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The Rock Cycle
Rocks: a solid collection of minerals or mineral materials.
Rock Types
Rock Cycle
Process of events in which rocks can be weathered, melted, altered, and
formed over millions of yrs.
Rock Cycle Processes
Weathering / Deposition of Sediments /
Cementation (lithification)  Sedimentary Rock
IGNEOUS ROCKS: On surface (extrusive) or
interior (intrusive)
Melting / Cooling & Solidifying  Igneous Rock
Metamorphic Rock: Usually inside Earth (@ depths
associated w. higher pressures & temps.)
Heat / Pressure  Metamorphic Rock
Weathering: wind & water breaks rocks down into fragments (Makes sedimentary)
Cementation: Water “glues” sediment together to make rock
Igneous Rocks
• From latin “ignis” = fire
• Formed when molten rock cools and solidifies.
• The slower rock cools  more crystals
Magma = Molten rock and trapped gases
mostly elements Si, O (Al, Fe, Ca, Na, K, Mg) +
gases like water vapor
Gases – kept in magma by pressure of surrounding
rock (want to escape)
• Magma rises b/c its less dense than surrounding rock.
(It’s warmer & has gases)
• As magma cools, elements combine to form (mostly
silicates) minerals
Extrusive vs. Intrusive
Intrusive – magma cools while trapped beneath
E’s surfaces
• uplift; weathering & erosion expose buried
intrusive rocks over time
• Cooling takes millions of yrs.; Bigger crystals
Characteristics: Bigger/more crystals = Coarse
grained (longer to cool)
• Ex: Granite
Extrusive vs. Intrusive
Extrusive – magma cools & crystalizes on E’s
Characteristics: air pockets/vesicles; glassy; few
or small crystals
• Ex: Basalt, Obsidian, Rhyolite, Scoria
Characteristics of Igneous Rocks
1. Coarse Grained
Slow cooling
Large crystals
Ex: Granites
Characteristics of Igneous Rocks
2. Fine-Grained Texture
• Cool quickly (no time for
big crystal growth)
• Small, interconnected
mineral grains
Characteristics of Igneous Rocks
3. Glassy
Cool too quickly for
minerals to organize and
Ex: obsidian, pumice,
Characteristics of Igneous Rocks
4. Porphyritic Texture
• Mix of large crystals
surrounded by finegrained minerals.
• Minerals crystallize @
diff. rates
• Cool deep w/in E.
Igneous Rock Textures
Igneous Rock Types: Composition
Granitic: ~70% silica
• Main minerals = Light
colored silicate minerals
(Quartz + feldspars)
• Less than 30% dark
minerals (biotite mica:
• Common in continental
• Ex: Granite
Minerals of Granites
In thin section under a microscope
Igneous Rock Types: Composition
• Mostly plagioclase feldspar(light
colored; gray/beige)
• ~ 25-45% dark silicate
minerals (biotite mica,
amphibole, pyroxene)
• Form @ volcanoes near
colliding plate boundaries
(ocean plate subducted under
continent; Andes in S. America)
• Ex: Andesite
Igneous Rock Types: Composition
• 45 – 85% dark minerals
magnesium and iron (Less
• Dense & dark
• Common in ocean crust
• Ex: Basalt & Gabbro
Igneous Rock Types: Composition
• 85% dark silicate minerals
(least silica)
• Dense & dark (green)
• Common in upper mantle
• Ex: peridotite = olivine +
pyroxene (green sand of
Classification of Igneous Rocks
Classification of Igneous Rocks
Palisades Sill, NJ (basaltic)
Giant’s Causeway, Ireland
Metamorphic Rock
• Formed when HEAT, PRESSURE, or
Hydrothermal solutions w/in E cause changes in
the texture and mineral content of rocks.
• Rock undergoes a change w/out melting over
millions of yrs.
• Minerals migrate, react with other minerals,
• Usually form at bases of Mts.; near magma, hot
springs, faults, or plate boundaries.
