Physical Geology lecture - geophysics and earth's internal structure

How do we 'see' into the earth and understand what it is made of?

  1. consideration of densities.
  2. seismic imaging.
  3. other.

1) Simple argument of comparative densities that provides insight into earth and planetary structure (all densities given below are in gm/cc which is grams per cubic centimeter).

First, what determines mineral/rock density?

Relative average densities of rocks in gm/cc:

Consider how the earth rotates. What is the relationship between the center of the earth and the rotation axis? What does this tell you about the distribution of density inside the earth?

Objects rotate about their center of mass. If they are internally composed of components of different mass, then you can say something about the internal distribution of mass from they way they rotate.

2) Seismic waves and seismic imaging:

Seismic image showing the Moho (the crust-mantle boundary) beneath part of the Himalayas and Tibetan plateau. There are different ways we can image the crust- mantle contact, which are shown here as dark lines (reflections), colors and red dots. Here the continental crust is thicker than normal because of the mountain building that resulted from the northward plate movement of India into Asia (more on that later). In addition, it is possible that the crust mantle contact is deformed here because of the plate tectonic movements. Credit: Tai-Lin Tseng, University of Illinois, Wang-Ping Chen, University of Illinois, Robert L. Nowack, Purdue University/IRIS Consortium Other images like this one can be found at the IRIS gallery page.

 

3) Other types of data to use in constructing a model for the interior of the earth?

Models for the large scale structure of the earth:

Image from USGS that summarizes larger aspects of earth's internal structure. From USGS site: Inside the Earth, http://pubs.usgs.gov/gip/dynamic/inside.html .

Model showing crustal and plate structure as deduced from seismic imaging for part of Indian continent. The blue versus the green is the upper versus the lower crust. Image source from USGS site: http://earthquake.usgs.gov/research/structure/crust/india.php .

Mantle convection:

A bit on comparative planetology.


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