Plate tectonics acts like a planetary thermostat, cycling greenhouse gasses from the deep Earth to the atmosphere. The results appear in the January 22, 2016 issue of the journal Science. All these conditions were fulfilled sometime in early Earth history, but were never met for other planets of the Solar System. As the continental crust erodes, land would disappear into the oceans – even without climate induced sea level rises. Others suggest a much more recent start within the last 800 million years.
The observations from the deepest ocean trench in the world have important implications for the global water cycle, according to researchers in Arts & Sciences at Washington University in St. Louis, Chen Cai, who recently completed his doctoral studies at Washington University.
“Previous estimates vary widely in the amount of water that is subducted deeper than 60 miles”, said Doug Wiens, the Robert S. Brookings Distinguished Professor in Earth and Planetary Sciences, which funded the study.
The main source of uncertainty in these calculations was the initial water content of the subducting uppermost mantle. The trench is where the western Pacific Ocean plate slides beneath the Mariana plate and sinks deep into the Earth’s mantle as the plates slowly converge. Under certain temperature and pressure conditions, chemical reactions force the water into a non-liquid form as hydrous minerals-wet rocks-locking the water into the rock in the geologic plate.
Previous conventions were based on active source methods, Cai says. He was referring to a type of seismic study that uses sound waves created with the blast of an air gun from aboard an ocean research vessel to create an image of the subsurface rock structure.
If other old, cold subducting slabs contain similarly thick layers of hydrous mantle, then estimates of global water flux into the mantle at depths greater than 60 miles must be increased by a factor of three. Sea levels have remained relatively stable over geologic time, varying by less than 1,000 feet. There’s been suggestions of that in Alaska and in Central America. This process, called subduction is the key characteristic of Earth’s crust into the deep mantle and for an efficient cooling of the Earth interior.
Scientists have found that 4.02 billion year old silica-rich felsic rocks from the Acasta River, Canada – the oldest rock formation known on Earth – probably formed at high temperatures and at a surprisingly shallow depth of the planet’s nascent crust. The quartz crystals show a high concentration of the element titanium content at just 11 miles.
The researchers took olivine and added melt (also known as basalt) to mimic how a new plate is created at a mid-ocean ridge. The research team’s results suggest the best way to model the plate thickness is based on the thermal profile. By analyzing trace element ratios that correlate to magnesium content suggests that plate tectonics began about 3 billion years ago.
“You can’t have continents without granite, and you can’t have granite without taking water deep into the Earth”, said Roberta Rudnick, former chair of the Department of Geology at UMD and senior author on the study. They used these data to construct a computer model of the early Earth’s geochemical composition.
Within a half billion years, that number had dropped to about 4 percent magnesium oxide by weight. During the first 1 or 2 billion years of the 4.5 billion years Earth’s history, the tectonic process was very different, probably similar to present-day Venus, where the lithosphere is not broken into plates and no subduction occurs.
Around 200 million years, the tectonic plates that carry America and Europe began their multi-million-year breakup in our region. The ancient continents of rock regions encircle the liquid outer core. These samples are made of igneous rock created from cooling lava.
According to new research, the transition to plate tectonics began influencing the early history of the Earth. The mantle, a thick layer of solid rock that flows under intense pressure and heat. As subducting plates descend into oceanic trenches, they carry oceanic crust, upper mantle rock, and sediments deeper into Earth, as well as seawater entrained in minerals in these materials.
Understanding the composition of specific rock masses near the core-mantle boundary helps geologists conceptualize ancient Earth-shaping processes that led to the modern-day mantle, according to the study’s authors.
