Intro to Rocks
Major Rock Types:
There are three major rock types
1. Igneous—Rocks formed from
cooling of magma or lava.
2. Sedimentary—Rocks formed
from sediments worn from other rocks.
3. Metamorphic– rocks formed
by changing the chemistry, mineralogy, or
texture of other rocks.
• Rock types are all connected in a cycle of
formation, change, and destruction which
we call the Rock Cycle.
• Let us start the rock cycle with molten rock
(magma), which cools and forms igneous
rocks. These rocks become uplifted as
mountains are formed (orogeny). There it
is attacked by the weather and starts to
• This weathered (eroded) material is carried
away by streams, rivers, wind, glaciers and
deposited elsewhere as sediments.
• The sediments are then buried and lithified
(turned into solid rock) being subjected to heat,
pressure, and fluids the sedimentary rocks
becomes metamorphic rocks.
• Metamorphic rocks maybe uplifted and eroded
or may become heated to the point that it again
What are Igneous Rocks?
• The term Igneous comes form the Latin
word ignis meaning fire. These are rocks
which form from cooling magma or lava.
• Magma: Molten or partially molten rock
materials and dissolved gases beneath
• Lava: Is also molten or partially molten
rock material and dissolved gases which
erupts at the Earth’s surface.
Classifying Igneous Rocks
• Two methods are used: Texture and
– Texture: a term which involves how a rock
looks. This method involves:
• 1. The size of the mineral grains (crystals)
• 2. Does the rock have holes (vesicles) in it. If it has
a lot of holes it is called a vesicular texture. This is
an indication that the rock was lava and at the
surface as it erupted or cooled.
• 3. Is the rock a coherent mass of mineral grains or
from smaller chunks of igneous rock which has
been cemented or welded together (pyroclastic
• Using the texture characteristics Igneous
rocks can be classified as either Intrusive
– Intrusive: Rocks composed of large crystals.
This indicates slow cooling below the earth’s
– Extrusive: Rocks composed of small,
microscopic, or no crystals (obsidian)
indicates rapid cooling at the earth’s surface.
• Based on the Chemical Composition
igneous rocks can be broken into 4
– Felsic: High in silica (65%+)
• Usually light colored
• Examples Rhyolite (extrusive) and Granite
– Intermediate: Lower silica content (55-65%)
• Darker than felsic, lighter than mafic
• Example Andesite/dacite (extrusive) and
– Mafic: low silica content (45-55%)
• Ususally dark colored
• Example basalt (extrusive) and gabbro (intrusive).
– Ultramafic: Extremly low silica content (less
• Usually dark colored, but high olivine content tend
to produce green colors. There are also other rare
• Example periodotite (intrusive)
Classification of Igneous Rocks
How does Magma form?
• Magma originates from melting rocks. But
rocks are made up of different minerals
which melt at different temperatures (not
uniformly). Some may not melt completely
resulting in Partial melting.
• Melting temperatures may be affected by
environmental conditions such as
pressure, amount of water. Higher
pressures increase melting temperatures,
presence of water lowers the melting
• Partial melting also results in Partial
freezing and called magmatic
differentiation. Freeze does not mean cold.
– Those minerals which have a higher melting
temperature will also freeze first which will
change the chemical composition of the
– This process continues until all the magma
has frozen or solidified.
• There are three general paths that igneous rock
may take as they cool depending on their
– Continuous Reaction Series: deals with those melts
that are calcium rich or sodium rich composition
• As the plagioclase feldspars crystallize, the first crystals are
calcium rich which leaves the sodium rich melt behind. This
continues till the melt reaches equilibrium again. Again
calcium crystals come out followed by sodium. This
continues till the entire melt is solid.
• Discontinous Series: As a mafic melt cools
slowly the first crystals that form are olivine,
followed by pyroxenes with the olivine being
converted to pyroxenes. As the temperature
farther lowers amphiboles crystallizes and all the
pyroxenes convert to amphiboles. Farther lower
of temperature results in Mica forming with the
amphiboles all convert to Mica.
• Discontinous series can be seen as separate
crystallizations and conversions, so that there is
only one type of mineral present at a time.
• Fractional crystallization says that after
crystals form, they are somehow
separated from the remaining molten
material and don’t re-enter the reaction.
• One possible way that this might occur is
the crystals as they form settle out due to
gravity. Or as crystals form the melt moves
to another location (squeezed out so to
Origin of Different types of Magmas
• Mafic Magmas (basaltic) has two sources:
– 1. Mid-Ocean Ridges, where mafic magma
rises up to form new ocean floor.
– 2. Mid-Plate Volcanoes are sites where mafic
magma rise up from great depths (where one
plate is being subducted under another plate)
this results in eruptions on the surface.
• Mafic to intermediate volcanism can also occur in
Origin of Different types of Magmas
• Felsic Magmas forms as oceanic plates move
along picking up mud, silt, and wet sediments.
As one oceanic plate is pulled under another
plate the wet sediment are also pulled under. As
the plate and sediments heat up due to pressure
the water is driven out and into the mantle. This
water lowers the melting point and rise (lower
density), resulting in mixing with the material
from the overlying continent (more melting
occurs). Finally the melt reaches the surface as
volcanoes or cools within the crust.
• You need to know the following:
– 1. Different classes of rocks.
– 2. How they are formed, and how they are tied together through
the rock cycle. Able to draw out the rock cycle and explain
– 3. What are igneous rocks?
– 4. Classification and Types of igneous rocks (p.124 text)
• Texture (intrusive vs. extrusive)
• Chemical classification
– 5. Magmatic differentation
• General idea
• Bowen’s series
– 6. Fractional crystallization
– 7. Where do different igneous rocks form (pp.122-123)
– 8. Name and definitions of various intrusive bodies (p. 125 text)
Thanks to Greg Anderson for use
of lecture notes.