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Lecture 17

EPSC 201 Lecture Notes - Lecture 17: Blueschist, Oceanic Crust, Schist


Department
Earth & Planetary Sciences
Course Code
EPSC 201
Professor
Anthony Williams- Jones
Lecture
17

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EPSC 201 Lecture 17 Notes
Why to stalactites form? They are made from 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 calci-
um carbonate precipitates.
The metamorphic rocks are formed through heat and
pressure. Igneous rocks are eroded, and form layers of
sedimentary rocks. Metamorphic rocks derive their
name from metamorph, Latin for change. If these rocks
are in an environment of heat and pressure, they are
able to undergo change. If igneous rocks on the surface
are buried and heated up, they change into metamorphic
rock.
Layers of rock have different composition. Sedimentary
rock subjected to heat and pressure changes the com-
position. Often layering effects will be observed, because the sedimentary rocks have pre-existing
layers. Specific sedimentary rocks give rise specific metamorphic rocks.
The first change rocks can undergo is re-
crystallization. Here we see sandstone, a
fine-grained sedimentary rock that has
been packed together. If sandstone under-
goes recrystallization, it turns into quartzite.
The recrystallization forces the sandstone,
which has no molecular order, to organize
its structure and become quartzite.
The crack represents a split in the crystal
structure. There is no molecular organiza-
tion 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 to be.
Key point – Crystals are in a low state of energy – therefore it is stable
Any constact between two crystals represents a zone of stability. Well all crystals can be broken
down into a combination of crystals. Crystals want to minimize their surface area, which represents

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instability. So the crystal grows, making current crystals bigger. The crystals want to minimize the
surface area. This minimizes the zones of instability.
Well isn’t the perfect shape a sphere? Yes, a
sphere does have the minimum surface area to vol-
ume ratio. But the molecules also need to be mini-
mized with respect to each other. To make sure
there is no space in-between space crystal, they
take polygonal shape. This allows for crystals to
connect to each other, and minimize surface area.
Limestone – CaCo3 – calcium carbonate
Limestone is transformed into marble by recrystal-
lization. Here we see that marble formed bigger
crystals to minimize surface area. So we are going
from something fine grained to some-
thing course grained.
There can sometimes be tiny films of
water left at the surface of crystals.
In a continent-continent collision there
are stresses. Lets imagine there is
shale, which forms in layers. Clay min-
erals tend to be sheet silica’s, which
prefer to be in horizontal layers. These
sheets can easily be broken in certain
directions.
Crystals grow at right angles (90 de-
grees) to stresses. In particular, sheet
crystals grow at 90 degrees to the
stress, to minimize directed stress.
Roofs are made from slate. The slates
brake at when a differential stress is ap-
plied.
Slaty cleavage – the plane along which
the rock will preferentially break. This is
the product of differential stress at low
metamorphic grade shale. Slaty Cleav-
age is a type of foliation expressed by
the tendency of a rock to split along par-
allel planes. This should not be confused
with bedding planes, which are a sedi-
mentary structure. Slaty cleavage results
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from the parallel orientation of microscopic platy miner-
als.
When a stress is applied, the new crystals will grow at
right angles to the stress.
The pressure would be applied over a long period of
time, because of the geological time scale. If the
stress is applied too rapidly, the rocks will break.
Think of squeezing a pack of cards. The cards
bend into a U shape.
If you take a flexible book and push on the ends, the book will fold, and your hands will get closer.
Same concept with rock, but slower timescale.
If chlorite and muscovite are present in the same rock under metamorphic conditions, they will form
garnet.
The important note for this reaction is that the formation
of minerals does not change the chemical composition.
It is simply the conversion of stable minerals at low tem-
peratures to minerals that are stable at high tempera-
tures. Overall, the product is more stable. Interfacial
energy has been minimized.
Garnet has lower chemical energy (more stable) then
chlorite and muscovite. Note that water is being given
off in the reaction. Metamorphic reactions usually
dehydrate the reactant. Water is given off, usually.
Chlorite is a
very wet mineral. This reaction is a dehydration reaction. If
CO2 was given off, it would be called a decarbonylation.
Calcite + Quartz (SiO2) Molastanite + CO2
The only reactions covered on the exam will involve water
and carbon dioxide leaving as product gases.
Schist – a lot of muscovite, with a wavy lamination to it.
These rocks break unevenly, and the break along the mus-
covite. Shistosity is defined by the muscovite content.
If Schist is cooked further (deeper in the Earth), then the rock produced is gneiss.
Slate is cooked into schist, and if cooked further, it turns to gneiss. All the sheet silicate minerals
have broken down by the time the rock is gneiss. At high metamorphic grades, the rock is very dehy-
drated. Each reaction removes water.
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