Download Test # 2 Study Guide Weathering What is Weathering? - in

Test # 2 Study Guide
Weathering
What is Weathering?
- in-situ process
- breaks down rocks and minerals at or near Surface of the Earth
- 2 types - Mechanical “Disintegration”
- Chemical “Decomposition”
Mechanical - Causes
- Frost Wedging
- Crystal Growth - Salt (NaCl) growth in cracks near the ocean
- Salt growth in cracks in paint on cars
- Exfoliation
- Root Growth
- Thermal Expansion/Contraction
- Abrasion
Chemical - Causes
- Dissolution
H2O + CO2
→ H2CO3
Water + Carbon dioxide → Carbonic Acid
CaCO3 + H2CO3 → Ca2+ + 2HCO3Calcite + Carbonic Acid → Calcium Ion + Bicarbonate Ion
- produces Karst Terrains
- Oxidation
FeS2 + 9.5O2 + 2H2O → 0.5Fe2O3 + 2H2SO4
Pyrite + Oxygen + Water → Hematite + Sulfuric Acid
- Hydrolysis
2KAlSi3O8 + 2H+ + 9H2O → Al2Si2O5(OH)4 + 4H4SiO4 + 2K2+
Potassium Feldspar + Hydrogen (from water) → Kaolinite Clay
Potassium Ions
What Factors Control the Rate of Chemical Weathering?
Climate – Rainfall, Temperature
Living Organism
Time
Mineral Composition
Sedimentary Rocks
Studied by Sedimentologists
+ Silicic Acid +
Formed at or near the surface of the Earth from organic or rock fragments or by
precipitation from solution. Lithified by compaction or cementation of sediments,
deposited in beds or layers
- Part of Rock Cycle
- Relative Abundance
Sedimentation Processes
1) Detrital material is deposited because of energy loss velocity of stream or wind
decreases
2) Chemical Precipitation - dissolved material (ions) are converted
to solids
Ca2+ + CO3 2- → CaCO3
(dissolved)
(solid)
3) Biochemical removal of CaCO3, PO4 or SiO2 to form shells or skeletal material (coral,
shellfish, diatoms, forams)
Mineral Composition
Sedimentary rocks are accumulations of minerals
- they are rarely composed of just 1 mineral, but 1 mineral
may predominate
- the most common minerals found in sedimentary rocks
are quartz, feldspar, mica, clay and calcite
Underlined minerals have the highest degree of SiO4 polymerization are the most
difficult to weather and therefore stay around to form sedimentary rocks
Clays - produced by the weathering of silicates (e.g. feldspars)
Calcite - main constituent of limestone
Calcite is produced when calcium and carbonate in solution combine to form calcite
Ca2+ + CO3 2- → CaCO3
(dissolved)
(solid)
The carbonate ion (CO3 2-) is produced when atmospheric CO2 reacts with water
CO2 + H2O → 2H+ + CO3 2(air)
(ion in solution)
Other Materials
Dolomite (Ca, Mg (CO3)2) if > 50% (Ca, Mg (CO3)2), rock is called dolomite
Iron oxides - (hematite, magnetite, goethite) product of ferromagnesian minerals
weathering
Evaporites - produced when H2O evaporates
- gypsum
CaSO4-4H2O
- anhydrite
CaSO4
- halite
NaCl
Volcanic matter - ash + cinders
Organic matter - gives dark color to rocks such as black
shales and coal
Origins of Sedimentary rocks
1. Detrital deposits - weathering products of existing rocks
(gravel, sand, silt, clay)
2. Chemical deposits - inorganic processes cause minerals to
precipitate from solution
- organic processes cause the precipitation
of minerals through the removal of ions
from solutions (e.g. seawater)
Sedimentation
- Source Material
1) Ultimate source of material is igneous rocks
2) Intermediate source is metamorphic and sedimentary rocks
Methods of Transport
1) Water (streams, rivers, glaciers, ocean currents)
2) Landslides (mass wasting)
3) Wind (aeolian deposits)
All these transport process are driven by gravity, the source of the
energy to transport sediments
Effects of Transport
1) rounding
2) sorting
Sediments are deposited in layers called strata or stratum. Thin sedimentary layers
(<1cm thick) are called laminae
