Download Ch. 2 rocks

Rocks and the Rock Cycle
The Rock Cycle
The continuous
and reversible
processes that
illustrates how
one rock changes
to another.
“One rock is the
raw material for
another”.
Rock Cycle Processes – Crystallization
Rock Cycle Processes - Weathering
Rock Cycle Processes - Lithification
Rock Cycle Processes Metamorphism
Characteristics of magma
• Magma is molten
rock that originates
in the mantle, at
depths of 100-350
km.
• Igneous rocks form
from molten rock
when it cools and
crystallizes into the
rock-forming
minerals.
• Magma that reaches
the surface is called
lava.
Igneous Rocks
Volcanic (extrusive)
igneous rocks form from
lava on the surface.
Plutonic (intrusive)
igneous rocks from magma
beneath the surface and
are exposed later
Volcanic Rock – Rapid rate of cooling
• Volcanic rocks
crystallize from
lava on the
surface.
• Lava contacts air
and water and
cools rapidly,
inhibiting large
crystal formation.
• The most
common volcanic
rock is basalt.
Plutonic Rock – Slow rate of cooling
• Plutonic rocks
crystallize from
magma in the interior
of the crust, which
are later exposed.
• Magma is insulated
and cools slowly,
allowing large
crystals to form.
• Most common
plutonic rocks are
from the granite
family.
Igneous Rock Classification
• Texture
– (overall appearance of rock, but
especially pertains to mineral crystal
size)
– linked to how rock formed since crystal
size depends on rate of cooling
• Mineral composition – chemical
makeup of magma or lava
Texture
– Size and arrangement of crystals
• Slow cooling rate promotes the growth
of fewer but larger crystals
• Fast rate forms many small crystals
• Very fast rate forms glass
– Overall appearance (vesicles, glassy
etc.)
Phaneritic - Coarse-texture
Granite
Aphanitic - Fine texture
Rhyolite
Porphyritic – Bimodal crystal size
• Large crystals,
phenocrysts,
form slowly from
magma.
• Magma erupts;
crystals are
suspended in an
aphanitic matrix,
the groundmass.
Glassy texture – indicates lack of
crystalline structure due to rapid cooling
Obsidian
Granitic (Felsic) Composition
• Composed of
primarily of light
silicates
• Contains high
amounts of silica
(SiO2)
• Major
constituents of
continental crust.
Basaltic (Mafic) Composition
• Composed of
dark silicates
and calcium-rich
feldspar
• More dense than
granitic rocks
• Comprise the
ocean floor as
well as many
volcanic islands.
Sediment
• solid rock or mineral
fragments transported
and deposited by water,
wind, ice or gravity
• dissolved minerals which
evaporate or precipitate
from water, or are
secreted by organisms
• accumulate over time as
loose unconsolidated
layer.
Sedimentary rocks form when sediment from
weathered and eroded rock is compacted or
cemented into solid rock
Fig. 3-2, p.46
Sedimentary rocks
• Formed by the lithification of
sediment
• compaction by burial
• cementation by minerals from ground water
• Sedimentary rocks contain evidence
of past environments
• Provide information about climate
• Often contain fossils
Clastic Sedimentary Rocks
composed of solid sediment from weathered rocks
sandstone
conglomerate
organic shale
Chemical Sedimentary Rocks
• Composed of minerals
precipitated from surface or
ground water (chemical
sediment).
• The upper picture shows
rock salt on the Bonneville
Salt Flats.
• The lower photo shows
veins of rock gypsum
embedded in mudstone.
• Both of these chemical
sedimentary rocks are
examples of evaporites.
Biogenic Sedimentary Rocks
• composed of sediment of
biological origin (e.g. shell
fragments, plant material).
• The upper photo is limestone,
composed of an ancient coral
reef in the Guadaloupe
Mountains of Texas.
• The lower photo is of
bituminous coal.
• COAL
Coal Formation
Metamorphic rocks form when heat, pressure or
chemical reaction alters existing rocks
Fig. 3-CO, p.44
Metamorphism
The transition of one rock into another by
temperatures and/or pressures unlike
those in which it formed.
Metamorphic rocks are produced from
• Igneous rocks
• Sedimentary rocks
• Other metamorphic rocks
Metamorphism
• Progresses incrementally (low to highgrade)
• Rock remains solid throughout process
• Agents of metamorphism
– heat (between 150° and 800° C)
– pressure (5 km below the surface the
pressure is about 1500 x greater than
atmospheric pressure
– chemically-active pore fluids.
Heat – from the Geothermal
Gradient or rising magma
Pressure from overlying rocks
Low grade to high grade
Metamorphism (left-right)
Types of Metamorphism
• Contact or thermal metamorphism –
usually caused by contact of a rising
magma body with host rock
• Regional metamorphism (mountain
building) – caused by tectonic plate
interactions
• Hydrothermal metamorphism – chemical
alterations from hot, ion-rich water
Changes due to metamorphism
• recrystallization of
mineral grains and
chemical
precipitates
• Formation of new,
stable minerals
• Foliation – parallel
alignment of
minerals due to
pressure.
Foliation
Granite is a common parent
rocks of Gneiss (“Nice”)
Igneous Granite
Metamorphic Gneiss
Sandstone is the parent rock for
quartzite
Sandstone - sedimentary
Quartzite - metamorphic
Limestone is the parent rock for marble
Limestone - sedimentary
Marble - metamorphic