Download Rocks and Minerals

Rocks and
Minerals
Igneous Rocks
Igneous Rocks
Fig. 5-12d, p. 145
Igneous Rocks
Fig. 5-5b, p. 138
Sedimentary
Rocks
Sedimentary
Rocks
Conglomerate
Sedimentary
Rocks
Meteetsee Spires – just SE of Red Lodge
Metamorphic
Rocks
Metamorphic rocks exposed at Mt. Everest.
Deformation occurs at various scales
A mica garnet schist
Metamorphic
Rocks
Earth Materials
• The Crust and its Composition
• Atoms and Minerals
• Igneous Rocks
• Sediments and Sedimentary Rocks
• Metamorphic Rocks
• The Rock Cycle
The Crust is the outer 8 to 75 km of the Earth
Composition of he Crust
oxygen and silicon account for about 75% of the earth's crust
metallic elements iron, aluminum and the base elements account for most
of the rest
The Crust and its Composition
the elements of the crust are combined in
inorganic chemical compounds called minerals
these minerals are mixed together in various
proportions to form different rock classes
rocks of the Earth's crust are grouped into three
major classes: igneous, sedimentary and
metamorphic rocks
The Nature of Minerals
• Mineral
– A naturally occurring
– inorganic solid
– that has an exact (or clearly defined
range) chemical composition
– with an orderly internal arrangement of
atoms
– generally formed by inorganic processes.
Minerals
• Internal structure
– Repetitive geometric pattern of atoms
– Expressed in physical properties
• Interfacial angles
• Cleavage
• Revealed in X-ray diffraction
Ionic Bonding
Periodic Table of the Elements
8.3
Polymorphs of Carbon Minerals
Physical Properties
•
•
•
•
•
Cleavage/Fracture
Optical Properties: Luster, Color, Streak
Hardness
Density
Other: Magnetism, reaction to dilute
HCl, salty taste, …
Perfect cleavage in 1 plane
Perfect cleavage in 2 planes
Perfect cleavage in 2 planes @ 90°
Perfect cleavage in 2 planes NOT @ 90°
Rock-Forming Minerals
• About 20 common minerals make up most
rocks
– Silicates dominate
– Quartz, Feldspars, Mica, Amphiboles, Pyroxenes
– Carbonates are common; limestones
– Evaporite minerals, salt mines, gypsum
– Secondary minerals formed during
weathering, e.g. Aluminum and Iron oxides
Silicate Minerals
• Most common minerals on Earth
– Comprise 95% of the volume of the crust
– Approximately 75% of the Earth’s mass is
made up of silicon and oxygen
– All silicate minerals are based on the
silica tetrahedron
• SiO4-4
Silica Tetrahedron
Silicate Structures
Isolated
Single chain
Double chain
Sheet
Solid
Clay Minerals
• Sheet silicates similar to mica
• Products of chemical weathering near
the Earth’s surface
• Usually microscopic crystals
– Kaolinite
SEM photograph of clay minerals: authigenic chlorite flake from
the Watahomigi Formation in Andrus Canyon, Supai Group, Grand
Canyon; x 20,900. Figure 05-D, U.S. Geological Survey Professional
Paper 1173.
Nonsilicate Minerals
• Usually form at low temperatures
(reactions that occur at the surface of the E arth )
– Carbonates (biologic)
• Calcite - Ca CO3
• Dolomite - CaMg(CO3)2
– Evaporite Minerals (seawater evaporation)
• Gypsum - CaSO4-2H2O
• Halite - NaCl
– Oxides (rust and weathering)
• Hematite
Igneous
Rocks and
the Rock
Cycle
The Rock Cycle
• A rock is a naturally
formed, consolidated
material usually
composed of grains of
one or more minerals
• The rock cycle shows
how one type of rocky
material gets
transformed into
another
Imagine the first rock and the cycles that it has
been through.