Lithification - the process by which unconsolidated sediment is formed into rock
There are 3 Lithifying Processes
Cementation - pore spaces between detrital grains are filled by
a binding agent (e.g. calcite, dolomite, quartz,
hematite)
Compaction - overlying weight of sediments forces H2O out
and mineral grains pack together
Chemical Diagenesis
- recrystallization
2CaCO3 + Mg2+
Calcite
→
CaMg(CO3)2 + Ca2+
Dolomite
- dissolution / precipitation
CaCO3 → CaCO3 or SiO2 → SiO2
- reaction rates increase with burial because:
1) Pressure increases with depth
2) Temperature increases with depth
Geothermal gradient 1°C/30 m or 30°C/km
Pressure gradient 1 bar/3 m or 300 bars/km (1 bar ~ 1 atmosphere)
Rock Types Produced
Sediment
Rock
mud (clays) shale
sand
sandstone
gravel
conglomerate
lime muds
limestone + dolomite
SEDIMENTARY ROCK TEXTURES
1) Clastic Texture - composed of mineral and rock fragments
- texture varies with depositional process or environment
(desert, glacier, delta, shoreline)
- chemical sedimentary rocks sometimes show clastic
texture (shell material)
-
grain size is useful in classifying sedimentary rocks
2. Nonclastic or Crystalline Texture
- composed of interlocking grains
- exhibited by chemical sedimentary rocks that precipitated
from a solution
- initial grain size of the precipitate is small
- “grains” settle out and accumulate as mud
- mud is compacted causing the grain size to increase
because of Pressure
Detrital Rocks
1. Conglomerate - >35% gravel size or larger
- breccia - angular fragments
- tillite - glacial deposits
2. Sandstone - > 50% sand-sized particles
- has four components
a) sand-sized grains
b) matrix - clays, micas, <1/16 mm
c) cement - SiO2, CaCO3
d) pore space - important for movement and storage of fluids
such as water, oil and gas.
3. Siltstone + Claystone (shales)
- claystone - finest, >50% is < 1/256 mm
- siltstone - >50% is between < 1/256 and 1/16 mm
- mudstone - mixture of sand, silt and clay none of which
makes up >50%
- shale - all fine-grained detrital rocks
Chemical Sedimentary Rocks
- subdivided by composition
- carbonates and evaporites are the most abundant
1. Limestone - Mostly CaCO3
~ 90% of Limestone is formed by living organisms
- Limestone is easily dissolved in H2O
CaCO3 → Ca2+ + CO3 2(solid)
(dissolved)
2. Dolomite - > 50% CaMg(CO3)2
- formed from limestone by partial replacement of Ca by Mg
3. Evaporites - minerals precipitate from lakes and seawater
- principal minerals are gypsum, anhydrite, halite
- least soluble minerals precipitate first
- as H2O evaporates, the remaining H2O becomes more saline
- when the salinity >3 times seawater, gypsum precipitates
- when the salinity >10 times seawater, halite precipitates
4. Coal - derived from ancient swamp-dwelling plants
- rich in carbon
- formed in areas of low relief, poor surface drainage, warm
temperatures, and abundant rainfall
- removal of water and volatiles with compaction results
in increasing coal rank
Relative Abundance of Sedimentary Rock Types
Shale
Sandstone
Limestone
~ 70 %
~ 20 %
~ 10 %
Sedimentary Features
Bedding
- horizontal bedding - parallel layers
- cross-bedding
- beds inclined at an angle
Graded Bedding - characterized by the progressive decrease in particle size upward
- formed by the sudden decrease in velocity of
sediment-laden currents
Ripple Marks - small ridges formed on the surface of sediments by
moving wind or water.
Mud Cracks - polygonal pattern formed on sediment surface from
drying and shrinking of freshly deposited mud. The
dried mud can also preserve rain drops and salt hoppers
Nodules, Concretions and Geodes - formed sometime after sediment
was deposited.
- irregular or disc-shaped body differing in composition
from the host sediment.
