The Layers of the Earth
The asthenosphere is the highly viscous mechanically weak region of the upper mantle of the Earth on which "float" the continental plates. It lies below the lithosphere, at depths between 60 and 120 miles below the surface, but perhaps extending as deep as 400+ miles. The lithosphere is a complex mixture of layers. For example the North American continent is not one thick, rigid slab, but a layer cake of ancient, 3 billion-year-old rock on top of much newer material probably less than 1 billion years old, according to a new study by seismologists at the University of California, Berkeley.
The lithosphere includes the crust and the uppermost mantle, which constitute the hard and rigid outer layer of the Earth which most people are most familiar with. The boundary between the lithosphere and the underlying asthenosphere is defined by a difference in response to stress: the lithosphere remains rigid for very long periods of geologic time in which it deforms elastically and through brittle failure, while the asthenosphere deforms viscously and accommodates strain through plastic deformation. The lithosphere is broken into tectonic plates. The uppermost part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process is called the pedosphere (which is where people live and ecosystems thrive).
The Earth's original continents started forming some 3 billion years ago when the planet was much hotter. The continental rocks rose to the surface and eventually formed the lithosphere, Earth's hard outer layer. These old floating pieces of the lithosphere, called cratons, apparently stopped growing about 2 billion years ago as the Earth cooled, though within the last 500 million years, and perhaps for as long as 1 billion years, the modern era of plate tectonics has added new margins to the original cratons, slowly expanding the continents.
The continental plates have been broken, glued back together and then broken up again, but the pieces of the very old lithosphere — very old pieces of continents — have been there for a very long time.
One of those original continents is the North American craton, located mostly in the Canadian part of North America. The study suggests that what continental lithosphere has been added since the original North American craton formed was scraped off of the ocean floor as it plunged beneath the continent, not deposited from below by plumes of hot material welling up through the mantle.
The top 25 miles of the lithosphere is crust that is chemically distinct from the mantle below, and while activities such as mountain building can dredge up deeper material, mountain building is rare in the planet's stable cratons. The deep interior of the North American craton is known only from so-called xenoliths — rock inclusions in igneous rock that have been delivered to the surface from deep below by volcanoes.
Seismologists also have the ability to probe the Earth's interior thanks to seismic waves from earthquakes around the globe, which can be used much like sound waves are used to probe the interior of the human body. Such seismic tomography has established that the bottom of the North American craton is about 150 miles deep at its thickest, thinning out toward the margins where new chunks have been added to the continental lithosphere.
Romanowicz and UC Berkeley postdoctoral fellow Huaiyu Yuan are testing a new technique, seismic azimuthal anisotropy, to look for the boundary between the lithosphere and asthenosphere. The technique takes advantage of the fact that seismic waves travel faster when moving in the same direction that a rock has been stretched than when traveling across the stretch marks. The difference in speed makes it possible to detect layers that have been stretched in different directions.
Surprisingly, they found a sharp boundary 100 miles below the surface that is not the lithosphere-asthenosphere boundary. The scientists believe that the sharp boundary is between two types of lithosphere: the old craton and the younger material that should match the chemical composition of the sea floor. Their interpretation fits with studies of xenoliths and xenocrysts, which indicate that there are two chemically distinct layers within the North American craton crust.
"One hypothesis was that the bottom part was formed by underplating," Romanowicz said. "You would have a big plume of material, an upwelling, that would get stuck under the root. But what we are observing is not consistent with that. The material would spread in all directions and you would see anisotropy that is pointing like spokes in a bicycle."
"We are seeing a very consistent direction across the whole craton. In the top lithospheric layer the fast axis is, on average, aligned northeast-southwest. In the bottom layer it is aligned more north-south. So underplating doesn't work," she said.
If subduction is adding to the continental lithosphere, on the other hand, the north-south strike of the subduction zones on the east and west sides of the North American craton is consistent with the direction Romanowicz and Yuan found.
For further information: http://www.universityofcalifornia.edu/news/article/23936