Igneous Rocks
• Form from Magma
– Hot, partially molten mixture of solid
liquid and gas
– Mineral crystals form in the magma
making a crystal “slush”
– Gases - H2O, CO2, etc. - are dissolved in
the magma
Igneous Rocks
• Magma vs. Lava
– Magma is molten rock beneath the surface
– Lava is molten rock that has reached the
surface
– Magma solidifies to form intrusive igneous
rocks
– Lava solidifies to form extrusive igneous
rocks
Distribution of igneous rocks in North America
Distribution of igneous rocks in Montana - Plutonic
Distribution of igneous rocks in Montana - Volcanic
Igneous Textures
• Texture - the size, shape and relationship of
mineral crystals in the rock
• Reflects cooling history of the magma or
lava
• Slow cooling rate
• Fast cooling rate
• Very fast cooling rate
>> Big crystals
>> Small crystals
>> glass
Phaneritic (coarse-grained) texture in granite
Aphanitic (fine-grained) texture in rhyolite
Glassy texture in obsidian
Porphyritic Texture
• Well formed crystals (phenocrysts)
• Fine grained matrix (groundmass)
• Complex cooling history
– Initial stage of slow cooling
• Large, well formed crystals form
– Later stage of rapid cooling
• Remaining magma crystallizes more rapidly
Porphyritic igneous rock:
Big xtals in a fine grain matrix
Pyroclastic Texture
• Produced by explosive volcanic eruptions
• May appear porphyritic with visible crystals
– Crystals show breakage or distortion
• Matrix may be dominated by glassy fragments
– Fragments also show distortion
– Hot fragments may “weld” together
Pyroclastic texture
Extrusive Rock Bodies
Volcanic
• Form of extrusive bodies influenced by
magma properties
– Composition
• Silica content
– Viscosity
• Volatile content
• Temperature
Aa flow
Pahoehoe flow
Figures 4.6 A & B
movies
F ountain.mov
Stromboli.proj.mpg
Lava4.mov
MainC rater.mpg
Redflow.mpg
PyroclasticFlow.mov
O ldfaith.av i
JuandeFucaSmok er.av i
Smoker.mov
Tube3.mov
Devil’s Tower; a volcanic neck, a feeder pipe
Shiprock, New Mexico; a volcanic neck
Rhumski, Cameroon; a volcanic neck
Sill; parallels layers in the country rock
Dike; cuts across layers in the country rock
Half Dome; part of the Sierra Nevada batholith
Beginnings of a
spatter cone
Large cinder cone
Flood basalts with several thick and
thin layers. Each layer represents a separate eruption.
pillow lavas
http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html
Formation of Volcanic Domes
Mt Fuji: Stratovolcano
Mt. St. Helen's prior to 1980 eruption,
a classic stratovolcano
http://www.youtube.com/watch?v=bgRnVhbfIKQ
Process of formation of ash flow caldera
- e.g., Crater Lake, Oregon or the super
Caldera of Yellowstone
Size comparison of various volcanic features
Intrusive Rock Bodies
Plutonic
• Less dense magmas rise through the
crust
• Rising magmas slowly cool
– Viscosity increases
– Density increases
• Intrusions form as magma solidifies
beneath the surface
Figure 4.18. Types of magmatic intrusions
Metamorphosis…
One of the oldest rocks in the world. A gneiss produced by
metamorphosis of an even older shale.
Metamorphism
• The transformation of rock by temperature
and pressure
• Metamorphic rocks are produced by
transformation of:
• Igneous, sedimentary and even other
metamorphic rocks
What causes metamorphism?
• Heat
• Most important agent
• Heat drives recrystallization - creates new,
stable minerals
• Pressure (stress)
• Increases with depth
• Pressure can be applied equally in all
directions or differentially, i.e. directed
Main factor affecting metamorphism
• Parent rock
• Metamorphic rocks typically have the same
chemical composition as the rock they were
formed from
• Different minerals, but made of the same
stuff.
Metamorphism
• Three types of metamorphic settings:
• Contact metamorphism – from a rise in
temperature within host rock
• Hydrothermal metamorphism – chemical
alterations from hot, ion-rich water
• Regional metamorphism -- Occurs in the cores
of mountain belts and makes great volumes
of metamorphic rock
Contact metamorphism
Produced mostly by local heat source
Hydrothermal Metamorphism
Circulation of hot fluids through cracks and porous
rock
Important source
of ores
Regional Metamorphism:
Subduction zones …..
