Environment

A cracking crust may have turned Earth into a giant snowball

An artist's impression of what our planet may have looked like during the "Snowball Earth" period, about 700 million years ago
An artist's impression of what our planet may have looked like during the "Snowball Earth" period, about 700 million years ago
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An artist's impression of what our planet may have looked like during the "Snowball Earth" period, about 700 million years ago
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An artist's impression of what our planet may have looked like during the "Snowball Earth" period, about 700 million years ago

Today Earth is a pretty temperate place, but about 700 million years ago ice covered the planet from pole to pole, in an extreme glaciation period often called "Snowball Earth." Scientists have proposed a whole range of theories for what might have caused the world to freeze over, and a new study has now suggested the root of them all could have been the beginnings of plate tectonics.

As residents of Earth, we've learned to accept that shifting plates and earthquakes are just an unavoidable part of life. But this activity is actually the exception rather than the rule: most other planets, except maybe Mercury, are far quieter.

"Earth is the only body in our solar system known to currently have plate tectonics, where the lithosphere is fragmented like puzzle pieces that move independently," says Robert Stern, co-author of the study. "It is much more common for planets to have an outer solid shell that is not fragmented, which is known as 'single lid tectonics'."

Things didn't play out that way on our planet. At some point in the distant past, the solid single lid broke apart to form plates that are still moving to this day. Just when that transition occurred is still up for debate, but it's generally believed that this monumental breakup took place between 3 and 3.5 billion years ago.

However, the new study proposes a much more recent transition, between 800 and 600 million years ago in the middle of the Neoproterozoic era. According to geologists at the University of Texas in Austin and Dallas, geological features that have previously been linked to plate tectonics only date back this far, and the Earth seems to have been relatively quiet for the billion years or so before that.

"If you look at the preserved record, diagnostic evidence for modern plate tectonics involving deep subduction is mainly Neoproterozoic and younger," says Nathaniel Miller, co-author of the study. "But most people think we had this much earlier in Earth history."

But there's another key piece of evidence for this revised timeline. A cataclysm such as the Earth's crust cracking into smaller pieces and rearranging themselves would no doubt have had global repercussions – and the results would likely have looked a lot like the Snowball Earth, which lines up perfectly with the newly-suggested time frame.

The UT researchers gathered 22 hypotheses that had previously been put forward as mechanisms that cooled the planet to Snowball Earth levels, including volcanic eruptions, changes to the planet's rotational axis, and rocks pulling more carbon dioxide out of the atmosphere and locking it away.

According to the team, the beginnings of plate tectonics could be the underlying cause of all of these scenarios. The breakup would have increased explosive arc volcanism and stimulated mantle plumes, which would have spewed huge amounts of material into the atmosphere. Meanwhile, the shifting plates could also have caused the Earth to wobble on its axis.

"We went through the literature and examined all the mechanisms that have been put forward for Snowball Earth," says Stern. "The start of plate tectonics could be responsible for each of these explanations. The fact that strong climate and oceanographic effects are observed in the Neoproterozoic time is a powerful supporting argument that this is indeed the time of the transition from single lid to plate tectonics. It's an argument that, to our knowledge, hasn't yet been considered."

The researchers acknowledge that their proposal goes against conventional thinking and that the possible link needs to be examined more closely. The Snowball Earth event may, for instance, just be the result of a period of increased tectonic activity, rather than the very beginning of it entirely.

The study was published in the journal Terra Nova.

Sources: UT Austin, UT Dallas (via Science Daily)

4 comments
Jerome Morley Larson Sr eAIA
Perhaps the ice caused the cracking of the crust as the snow compressed and expanded into ice? Or is my physics phuzzy? Jerome Morley Larson Sr eAIA ARCHITECT
Douglas Bennett Rogers
The "green goo" theory seemed entirely adequate. It is hard to see how the advent of plate tectonics causes cooling.
ljaques
So, we now have 23 hypotheses, and likely none of them is correct. As they used to say on Laugh In, "velly intelesting."
Nik
Geologists need to get their noses out of the ground, and look upwards. The Earth orbits the sun, and the sun orbits the centre of the Galaxy. Every 150 million years or so, the Solar system passes through one of the arms of the galaxy. When this occurs, interstellar dust and debris in the arms, attenuates the solar radiation reaching Earth. This causes a 'Galactic' ice age. Other phenomena due to gravitational effects can be increased earthquake and volcanic activity. One of these notable events was the Permian extinction. The solar system has been experiencing one of these events for the last 30 or so million years, and the average global temperature now is very similar to that time, and is presently at its lowest since then. These galactic ice ages can last for 50 to 100 million years. If the reduced solar radiation is added to, by increased volcanic action, and consequent additional atmospheric 'clouding' with ash etc, then the Earths temperature will plummet. In the past, just one violent volcanic action was sufficient to cause a catastrophic ''year without a summer,'' when thousands starved. If the period of the solar orbit around the Galaxy is considered, then this present 'Galaxian Ice Age' may well be a repeat of the ''Snowball Earth'' event. If this combines with a ''Malenkovitch'' ice age, then that event may extend into a Snowball Earth. The Graph below shows the climate for the last 600 million years, since the end of the Snowball Earth event, and the 150 million year events are clear, as is the extended cold period of the Permian extinction. Its notable that the atmospheric CO2 was at a minimum during the Permian extinction, and is at a very similar level now. Cold reduces the CO2 released by the Oceans. http://www.geocraft.com/WVFossils/PageMill_Images/image277.gif