permeable. This could
introduce new levels of uncertainty over how changes in the Arctic are
likely to impact wider global warming on the planet.
“The craters are a very shocking indicator
of what is happening in the Arctic more widely,” says Natali. “When you look at
changes that are happening across this landscape, some are occurring gradually
and others abruptly. Very few are occurring explosively, but it brings
attention to how all these changes contribute to the greenhouse gases in the
atmosphere.”
While the mystery of Yamal’s craters is
still to be completely solved, what has been unraveled so far suggests that
perhaps we should be watching them carefully in the future.
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