Download Igneous, sedimentary and metamorphic rocks Handout

Igneous, sedimentary and
metamorphic rocks
The building blocks of landforms
Silicate Minerals
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Most igneous rocks contain
silicate minerals
Silicate minerals are
combinations of Si-O tetrahedra
plus one or more metallic
elements
A silicon-oxygen tetrahedron is
an anion that consists of a
single silicon atom surrounded
by four oxygen atoms (SiO4-4)
arranged so that they define the
corners of a pyramid
From: Marshack, S. 2001:Earth,Portrait of a Planet.
WW Norton & Company, New York. Fig 5.21
Rocks
• We have now seen the relationship between
elements and minerals:
• Elements combine chemically to form
minerals
• How does this fit into our understanding of
what rocks are?
• Rocks are aggregates of different minerals
Silicate minerals (cont.)
• Silicate minerals containing feldspar and silicon
combinations are referred to as felsic minerals
• Feldspars are alluminium silicates in combination with Ca,
Na, and K
• Silicate minerals containing magnesium and iron(Fe)
combinations are referred to as mafic minerals
• Most igneous rocks are built up of one or more of only
seven major silicate minerals or silicate mineral groups
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Major silicate mineral groups
Major silicate mineral groups (cont.)
Mafic Group
Felsic Group
1.Quartz (SiO2)
2.Potash Feldspar (K,Na)AlSi3O8
Density
2.6g/cc
2.6g/cc
3.Plagioclase Feldspar NaAlSi3O8
2.6g/cc
CaAl2Si2O8
2.8g/cc
4. Mica (K,Mg,Fe)AlSi3O8
(Biotite, Muscovite)
5. Amphibole(Ca,Mg,Fe)AlSi3O8
(Hornblende)
6. Pyroxene (Ca,Mg,Fe)AlSi3O8
(Augite)
7. Olivine (Mg,Fe)SiO4
(Olivine)
Density
2.9g/cc
3.2g/cc
3.2g/cc
3.3g/cc
Additional Minerals
Magmas
• Besides silicate minerals, igneous rocks also
have additional minerals that are not
silicates
Magnetite (Fe2O4)
Ilmenite (FeTiO3)
• These are known as heavy minerals because
they have high densities of 4.5 – 5.5g/cc
• Igneous rock solidifies from molten rock material
consisting mainly of silicate magmas under high
pressure(6000-12000 atmospheres) and high
temperature(500-1200ºC)
• Included in silicate magmas are the volatiles,
which are mainly gasses and liquids like water
vapor (H2O), carbon dioxide (CO2), sulphur
dioxide (SO2), nitrogen (N2) and Hydrogen (H2)
• Most of the volatiles are released during
crystallization
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Classification of igneous rocks
Texture
Extrusive
Intrusive
• The physical characteristics of an igneous rock are
closely related to the size, form and orientation of
individual minerals in the rock
• Intrusive rocks (plutonic) form at great depth,
slow crystallization, large crystals therefore they
will have coarse texture
• Extrusive rocks form at shallow depth, rapid
crystallization, small crystals, therefore they will
have fine texture
• TEXTURE and COMPOSITION are combined
to order or classify igneous rock
(Diagram from: Marshack, S. 2001:Earth, Portrait of a Planet. W Norton & Company,
New York. Fig 6.17
Chemical alteration of igneous
rocks
• Igneous rock forms under high temperature and
pressure somewhere in the crust as a magma cools
down
• Exposure of igneous rock at the surface brings it
into a totally different environment of low
temperature and pressure
• It comes into contact with water and solutions
containing oxygen and carbon dioxide that will
react with some of the minerals in the rock
• This chemical change in response to an alien
environment is called mineral alteration, a process
of weathering
Essential process
• It is essential that rocks be broken into
smaller fragments for chemical reactions to
function properly
• WHY?
