Igneous Rocks

Topics: Magma, Granite, Petrology Pages: 3 (643 words) Published: June 22, 2015
NOTE: The information in these handouts are
not my own but are from several references.
These handouts are made for reference to
points and not as a means to an end. Reading
from books and others references is still more
beneficial to learning.
ROCKS
What is a rock?
 a naturally-occurring aggregate of one
or more minerals; may or may not
contain mineraloids, natural glass and
organic matter.
 Types of rocks vary based on
composition, color, texture, structures,
etc.
The Rock Cycle
What are igneous rocks?
• Ignis = fire
• Formed from solidification of magma
(intrusive) or lava which flows out from
depths (extrusive)
What is magma?
• Molten material which may contain
suspended crystals and dissolved
volatiles (gases e.g. water vapor, CO2,
SO2)
• Composed of mobile ions of the 8 most
abundant elements in the Earth’s crust:
 O, Si, Al, K, Ca, Na, Fe, Mg
Magma generation
• role of heat
• role of pressure
• role of volatiles
Sources of heat for melting in the crust
• Original heat
• Radioactive decay
• Conduction




Mantle plumes
Frictional heat

ORIGIN AND FORMATION OF MAGMA
Magma forms at:
• Mid-Oceanic Ridges (MOR)
Basaltic magma
• Subduction Zones
Andesitic magma
• Hotspots
Basaltic magma

Classification of Magma
Silica Content

Viscosity

Gas Content

Mafic/Basaltic

Low

Low

Low

Intermediate/
Andesitic

`Intermediate

Felsic/Silisic/
Rhyolitic

High

Intermediate Intermediate
High

High

Basaltic magma

Granitic/rhyolitic magma

Higher density

Lower density

Crystallizes at ~1000 1200ºC

Crystallizes at ~600ºC
19

VARIATION IN COMPOSITION OF MAGMA
Magmatic differentiation
- Any process that causes magma
composition to change
Processes:
Assimilation of country rock
Magma mixing
Partial melting
Fractional crystallization

FORMING IGNEOUS ROCKS
 Igneous Rocks
Extrusive (volcanic) – molten rock formed at
the surface.
 Ex. Basalt, Andesite, Rhyolite...
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