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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…