EPSC201 2004 Final Exam Review Section 5&6
1. Describe the mineralogical changes that occur during the chemical weathering of
granite. What conditions favour such weathering?
Granite is much less susceptible to chemical weathering, compared to marble. This is easily observed in
cemeteries, where headstones are made of both. Granite will preserve the names much longer, whereas
marble weathers very easily.
The reaction of marble with acid rain is widely known. Any exposure to acid will dissolve the marble. This
is seen in Europe, where any buildings of marble are badly weathered due to acid rain from the early
1900s. The acid responsible for this weathering is carbonic acid. This was produced in the industrial
revolution from burning coal.
Water is a polar molecule. This is because the oxygen is electronegative, and has a tiny net negative
charge. This gives the hydrogen’s a net positive charge. The molecule behaves like a mini bar magnet.
The plus hydrogen’s are directed towards the green chlorine (negative) ions, and help it dissolve. The
negative oxygen’s on water are directed towards the positive potassium ions. This is why water can
dissolve salts. The charged ions from salt are able to dissolve in the polar water molecules, because the
water molecules are charged and orientate themselves to stabilize individual ions.
So the acid hits the limestone / marble and produces these ions, and then the water carries them away.
2. What is limestone and how does it form?
Limestone is basically calcium carbonate. Water drops from the ceiling, and calcium carbonate
precipitates. Carbon dioxide is dissolved in the water. When CO2 is in contact with water, you get
carbonic acid. When the water drops, it loses its CO2, which means the water is no longer acidic, and the
calcium carbonate precipitates.
Acid rain can dissolve rocks. Limestone that has been metamorphosed in the rock cycle forms marble.
Marble is vulnerable to acid rain. Acid on marble will ﬁzz, and carbon dioxide bubbles off.
3. Explain why recrystallization occurs and new minerals form during metamorphism.
The ﬁrst change rocks can undergo is recrystallization. Here we see sandstone, a ﬁne-grained
sedimentary rock that has been packed together. If sandstone undergoes recrystallization, it turns into
The recrystallization forces the sandstone, which has no molecular order, to organize its structure and
The crack represents a split in the crystal structure. There is no molecular organization on the crack.
Because of that, the crack is a region of instability.
When things are heated up and subjected to pressure, the rocks are trying to form the most stable form.
This is why things crystallize. The heat gives them the energy to reorganize themselves once cooled.
The energy of the crystal rock is at a minimum. The higher the energy, the less stable the state is going
Key point – Crystals are in a low state of energy – therefore it is stable 4. Explain the metamorphic facies concept. Name three facies.
Metamorphic facies – a domain in the pressure temperature graph. The facies is dependant on the
composition of the rock.
There are different environments that these reactions can take place. Depending on the tectonic
environment, you will take different tracks through the graph.
Bluechist – low temperature, variable to high pressure
Greenschist – medium temperature and medium pressure
Amphibolite – high temperature and medium pressure
5. Explain the idea of paired metamorphic belts. Where do you ﬁnd them and why?
If erosion occurs on the surface, some time later, we can see rocks that used to be underground. These
rocks exhibit these nice trends, were changes in temperature are reﬂected by observing nice shifts
between different facies.
Paired metamorphic belts – juxtaposed facies are called metamorphic belts because they are found
next to each other.
6. What are horsts and grabens? Why do they form?
Graben – formed from two normal faults – where things are
Horst - a horst is the raised fault block bounded by
normal faults or graben. A horst is formed from extension of the