From 1.7 to 0.7 billion years ago nothing much happened on Earth. It was “characterized by environmental, evolutionary and lithospheric stability that contrasts with the dramatic changes in preceding and succeeding eras,” as a paper in Geology puts it. But the paper goes further, offering an explanation for this pause, while complementary evidence suggests continental drift may be speeding up.
You’ve got to love the geologists willing to label a planetary era the “boring billion”. Others call it the Earth’s middle age, but however you want to title it an explanation is required. Plenty have been offered without gaining overwhelming support.
The pause is thought to have been brought to an end by the a series of glaciations, where the entire Earth may have frozen, after which the wonderfully named Avalon Explosion saw complex multicellular organisms appear in the fossil record.
However, it is not just biology that stalled during the Mesoproterozoic era and Tonian period. The paper notes, “The period is marked by …a lack of orogenic gold and volcanic-hosted massive sulfide deposits, and an absence of glacial deposits and iron formations.”
The paper’s authors, Professors Peter Cawood and Chris Hawkesworth, both of the University of St Andrews, think these are related. They argue, “These trends are attributed to a relatively stable continental assemblage that was initiated during assembly of the Nuna supercontinent by ca. 1.7 Ga and continued until breakup of its closely related successor, Rodinia, ca. 0.75 Ga.”
“Before 1.7 billion years ago, the Earth’s crust would have been substantially hotter, meaning that continental plate movement may have been governed by different rules to those that operate today,” said Cawood. The mantle was too hot for continental material to slide into it as now happens at plate boundaries. Consequently, once a supercontinent formed nothing could break it apart.
A single giant continent and even larger ocean didn’t offer the diverse niches that tend to drive evolution. Moreover, the creation and destruction of mountain ranges that come with continents in collision release pulses of nutrients into the oceans with major effects on the diversity of life there.
Cooler temperatures are normally associated with reduced movement, but in this case Cawood says we saw the reverse.
“Seven hundred and fifty million years ago, the crust reached a point where it had cooled sufficiently to allow modern day plate tectonics to start working, in particular allowing subduction zones to form (where one plate of the crust moves under another),” says Cawood. Meanwhile the continent was growing downwards, adding bulk that meant that when it pulled apart there was even more of it to be driven into the mantle.
“This increase in activity could have kick-started a myriad of changes including the break-up of Rodinia and changes to levels of key elements in the atmosphere and seas, which in turn may have induced evolutionary changes in the life forms present.” says Cawood. The continental changes could also have affected the climate.
At the Goldshmidt geochemistry conference this month, Professor Kent Condie of New Mexico Tech presented evidence from independent research in keeping with Cawood and Hawesworth’s conclusion. “Our results challenge the view that the rate of plate movement is stable over time,” said Condie. “The interpretation of data from many other disciplines such as stable isotope geochemistry, palaeontology and paleoclimatology in part rely on the assumption that the movement rate of the Earth’s crust is constant.” Instead he found evidence from the geomagnetic record to suggest continents have sped up with time, and may be continuing to do so.
“We now urgently need to collect further data on critical time periods to understand more about the constraints on plate speeds and the frequency of collision between continental blocks,” Condie added.
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