- formed parallel to bedding planes
- usually <1/3 m in diameter (e.g. chert nodules in Limestone)
Concretions - spherical or disc-shaped sedimentary rock that is more
firmly cemented than the surrounding host rocks
- the cement is usually calcite, silica or iron oxide
- the spheres are usually up to <1 m in diameter
(some <3 m)
Geodes - hollow in the center and commonly lined with crystals of
quartz or calcite
- common in Limestone
- up to 30 cm in diameter
Fossils - preservation or imprints of hard parts of plant and animal
remains
- abundant in Limestone, Shale and some Sandstone
- useful in determining the age of the rock
Metamorphic Rocks
Metamorphism - solid state change in response to temperature,
pressure and chemical environment.
- takes place within the Earth’s crust below the
zone of weathering and cementation and above
the zone of remelting.
What are the agents of Metamorphism?
1. Heat - Temperature range from 100° to 900°C
- reactions involved in the formation of new minerals
proceed faster at higher temperatures
2. Pressure - Pressure range from 1 to 10,000 bars (What’s a bar?)
- effects of pressure
(1) causes recrystallization and formation of new
minerals (closer atomic packing)
(2) causes rocks to flow and creates linear features
(foliation)
3. Chemically Active Fluids - hydrothermal solutions heated by
or released from magma
- fluids are important in dissolving, transporting
and precipitating minerals
- fluids tend to increase reaction rates
Does the appearance of the rocks change when metamorphosed?
Metamorphic Textures
1. Foliated - planar or parallel orientation of mineral grains
- foliation imparts a rock cleavage
- types of foliation
a) slaty - smooth, regular cleavage planes on a microscopic scale
slaty cleavage
b) phyllitic - more irregular cleavage planes barely visible to the
naked eye
c) schistose - rough, clearly visible cleavage surfaces, flakes
d) gneissic - coarse foliation or bands of different mineral composition
2. Unfoliated - no preferred orientation of breakage
Can a rock containing only one mineral be metamorphosed?
Composition
1. Monomineralic - composed dominantly of 1 mineral
- marble (calcite)
- quartzite (quartz)
- unfoliated or weakly foliated
2. Multimineralic - most contain only 3 minerals
- includes most metamorphic rocks
- common minerals are those of igneous and sedimentary rocks
- new minerals are formed as a result of metamorphism:
diopside
tremolite
garnet
sillimanite
chlorite
How do we name Metamorphic rocks?
Texture
Monomineralic
Unfoliated
(Contact Mm)
Foliated
(Regional Mm)
Metamorphic Rock Types
quartzite
marble
MultiMineralic
hornfels
slate
phyllite
schist
gneiss
1. Slate - metamorphosed shale (low grade metamorphism)
- increasing P+T convert clays to chlorite and micas
2. Phyllite - metamorphosed slate
- brighter, coarser grained than slate
- at 250° - 300°C, chlorite and mica develop large flakes
that gives the rock a silky sheen on fresh surfaces
3. Schist - contains visible flakes of minerals such as micas, talc,
chlorite, or hematite
- rock cleaves parallel to platy or fibrous minerals
- most abundant of the regionally metamorphosed rocks
- generally named for the predominant mineral in rock
(e.g. chlorite schist, biotite schist, etc.)
- characterized by > 50% platy minerals
4. Amphibolite - medium to high-grade regional metamorphic rock
composed largely of ferro-magnesian minerals.
- composed chiefly of hornblende and plagioclase
- typically dark colored (green, gray, black)
- may be meta-igneous or meta-sedimentary
5. Gneiss - coarse grained, banded
- high grade regional metamorphism
- segregation of minerals into distinct bands
(mica; quartz + feldspar)
- rocks named for igneous counterpart (granite gneiss,
diorite gneiss, etc.)
