Studies show movements of continents speeding up after slow ‘middle age’

Two studies show that the movement rate of plates carrying the Earth’s crust may not be constant over time. This could provide a new explanation for the patterns observed in the speed of evolution and has implications for the interpretation of climate models. The work is presented today at Goldschmidt 2014, the premier geochemistry conference taking place in Sacramento, California, USA.

The Earth’s continental crust can be thought of as an archive of Earth’s history, containing information on rock formation, the atmosphere and the fossil record. However, it is not clear when and how regularly crust formed since the beginning of Earth history, 4.5 billion years ago.

Researchers led by Professor Peter Cawood, from the University of St. Andrews, UK, examined several measures of continental movement and geologic processes from a number of previous studies. They found that, from 1.7 to 0.75 billion years ago (termed Earth’s middle age), Earth appears to have been very stable in terms of its environment, with little in the way of crust building activity, no major fluctuations in atmospheric composition and few major developments seen in the fossil record. This contrasts markedly with the time periods either side of this, which contained major ice ages and changes in oxygen levels. Earth’s middle age also coincides with the formation of a supercontinent called Rodinia, which appears to have been stable throughout this time.

Professor Cawood suggests this stability may have been due to the gradual cooling of the earth’s crust over time. “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 Professor Cawood. “0.75 billion 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). 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.”

This view is backed up by work from Professor Kent Condie from New Mexico Tech, USA, which suggests the movement rate of the Earth’s crust is not constant but may be speeding up over time. Professor Condie examined how supercontinents assemble and break up. “Our results challenge the view that the rate of plate movement is stable over time,” said Professor 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.”

Results from these fields may now need to be re-examined in light of Condie’s findings. “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,” concluded Professor Condie.

A billion-year-old piece of North America traced back to Antarctica

The Franklin Mountains in West Texas were once part of Coats Land in Antarctica, according to Staci Loewy, a geochemist at California State University, Bakersfield, et al. This photo shows Coats Land with its only rock outcrops, Littlewood (L) and Bertrab (B) nunataks. -  Ian Dalziel
The Franklin Mountains in West Texas were once part of Coats Land in Antarctica, according to Staci Loewy, a geochemist at California State University, Bakersfield, et al. This photo shows Coats Land with its only rock outcrops, Littlewood (L) and Bertrab (B) nunataks. – Ian Dalziel

An international team of researchers has found the strongest evidence yet that parts of North America and Antarctica were connected 1.1 billion years ago, long before the supercontinent Pangaea formed.

“I can go to the Franklin Mountains in West Texas and stand next to what was once part of Coats Land in Antarctica,” said Staci Loewy, a geochemist at California State University, Bakersfield, who led the study. “That’s so amazing.”

Loewy and her colleagues discovered that rocks collected from both locations have the exact same composition of lead isotopes. Earlier analyses showed the rocks to be the exact same age and have the same chemical and geologic properties. The work, published online 5 August (ahead of print) in the September issue of the journal Geology, strengthens support for the so-called SWEAT hypothesis, which posits that ancestral North America and East Antarctica were joined in an earlier supercontinent called Rodinia.

The approximately 1.1 billion year old North American Mid-continent Rift System extends across the continent from the Great Lakes to Texas. Volcanic rocks associated with the rift, which appears to represent an aborted tectonic attempt to split the ancestral North American continent of Laurentia, are well exposed in the Keweenaw Peninsula of the Upper Peninsula of Michigan from which they take their name, the Keweenawan large igneous province. The rift extends in the subsurface beneath Minnesota, Iowa, Nebraska, Kansas and Oklahoma to the Franklin Mountains near El Paso, Texas where related rocks are exposed. In this latest report, Loewy, Ian Dalziel, research professor at The University of Texas at Austin, Richard Hanson of Texas Christian University and colleagues from several overseas institutions, find that rocks barely peeking through the ice in Coats Land, a remote part of the Antarctic continent south of the Atlantic Ocean basin, reflect a former continuation of the North American rift system. Loewy began her collaboration with Dalziel several years ago as a graduate student at the University of Texas at Austin.

Loewy et al. use new lead (Pb) isotopic data from the 1.1-billion-year-old rocks from Coats Land, to constrain the positions of Laurentia (ancestral North America) and Kalahari (ancestral southern Africa) in the 1-billion-year-old supercontinent, Rodinia. The Coats Land rocks are identical in age to both the Keweenawan large igneous province of the North American mid-continent rift and the contemporaneous Umkondo large igneous province of southern Africa. Comparison of the isotopic compositions, however, unequivocally links the Coats Land rocks with the Keweenawan province. Together with paleomagnetic data this suggests that the Coats Land block was a piece of Laurentia near west Texas 1.1 billion years ago. Furthermore, the Coats Land block collided with the Kalahari Precambrian craton of Africa during a 1-billion-year-old collision. Based on this reconstruction, Laurentia collided with Kalahari along Antarctica’s Maud mountain belt, which would represent a continuation of the 1-billion-year-old Grenville mountain belt of eastern and southern North America.

Thus the tiny Coats Land block of Antarctica is a ‘tectonic tracer’ providing critical clues to the geographic relationships between three of the major continents of the planet in the time interval 1.1 – 1.0 billion years ago, just prior to the opening of the Pacific Ocean basin, the hypothesized ‘Snowball Earth’ glaciations, and the rise of multi-cellular life.

