ANSWERS TO “TEST YOUR KNOWLEDGE” ON PAGE 36 Answers:
1. The answer is choice “a” or “series”. “System” is the chronostratigraphic (time-rock) unit that relates to the geochronologic (time) unit we call “period.” “Formation” is a lithostratigraphic (rock) unit, not a time unit. Lithostratigraphic units can cross time boundaries.
2. The answer is choice “a” or “slump.” “Falls” typically involve failure of solid bedrock along cliffs and steep slopes. “Flows” involve the down-slope movement of earth materials behaving as viscous fluids.
3. The answer is choice “b” or “false.” The statement is generally true for faults, but not for folds. For the latter, fractures do not necessarily reflect the regional stress field that gave rise to the fold. More commonly, they are genetically related to the folding process itself. David W. Stearns recognized and described four distinct sets (sets I through IV).
4. The answer is choice “b” where albite is sodium plagioclase
feldspar [NaAlSi3O8] and kaolinite is a clay mineral and a hydrous aluminum silicate [Al2Si2O5(OH)4]. Feldspars commonly weather into clay minerals.
The mineral gibbsite is aluminum hydroxide [Al(OH)3]. The mineral anorthite is calcium plagioclase feldspar [CaAl2Si2O8].
5. The answer is choice “c” and the proof now follows:
A stress tensor T can be rewritten as the sum T = S + D. Now, S is the spherical or hydrostatic stress tensor (that part of the state of stress that causes hydrostatic conditions and only change in volume with no distortion), whereas D is the deviator stress (that part of the state of stress that deviates from hydrostatic and causes distortion). The principal diagonal
of the spherical stress tensor has values equal to I1/3 where I1 is the sum of the principal diagonal of the original stress tensor. In our case, I1 = 12 + 9 + 3 = 24 and I1/3 = 8. Then the spherical tensor (S) is:
S =
It now follows that the deviator stress tensor (D) must be: D =
The above corresponds to our choice “c” and is the answer that we seek. Note that: S + D = T.
Awesome, isn’t it? Well, maybe not “awesome” to some, but if you are into geomechanics, rock deformation and structural geology, this is really quite important.
38 TPG •
Jul.Aug.Sep 2019
www.aipg.org
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