MadSci Network: Earth Sciences

Re: What is the difference between the Earth Crust and Lithosphere?

Date: Mon Oct 29 14:06:43 2001
Posted By: David Smith, Faculty Geology, Environmental Science
Area of science: Earth Sciences
ID: 1002424847.Es

The crust and the lithosphere both are names for the outermost part of the solid earth, but they are defined in different ways. See an excellent illustration from an online book on plate tectonics (published by the United States Geological Survey) at:

The crust and the next layer down, the mantle, are defined by their composition, but we don't have many opportunities to take a direct sample and analyze it, so one of the ways we use instead is to use properties that reflect composition. One of these is the speed of earthquake waves (or manmade seismic waves from explosions, etc.). At the boundary between the crust and the mantle, there is a big jump in the speed of seismic waves. That corresponds to a change from mostly mafic rocks (such as gabbros and diorites) in the crust to mostly ultramafic rocks (such as peridotites) in the mantle. The speed of seismic waves normally increases as you go down in the earth because the pressure increases, but the increase is slow and steady. At the crust-mantle boundary, however, the increase is very rapid, creating a discontinuity in the speed versus depth graph. The discontinuity is named for the discoverer, Mohorovicic (see, but people get tired of saying Mohorovicic discontinuity all the time, so we just call it the Moho, for short.

The lithosphere in defined by physical properties, rather than chemical properties. Confusingly, these can also be measured by the speed of earthquake waves. The lithosphere is the stiff part of the earth. It includes all of the crust and some of the upper mantle as well. Below the lithosphere is another part of the mantle, known as the asthenosphere, which is sort of a stiff mush and behaves like plastic. Remember how I said that seismic velocity normally goes up slowly with depth and that it takes a big jump up at the Moho? Well, at the base of the lithosphere (which is the top of the asthenosphere), there is a big _decrease_ in seismic velocity. This decrease reflects the presence of a tiny amount of melt between the mineral grains of the mantle. This melt lubricates the grains and make it possible for them to flow. As a result, the stiff and brittle shell of the earth (the lithosphere) is carried around on the moving asthenosphere.

The other questions you ask, are also related to quakes. The shadow zone is related to the structure of the earth. Earthquake waves come in two flavors, P or pressure waves, and S or shear waves. P waves can move through anything, but S waves can only move through solid or somewhat solid materials. When a quake happens, there is a big area on the opposite side of the earth where seismometers do not pick up the S waves. That area is the shadow zone. In order to block S waves, you need something that is not solid. Since S waves _are_ blocked, we believe there must be a zone of liquid down near the center of the earth. We call this liquid material the outer core and know from P waves and other data that it is made of molten Fe, Ni, and S. The shadow zone is not related to the Moho in any way. The outer boundary of the outer core is thousands of kilometers deeper than the Moho.

Seismic gaps are near-surface features. When scientists study earthquakes along plate boundaries (where the fractured pieces of the lithosphere rub together as they move around), they sometimes find a part of the plate boundary where there have not been any big quakes in a long time. If you put all the quakes on a map, there would be a space with none or very few quakes. This space is the "seismic gap". What I think you mean by the gap hypothesis is the idea that if there is a gap, a place without many quakes for some time, then that place is at a higher risk for a big earthquake than adjacent areas which have been active. So the gap is the area itself and the hypothesis is that such an area is a high-risk area for future quakes. Gaps can also occur, however, where the plates are moving slowly and steadily (known as "creep"). Those areas are not at risk because the steady movement keeps releasing energy before it can be stored.

I hope this helps. Sorry for the delay, you caught me at midterm exam time.

Dave Smith

La Salle University, Philadelphia, PA

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