What are igneous rocks?
SWBAT compare and contrast intrusive and
extrusive igneous rocks; describe the
composition of magma; discuss the factors
that affect how rocks melt and crystallize
Get your lab notebook
• Write a description of the rocks you are shown:
• Using the hand lens take a closer look, and
describe what you see:
• Draw a picture of what you saw through the
hand lens.
• How many different minerals did you see?
• What minerals can you identify?
• Do you see any evidence that these minerals
crystallized form molten rock? Explain.
How are igneous rocks formed?
• Magma
– Molten rock under Earth’s surface
• Lava
– Magma that flows out onto Earth’s surface
• Igneous rocks are formed when magma
Types of Igneous Rocks
• Extrusive
– Rocks that form from lava solidifying on
Earth’s surface
– They are generally fine grained due to quick
• Intrusive
– Rocks that form from magma cooling
– Generally coarser grained because of the
longer cooling time
• What is magma composed of?
– Slushy mix of molten rock, gases, and mineral
• What elements does it contain?
– Oxygen, silicon, aluminum, iron, magnesium, calcium,
potassium, and sodium
• What are the 3 types of magma?
– Basaltic – 50 % SiO2
– Andesitic – 60% SiO2
– Rhyolitic - 70% SiO2
• What affects magma characteristics?
– Silica - SiO2
List and describe the 4 factors that
affect magma formation
Temperature increases with depth in
crust (geothermal gradient)
Pressure increases with depth in crust
due to overlying rock. As pressure
increases, so does the melting point
As water content increases, the
melting point of a rock decreases
Water content
Different minerals have different
melting points
• Partial melting
– Not all parts of a rock melt at the same time
because different minerals have different
melting points
• Fractional crystallization
– Different minerals form at different
temperatures as a rock cools (opposite order
that they melt in)
• Layered Intrusions
– Minerals form into distinct bands in the order
shown in Bowen’s reaction series
Bowen’s reaction series
• Illustrates the relationship between cooling
magma and crystal formation.
– First pattern
• Characterized by a continuous gradual change of
mineral composition in the feldspar group
– Second pattern
• Characterized by an abrupt change of mineral type
in the iron-magnesium groups

What are igneous rocks?