Igneous Rocks
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Igneous Rocks
• Rock heated to liquid (molten)
state is called magma.
• When magma cools and
hardens it becomes Igneous
rock.
–Different minerals solidify (or
melt) at different temperatures
• Usually 600 - 1000 degrees Celsius
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Igneous Rocks
• Slowly cooling magma forms rock
with large mineral crystals
–Ex. Granite
 On planet Earth where does
magma cool slowly?
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Igneous Rocks
• Rapidly cooling magma forms rock
with small mineral crystals
–Often too small to see
• Ex. basalt
 On planet Earth, where does
magma cool rapidly?
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Igneous Rocks
• When magma cools very fast
crystals have no time to form
–Ex. Obsidian
 Where might obsidian form?
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 Igneous Rock Formation:
Crystal Size vs. Cooling Rate of Magma
Draw a line!
Tiny
Large
Crystal Size
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Intrusive Igneous Rocks
• Are formed deep underground
• Usually have large, well formed
crystals
–Also called plutonic igneous rocks
–Example: granite
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Extrusive Igneous Rocks
• Are formed at or near the Earth’s
surface
• Usually have small mineral crystals
–Also called volcanic igneous rocks
–Example: obsidian
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 Igneous Rock Formation:
Crystal Size vs. Depth beneath Earth’s surface
Draw a line!
Tiny
Large
Crystal Size
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Igneous Rock Characteristics
• Do not usually contain layers
• Varied in color
– Color depends on minerals present
• Color and crystal size are used
to identify igneous rocks
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 Describe a situation in which
both small and large crystals would
be found in the same igneous rock.
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Igneous Rocks - Color
• Light colored igneous rocks are usually
rich in feldspar (fel) and silica (s)
– Called felsic
• Dark colored igneous rocks are usually
rich in magnesium (Ma) and iron (Fe)
– called Mafic igneous rocks
• Exceptions
– Obsidian is felsic but appears dark in
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color
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Mafic Igneous Rocks
• Known as basaltic
• Dense
• Dark colored
• Make up Earth’s ocean floors
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Felsic Igneous Rocks
• Known as granitic
• Less dense than mafic igneous
rocks
• Light colored
• Form Earth’s continents
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Cooling Magma
• volcanic or extrusive
• crystal size is small
- fine texture
• Plutonic or intrusive
• crystal size is large
- coarse texture
 2 examples:
 2 examples:
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3 steps for identifying
igneous rocks
1. Determine texture (you have to look
at the rock!)
2. Determine color
3. Determine what minerals are
present, if you can see them
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Igneous Rock Identification--step 1
Determine the texture (crystal size):
Can I see the crystals by eye?
coarse textured
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fine textured
Igneous Rock Identification--step 2
Determine the relative color:
light
colored
(felsic)
 Example:
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In-between
 Example:
dark
colored
(mafic)
 Example:
Igneous Rock Identification--step 3
Determine mineral composition - what
minerals are in the rock?
 Use physical characteristics and special
properties of minerals such as:
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