Earth Observatory Blog
Magnetic Anomalies in the Wharton Basin (Part 1 of 2)
Between roughly 84 and 118 million years ago, during the Cretaceous Period, north was north and south was south, just like it is today. But around 83 million years ago, the planet’s polarity reversed, which means if you had been alive at that time and had held a compass in the palm of your hand, the north needle would have pointed south. Since then, the Earth’s polarity has reversed more than 40 times, sometimes for stretches lasting millions of years, other times for comparatively short slivers of geological time. Long or short, the records of these changes on the seafloor are known as magnetic anomalies.
Dr Jerome Dyment, a CNRS (Center for National Science Research) senior researcher at IPGP (Institut de Physique du Globe de Paris), has been studying magnetic anomalies for much of his 30-year career. Here in the Wharton Basin, as a scientist participating in the MIRAGE, Dr Dyment is using his knowledge of magnetic anomalies — as measured by a magnetometer towed off the stern of the Marion Dufresne — to help his colleagues better understand the patchwork quilt of fractures and faults that characterise this seismically active area.
“All seafloor is basically created from molten material rising from deep within the earth at mid-ocean ridges, also called spreading centres,” Dr Dyment said. “When this material cools and eventually freezes, it acquires the direction of the earth’s polarity at that time. This is important because we know that the magnetic field of the Earth has reversed itself over geological time. Fortunately, these reversals have happened in a very uneven way. This creates sequences that are easily recognisable in the magnetic anomalies, and these sequences help us to date the seafloor.”
Turns out, knowing this sort of ancient geological history is quite relevant to the study of earthquakes in the Wharton Basin and nearby Sunda subduction zone over the past couple of decades. “On this cruise,” Dr Dyment said, “we are looking at recent features such as fault lines, but the material that makes up those features formed much earlier than the present forces.”
How much earlier? Well, based on the locations of the magnetic anomalies in the Wharton Basin, Dr Dyment believes the area was once one of those spreading centres he mentioned earlier, perhaps as recently as 40 million years ago. This is not particularly remarkable in and of itself, but the Wharton Basin’s history as a spreading centre may help scientists understand what is happening in the Sunda subduction zone where events, like the catastrophic magnitude-9.2 earthquake of 2004, have been occurring with disturbing regularity.
Continued in Part 2.
To continue to follow the progress of MIRAGE, please check the EOS blog throughout the month of July, and spread the word using #MIRAGEcruise.
All photographs are taken by Ben Marks, unless otherwise stated.