This week, the lab was buzzing with anticipation as we approached a seamount (an underwater mountain formed by volcanic activity), a small section of which had been mapped during last year’s MEGATERA cruise. The presence of the seamount was hardly a mystery, but details about its bathymetry were. We were about to get the first good look at this distinctive feature of the Wharton Basin.
When the prospect of joining the MIRAGE team was dangled before me this spring, I was briefed on the nature of the survey we’d be conducting, the importance to the region of understanding why enormous earthquakes were occurring in the middle of a seafloor plate, and the impressive resumes of the scientists who would be on board. But in every conversation, there was always this promise — it’s a French ship, so the food is going to be amazing.
The less said about yesterday’s equator ceremony the better. Unfortunately, as the Communications Officer for the MIRAGE, I am duty bound to describe the events as they transpired.
Obviously, no one on deck gets to see this momentous event since it’s happening 4.5 km below sea level, but the pipe’s collision with the seafloor can be followed on a monitor that tracks the tension of the polymer cable. As the coring unit makes its three-to-four hour descent, the tension on the cable is about 6.5 tons — it drops to zero when the pipe hits the seafloor, then spikes to about 11 tons as it’s pulled from the sticky mud. On the way back up, which takes another three-to-four hours, the tension is greater than it was on the way down, thanks to the weight of the mud now trapped inside the pipe.
In my first post about coring, I explained why we selected our two coring sites, as well as what MIRAGE researchers hope to find once the samples we retrieved are analysed. But I left out the most fascinating part — how it’s done.
Coring is difficult work, requiring days of planning, specialised equipment, and no small amount of physical prowess. Unlike bathymetry, which is primarily experienced by staring at computer monitors for hours upon end, coring happens on deck — day or night, rain or shine. This combination of engineering know-how and man doing battle with the elements makes coring fascinating to observe.
Stepping onto the bridge of the R/V Marion Dufresne is an intimidating experience, or at least it was for me. I had been there once before with a friend, climbing the stairs to “H” deck before making a quick jog into a small hallway that leads to several more steps and a door that bears the following sign: “RESTRICTED AREA”.
A type of volcanic crater called a caldera forms when a large eruption drains a magma chamber beneath a volcano, causing the ground to collapse. These eruptions can eject anywhere from 10 km3 to over 1,000 km3 of magma, devastating the local area and sometimes cooling the entire planet.
One of the root causes of all this activity could be the age of the lithosphere, that ever-spreading, always-moving seafloor crust. “South of eastern Java,” Dr Dyment said, pointing to a brightly coloured map on his computer, “the lithosphere is about 120 million years old. South of western Java, it’s younger, maybe 80 million years old. But alongside Sumatra, the crust is much younger, as young as 45 million years old. And then, of course, on the other side of that, things start aging again.
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.