|theImage.com     Notes on Basic Geology|
|Notes created & information organization based on the book:
"The Dynamic Earth - an introduction to physical geology"
Brian Skinner & Stephen C. Porter   (further book information here)
also look at www.wiley.com for additional resource information
|EarthQuakes - Page 5|
|Seismic Wave cannot tell us the chemistry|
|Velocity can tell us
something about density though. The crust appears to be
about 3.3 gm/ cm3 on average. The mantle is
heavier at about 5.5 gm/cm3. The core appears to
be quite heavy at between 10 and 11 gm/cm3. The only common element that comes close to fitting this density is "iron".
The S- wave, P wave combination show an outer liquid area and solid inner core. Apparently just below the mantle, the pressure temperature curve favors molten iron and at about 5350 km the pressure wins out and the material solidifies.
|There are other discontinuities that have been found over time. Not all have been explained.|
|(60 - 70 km) Crust||Thicker in continents than under the ocean.|
|(100 km) (8.3 km/sec) Top of Mantle
|Asthenosphere||Zone where velocity drops. Apparently due to partial melting and an increase in elastic properties. This is defined by measuring the P-wave velocities.|
|(350 km) 8 km/sec (drop in speed)|
|(400 km) Small speed increase (POLYMORPH)||Change in density due to atomic rearrangement.|
|(670 km) Velocity increase (UNKNOWN)||Lots of theories no good reasons|
|(2880 km) 14 km / sec (Bottom of Mantle)|
Areas over old ice sheets
There is an instrument called a "gravimeter" which can measure very small differences in gravity.
First, gravity is directly related to the MASS of an object. The more mass, the more gravitational pull. Hence a 1" diameter iron sphere has more gravitational pull than 1" ice sphere.
The gravitational pull over the earths surface is not a constant. Therefore the material (rock) between the measurer and the center of the earth must be different in these locations.
One might expect there to be higher gravity at the top of a mountain than near the sea shore as there is far more rock under the mountain. (This is, however, not always the case!) Since gravity is directly related to the density of the rock (mass), the type of rock plays an important role.
Continental rock is less dense than mantle rock, and as it happens, the Principle of Isostacy plays a pivotal role. A mountain has more continental rock under it, but it displaces mantle rock as it sags into the mantle. Hence, the thickness is higher, but it is made up of less dense rock. The gravimeter can actually shows less gravity above a mountain.
Remember the glacial ice sheets. They too depress the lithosphere and hence move more material out of the way. Measurements above the ice sheets or where there used to be ice sheets both show lower gravity readings. This is due to the slow rebound of the depressed areas. It takes time for the asthenosphere to "flow" back in and refill the voided area.
The "take home message" from all of this is that the Lithosphere "floats" on the asthenosphere, and will eventually achieve isostatic equilibrium.