Transcript Ch. 6 Lecture

Physical Geology
Chapter 6
Part 1
The Rock Cycle
Rocks are solid pieces of the earth of
any size. They can be made up of one
or more minerals, or they can be made
up of solid organic matter.
3 Types by Origin
• Igneous – “from fire”; forms by cooling of
magma or lava
• Sedimentary – formed from compaction
and cementation of rock fragments
(sediments)
• Metamorphic – “changed form”; from
alteration of existing rock by heat,
pressure and time
Rock Cycle
• Igneous may become sedimentary,
metamorphic, or igneous
• Sedimentary may become metamorphic,
igneous, or sedimentary
• Metamorphic may become igneous,
sedimentary, or metamorphic
Rock Properties
• Bowen’s Reaction Series – As magma
cools, certain minerals tend to crystallize
first. As they form out, they remove
certain elements from the magma, which
changes the composition of it. Thus
different minerals form at different times
during the cooling and solidification, and
they generally always form in the same
order.
The Canadian geologist N.L. Bowen found that
minerals tend to form in specific sequences in
igneous rocks, and these sequences could be
assembled into a composite sequence.
In Summary
Chemical Stability
• Tendency to maintain original chemical
composition
• Based on strength of chemical bonds
• Highest # of bonds between Si and O are
most resistant
Physical Stability
• Reduced by zones of weakness
– Layer joints
– Joints between crystalline formations
– Joints and fractures from release of formative
pressures
Part 2
Igneous Rock
• Magma is melted rock below the surface; liquid
or fluid
• Melting depends upon
– Temperature
– Pressure
– Presence of fluids
• Melt temperature depends upon the chemical
composition of the rock
• Pressure heightens melting point; release
lowers
• Fluids decrease some melting points
Partial Melting
• Different components have different
melting points
• As each melts, magma composition of the
magma is altered
• Opposite effect of Bowens Series
Fractional Crystallization
• Opposite of partial melting
• Minerals crystallize and change makeup of
magma chemically
• In same order as Bowens Series
• Longer formation time = larger crystals
• Shorter time = smaller crystals
• Inner/outer crystals may be different due
to differences in the magma as time goes
by
Igneous Rock Textures
• Coarse grained – slow loss of heat by
subsurface magma results in large, welldeveloped crystals (rough granite)
Igneous Rock Textures
• Fine grained – rapid heat loss from
superficial lava results in small, poorly
developed crystals (smooth basalt)
Igneous Rock Textures
• Porphyritic – magma cools somewhat
slowly, then speeds up near or on surface
resulting in a mixed appearance/texture
Igneous Rock Textures
• Glassy – viscous magma cools quickly
with few dissolved gases forming few, if
any crystals
Obsidian
Igneous Rock Textures
• Vesicular - viscous magma cools quickly
with a lot of dissolved gases forming few, if
any crystals, but many bubbles (vesicles)
Pumice
Igneous Rock Composition
• Felsic – lots of silica; light color; has
minerals feldspar, mica and quartz; e.g.,
granite, rhyolite, obsidian, pumice
• Mafic – rich in Fe and Mg; dark color;
minerals pl feldspar, pyroxene,
ferromagnesians (hornblende); e.g., basalt
and gabbro
• Intermediate – intermediate silica
content;plagioclase, hornblende,pyroxene,
and biotite; e.g., diorite and andesite
Igneous Rock Composition
• Felsic – lots of feldspar (fel); lots of silica
(sic)
• Mafic – rich in Fe (f) and Mg (Ma) or
ferromagnesians
• Intermediate – intermediate blend
Intrusive formations
batholith
Extrusive formations
Some uses of Igneous rock:
GRANITE: An igneous-plutonic rock, medium to coarse-grained that is high in silica,
potassium, sodium and quartz but low in calcium, iron and magnesium. It is widely used
for architectural construction, ornamental stone and monuments.
PUMICE: An igneous-volcanic rock, it is a porous, brittle variety of rhyolite and is light
enough to float. It is formed when magma of granite composition erupts at the earth’s
surface or intrudes the crust at shallow depths. It is used as an abrasive material in hand
soaps, emery boards, etc.