• Limestone + heat + pressure  Marble
• Mudstone/Shale + heat + pressure  Slate
2 Types of Metamorphism
1. Contact Metamorphism
Rocks near magma intrusions; minerals migrate
and rearrange; “Low Grade”
2 Types of Metamorphism
2. Regional Metamorphism
Happens over large area; usually near
colliding plate boundaries; at great depths;
Mt. Building areas; “High Grade”; minerals
recrystallize into banding patterns
Classification of Metamorphic Rocks
1. Foliated:
•Platy minerals realign and b/cm more compact and dense
(slates) *Platy minerals = flat = clays, micas
•Minerals rearrange into bands or stripes(= Foliations)
•Ex: Shale (sedimentary mud stone)  slate
Granites (igneous)  schists & gneisses.
Rocks viewed with a microscope
View of
Minerals are
Microscope View of GNEISS
Minerals are NOT randomly arranged.
(Green/Blue minerals are lined up in
parallel layers or foliations)
Green, platy, sparkly
Classification of Metamorphic Rocks
1. Non-Foliated:
No banded texture; contain only one mineral
Ex: limestone (calcite)  marble
Forming Sedimentary Rocks:
• Formed when pieces of rock or sediment
get squeezed and cemented together (=
Sediments: Fragments made by weathering
& erosion of existing rocks
Erosion = breaking up rock fragments &
transporting them.
Agents of Erosion (What erodes rock?)
Water; Wind; Ice; Gravity
Deposition, Compaction &
• Deposition: sediments settle in new
locations (river beds, ocean bottoms, new
– Smaller sediments travel farther than larger
• Compaction: buried sediments get
squeezed by overlying sediments; water
b/w fragments is forced out
• Cementation = minerals in solution
precipitate in pore space b/w fragments =
Rocks viewed with a microscope
Sandstones (Sedimentary): Grains of sand cemented
together with pore space between.
Features of Sedimentary Rocks
• Visible clasts (pebbles, sand grains,
• Fossils = imprints or remains of
• Ripple marks
• Mudcracks
Features of Sedimentary Rocks
Conglomerate – made of pebble clasts
Sandstone – made of sand clasts
Ripple marks – sand deposited by stream
Features of Sedimentary Rocks
Mud cracks: seasonal
lake/desert enviro.
oolites – tidal enviro.
Stromatolites – fossils
of bacterial organisms
common in shallow
Fossils in Sedimentary Rocks
Fossils in sedimentary help to
1. Date/determine age or rocks. Age or rock
correlates to time when those organisms lived.
2. Provide info on depositional enviro (shallow sea?
Tidal area? Swamp? Desert?)
Ex: Rocks w/ trilobite fossils are from Precambrian time (540 mya or older); Crinoid fossils =
Paleozoic (540 – 254 ma); Ammonite fossils = Mesozoic (64 -254 ma)
Types of Sedimentary Rocks
A. Clastic: form fr. broken fragments of other
rocks (held together by cement)
Ex.: Conglomorate, sandstone, mudstone
B. Chemical: form fr. minerals that
precipitate out of solution.
Ex.: gypsum, rock salts, limestone
C. Organic(biochemical): form as a result of
organic (living) processes
Ex. :Fossiliferous limestone made from shells of organisms or rocks w/
fossils in them or Anthracite (coal) made from plant matter that did not
get decomposed.
Conglomerate: Clastic
Tufas (salt
Cliffs of
Sedimentary Rock Classification
Where does each rock form?
Igneous: surface (extrusive) or deep in
earth (intrusive); Heat from Earth’s interior
creates igneous rocks
Metamorphic: usually deep in the crust
(heat from Earth’s interior creates)
Sedimentary: on surface; Sun (drives
weather) and gravity provide energy for
sedimentary processes.
Rock ID Check Up
Answer these questions on lined paper in complete sentences.
Which rocks were igneous? List # and name.
Which of igneous rocks would be considered EXTRUSIVE?
Which rock was an intrusive igneous? What characteristic can you
observe to confirm that it is intrusive?
Did granite form quickly or slowly? How do you know?
Which rocks were sedimentary? List # and name.
Which sedimentary rocks were clastic?
Which sedimentary rock was organic or chemical?
Which rocks were metamorphic? List # & name.
Shale can turn into slate. What conditions would change shale
into slate?
Which rock (or rocks) could you find fossils in? Support your