exam questions answers deformation and metamorphosis of the earths crust geo2 1209
crustal deformation and metamorphosis
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Deformatie en metamorfose van de korst (GEO21209)
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Practice tests
TEST 1
7. Give short answers to the following questions;
a) What is the definition of metamorphism and what grain-scale processes are involved?
Metamorphisms is the sum of all changes (minerals, grain size and grain shape) in a rock due to changes in the
environment; such as temperatures, pressure and pore fluid interactions. When a rock undergoes metamorphism, it
eventually looses it's foliation and the grains will get larger when they merge together (since merging together of grains
decreases the free energy; and a metamorphic rock is a thermodynamic equilibrium assemblage.
b) Give a short description of a schist, a gneiss and a migmatite.
Schist = type of rock with continuous foliation (when there is more mica - there will be more foliation)
Gneiss = partly foliated rock, consisting of mica and other elongated minerals (this rock contains more quartz and
feldspar compared to schist - therefore it is less foliated)
Migmatite = when the rock has started to melt, a migmatite will form → this is a combination of igneous and metamorphic
rock. And foliated layers of residue from melting.
c) What is the pressure at the base of the crust at a depth of 30 km. Note that 32crustal density is about 2700
kg/m , g =9.8 m/s , SI units of pressure are Pa and 1 kilobar (kb) = 100 MPa.
Pressure = density * depth * g
Pressure = 2700 (kg/m3) * 30 (km) * 9,81 (m/s2)
Pressure = 794610 Pa = 794MPa = 7,9 kb.
d) How can you distinguish metamorphic rocks from igneous and sedimentary rocks in the field?
The mineral composition. Sedimentary rocks do not cary a lot of minerals (since they will not produce them).
Metamorphic rocks do carry minerals, and they are different from igneous rocks.
Igneous rocks are made of the crystallization go silicate melts - formed by volcanic processes.
Metamorphic rocks are produced when environment changes (pressure and temperature)
Metamorphic rocks have foliations and porphyroblasts. Sedimentary rocks have bedding, sedimentary structures and
clastic grains. Igneous rocks have interlocking grains and usually no bonding or foliation.
e) What is the difference between dynamic metamorphism and regional orogenic metamorphism?
Dynamic metamorphism is the process of metamorphism under only deformation circumstances (local high strain zone).
Regional (orogenic) metamorphism on the other hand is the metamorphism under both deformational conditions as well
as extra heating (in mountain range). So the difference is wether there is heating or not.
8. Some rocks in a metamorphic belt contain the following minerals: jadeite = NaAlSi2O6, quartz = SiO2 ,
Andalusite Al2SiO5, Silliminite Al2SiO5, and albite NaAlSi3O8
a) How many chemical components are needed to describe these rocks and minerals?
NaAlSi2O6
SiO2
Practice tests 1
, Al2SiO5
NaAlSi3O8
Amount of elements = Na + Al + Si + O = 4
Amount of oxides = SiO2 + Al2O3 + H2O + Na2O = 4 → the number of oxides is the number of components (in this case
the number of components is 4!)
RIGHT ANSWER = there are 3 components (oxides); SiO, AlO and NaO.
b) What chemical reactions and polymorphic reactions can occur in this system?
Polymorphic reaction → from andalusite to sillimanite
c) Define the phase rule and use this rule to determine how many minerals will be present in this system in a
divariant mineral assemblage and an invariant assemblage in this chemical system.
F = amount of freedom (what can be changed; the pressure, the temperature or the ,mineral composition?
C = the amount of elements (so the amount of oxides) + H2O
Ph = the amount of phases (the number of minerals in a rock)
F = C + 2 - Ph
F = 4 oxides + 2 - amount of minerals 5 (also adding water) ??? wel of niet de polymorphs meetellen????
F=4+2-5=1
?????
RIGHT ANSWER
F (degrees of freedom) = (chemical components +2) - phase number. As said, the amount of chemical components is 3.
For a divariant the F=2 and therefore there will be 3 phases and for an invariant the F=0 and therefore there will be 5
phases.
d) The diagram shows a plot of the Gibbs free energy
for sillimanite and andalusite, with increasing
temperature, at constant pressure. Which mineral will
be stable at high temperatures?
Increasing temperature and constant pressure means that
we are looking at the entropy phases. The high entropy
phase(s) are stable at high temperatures. The Gibbs free
energy decreases with temperature. The mineral with the
highest free energy and the highest entropy is the one
that will not be stable at low temperature - only at high
temperatures. Therefore, the mineral with the lowest free
energy will be stable at low temperature. In this case,
andalusite has a higher entropy compared to sillimanite.
The mineral with the highest Gibbs free energy is the one
that is stable at high temperatures. In this case; that is
Practice tests 2
, Andalusite (the highest Gibbs free energy as well as the
highest entropy)
The phase with the lowest Gibbs free energy will be
stable; so sillimanite
e) What thermodynamic parameter controls the slope of the Gibbs free energy versus temperature plot in the
figure above.
The entropy; dG/dT = -S.
9. The figure below shows an AFM diagram for the staurolite metamorphic zone from NE Scotland. This zone
occurs in a sequence of garnet zone, staurolite zone, kyanite zone and sillimanite zone.
1. a) Why are the minerals like biotite plotted as a band of compositions?
This is because these minerals are all projected on the mineral muscovite. Therefore, they appear in 'bands'. Because
biotite can be made of FeO and MgO → biotite appears as band, with Fe-rich minerals at the left and Mg-rich minerals at
the right. Biotite is a solid solution with a variable Fe/Mg ratio.
2. b) Why does garnet (G) plot on the Fe rich side of the diagram and chlorite (Chl) on the Mg-rich side of the
diagram?
That is because garnet contains Fe-rich biotite and chlorite contains Mg-rich biotite. Fe fits better into garnet structure
and Mg better into Chlorite structure.
3. c) Make a list of the all of the mineral assemblages containing staurolite (St) that can occur in this zone.
4. d) Draw AFM diagrams for the garnet zone and the kyanite zone. The reactions at each isograd are: at the
strauolite “in” isograd, staurolite + biotite = garnet + chlorite + water; and at the kyanite “in”isograd, staurolite +
chlorite = kyanite + biotite + water.
Here you should use the staurolite and made kyanite and garnet diagrams from the reactions that are given.
Practice tests 3
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