GABBRO: An igneous-plutonic rock, generally massive, but may exhibit a layered
structure produced by successive layers of different mineral composition. It is widely
used as crushed stone for concrete aggregate, road metal, railroad ballast, etc. Smaller
quantities are cut and polished for dimension stone (called black granite).
BASALT: An igneous volcanic rock, dark gray to black, it is the volcanic equivalent of
plutonic gabbro and is rich in ferromagnesian minerals. Basalt can be used in aggregate.
Practice questions for igneous rocks
Part 3
Sedimentary Rock
Beginnings - sediments
……then…..
Lithification in this case means
compaction (squeezed together
tightly reducing pore space) and
cementation (glued together by
chemicals in the water) of the
sediments into rock
Classes by origin
• Chemical
• Organic
• Clastic
Chemical
• Dissolved minerals
• Precipitate to layers as evaporites or
chemical reaction products
• Examples: halite, gypsum
Bonneville Salt Flats, Utah
Organic
• Remains of living things
• Heavy deposits
• Examples: coal, chalk and organic
limestone
Clastic
• Imported fragments that are lithified
• By fragment size
– Conglomerate (breccia) – large fragments
cemented by small
– Sandstone – quartz fragments (sand)
– Shale – clay sized fragments usually
compacted into flat layers
C
l
a
s
t
I
c
Characteristics of Clastic
Sediments
• Sorting – tendency of currents to separate
sediments according to size
• Angularity – collisions during movement of
particles tends to round them off. Little
movement = angular; more movement =
more rounded/smooth; great movement =
most rounded/smooth particles
very angular
Angularity
Patterns
angular
sub-angular
rounded
well-rounded
Sedimentary Rock Features
•
•
•
•
•
•
•
Stratification
Cross bedding
Graded bedding
Ripple marks
Mud cracks
Fossils
Concretions
Stratification – in layers or beds
Cross beds – slanting layers
Graded beds – bed of sediments
assorted by size
Ripple marks & Mud cracks
Fossils
Concretions and geodes
•Sedimentary rocks have great economic importance •Oil, natural gas, coal, and uranium, our major energy resources, are
formed in and come from sedimentary rocks.
•Sand and gravel for construction come from sediment.
•Sandstone and limestone are used for building stone.
•Rock gypsum is used to make plaster.
•Limestone is used to make cement.
•Salt is used for flavoring.
•Phosphate-bearing sedimentary rocks are used for fertilizer.
•Quartz sand is used to make glass.
Practice questions on sedimentary rocks
Part 4
Metamorphic Rock
Metamorphism
• The process by which heat, pressure, or
chemical processes change one type of
rock into another
– Change into other minerals
– Change in size or shape
– Separate into bands
– Change mineral/chemical composition by
addition/subtraction of materials
• Most deep within the crust
– Contact with hot magma
– Regional from heat/pressure of tectonic
forces
Metamorphic Classification
• Foliated
– Extreme pressure causes crystals to realign
or regrow in parallel bands
– Minerals of different compositions separate to
produce a series of bands
• Non-foliated
– No banding
– Original rock has basically one mineral so no
separation or banding
– Original rock has round/square grains that
cannot realign when subjected to further
stress
A rock that contains parallel- or sub-paralleloriented mineral grains is said to be foliated
Foliation promotes cleavage in rocks (they
break easily along parallel planes).
•If mineral crystals are disc or pencil shaped, their parallel orientation is easily
discernible and the rock is clearly foliated. If all mineral crystals are equant (length,
width, and height are equal) there can be no discernible parallel orientation.
•Nonfoliated rocks - mineral crystals are equant in shape.
Made of calcite = marble
(metamorphosed limestone)
Made of quartz = quartzite
(metamorphosed sandstone)
Uses for metamorphic rocks -
•Slate is used to make
roofing tiles and in earlier
periods of our history was
used as "blackboards" in
classrooms. As a resource,
slate is nonrenewable, but
common. Man-made roofing
tiles are a suitable
substitute.
•Marble is used as a
building stone and as
ornamental rock, such as
for carving statues. As a
resource, marble is
nonrenewable, but
common. Man-made
building stones are a
suitable substitute.
Practice questions on metamorphic rock