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Chemical alteration processes
Clay minerals
• Oxidation – a process whereby minerals react with
oxygen dissolved in soil or groundwater
• Oxidation converts silicate minerals to very stable
oxides found abundantly in many rocks and
sediments
• Hydrolysis – a chemical process where water
reacts with minerals producing totally new and
very stable minerals
• Combinations of oxidation and hydrolysis very
often produce clay minerals
• Most important component of soils
• Clay minerals have plastic properties when
moist meaning they can be deformed
• Deformation is possible because thin flakes
of mica become lubricated by layers of
water molecules in between them
Chemical alteration processes
(cont.)
• Carbonation – a process of solution where
atmospheric CO2 dissolves in rainwater to
form a weak acid, (carbonic acid H2CO3)
that will react with any carbonate such as
Calcium carbonate CaCO3
• Any limestone or dolomite area is therefore
a target for carbonation. WHY?
Sediments and sedimentary rocks
• Sediments are the unconsolidated mineral
matter produced by weathering that is
capable of being transported by a fluid
medium such as air, water and glacial ice
• Sediments can be subdivided into:
– Clastic sediments
– Non-clastic sediments
– Piro-clastic sediments
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Sediments
• Clastic sediments consist of particles
derived directly from the original bedrock
without being altered
• Non-clastic sediments are newly created
mineral matter derived from chemical
precipitation or organic activity
• Piro-clastic sediments consist of solid
particles derived from volcanic action
Clastic sediments
• Are composed of quartz, feldspar, heavy
minerals, clay minerals and any other rock
fragments
• Particle size is an important role player
during sediment transport and during the
settling of sediments
Wentworth Scale for grain size
Sorting
• Due to the difference in
settling velocity of
particles of different
grain size, sorting of
sediments will occur
• Sorting is the process
whereby similar grain
sizes group together
From: Marshack, S. 2001:Earth,Portrait of a Planet.
WW Norton & Company, New York. Fig 7.18
From: Boggs, S., 1995: Principles of Sedimentology and Stratigraphy,
Prentice Hall. New Jersey. Table 4.1
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Flocculation
• Particles can only settle in water when they
are “heavy” enough
• They will stay in suspension indefinitely
unless they clot together, a process called
flocculation
Igneous vs. sedimentary
landscapes
Lithification
• To change from a sediment to a sedimentary rock,
sediments need to undergo a process of compaction
and hardening called lithification
• Sand will change to sandstone
• Mud will change to mudstone
• Clay will change to claystone
• Finely laminated mudstone is called a shale when it is
fissile
• Sedimentary rocks are characterized by their layered
appearance and the layers are called strata
Non-clastic sediments
• Are divided into two groups
Igneous Bornhardt Namibia
– Chemical precipitates derived from the
evaporation of water (also called hydrogenic
sediments or evaporites)
– Organically derived sediments created by the
life processes of plants and animals (also called
biogenic or carbonates)
Colorado Plato Utah, USA
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Examples
• Examples of evaporites:
Gypsum(CaSO4·2H2O)
Halite (NaCl)
Anhydrite (CaSO4)
• Examples of carbonates:
calsite (CaCO3)
Aragonite (CaCO3)
Dolomite (Ca,Mg(CO3)2)
Salt playa,Eastern California
Limestone, Yugoslavia
Metamorphic rocks (cont.)
• Since new minerals are created during
metamorphism, the end product (metamorphic
rock) will also be new
• Shale
slate
schist
• Schists are the most advanced grade of
metamorphic rocks
• Schists have a texture called foliation which mean
they have thin but irregular planes of parting in the
rock (mica minerals are arranged more or less
parallel to each other).
Metamorphic rocks
• Metamorphic rocks are rocks from
sedimentary and igneous origin that have
been changed through high pressures and
temperatures during orogenesis (mountain
building) or during igneous intrusion
(contact metamorphism)
Some rocks and their
metamorphic equivalents
Sandstone
Siltstone
Conglomerate
Limestone
Granite
Quartzite
Marble
Gneiss
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The Rock Cycle
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