6. Marble - contact or regional metamorphism of limestone or dolomite
- No rock cleavage or foliation
- interlocking grains of carbonate
7. Quartzite - metamorphosed quartz-rich sandstone
- unfoliated
- distinguished from sandstone by:
a) no pore spaces and no cement
b) quartzite breaks through grains not between them
8. Hornfels - contact metamorphism of Shale, Limestone, Sandstone,
Tuff (or basalt)
- fine-grained, hard, unfoliated
- form at Temperatures > 550°C
9. Anthracite - Coal with high Carbon content because Pressure and
Temperature drive off water and other volatile gases
- characterized by a conchoidal fracture
10. Migmatite - rock formed by the highest grade of regional
metamorphism ~ equal to partial melting of the rocks
- First rocks to melt are layers with a granitic composition
- melting occurs starting at 700 to 900°C
11. Blueschist - rocks form in high Pressure low Temperature conditions
- these conditions occur in subduction zones
- blueschist rocks are named for the diagnostic blue amphiboles
and layered-silicates that grow under these conditions
Is there more than one type of Metamorphism?
Types of Metamorphism
1. Cataclastic / Dynamic - mechanical, localized deformation
- this type of metamorphism is caused by shearing and/or
grinding associated with intense folding or faulting
- recrystallization and chemical changes are not common
in these rocks
- rocks with this type of metamorphism include:
friction breccias - angular, fragmented rocks
mylonites - finely granulated rocks
2. Contact Metamorphism - rocks in contact with hot, igneous intrusion
- occur in High Temperature, Low Pressure environments
T = 150 - 800°C
P = 1 - 3000 bars
- metamorphism restricted to zones called aureoles (halos)
around the intrusion
- isochemical process in which no ions are
added or lost, they are just rearranged.
2b. Contact Metasomatism - ions are added and lost by exchange
between the fluids and the rocks
- escaping gases and magmatic solutions cause ionic transfer
into the surrounding rocks or heated groundwater around the
intrusion can cause the same thing to occur.
- process results in recrystallization / alteration of country rocks
- develops in late stages of mountain building and occurs at
shallow depths
- often results in the formation of ore deposits (Cu, Zn, Pb, Fe)
2. Regional Metamorphism - is extensive in nature and can involve
areas that are 100’s of km by 100’s km
- occurs deep in the Earth at high Pressure and Temperature
- associated with roots of folded mountains formed by plate
collisions + continental shields
- results in the formation of new silicate minerals
- zones of regional metamorphism (at same pressure)
low grade
300-500°C
medium grade 500-600°C
high grade
>600°C
- metamorphic grade is reflected by mineral assemblages
(index minerals)
Where do Metamorphic Rocks Occur on Earth?
Geologic Time and Dating
- Length of Geologic Time
- Relative Dating
- Absolute Dating
Absolute Dating
- K-Ar (Potassium-Argon) method is the most widely used
(half life of 1.25 b.y.)
- K (potassium) occurs in many different minerals
- works best for Igneous and Metamorphic rocks that have
formed or cooled quickly
- works by trapping Argon in Crystal structure
- Method is no good for rocks / minerals that have cooled
slowly or have been reheated.
- Method can be precise to +/- 5 m.y. for rocks 2.5 b.y. old
Carbon - 14 Dating
- half life 5730 years
- produced by cosmic radiation in upper atmosphere
- incorporated into all organic matter while living
- upon death
14
C begins to decay to
14
N by beta decay
- method good for material up to 50,000 years old (e.g. 15,000
year old moccasins from Missouri cave)
-
1 to 4% variation in production rate of
14
C is a small problem
Problems with Absolute Dating
1) most radioactive elements only occur in trace amounts and do
not form minerals of their own.
2) typical concentrations of radioactive elements in rocks and
minerals is in the parts per million (ppm) range (e.g Uranium
and Thorium concentration in granite is ~ 5 ppm).
3) the low concentration of radioactive parent and daughter
elements can affect how precise an age we can determine.
Instruments can measure the abundance of elements in the
parts per billion range
4) the accuracy to which a parent decays to a daughter element
(half-life) is know can also affect the precision of and date.
5) the parent → daughter half-life for Uranium 235 → Lead 207 is
considered accurate to < 1%.
6) parent and daughter isotopes can can move after the radiometric
clock has started.
