It's often said we know more about the surface of the Moon, or even Mars, than the depths of Earth's oceans. It seems this isn't just true of the ridges and craters we miss by not surveying large parts of the ocean floor in detail, it even applies to the composition of the rocks on which the ocean lies. A never-seen-before type of basalt has been found whose chemistry points to eruptions larger and hotter than any geologists were aware of, proof of the power the planet can unleash.
The JOIDES Resolution research vessel is slowly addressing the ocean-sized gaps in our knowledge, including drilling the Amami Sankakus Basin (ASB) south of Japan, part of the volcanic “Ring of Fire”. Six kilometers (3.6 miles) beneath the surface, the Resolution drilled 1.5 kilometers (0.9 miles) further into rocks. "This was one of the deepest waters ever to be considered for drilling, using a research vessel specifically designed for such challenging deep-sea environments,” Dr Ivan Savov of the University of Leeds, UK, said in a statement.
The efforts were rewarded. Basalt is among the most common rocks on Earth, and indeed is present on other rocky worlds with volcanic pasts, the Moon and Mars included. Nevertheless, it has many different forms, depending on the ingredients that compose it and the temperatures at which they are forged.
"The rocks that we recovered are distinctly different to rocks of this type that we already know about,” Savov said. “In fact, they may be as different to Earth's known ocean floor basalts as Earth's basalts are to the Moon's basalts.”
The new type of basalt, which has not yet been given a name, formed in an immense eruption around 49 million years ago, following the start of the subduction event when the Pacific Plate started sinking beneath the Mariana Plate. It covered an area the size of western Europe but the whole event was over in 1-2 million years – short by geological standards. Basalts produced at sites like this are different from those produced at mid-ocean ridges and also those produced on ocean islands like Hawaii.
The ASB basalt is low in titanium and potassium compared to other basalts and produced from the upper mantle at extremely high temperatures in an eruption on a scale beyond anything we see in the recent past. More than a kilometer of sediments have piled up on top since, explaining why we have not observed this type of basalt before, despite it covering such an immense area of the sea floor.
"Now that we know where and how this rock type is formed, we anticipate that many other rocks that we know were originally formed by ocean floor eruptions will be re-examined and potentially alter our wider understanding of the basalt formation,” Savov said.
