Assistant Professor Julie Bowles recently returned from a two-month expedition that’s part of a renewed effort to drill through Earth’s outermost crust and into the mantle below. The expedition on board the research vessel JOIDES Resolution is part of an international effort to discover the nature of the transition from crust to mantle, which has never been directly sampled before, despite efforts dating back to 1960s. Thirty scientists plus approximately 80 technicians, drilling experts, and ship’s crew departed from Colombo, Sri Lanka on December 5 and arrived on site at the Southwest Indian Ridge twelve days later. By the time they departed for the tiny island nation of Mauritius, they had drilled 2,588 feet into the crust. That’s the fifth deepest ever drilled into hard ocean crust and the deepest ever drilled in a single two-month expedition.
Our estimates of how thick ocean crust is come mostly from indirect observations of seismic waves. Current thinking is that at in some settings, these estimates are too deep. The hope is to drill to the crust/mantle transition (where Earth’s crust transitions into the mantle) to test this hypothesis. The site in the Indian Ocean was chosen because Earth’s crust is particularly thin at that spot, and it should be easier to reach the transition. In spite of that, it was never expected that the crew would make it all the way to the crust/mantle transition during this trip, which was designed to be the first of three visits to the same site. Further scientific goals of the current expedition included characterizing the variability in the chemistry and structure of lower oceanic crust, which has implications for how heat and chemicals are transferred between the solid Earth and the oceans and the atmosphere. A team of microbiologists were kept busy trying to understand what type of microbial life might be present deep in Earth’s crust. And Bowles and two other experts in Earth’s magnetism characterized the magnetic properties of the cores.
When the rocks that form Earth’s crust cool from high temperatures, electrons in certain minerals like magnetite record the direction of Earth’s magnetic field at that time, serving as tiny fossil compasses. By measuring deviations from the expected ‘compass’ direction in the rocks, it is possible to reconstruct how this section of ocean crust has rotated or tilted over time. By additionally using magnetic techniques to measure the physical alignment of the magnetic minerals, we can understand how the rocks deformed at higher temperatures.
One of the major goals of the expedition was to drill through a section of rocks that recorded a reversal in the polarity of Earth’s magnetic field. Although the current structure of Earth’s field causes your compass needle to point north, at times in Earth’s past your compass needle would have pointed south. Sampling this transition directly in Earth’s lower crust would enable geophysicists to better interpret the magnetic data collected by ships at the oceans’ surface. This magnetic data, which is related to field reversals, is the primary way we know how old ocean crust is around the world. Unfortunately, that drilling target remained elusive on this expedition, but the scientists have great hopes for the next leg!