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Lab 05 ONLINE
LESSON
Use down or up arrows to navigate
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DO TAKE NOTES WHILE PERUSING
ALONG…
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Lab 05
Igneous Rocks
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But first, what is a rock?
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A rock is a collection of minerals…
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There’s a mineral
There’s a mineral
There’s a mineral
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As you may recall, a mineral
is…
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1. An inorganic compound....formed
through physical forces and not biological forces
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2. A naturally occurring object...
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3. Has a specific chemical
composition...eg…
CaCO3
Fe2O3
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4. A distinctive set of physical
properties...hardness, streak,
luster etc. etc. etc...
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5. And has a crystalline
structure... with a 3 dimensional
arrangement of atoms
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6. And is a solid…no liquids or gas…
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So a rock is just a collection of
minerals
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The Rock Cycle can be used to illustrate the life of rocks…
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Igneous rocks start out as a melt below the surface of our planet…this is magma
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Magma can be pushed up to the surface where it is called lava…
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Magma and lava can solidify in place to form plutonic and volcanic rocks…
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Eventually, igneous rocks break apart to form sediment…
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Sediment then can bind together to sedimentary rocks…
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Sedimentary rocks can then be reprocessed by applying changes in temperature and pressure to form metamorphic rocks…
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Enough changes in pressure and temperature, rocks can then re-melt back to igneous rocks…then the whole process starts again…
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This is The Rock Cycle…
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There are three rock groups…
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AC/DC
Metallica
Led Zeppelin
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Ha ha ha ha…that was supposed to be funny…ha ha ha ha ha hah ho hoho…uh
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Was it funny?
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Somebody say yes.
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Good
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The three rock groups are…
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Igneous
Sedimentary
Metamorphic
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Igneous rocks will be our focus today…
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Igneous Rocks are produced from a melt
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Igneous rocks start out as a melt (liquid rock) below the surface of the earth. This is called magma….
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Eventually, this magma pushes up to the surface and becomes lava
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Lava and magma are chemically the same just one is below ground and the other is above…
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Igneous rocks that form below ground are called plutonic rocks
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Igneous rocks that form above ground are called volcanic rocks…
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Igneous rocks that form below ground are also called intrusive rocks and igneous rocks that form above ground can be called extrusive rocks…
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Volcanic/Extrusive
Plutonic/Intrusive
Plutonic/Intrusive = below groundVolcanic/Extrusive = above ground
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Both magma and lava cool to form
crystals of minerals…a rock is
a collection of minerals
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Press the play button to see magma in action
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Now that we know a little about the
source material for igneous rocks…let’s take a look at some
characteristics…
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To identify igneous rocks…we need to
look at
1.Texture 2.Mineral content
3.Gases
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Texture of igneous rocks refers to the
crystal size…
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Crystal size is determined by the rate of cooling of magma or lava…
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Rate of coolingFast Cooling Slow Cooling
Aphanitic Texture
Phaneritic Texture
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A fast cooling magma produces
small crystals
Or an aphanitic texture
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This rock possesses aphanitic crystals…
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A slow cooling magma produces
large crystals
Or an phaneritic texture
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Aphanitic igneous rocks form in a
volcanic environment…or above ground…
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This rock possesses phaneritic crystals…
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Aphanitic crystals cannot be seen with the naked eye…they
are too small…
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Phaneretic igneous rocks form in a
plutonic environment…or below ground…
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Phaneretic crystals can be seen with the
naked eye…
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Another texture is called porphyritic… this is a collection of
large and small crystals…
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This is caused by originally slow
cooling then an episode of fast
cooling…
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Producing a rock with various sized
crystals…
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So texture is based upon crystal size
which is dependant upon the
environment of formation…
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2. Mineral content is the proportion of minerals in an igneous rock…
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There are 8 minerals that make up most igneous rocks…
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1.Quartz 2.plagioclase feldspar 3. potassium feldspar
4. muscovite
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These are FELSIC minerals because of their light coloration. Grays,
whites and pink…
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FELSIC minerals are predominantly silica or
quartz rich…
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5. Biotite 6. Pyroxene 7. Olivene
8. Amphibole
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These are MAFIC minerals because of their
dark coloration. Dark grays, green and black…
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MAFIC minerals are also called ferromagnesian
minerals…they are rich in iron and magnesium…
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Igneous rocks can be identified by their mineral content and
texture…
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Igneous rocks are identified by their MAFIC or FELSIC mineral content…
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This chart from your lab manual
illustrates how mineral
content is used to identify igneous
rocks
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The identification of igneous rocks is just a contest between
MAFIC and FELSIC minerals…
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So in general, if a rock posesses 0%
to 15% MAFIC minerals, then it
plots in this range
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If a rock posesses 45% to 85% MAFIC
minerals, then it plots in this range…
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If an igneous rock is rich in quartz, and
feldspar and posesses a
phaneritic texture, then it is called
GRANITE…it is also a FELSIC igneous
rock…
GRANITE is also an intrusive and
plutonic rock…
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If an igneous rock is rich in augite and
plagioclase feldspar, and posesses an aphanitic texture, then it is called
BASALT…it is also a MAFIC igneous
rock…
BASALT is also an extrusive and volcanic rock
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The significance of FELSIC vs MAFIC rocks can be seen in the
chart below
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This chart is referred to as Bowen’s Reaction Series…
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MAFICs are high temperature rocks…eg olivene and
pyroxene…
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FELSICs are low temperature rocks…eg quartz, K-feldspar and
muscovite…
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Mineral content is an indicator of the
temperature environment in
which igneous rocks form…
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3. Gases are used in the identification of igenous rocks…
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Melted rocks contain volatiles such as water and carbon
dioxide…
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As melted rocks reach the surface of the earth, gases are released producing bubbles that solidfy
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Pumice is a very good example of escaping
gases…
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There are a lot of openings where gases
escaped…
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These openings are called vesicles…
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Vesicles appear only in volcanic, extrusive, aphanitic igneous
rocks……
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Vesicles do not appear in plutonic, intrusive rocks…
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It is now time for a short quiz….ask Bob for a
worksheet…
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