…and/or deep burial
Metamorphic textures
• Foliation
• Foliation can form in various ways:
– Rotation of platy or elongated minerals
– Recrystallization of minerals in a preferred
orientation
– Changing the shape of equidimensional grains
into elongated and aligned shapes
Development of foliation due to
directed pressure
Foliated vs. Nonfoliated textures under
the microscope
Flattened Pebble Conglomerate = flattening
Progressive metamorphism
of a shale
Shale
Slate
Phyllite (left) and Slate (right)
lack visible mineral grains
Phyllite
Schist
Schist
A mica garnet schist
Gneiss
Gneiss displays bands of light and dark
minerals
Gneiss
Change in metamorphic grade with depth
Metamorphic rocks exposed at Mt. Everest.
Deformation occurs at various scales
Outcrop of foliated gneiss
Common metamorphic rocks
• Nonfoliated rocks
• Quartzite
– Formed from a parent rock of quartz-rich
sandstone
– Quartz grains are fused together
– Forms in intermediate T, P conditions
Sample of
quartzite
Thin section
of quartzite
Common metamorphic rocks
• Nonfoliated rocks
• Marble
– Coarse, crystalline
– Parent rock usually limestone
– Composed of calcite crystals
– Fabric can be random or oriented
Marble (Random fabric = annealing; nonfoliated)
Marble
Question:
Where do we see metamorphic rocks in outcrops?
North
American
Craton
Shield
Western North
American
Mobile Belt
Platform
Eastern Nor
American
Mobile Be
Answer:
In continental shields and uplifted basement rocks
Sedimentary
Rocks
The Nature of Sedimentary Rocks
• Sedimentary rocks are composed of:
– Fragments of other rocks (detrital or clastic)
– Chemical precipitates
– Organic matter or biochemically produced
materials
Types of Sedimentary Rocks
Detrital
Chemical
Biologic
Clastic
Texture
Crystalline
Texture
The Nature of Sedimentary Rocks
• Sedimentary rocks are common at the
Earth’s surface
– Cover ~75% of the continents
– Cover nearly all of the ocean floor
– Easily eroded
– Occur in distinct layers (strata)
The Nature of Sedimentary Rocks
• Layers are easily identified
– Majors layers (formations) easily recognized
over large distances
– Smaller layers within a formation are
separated by bedding planes
– Gradation in grain size, composition or
physical features may vary
Sedimentary layers may extend for many miles
Identifying and correlating the layers is
Stratigraphy. More on that later.
Rock Identification is based on:
• Composition
What minerals make up
the rock?
These can easily be confused
• Texture
What is the shape, size and
orientation of the mineral
grains that make up the
rock?
Major Classes:
Clastic
Crystalline (chemical
and/or biochemical)
Biologic (coal,
fossiliferous limestones, etc.)
Clastic Sedimentary Rocks
• Made of rock & mineral fragments or
clasts
• Clasts are broken and worn particles
transported by water, wind or ice
• Clastic rocks are subdivided by grain size
Clastic Sedimentary Rocks
• Grain size is controlled by:
– Size and mineralogy of grains in source rock
– Carrying capacity of transport process
– Weathering and erosion that occurs during
transportation
– Energy of the depositional environment
Grain size ranges for classification of
common clastic sedimentary rocks
Clastic Sedimentary Rocks
• Common clastic sedimentary rocks
– Conglomerate
– Sandstone
– Mudstone or Shale
Conglomerate
Sandstone
Shales
Shales
Shales erode very easily
and form slopes
Chemical/Biochemical Sedimentary Rocks
• Formed by a process that takes ions from
solution to form a solid
– Chemical Sediments
• Precipitates from water by an inorganic
process, e.g. evaporites
– Biochemical Sediments
• Formed during the growth of some organism
Chemical/Biochemical Sedimentary Rocks
• Subdivided by composition and mode of
formation
• e.g., Limestone
– Biochemical formation by algae, coral, etc.
– Direct chemical precipitate from warm sea water oolites
– Chemical precipitate from springs and in caves
Chemical/Biochemical Sedimentary Rocks
• Common Chemical/Biochemical rocks:
– Dolostone - composed of dolomite
– Chert - microcrystalline quartz
• Various modes of formation
– Evaporites
• Rock salt - halite
• Gypsum
Limestones
Limestones
Limestones
Oolitic
Limestone
Chalk
(Coccolithophores)
Travertine
(Limestone)
Dolostone
Chert (Flint, Jasper, Agate…)
Evaporites:
Bonneville Salt Flats, Utah
Rock
Gypsum
Rock Salt