January 19, 2011

Geology Word of the Week: L is for Lithosphere

Posted by Evelyn Mervine

Mantle Hills, Oman, January 2009.

def. Lithosphere:
1. The cool, outer layer of the Earth that deforms in a brittle manner.
2. Although often mistaken for the crust, a layer of the Earth that actually consists of both the crust and part of the upper mantle.

Last week I blogged about the notorious PhD general or qualifying exam. A common question that comes up in the general knowledge section of the geology exam is:

“What is the lithosphere?”

Or sometimes:

“What is the difference between the lithosphere and the crust?”

Or sometimes, if the committee members want to trick the student:

“Where in the lithosphere is the MOHO located?”

The lithosphere is a fundamental, but somewhat confusing, concept in geology. The lithosphere is sometimes mistaken for the crust, even by some geology students. However, the lithosphere and the crust are not the same. This is because the lithosphere is defined by physical properties while the crust is defined by chemical (or compositional) properties. The lithosphere consists of the crust AND part of the upper mantle.

When I think about the lithosphere, I think about two sets of words:

Lithosphere and Asthenosphere


Crust and Mantle

The lithosphere and the asthenosphere are layers of the Earth which are defined by physical properties. Specifically, the lithosphere consists of “the crust and upper mantle (viscosity >10^21 Pa s) which deforms in a brittle fashion when subjected to a stress of ~100 MPa (Keary, 1996)” while the asthenosphere is “a mechanically weak layer of the mantle immediately beneath the lithosphere, corresponding to the depth range within the Earth where the melting temperature is most closely approached (Keary, 1996).”

In a nutshell: when stressed, the lithosphere breaks but the asthenosphere flows. Basically, the lithosphere is the cool, rigid, outer shell of the Earth that breaks (deforms in a brittle fashion) when stressed. All of Earth’s crust deforms in a brittle fashion. Part of the upper mantle also deforms in a brittle fashion. Thus, both the crust and part of the upper mantle comprise the lithosphere. The asthenosphere, on the other hand, is hot and soft enough to flow (very slowly) rather than break when stressed. The asthenosphere is a solid, but it moves very slowly, like honey or tar.

The lithosphere and asthenosphere make plate tectonics possible. The lithosphere breaks up into tectonic plates, which slowly move over the tar-like asthenosphere.

The depth of the lithosphere-asthenosphere transition varies widely throughout the Earth as it is dependent on the thermal regime. The lithosphere may extend only 2 or 3 kilometers beneath young, hot, thin oceanic crust. However, beneath old, cool, thick continental crust, the lithosphere may be as thick as 250 or even 500 kilometers. Under some very old, very cold, very thick continental cratons, the asthenosphere  may not exist at all.

The crust and the mantle, on the other hand, are layers of the Earth which are defined by chemical (or compositional) properties. Specifically, the crust consists of less-dense crustal rocks (e.g. granites, basalts, gabbros) while the mantle consists of denser rocks (mainly peridotite). Oceanic crust is thin (less than 10 kilometers) while continental crust can be much thicker (70 kilometers or more).

Geologists have actually never directly observed the crust-mantle boundary. No one has drilled a hole deep enough to reach the crust-mantle boundary.  The deepest hole ever drilled on Earth is the Kola Superdeep Borehole, which reached about 12 kilometers in depth. However, because this hole was drilled into thick continental crust, the hole came nowhere near the crust-mantle boundary. I believe that the deepest hole ever drilled in oceanic crust is this approximately 2 kilometer deep hole off the coast of New Zealand. However, since this hole was drilled on the thick, sediment-covered continental shelf, I don’t think this hole came anywhere close to the mantle.  However, other holes in other pats of the oceanic crust have almost– but not quite– reached the elusive mantle.

As far as I know, no hole in either continental or oceanic crust has reached the crust-mantle boundary. Geologists haven’t even managed to drill through the thin (5-10 kilometers) thick oceanic crust to reach the mantle. Really, geologists have only scratched the surface of the planet. They haven’t even directly observed the mantle! The only places where scientists have been able to “see” the crust-mantle boundary are the rare places, such as in Oman, where ocean crust and part of the underlying  mantle has been uplifted onto land through natural tectonic processes.

Since geologists cannot directly observe the crust-mantle boundary (or, for that matter, the lithosphere-asthenosphere boundary), the boundary is defined by geophysical observations. The crust-mantle boundary is called the Mohorovicic Discontinuity or the MOHO for short. The MOHO is a place where seismic P-waves suddenly increase in velocity, presumably because they are able to travel faster through the denser mantle rocks.

So, for those geology students about to take a test or a PhD qualifying exam, remember:

The lithosphere is NOT the same as the crust. Rather, the lithosphere is comprised of both crust and upper mantle.

The MOHO is NOT the lithosphere-asthenosphere boundary. Rather, the MOHO is the crust-mantle boundary.

Keary, Philip. 1996. Dictionary of Geology. London: Penguin Books.