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Rocks
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Classifying Rocks
• Based on Composition
– Indicates the minerals a rock contains
– Not always reliable (color changes)
• Based on Texture
– Better to look at size, shape, and
arrangement of crystals and other particles
that make up a rock’s texture.
Igneous Rock
• Rock that forms from
magma.
•
Magma consist mainly of the
minerals found in silicates and water
vapor.
• Molten material cools
and solidifies either:
– Inside =
Intrusive/Plutonic
– Outside/Surface =
Extrusive/Volcanic
Intrusive
• Forms underground from hardened magma.
• Cools slowly underground.
– Allows crystals to grow large.
– Have coarse-grained texture. (Pheneritic)
Granite – Coarsegrained intrusive
igneous rock
Extrusive
• Forms at Earth’s
surface
• Cools quickly at
surface.
– Crystals don’t grow
much. (Aphanitc –
fine grained)
Obsidian – extrusive igneous rock
Igneous Rock Textures:
Texture reveals a great deal about the environment in which
the rock developed.
Igneous Rocks that cool rapidly have a fine-grained texture with
too small to be seen with the unaided eye.
Voids or vesicles are formed by gas bubbles that formed as
the lava solidified.
Igneous Rocks to cool very slowly exhibit coarse-grained textures.
If magma is cooling slowly—develops large crystals– and
suddenly erupts, the remaining crystals will cool very rapidly.
Thus you have large crystals embedded inside the smaller crystals
This is said to have Porphyritic Texture
Glassy textured igneous rocks cool so fast that their crystals do
not have enough to form.
Classifying Igneous Rocks: This chart should be copied
Classified by their texture and mineral composition
(Andesitic)
Felsic rocks contain light colored silicate – quartz and feldspar
Mafic rocks contain darker minerals– olivine and iron
Andesitic rocks contain a mixture of light and dark minerals
Sediment
• Small, solid pieces of material that comes
from rocks or living organisms.
• Carried away by running water or wind,
and settles to the bottom.
Sedimentary Rock
Forms over time as
sediment is squeezed
and cemented
together.
(LITHIFICATION)
Sedimentary rocks are divided into two broad
classes, detrital sedimentary rocks and chemical
sedimentary rocks
Types of Sedimentary Rock
• Detrital/classtic
are composed of pieces of rock, minerals,
that have been cemented together.
Sandstone is an example.
• Chemical
form by precipitation or the growth of
new minerals in water
Detrital and Chemical Rock Chart
This chart should be copied
Detrital Rock
• Form from broken fragments.
• Fragments are usually held together by
cement.
– Conglomerate: made
of gravel and pebbles
(rounded pieces)
– Breccia: made of
sharp-edged
fragments (from
volcanic
explosions)
– Sandstone: made
of small grains of
sand
Chemical Rock
• Sedimentary rock that forms when minerals
precipitate out of solution
– Limestone: forms from precipitation of calcium
carbonate in the oceans.
Metamorphic Rock
• Rock that has been changed by
temperature, pressure, or reactions with
hot water.
• Changes texture of rock from its original
(recrystallizes it)
Metamorphic Rock Chart—Please copy this
• Slate: original rock
is shale
• Gneiss: original rock
is schist.
Foliated rocks: many metamorphic rocks
end up with this stripped appearance.