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GEOLOGY 12
SEDIMENTARY ROCKS
Assignment # 27
April, 2006
METAMORPHIC ROCKS
- 3rd category of rocks (igneous, sedimentary)
- form from pre-existing rocks, are one of the oldest rock types
- metamorphic = ‘changing of form’ due to deep burial, tectonic forces and/or high
temperatures (not surface conditions), the rock texture is completely different from parent rock
e.g. limestone metamorphosed
clay
shale
marble
- when rock is metamorphosed:
- the old rock crystals change into new crystal formations due to
heat and pressure
- rock has to achieve equilibrium with its new environment by
changing its crystal structure
- heat and pressure make the original rock ductile
- sedimentary rock that is layered, due to pressure and heat changes its
crystal structure and usually ends up in a ‘banded’ - ‘twisted’ character
- metamorphism =
changes that occur to rocks deep within earth’s interior
- changes occur in solid state: i.e. rock doesn’t melt
- influences important in metamorphism:
- chemical composition of rock
- temperature
- pressure
- water present
- hydrothermal ore deposits: important to world supply of metals
- the core of continents are largely metamorphic rocks and granite plutons
- erosion of mountain belts + isostatic recovery
expose large areas of metamorphic rock
- Canadian shield = exposed metamorphic and granitic intrusive rocks
FACTORS CONTROLLING THE CHARACTERISTICS IF METAMORPHIC ROCKS:
- four factors that influence formation of metamorphic rocks are:
1) composition of original rock material
(no new chemicals are added to parent rock)
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2) the amount of temperature and pressure
(the deeper - the hotter it is: 30’C/km depth)
- rock minerals become unstable with temperature
- the greater the temp. the more crystal ions ‘vibrate’
the more the
ions vibrate the more the bonds between the mineral crystals break
changes in crystal texture of rock (at some point with enough heat, rock
will melt becoming magma - results in igneous rock)
- Note: metamorphism occurs before rock melts i.e. rock = ductile
pressure = pressure equally applied to all surfaces of buried material
- lithostatic pressure = pressure from all sides
- pressure squeezes grains of rock together, rock becomes more
dense as atoms squeezed together
- pressure increases 1000 atmospheres/3.3 kms in depth
Page 157 Fig. 7.4
foliation = parallel alignment of rock material/crystals
- minerals align themselves according to direction of pressure
- foliation is used to classify metamorphic rocks
- 3 types of foliation=
1) slaty = rock splits along flat parallel planes
2) schist = coarse grained minerals oriented approximately parallel
Page 157 Fig. 7.6
3) gneiss = rock composed of light and dark layers as minerals
separate into distinct bands
Page 164 Fig. 7.14
- other factors that influence formation of metamorphic rocks are:
3) effects of tectonic forces
4) effects of fluids i.e. water
- water is trapped in sedimentary material
- as rock squeezed water squeezes through pores in rock material
- water dissolves and carries ions between grains as material is squeezed
creating new types of crystals
different textures
time - the process of metamorphism takes millions of years for pressure + temp to change the
parent rock
Assignment Page 171 Questions #1, 2, 4
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Classification of Metamorphic Rocks:
- 2 most common classifications are:
1) contact metamorphism (thermal metamorphism):
- temperature dominant factor
- occurs near surface of crust (not deep), less then 10 kms deep
- occurs adjacent to a pluton: adjacent rock to pluton is slowly ‘baked’
- aureole = zone of contact between pluton and adjacent material
- usually 1 - 100 meters wide that is changed (metamorphosed)
Page 161 Fig. 7.9
e.g. limestone changes to marble (marble quarries from
which marble ‘mined’ aren’t huge since they represent
the contact area of a pluton and sedimentary rock)
2) regional metamorphism (dynamothermal metamophism)
- represent the majority of metamorphic rocks on earth
- takes place at greater depths (greater then 5 kms deep)
- rocks always foliated (= differential stress)
- often found in most deformed sections of mountain ranges (rocks exposed
as erosion of range occurs due to isostacy, formed in ‘roots’ of mountain systems)
- temp due to depth - pressure due to depth + tectonic forces (isostatic effects)
e.g. deposits of clay
(sedimentary rock) shale
(slate = blackboards, pool tables)
metamorphism = slate
Page 161 Fig. 7.2 (examples of rocks changes)
migmatite = a micture of igneous and metamorphic rock
- partial melting and as a result magma intrudes into metamorphic rock
- occurs at narrow transition zone between pluton and surrounding material
Plate Tectonics and Metamorphism:
- convergent boundary
between two plates
areas of differential stress on rocks as rock material squeezed
- subduction plate as it slides under other plate = zone of shearing and varying heat zones
- at converging plates heat zones vary greatly since equilibrium with surrounding heat
areas has not been achieved
Page 165 Fig. 7.16
isotherms = lines connecting areas of same heat
- variety of different metamorphism at convergent boundary due to pressure and heat variations
Assignment Page 171 and 172 #5, 7 to 13
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Hydrothermal Processes:
(Influence of water in metamorphism)
- water in metamorphic process affects ion transportation and formulation of different crystals
hydrothermal rock = a rock formed from an accumulation of hydrothermal crystals
role of water = 1) water transports ions between crystals (= metamorphism)
2) water brings ions from surrounding rock material (= metasomatism)
3) precipitates minerals from rock material (=hydrothermal rocks)
Page 167 Fig. 7.3
Divergent boundary:
- water moves through crust, dissolves minerals, minerals and water are transported up (have hot
springs)
metasomatism = water movement between rock materials
- at contact zones between plutons (magma) and surrounding rock water
transports ions and can accumulate minerals e.g. iron, tungsten, copper, lead, zinc,
silver
hydrothermal veins - quartz veins = (gold accumulation) as water percolates up
toward surface along pore spaces, hot water cools off the closer it gets to the
surface, cooling
water stops moving up and crystalize minerals. = how
zones of mineralization can occur, how gold veins occur, how deposits of copper
etc occur.
disseminated ore deposits = water percolates minerals throughout an area, not
simply in veins
- most mineral deposits are the result of hydrothermal viens (explain episode in
Australia @ copper deposit and Jack Snow
Page 168 Fig. 7.20
Assignment Page 172 #’s 15, 16, 17, 18
Expanding Your Knowledge: #2 and #3