The process of dripping lithosphere below the surface of the Earth

There are some parts of the world where the kinetic activity below the surface of the Earth cannot be detected.

Volcanoes, earthquakes, and even the sudden rise or fall of the ground are the results of deep underground vibrations. However, according to a study by geologists at Arizona State University (ASU), the dynamic activity deep beneath us is not always expressed on the surface.

The Great Basin in the western United States is a desert area with virtually no surface changes. The area includes small mountains separated by valleys and occupies most of Nevada, the western half of Utah and part of nearby states.

For 10 million years, the Great Basin has undergone extensive activity - the stretching of the Earth's crust.

When studying this area's expansion, ASU geographer John West was surprised to find that something unusual existed beneath the surface.

West and colleagues discovered that some parts of the lithosphere - the Earth's upper crust - have subsided into a loose upper shell and formed a cylindrical mass of cold material deep below central Nevada. .

This is a surprising finding in the area that does not show any changes in surface topography and volcanic activity.

West compared unusual results to his geographic model - CAT scans inside the Earth - done by geographer Jeff Roth. West and Roth are graduate students at ASU, currently working with counselor Matthew Fouch. The team concluded that they found the lithosphere drip.

The results of the study, funded by the National Science Foundation (NSF), were published in Nature Geoscience on May 24.

Greg Anderson, program director at NSF's Earth Science Unit, said: 'These results provide important knowledge about the small scale convection process of the Earth's crust, as well as the relationship. between these processes with volcanic phenomena and mountain formations on the surface '.

The drip of the lithosphere can be visualized as honey falling from a spoon,

When a small and dense mass lies at the bottom of the crust, and the area is warmed, this dense mass becomes heavier than the surrounding areas and begins to sink. When it subsides, the material in the lithosphere begins to flow into the newly created path.

The road runs along the Great Basin Desert in Central Nevada: 250 miles of basin.The area includes small mountains separated by valleys and occupies most of Nevada, the western half of Utah and part of nearby states.(Photo: iStockphoto / Erik Gauger)

Seismic images of the shell structure below this area provide additional evidence, showing that there is a large cylindrical block 100 km wide and at least 500 km high.

Fouch commented: 'I have always been against this' dripping' view from the early days of science. The idea of ​​lithosphere drip has been used many times over the years to explain phenomena such as volcanism, surface elevation, but you cannot assert - and so far no one has encountered one. The process of 'dripping' is active '.

Initially, the team did not think that any visible signs appeared on the surface.

Fouch said: 'We wonder how a process like' drip 'takes material into its center while the surface of the whole area is expanding'.

'But the fact that there is an area just above the process of' dripping ', the only area in the Great Basin, is shrinking. John's discovery of the drip process opens up a new model of the Great Basin 'transformation process.

Scientists already know about this contraction, but still argue about its cause.

When the drip forms, the surrounding material is sucked out after it; means that the surface will shrink towards the center of the basin. Because contraction is the natural result of drip, but what is observed in the Great Basin can be explained by the lithosphere drip.

West said: 'A lot of people in the scientific community think that it cannot be drip because there is no change in elevation or expression of the surface. Dripping is always related to big changes on the surface '.

'But those phenomena are not absolutely necessary for the dripping phenomenon to occur. Under certain conditions, such as in the Great Basin, drip can form without corresponding changes in surface topography or volcanic activity. '

All data models performed by the research team show that this drip does not create phenomena like earthquakes, subsidence or rapid rise.

There will be little or no impact on those who live above. The team believes that this drip phenomenon is a temporary process that began 15-20 million years ago.

"We will not be able to detect this without the seismic data of EarthScope Devices moving along the western United States," West said.

'We can access data from a number of stations in the region, but EarthScope's rich data and 75-km grid system are the key to these findings.'

Fouch commented: 'This is a great example of' science in action '.

'We started the study without expecting to find this. Instead, we hypothesize that no one has ever proposed this area, then we test that hypothesis with a variety of searchable data. '

'We are excited to wait for the role of this discovery in developing new ideas about the geological history of the western United States'.

References:
John D. West, Matthew J. Fouch, Jeffrey B. Roth, Linda T. Elkins-Tanton.Vertical mantle flow associated with a lithospheric drip beneath the Great Basin.Nature Geoscience, 2009;DOI: 10.1038 / ngeo526