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.
NOTES FOR EDITORS
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The origin of the continental crust and its impact on the Earth system
PETER A. CAWOOD1, CHRIS J. HAWKESWORTH1 AND BRUNO DHUIME1,2, 1Earth Sciences, University of St Andrews, UK, 2Earth Sciences, University of Bristol, UK
The continental crust is the archive of Earth history and its record of rock units and events is heterogenous with distinctive peaks and troughs of ages for igneous crystallization, metamorphism, continental margins and mineralization. This temporal distribution is argued to largely reflect the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. In contrast there are other signals, such as the Sr isotope ratios of seawater, mantle temperatures, and redox conditions on the Earth, where the records are regarded as primary because they are not sensitive to the numbers of samples of different ages that have been analyzed. Models based on the U-Pb, Hf and O isotope ratios of detrital zircons suggest that at least ~60-70% of the present volume of the continental crust had been generated by 3 Ga. This volume contrasts markedly with the <10% of crust of that age apparently still preserved requiring ongoing recycling of early formed crust and subcontinental mantle lithosphere back into the mantle. Between 1.7 to 0.75 Ga, the tempo of Earth processes was characterized by environmental, evolutionary and lithospheric stability that contrasts with the dramatic changes in preceding and succeeding eras. The period is marked by a paucity of passive margins, an absence of a significant Sr anomaly in the paleoseawater record or in the epsilon Hf(t) in detrital zircon, a lack of orogenic gold and volcanic-hosted massive sulphide deposits, and an absence of glacial deposits and of iron formations. In contrast, anorthosites and kindred bodies are well developed and major pulses of Mo and Cu mineralization, including the world's largest examples of these deposits, are features of this period. These trends are attributed to the combined effects of lithospheric behaviour related to secular cooling of the mantle and a relatively stable continental assemblage that was initiated during assembly of the Nuna supercontinent by 1.7 Ga and continued until breakup of its closely related successor, Rodinia, around 0.75 Ga. Since ~0.75 Ga, modern plate tectonic processes have dominated the Earth system.
Is there a secular change in supercontinent assemblies?
K. C. CONDIE1*, S. A. PISAREVSKY2 AND J. KORENAGA3, 1New Mexico Tech, Socorro, NM 87801 USA 2Curtin University, GPO Box U1987, Perth, WA 6845, Australia 3Yale University, PO Box 208109, New Haven, CT 06520, USA
High frequencies of craton collision occur during supercontinent assembly at 1800, 1100, 650-300 and <100 Ma and low rates during breakup at 2100, 1300-1200 (?), 750, and 200-100 Ma. Angular plate velocities as weighted by craton area range from 20 to 80 deg/100Myr with two peaks at 450- 350 Ma and 1100 Ma, both of which correlate with the initial stages of supercontinent assembly. The number of cratons decreases from ≥ 15 before 1900 Ma to < 10 after this time. Orogens and passive margins show the same two cycles of ocean basin closing at 2700 to 2000 Ma and at ≤ 2000 Ma. The younger cycle shows decreasing durations of ocean basin closing until about 1000 Ma. Supercontinent assembly and breakup durations are 200-300 Myr and 100-200 Myr, respectively. Except for 1200-700 Ma, duration of ocean-basin closing is ≤150 Myr. Time-averaged plate speeds suggest more sluggish plate tectonics in the past, which is consistent with a possible increase in craton collision frequency in the last 1000 Myr. If Gondwana and Pangea are counted as stages of the same supercontinent, the supercontinent cycle has a period of about 500 Myr and there is no clear evidence for it speeding up or slowing down with time.