Geology - Guide Section 10.2 (first part)
IGNEOUS ROCKS
Name_____________________________
Period _______
Cools; solidifies
Process: Magma or Lava
Igneous Rock
1. Describe the two main types of igneous rock:
1) Intrusive - ___________________________________________________________________
______________________________________________________________________________
2) Extrusive - ___________________________________________________________________
______________________________________________________________________________
Texture of Igneous Rock
INTRUSIVE IGNEOUS ROCK
2. Why do intrusive igneous rocks have larger mineral crystals? _________________________
______________________________________________________________________________
(This is called “coarse-grained”)
A common example of intrusive igneous rock is ___________________
Other examples – Diorite, Gabbro
Look at the examples of intrusive igneous rock in the room – describe their texture (grain size) –
Can you see the separate crystals of different minerals?
__________________________________________________________________
EXTRUSIVE IGNEOUS ROCK
3. Why do extrusive igneous rocks have smaller mineral crystals? _________________________
________________________________________________________________________________
(This is called “fine-grained”)
A common example of extrusive igneous rock is ____________________
Other examples - Obsidian, Pumice
Look at the examples of extrusive igneous rock in the room – describe their texture (grain size) –
Can you see the separate crystals of different minerals?
__________________________________________________________________
4. What is “porphyry”? ____________________________________________________________
________________________________________________________________________________
This is kind of a mixture of intrusive and extrusive – large crystals embedded in fine-grained rock.
Formed when magma cools slowly at first, then more quickly as it gets closer to the surface.
Look at the example of porphyritic rock on the front counter (and shown on the screen in the front
of the room). This is a fined-grained rock (basalt?) with large crystals of several minerals, including
olivine. Draw a picture below:
5. Examine the sample of obsidian in the room. Any visible crystals? ____________
Describe this rock: _______________________________________________________
How is obsidian formed? __________________________________________________
Is it intrusive or extrusive? ___________________
Describe the sample of pumice: _____________________________________________
How is pumice formed? ____________________________________________________
________________________________________________________________________
Composition of Igneous Rock
We’ll talk about this in class – for now, READ the section and give definitions of the following
words:
Felsic: __________________________________________________________________________
Mafic: __________________________________________________________________________
Generally, what is the coloring of felsic rock - light or dark? ____________________
What ELEMENT is a large part of felsic rock? ______________________
Generally, what is the coloring of mafic rock - light or dark? ____________________
What ELEMENT is a large part of mafic rock? ______________________
Igneous Rock Structures - Later
Mineral composition of igneous rocks
Course-grained 
(intrusive)
Granite
Diorite
Gabbro
Fine-grained 
(extrusive)
Rhyolite
Andesite
Basalt
Main minerals 
Quartz,
Plagioclase feldspar
Pyroxene
K-feldspar (pink)
Biotite mica
Plagioclase feldspar
Amphibole
Color 
Main elements 
Light
Medium
Dark
(white/pink)
(gray/green)
(dark gray/black)
Si, K, Na
(silica-rich up to 75% SiO2)
Intermediate
Si, Ca, Fe, Mg
(less silica –
45-55%SiO2)
“Felsic”
Intermediate
“Mafic”
Type of rock 
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Igneous rock formations (Overheads, and picture on p. 181)
Intrusions - _____________________________________________________________________
Batholiths _________________________________________________________________
__________________________________________________________________________
Stocks ____________________________________________________________________
__________________________________________________________________________
Others – Laccolith ___________________________________________________________
Sill _______________________________________________________________
Dike ______________________________________________________________
Extrusions - ____________________________________________________________________
Volcanic neck ______________________________________________________________
Lava plateau _______________________________________________________________