Dear Greg
See www.mantleplumes.org, there are several discussions and opinions in
the section “seismology”, also about the “red = hot, blue = cold” your
students ask
> Awesome answer! Thank you, Ernie! That is just what I was looking for!
>
> Best regards,
> Greg
>
> On 3/17/2011 11:58 AM, Ernest Rutter wrote:
>> It is a fact that velocity increases with the density of a suite of
>> (different) rocks, but for constant elasticity velocity decreases with
>> density. It is just that going from say, granite to peridotite, the
>> increase in density is more than offset by the increase in stiffness.
>> Denser rocks tend to be stiffer. Hotter rocks have lower densities AND
>> lower stiffnesses. Stiffness is a much more variable property than
>> density, so you can have denser rocks being faster than less dense
>> rocks.
>>
>> Ernie Rutter
>>
>> -----Original Message-----
>> From: Tectonics& structural geology discussion list
>> [mailto:[log in to unmask]] On Behalf Of Gregory Dumond
>> Sent: 17 March 2011 16:45
>> To: [log in to unmask]
>> Subject: P-wave velocity, Density, and Modulus
>>
>> There is a lot of remarkable work being done in North America, Tibet,
>> and all over the globe using seismic tomography! Consequently, I receive
>> questions from many students that are similar to "What do the red and
>> blue colors mean?!"
>>
>> I first admit my unfortunate lack of anything but a basic education in
>> geophysics. In my non-expert opinion, the equation for P-wave velocity
>> (e.g., in Fowler (2005-2nd ed. on p. 102) appears at first to be
>> nonintuitive, based on the following:
>>
>> (1) Classic experiments by Christensen and Mooney (1995: JGR-Solid
>> Earth) and others demonstrate a general trend of P-wave velocity
>> increase with increasing density. One of my illustrations for students
>> is to hand them similarly-sized hand samples of basalt and eclogite to
>> convey to them a sense of how phase changes and densification can occur
>> in a fixed bulk composition. Coincidentally, the average P-wave velocity
>> and density for mafic eclogite (7.984 km/s for a density of 3515 kg/m3)
>> is greater than basalt (5.873 km/s for a density of 2926 kg/km3), at
>> 762C and 50-km-depths (Christensen and Mooney, 1995).
>>
>> (2) The density term is in the denominator of the P-wave velocity
>> equation, implying that density is (by definition) inversely
>> proportional with seismic velocity. Yet, Birch's Law tells us that
>> denser rocks DO have higher seismic velocities. This inspired me to look
>> for references that have plotted Modulus as a function of Density. Web
>> of Science provided few results. The most intriguing one involve a 2007
>> publication in Nature Materials on the direct relationship between
>> modulus and density in bone and some nanomaterials (Fan et al., 2007).
>>
>> I really like Professor Fowler's explanations on pp. 102-104 in her
>> awesome "The Solid Earth..." textbook, but I have still encountered
>> difficulties with regards to getting junior-senior undergraduate
>> students (and some colleagues) to fully understand these relationships.
>>
>> I would sincerely appreciate any advice or guidance on this topic.
>>
>> With warmest regards,
>> Gregory Dumond
>> Department of Geosciences
>> University of Arkansas, USA
>
> --
> Gregory Dumond
> Assistant Professor
> Department of Geosciences
> University of Arkansas
> 18 Ozark Hall
> Fayetteville, AR 72701
> Phone: 479.575.3411
> Email: [log in to unmask]
> Web: http://geosciences.uark.edu/3858.php
>
> --
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>
Rubén Somoza
Dpto. Ciencias Geológicas, FCEyN, UBA
Ciudad Universitaria, Pabellón 2
C1428EHA, Buenos Aires, Argentina
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