Tom recently went to Cape Verde together with Dr. Ricardo Ramalho to sample tsunami deposits of an ancient mega-tsunami. These deposits, first found by Ricardo and published in Science Advances, were created 73,000 years ago during a flank collapse of nearby Fogo Volcano that caused a tsunami wave of at least 170 meters height. At PKU we are now working on a new method to date tsunami deposits that were previously impossible to date using conventional methods such as cosmogenic dating.
This method makes use of the viscous remagnetization (VRM) these boulders acquired after the tsunami: When the rocks formed, magnetic inclusions in the rocks were magnetized in the direction of Earth’s magnetic field at that time. Like a compass needle, all the magnetic grains pointed north. But when the rocks were transported by the flood, they tumbled and rotated so that the grains no longer pointed north. As time passes, some of these grains get remagnetized and start pointing northward again. By comparing the proportions of remagnetized to non-remagnetized grains in samples taken from the boulders, the age of the flood can be calculated.