Identification of the oldest preserved pieces of Earth’s crust in southern Greenland has provided evidence of active plate tectonics as early as 3.8 billion years ago, according to a report by Scripps geophysicist Hubert Staudigel and an international team of geoscientists in the March 23 edition of Science magazine.
The finding pushes back the date of continent-forming processes previously determined as 2.5 billion years ago to a much earlier era considerably closer to Earth’s formation some 4.5 billion years ago. Geochemical analysis of rocks has previously suggested an earlier date for plate tectonics, but this is the first study to find physical evidence of tectonics among Earth’s oldest known rock structures, according to Hubert Staudigel of the Cecil and Ida Green Institute of Geophysics and Planetary Physics.
“The fact that this rock structure is so well preserved is particularly lucky,” Staudigel said. “The materials were formed as seafloor along a spreading center and accreted to a continental plate and just stuck there, surviving almost unscathed for as long as 3.8 billion years.”
The study focuses on an area near the southwestern coast of Greenland where there is a rare outcrop of ancient rock, called the Isua Supracrustal Belt, which have been dated at 3.8 billion years old. The new study reveals the geological structure at Isua contains both seafloor pillow lavas and dikes, or sheets, of basalt that intruded into the pillow lavas after they formed. These features and the chemistry of the rocks indicate that the area was formed as the result of seafloor spreading.
The paper also sheds light on the ongoing debate about the oxygen isotope composition of seawater through geological time periods. The reactions of seafloor and seawater largely control the ocean’s oxygen isotope makeup, but scientists have been polarized between those that maintain the oxygen isotope content has remained relatively constant and those that argue that it has varied. This research work shows that the early ocean had the same or slightly heavier oxygen isotope composition as that of the modern ocean.
Staudigel said the science team was sampling the Isua Supracrustal Belt looking for chemical or isotopic traces of early life in the pillow lavas when they realized the area supplied geological structures proving plate tectonics from the earliest history of Earth.
Coauthors of the report are Harald Furnes of University of Bergen, Norway; Maarten de Wit of University of Cape Town, South Africa; Minik Rosing of the University of Copenhagen, Denmark; and Karlis Muehlenbachs of the University of Alberta, Canada.
— Chuck Colgan
View the research paper “A Vestige of Earth’s Oldest Ophiolite,” Science, 23 March 2007, at http://www.sciencemag.org/cgi/content/abstract/315/5819/1704?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=furnes&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT