Plate tectonics controls reversals in the Earth’s magnetic field -------------------------------------------------------------------------------------------------------------- Scientists know that the Earth’s magnetic field is produced by the flow of liquid iron in its core—3,000 kilometers beneath our feet. But what has caused this field to reverse so many times throughout our planet’s history? To help solve the mystery, physicists and geophysicists determined the distribution of the continents on either side of the equator over the past 300 million years. They found that the more uneven the distribution, the faster the rate of reversals. Over the last five years, modeling work and experiments have shown that reversals occur when the motion of molten metal in the core is no longer symmetrical to the equatorial plane. This led to the conclusion that asymmetry in the core could be caused by the motion of the mantle. French researchers’ recent findings go further: the phenomenon could be due to the motion of tectonic plates. Geophysical Research Letters October 2011 Position of the continents, with their convex envelope, at various periods. From top to bottom: present day—65 million years ago—200 million years ago—260 million years ago. The hill of Piraeus was once an island The first century AD Greek geographer Strabo was convinced that the hill of Piraeus had once been an island. To confirm his intuition, a French-Greek team collected around ten geological core samples from boreholes over 20 meters deep in the plain located between Piraeus and Athens. By comparing the dating of these sedimentary records with available historical records, the researchers showed that between 4,800 and 3,400 BC, sea levels had risen enough to separate the hill of Piraeus from the mainland. Warning signs of earthquake detected Geology May 2011 for the first time By analyzing seismic recordings obtained near the epicenter of the 1999 Izmit earthquake in Turkey, seismologists have identified a very special seismic signal for the first time. Preceding the earthquake by 40 minutes, the signal was caused by slow, discontinuous slip of the fault in the area where the earthquake finally occurred. If this precursor signal is confirmed for other earthquakes, it could make it possible to predict some of them several tens of minutes before the fault ruptures. Science February 2011 Stratigraphic section at Karnak, Egypt. Carbon-14 dating of the deposits was used to obtain an accurate chronology of the sedimentation phases of the Nile. Developing more accurate cold-atom Karnak temples weren’t always accelerometers on the banks of the Nile accelerometer enhances our understanding of the composition of the local gravitational field using a cold-atomDetailed mapping Built around 2,000 BC, the Karnak temples were not originally on the of the Earth’s crust. The downside is that, although cold-atom banks of the Nile. By dating river sediments collected near the western accelerometers are highly accurate, they are extremely sensitive entrance to the temple complex, geographers have for the first time to variations, which makes them difficult to use outside the lab. To succeeded in working out how the surrounding landscape has changed. get round the problem, physicists have developed an accelerometer Their analysis shows that the first Karnak temples were built, not on an that combines data from conventional and atom accelerators and island in the middle of the Nile as previously thought, but several hundred that can operate under difficult conditions. A portable version meters from the river. It was only around 1,600 BC that the riverbed will soon be put on the market, and could equip geophysics shifted to its present location at the foot of the temples. laboratories. Journal of Archaeological Science June 2011 Nature Communications September 2011 43 2011 A year at CNRS
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