March GSA Today: The case for a neoproterozoic oxygenation event

The Cambrian “explosion” of multicellular animal life is one of the most significant evolutionary events in Earth’s history. But what was it that jolted the Earth system enough to prompt the evolution of animals? While we take the presence of oxygen in our atmosphere for granted, it was not always this way.

The Neoproterozoic era that preceded the Cambrian explosion of life was witness to a dramatic rise in oxygen levels. It has been widely assumed that the rise in atmospheric oxygen was the essential precursor to the evolution of animals. But the work of Graham Shields-Zhou and Lawrence Och of University College London shows that the rise of oxygen was chaotic and nonlinear. Tectonically, the Neoproterozoic Earth was in the throes of the breakup of a supercontinent, Rodinia, and climatically, it had plunged into a snowball state, with ice-covered oceans extending from pole to pole.

In their March GSA Today article, Shields-Zhou and Och summarize geochemical and biological data that suggests that oxygen-depleted waters characterized the scattered seas that lay trapped beneath this global ice sheet. It may well have been the ability to survive in this harsh and variable climate that constituted the vital first step in the evolution of animals.

A Single Boulder May Prove that Antarctica and North America Were Once Connected





The Transantarctic Mountains where the boulder was found. - Credit: John Goodge / University of Minnesota-Duluth
The Transantarctic Mountains where the boulder was found. – Credit: John Goodge / University of Minnesota-Duluth

A lone granite boulder found against all odds high atop a glacier in Antarctica may provide additional key evidence to support a theory that parts of the southernmost continent once were connected to North America hundreds of millions of years ago.



Writing in the July 11 edition of the journal Science, an international team of U.S. and Australian investigators describe their findings, which were made in the Transantarctic Mountains, and their significance to the problem of piecing together what an ancient supercontinent, called Rodinia, looked like. The U.S. investigators were funded by the National Science Foundation (NSF).



Previous lines of scientific evidence led researchers to theorise that about 600-800 million years ago a portion of Rodinia broke away from what is now the southwestern United States and eventually drifted southward to become eastern Antarctica and Australia.



The team’s find, they argue, provides physical evidence that confirms the so-called southwestern United States and East Antarctica (SWEAT) hypothesis.



“What this paper does is say that we have three main new lines of evidence that basically confirm the SWEAT idea,” said John Goodge, an NSF-funded researcher with the Department of Geological Sciences at the University of Minnesota-Duluth.



Added Scott Borg, director of the division of Antarctic sciences in NSF’s Office of Polar Programs, “this is first-rate work and a fascinating example of scientists at work putting together the pieces of a much larger puzzle. Not only do the authors pull together a diverse array of data to address a long-standing question about the evolution of the Earth’s crust during a critical time for biological evolution, but the research shows how the ideas surrounding the SWEAT hypothesis have developed over time.”



As a field researcher during the late 1980’s and early 1990’s, Borg authored papers on the SWEAT hypothesis.



The boulder find came by serendipity while the researchers were picking though rubble carried through the Transantarctic Mountains by ice streams-rivers of ice-that flow at literally a glacial pace from East Antarctica.



Goodge and his team were searching for rocks that might provide keys to the composition of the underlying continent crust of Antarctica, which in most places is buried under almost two miles of ice.



“We were picking up boulders in the moraines that looked interesting,” Goodge said. “It was basically just a hodge-podge of material.”



One rock in particular, small enough to heft in one hand, found atop the Nimrod Glacier, was later determined to be a very specific form of granite with, as Goodge describes it, “a particular type of coarse-grained texture.”



Subsequent chemical and isotopic tests conducted in laboratories in the United States revealed that the boulder had a chemistry “very similar to a unique belt of igneous rocks in North America” that stretches from what is now California eastward through New Mexico to Kansas, Illinois and eventually through New Brunswick and Newfoundland in Canada.


That belt of rocks is known to have been a part of what is called Laurentia, which was a component of the supercontinent of Rodinia.



“There is a long, linear belt of these igneous rocks that stretches across Laurentia. But ‘bang’ it stops, right there at the (western) margin where we knew that something rifted away” from what is now the West Coast of the United States,” Goodge said.



“It just ends right where that ancient rift margin is,” Goodge said. “And these rocks are basically not found in any other part of the world.”



That it should turn up on a glacier high up in the mountains of Antarctica is strong evidence in support of the SWEAT model that parts of North America continue into part of the frozen continent at the bottom of the Earth.



“There’s no other explanation for how it got where we found it,” Goodge said. “It was bull-dozed over from that interior region of Antarctica.”



The find itself is compelling to geologists, Goodge noted, because little other physical evidences exists to allow them directly to put together the jigsaw puzzle of the long-disappeared Rodinia.



But because the supercontinent existed at a key time in the development of multi-cellular life on Earth, it also helps provide a geological context in which this massive biotic change took place.



“During the Cambrian explosion about 520 million years ago we started seeing this huge expansion in the diversity of life forms,” Goodge said. “This was also a time when the Earth was undergoing tremendous geologic changes.”



He added that “something helped trigger that big radiation in life.”



The shifting configuration of the continents, accompanied by collisions between landmasses, erosion and the influx of chemicals into the seas may well have provided the nutrients to that growing diversity of lifeforms.



“There are ideas developing about these connections between the geo-tectonic world on the one hand and biology on the other.



The job of geoscientists in this context, he said “is to reconstruct what the world was like at the time.”