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Igneous Rocks
The Rock Cycle
Formation
All igneous rocks form from the cooling
of melted or molten rock.
Lava is
molten rock
that has
exited the
Earth
through a
volcano
Lava
Mt. Etna
Lava
Formation
Magma is molten
rock that exists
underground in large
chambers.
Magma
Extrusive Rocks
Volcanic rock is rock that erupted on the
surface and cooled quickly. This could
take few minutes to a few hours.
Volcanic rocks or
extrusive rocks
are Fine Grained
or have small
mineral crystals
Intrusive Rocks
Plutonic Rock is rock that solidifies within the
Earth from magma that cools very slowly. This
cooling could take thousands of years.
Plutonic rocks have a
large grain size
because the crystals
have a long time to
grow
Plutonic Rocks -Coarse Grained
Porphyritic Texture
Large Crystals in a fine-grained Setting.
This indicates there were two different
cooling rates. One slow and one fast.
Composition
• Composition refers to the chemical make-up of
the rock. This is based on the dominant
mineral content.
Mafic Rocks
Mafic igneous rocks contain large amounts of iron(Fe)
and Magnesium(Mg). They have high percentages of
minerals like amphibole, pyroxene, magnetite and
olivine. This makes them dark in color.
Felsic
Felsic igneous rocks have high amounts of
Aluminum a Silicon. As a result these rocks are
light in color.
Common minerals are quartz, Potassium feldspar,
and muscovite mica.
The Scheme for
Igneous Rock Identification
A Brief Tour
Grain Size
Crystal size
Description
Igneous rocks have
“Intergrown Crystals”
Intergrown
Intergrown
Intergrown
Not Intergrown
Vesicular
Outside
theEarth:
volcano:
Extrusive
Inside the
Intrusive
Where it was formed
Bubbles?
Yes= Vesicular
Bubbles?
No=Non-vesicular
Very Light
Neither Light nor Dark
Color
NotVery Light Not Very Dark Very Dark
Light
Dark
Darkest
Lightest
Very Light
Medium Density
Density
Light
Dense Very Dense
Practice
Name a light-colored, fine-grained
rock with no bubbles.
Name a coarse-grained, dense rock.
Name a very light-colored, glassy,
extrusive rock with bubbles.
Finding The Minerals
Identify the rock.
Unless you have other information,
work in the middle of the rock’s box.
This is the amount of Potassium Feldspar in the rock.
This is the amount of Quartz in the rock.
This is the amount of Plagioclase Feldspar in the rock.
Use tick marks on a scrap paper to measure the percentage.
Potassium Feldspar 25%
Quartz 40%