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UNIT 3 ROCKS AND THE ROCK CYCLE
A Rock is composed of one or more minerals that
are naturally occurring.
There are three rock types
a. Igneous-formed by cooling magma.
b. Sedimentary-formed by the breakdown of other
rocks.
c. Metamorphic-formed when preexisting rocks are
heated under pressure.
• Some rocks are composed entirely of one mineral
ex. Limestone (mineral) calcite.
• Most rocks have more than one kind of mineral
ex. Granite
• Some rocks contain non-mineral matter.
• Ex coal organic debris, and obsidian volcanic rocknot crystalline.
rock
minerals
mineral
Example:
Granite & its
constituent
minerals:
Quartz
Amphibole (hornblende)
Feldspar
A. THE ROCK CYCLE
1. Igneous- formed by the crystallization of molten
rock material called magma.
2. Sedimentary-formed from pre-existing rocks by
weathering-chemical and physical breakup,
erosion, and transport.
3. Metamorphic-formed by textural and
compositional changes that occur when preexisting rocks are buried and subjected to
increased temperatures and pressures.
• The rock cycle connects the three rock groups to
each other by processes.
• The rock cycle is embedded within the
hydrological and plate cycles.
Weathering
SEDIMENT
Erosion
Transport
Deposition
Volcanic
IGNEOUS
SEDIMENTARY
Plutonic
Increased P&T
METAMORPHIC
Crystallization
Melting
Uplift
Burial
MAGMA
7
• B. Igneous Rocks
• Molten rock inside the Earth is called Magma.
• Magma is buoyant, rises to the surface and
sometimes breaks through as volcanic eruptions.
• When magma reaches the surface it is called
lava.
• An igneous rock is formed when magma or lava
cools and solidified.
• Igneous rocks make up the bulk of the Earth’s
crust.
• Earth’s mantle is composed entirely of igneous
rock.
• Igneous rocks are important economically as
building stones and as host rocks for a variety of
mineral (ore) deposits.
Volcanic activity is a geological hazard, igneous rock
can tell us both the nature of past volcanic
eruptions and the potential for future eruption
hazards.
Igneous rocks that form by
magma at the surface are called
volcanic or extrusive.
Fragmented materials are
called pyroclastic and consist of
ash & cinders.
In igneous rocks, texture is controlled by the cooling
rate of the magma.
Cooling Rate
Crystal Size
Slow cooling
larger crystals
Fast cooling
small or no crystals
Plutonic Igneous Rocks
Igneous rocks that form
deep below the surface
are called
plutonic (intrusive)
igneous rocks.
To see them, they must
be uplifted to surface
and the overlying rock
eroded away.
As a magma cools, atoms arrange themselves
into orderly crystalline structures called
minerals. This process is called:
Crystallization
Plutonic igneous rocks cool slowly at
depth and are therefore coarser grained!
Microscopic views of plutonic igneous rocks
Subsurface
intrusion called
a dike
All rocks are classified according to their texture
and mineral composition.
Texture involves the size, shape, and arrangement
of the minerals making up a rock.
Types of Igneous Textures
Fine-grained
Coarse-grained
Porphyritic
Glassy
C. Types of igneous textures
1. Fine-grained-fast cooling magma/lava forms at or
near surface sometimes gas holes present, hard to
see individual crystals.
2. Coarse-grained-forms deep below the surface
slowly cooling. The crystals are coarse and
intergrown.
3. Porphyritic- magma cooled slowly for a while
then erupted. Minerals crystallized at different
temperatures and rates over a period of time,
• 4. Glassy-rapid cooling at the surface. Cannot
form orderly crystalline structures.
Fig. 6.12
W. W. Norton
Zooming in:
Basalt
Gabbro
Zooming in
Andesite
Diorite
Obsidian
Volcanic Glass
Pumice
Volcanic “Froth”
Basalt eruptions on land produce flows
that travel great distances.
Fig. 6.20a
W. W. Norton
Columbia River
basalts
Fig. 6.21a
W. W. Norton
Fig. 6.21b
Stephen Marshak
Fig. 6.02
W. W. Norton
• D. Explosive Volcanic Eruptions.
• Violently explosive volcanic eruptions produce
rock fragments of all sizes, finely-fragmented ash,
and molten bombs. They accumulate to form
pyroclastic volcanic rocks.
• Rock types from these volcano’s include:
• a. Tuff-composed of ash, finely fragmented
volcanic rock.
• b. Scoria-red or black, frothy lave, denser then
pumice.
• c. Volcanic Breccia-coarse, angular rock
fragments, usually in a matrix of fine to coarse
ash.
Fig. 6.08a
W. W. Norton
Types of Igneous
Plutons
Volcano
Dikes
Laccolith
Sills
p.140-141d
Volcanic Pipe
Original artwork by Gary Hincks
Dikes
Batholith
Fig. 6.08b
Stephen Marshak
Fig. 6.10
W. W. Norton
Fig. 6.11a
Stephen Marshak
Fig. 6.11de
Paul Hoffmann
Fig. 6.18
W. W. Norton

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