Igneous Rock Classification
Dr. David Steer
Rock cycle
Analysis Time
 Given:
Bag rocks
Group these rocks in some manner
Color (Proxy for silica content)
Grouping items
Size (Proxy for grain size)
 Group these objects in some logical way.
Grouping items
 Group these objects in some logical way.
Analysis time
 Given:
Bag rocks
Group these rocks in some manner
Rock Cycle
Igneous Rocks
 Volcanic (extrusive) igneous rocks
Cool
fast, small crystals
form when molten rock (magma) in the earth’s interior rises to
the surface through pipes or fractures in the crust.
Igneous Rocks
 Plutonic
Cool
form
(intrusive) igneous rocks
slow, large crystals
when magma cools within the earth. Igneous rocks
that cool below the earth's surface are termed plutonic (or
intrusive) igneous rocks.
Classification of Igneous rocks
 Texture size of crystals most important
Cooling rates
– Surface, fast
 Small crystals
Which formed at surface?
– Below surface slow
Both are the same chemistry
 Larger crystals
a)
b)
Rhyolite
Granite
Chemical Composition

Color
Indicative of chemistry and temperatures of formation
– Dark colors
 High temperatures (1000 to 1200 oC)
 Low silica content
– Light colors
 Lower temperatures
 High silica content
a)
Which is the high silica rock?
Both formed below the surface
b)
Gabbro
Granite
Texture

So explain this!!!!
Six common Igneous Rocks
1000 C
Solidifying Temperature
Increasing Grain Size
500 C
Minerals
Andesite intermediate
feldspar,
amphibole,
pyroxene, biotite
mica
Diorite
Rhyolite
feldspar, quartz,
muscovite mica,
& amphibole
Granite
high
Present (in order of
abundance)
pyroxene,
olivine, feldspar,
& amphibole
Plutonic
Rocks
Gabbro
Lighter Color
Volcanic Silica
Rocks
Content
Basalt
low
Draw a concept map that guides you through classifying
an igneous rock.
Use these terms to: Classify Igneous Rocks
Basalt
Cool Slow
Gabbro
Large Grain
Plutonic
Small Grain
Color
Dark
Granite
Low Silica
Texture
Cool Quick
Extrusive
High Silica
Light
Rhyolite
Identify the rock type associated
with the diagram below.
Question
1)
2)
3)
4)
a.
b.
c.
d.
Plot 1
Plot 2
Plot 3
Plot 4
andesite porphory
gabbro
basalt
obsidian
5) Imagine that all minerals found in
rocks were the same color. What
information would you no longer be able
to infer?
a. texture
c. composition
b. cooling rate
Analysis Time
 Given:
Bag rocks
Identify the rocks