(Phys.org) — We’re familiar with the theory that the Earth’s crust is composed of tectonic plates that move, sometimes dramatically to create earthquakes and tsunamis – but until recently, nobody knew how long this movement has been going on.

An international team of researchers, including Dr. Anthony Kemp from The University of Western Australia, believes they have found out and their work is published in Nature today.

Dr. Kemp, an Australian Research Council Future Fellow in UWA’s School of Earth and Environment, said as far as we know, Earth is the only planet in the solar system with active plate tectonics.

“The purpose of this study was to examine the earliest rock record to find out when plate tectonics started and when the continental crust began to form,” he said.

“We analyzed zircon crystals from ancient rocks of Greenland.  These rocks included some of the oldest and best-preserved parts of the Earth’s crust and were between 2.85 and 3.9 billion years old.

“In much the same way that tree rings record the growth of a tree, zircons provide important insights into the nature and composition of the magma (molten material deep within the Earth’s crust) from which the zircon crystallised.”

The researchers analyzed the isotopes of oxygen and hafnium in the zircon to learn more about the crystals’ growth bands and discovered that the Greenland crust had evolved in two stages.  The first involved a simple re-melting of 3.9 billion year-old rocks, followed by a second more complex period after about 3.2 billion years ago involving more diverse magma sources associated with re-melting and the formation of new continental material.

“We attribute this transition to the onset of plate tectonics at approximately 3.2 billion years ago and infer that significant volumes of crust began to be stabilized as continents only after that time,” Dr. Kemp said.

“We’re now trying to verify and extend these findings by studying rocks from the Pilbara, which span the same key time period from 2.8 to 3.6 billion years ago.  A broader aim is to identify the triggers for plate tectonics.

“High-precision isotope measurements will be done at the Advanced Geochemistry and Mass Spectrometry Facility recently established at UWA by Winthrop Professor Malcolm McCulloch.”

The project was led by Dr. Tomas Naeraa from the Geological Survey of Denmark and Greenland, and also involved researchers from Sweden and Germany.

Journal reference:Nature

Provided byUniversity of Western Australia

(Science Daily) The Landsat satellite image at left shows a huge lake on the Tsangpo River behind a dam created by a landslide (in red, lower right of the lake) in early 2000. The image at right shows the river following a catastrophic breach of the dam in June 2000. (Credit: U.S. Geological Survey/NASA)

Some of the steepest mountain slopes in the world got that way because of the interplay between terrain uplift associated with plate tectonics and powerful streams cutting into hillsides, leading to erosion in the form of large landslides, new research shows.

The work, presented online May 27 in Nature Geoscience, shows that once the angle of a slope exceeds 30 degrees — whether from uplift, a rushing stream carving away the bottom of the slope or a combination of the two — landslide erosion increases significantly until the hillside stabilizes.

“I think the formation of these landscapes could apply to any steep mountain terrain in the world,” said lead author Isaac Larsen, a University of Washington doctoral student in Earth and space sciences.

The study, co-authored by David Montgomery, a UW professor of Earth and space sciences and Larsen’s doctoral adviser, focuses on landslide erosion along rivers in the eastern Himalaya region of southern Asia.

The scientists studied images of more than 15,000 landslides before 1974 and more than 550 more between 1974 and 2007. The data came from satellite imagery, including high-resolution spy satellite photography that was declassified in the 1990s.

They found that small increases in slope angle above about 30 degrees translated into large increases in landslide erosion as the stress of gravity exceeded the strength of the bedrock.

“Interestingly, 35 degrees is about the same angle that will form if sand or other coarse granular material is poured into a pile,” Larsen said. “Sand is non-cohesive, whereas intact bedrock can have high cohesion and should support steeper slopes.

“The implication is that bedrock in tectonically active mountains is so extensively fractured that in some ways it behaves like a sand pile. Removal of sand at the base of the pile will cause miniature landslides, just as erosion of material at the base of hill slopes in real mountain ranges will lead to landslides.”

The researchers looked closely at an area of the 150-mile Tsangpo Gorge in southeast Tibet, possibly the deepest gorge in the world, downstream from the Yarlung Tsangpo River where the Po Tsangpo River plunges more than 6,500 feet, about 1.25 miles. It then becomes the Brahmaputra River before flowing through the Ganges River delta and into the Bay of Bengal.

The scientists found that within the steep gorge, the rapidly flowing water can scour soil from the bases, or toes, of slopes, leaving exposed bedrock and an increased slope angle that triggers landslides to stabilize the slopes.

From 1974 through 2007, erosion rates reached more than a half-inch per year along some 6-mile stretches of the river within the gorge, and throughout that active landslide region erosion ranged from 0.15 to 0.8 inch per year. Areas with less tectonic and landslide activity experienced erosion rates of less than 0.15 inch a year.

Images showed that a huge landslide in early 2000 created a gigantic dam on a stretch of the Po Tsangpo. The dam failed catastrophically in June of that year, and the ensuing flood caused a number of fatalities and much property damage downstream.

That event illustrates the processes at work in steep mountain terrain, but the processes happen on a faster timescale in the Tsangpo Gorge than in other steep mountain regions of the world and so are more easily verified.

“We’ve been able to document the role that landslides play in the Tsangpo Gorge,” Larsen said. “It explains how steep mountain topography evolves over time.”

The work was financed by NASA, the Geological Society of America, Sigma Xi (the Scientific Research Society) and the UW Quaternary Research Center and Department of Earth and Space Sciences.

SEATTLE – The Seattle fault zone, a series of active-east-west trending thrust faults, poses seismic threat to the Puget Sound region. The 900-930 AD rupture is the only known large earthquake along the Seattle Fault, making geological records of prehistoric events the only clues to the earthquake potential of the fault. While a graduate student at the University of Washington, Maria Arcos looked at tsunami and debris flow deposits – both evidence of a paleo-quake – in the coastal marsh at Gorst, Washington. She also identified evidence of at least three meters of uplift that preceded a tsunami, which was followed by a sandy debris flow from Gorst Creek, and suggests that the 900-930 AD earthquake covered a greater geographic area than previous fault interpretations. The revised height and width of deformation caused by the quake may influence current interpretations of the Seattle fault’s structure. This study found a minimum of three meters of uplift at Gorst, which is double the amount of previous fault models for the same location. A broader zone of deformation, says Arcos, may indicate either a wider zone of slip along the dip of the fault, a shallower dip or splay faults farther to the south. –Physics