Transcript Classification of Rocks

Classification of Rocks
Rocks belong to one of 3 major
rock groups:
A. Igneous rocks
B. Sedimentary rocks
C. Metamorphic rocks
Igneous rocks
Chapter 3
• Igneous rocks (fire rocks) are derived from
the cooling of magma or lava.
• Igneous rocks consist of mostly the
elements found in the silicate mineral
group along with some gases and water
vapour.
• Without igneous rocks, sedimentary and
metamorphic rocks would not exist.
Sedimentary rocks
• Sedimentary rocks are derived from the
sediment of previously existing rocks.
• The previously existing rocks may be
igneous, metamorphic and other
sedimentary rocks
Metamorphic rocks
• Metamorphic rocks are
derived from the
transformation of preexisting rocks by heat,
pressure and chemically
active fluids.
Group A Igneous rocks
• Two classes of igneous
rocks
1. Extrusive (volcanic) – forms
from lava reaching the
surface and cooling quickly.
2. Intrusive (plutonic) – forms
from magma cooling slowly
beneath the surface.
Identification clue!!
• Rocks with interlocking
crystals are usually
igneous. The minerals look
like a jig saw puzzle.
Identification Clue # 2
• Rocks that show gas
bubbles (white
roundish inclusions)
or holes where
bubbles used to be
are clues that the
rock is an extrusive
igneous rock
Classification of igneous
rocks
• Read pages 48 to 53 in
our Earth Science
textbook then describe
the characteristics that
geologists use to classify
igneous rocks
• Igneous rocks are
classified on the basis of:
A.Texture (size of the
crystal)
B. mineral composition
(color of the rock)
texture
• Texture is used to describe
the overall appearance of
the rock based on the size
and the arrangement of its’
interlocking crystal. (The
rate of cooling influences
texture)
Types of textures
• aphantic - a very fine texture
• phaneritic – a coarse texture
• Porphyritic – Large crystals
surround by a matrix of small
crystals. The large crystals are
called phenocrysts
Identification clue!!
• Igneous rocks with large
interlocking crystals (easliy seen
by the unaided eye) are intrusive
igneous rocks.
• Igneous rocks with small ( very
difficult to see) interlocking
crystals are extrusive igneous
rocks.
Mineral Composition
• Igneous rocks are classified as
being either:
• A. mafic – dark in color, high in
Fe and Mg.
• B. felsic – light in color, high in
feldspars and quartz. (also
called granitic)
Identification clue!!
• Two igneous rocks may
have the same mineral
composition but have
different textures hence
a different name.
Crystallization of
Magma
• The rate of cooling strongly
influences the size of the crystals.
• Rapid cooling = small crystals
(extrusive)
• slow cooling = large crystals
( intrusive)
Factors that influence
crystallization
• Three factors influence
crystallization:
• A. rate of cooling
• B. mineral composition of magma.
• C. amount of volatile (explosive
gases) material
Identification clue
• Rocks that show gas bubbles or
vesicles (holes where a gas used to
be) are usually igneous extrusive
rocks like basalt.
Don’t fear me…I’m Volcano Man!