- causes
- weathering of minerals
- alteration of minerals (hydrothermal)
- heating of samples during burial or
metamorphism
7) many rocks cannot be dated because:
- rocks are too young to date
- rocks are too old to date (parent element has all decayed)
- they contain insufficient concentrations of radioactive
elements (e.g. quartz does not accommodate radioactive
elements)
Absolute Dating Safeguards
- run duplicate analyses
- use several different radioactive isotopes to determine an age
- if different methods agree you probably have a good age
-
compare absolute ages to relative ages
Other Methods for Measuring Absolute Time
- Tree ring growth
- oldest tree is a huon pine in Tasmania ~ 10,500 years
(covers 2.5 acres and may be 30 - 40,000 years old)
- Ice Layers in Glaciers
- glaciers in Greenland, Antarctica and highest mountains
are up to 3,000 m thick (2 miles)
- glaciers are built up by annual deposition of snow and
packing of this into a more dense material called firn.
- there is a nearly continuous record over 65,000 years
- composite cores provide us with a record extending back
165,000 years
- Varves - layers of clay deposited in glacial lakes
(1 dark and 1 light colored layer = 1 year)
- layers in Baltic Sea extend back 20,000 years
- Carbon-14 dating of organic matter in dark layers
helps confirm age of layers (if not enough organic
matter we can only establish total - not absolute time
Folds, Faults and Mountains
Folding, Faulting + Mountain Building are studied by Structural Geologists
Why do rocks fold or break ?
It has to do with deformation characteristics
What processes cause rocks to be deformed ?
- compression
- Tension
- Shearing
Where on Earth do rocks become deformed ?
What are Synclines and Anticlines ?
How do we measure their orientation?
- strike
- dip
What controls the structures that form?
1) rock composition
2) amount + orientation of pressure
- symmetric
- asymmetric
- overturned
- recumbent
Plunging Synclines and Anticlines
Domes + Basins
Faults
What is a Fault ?
- Hanging Wall
- Footwall
Types
- Normal
- Oblique
- Reverse / Thrust
- Strike-Slip
Why do we want to understand Faults?
Implications of Faults
- Ore Deposits
- Landslides
- Dam construction
- Earthquakes
Recognizing Faults
- scarps
- Valleys and Lakes
- Offset Hills / streams
- Surface rupture
Where do Faults occur on Earth?
Folds + Faults in Oil Exploration
- Structural Traps
Mountain Building
What is a Mountain?
- part of the crust of the Earth that stands >300 m (1000’) above the surrounding
land.
How do Mountains form ?
- By a process called Orogenesis
- Volcanic Activity
- Folding + Thrusting
- Upwarping
- Faulting
Mountain Unbuilding
Earthquakes
What is an Earthquake ?
- it is the vibration of the Earth produced by the rapid release of energy
- the vibrations radiate in all directions from the focus
What causes an Earthquake ?
- it is caused by the rapid release of elastic energy stored in rocks on either side of a
fault.
- discovered by H.F. Reid who studied 1906 San Francisco Earthquake
What is elastic energy ?
What evidence is there that earthquakes occur along faults ?
How do we measure Earthquake activity ?
- measured using the Mercalli Intensity Scale
- Mercalli Scale ranges from I to XII
-
it measures the effect that seismic waves has on things
- measured using the Richter Scale
- Open ended logarithmic scale that starts at 0
-
it measures the amplitude of waves measured by a seismograph
Richter Scale - frequency of Earthquakes
What type of seismic waves are generated by an Earthquake?
Body Waves - Primary (P) + Secondary (S)
Surface Waves – Love + Rayleigh
-
What can we learn from the different seismic waves ?
-
Location of the Epicenter of the Earthquake
What factors contribute to damage caused by Earthquakes ?
1) Proximity to the Epicenter
2) The intensity of the Earthquake
3) Duration of the vibrations
- most earthquakes last for < 1 minute
- Loma Prieta, CA, 1989 - 15 seconds
- North Ridge, CA, 1994 - 40 seconds
- Alaska, 1964 - 3 to 4 minutes
4) Proximity to Faults
5) Nature of the material on which a structure is built
- Liquefaction
- Amplification of waves
- Landslides / Subsidence
6) The design of the structure
- materials used
- reinforcement
- joints between walls and ceiling
- connection between building and foundation
- in support elements
- height of building
7) Fires after the Earthquake
8) The presence of large bodies of water
- Tsunami
Can we predict when Earthquakes will occur ?
No !
“Prediction provides a happy hunting ground for amateurs, cranks, and outright
publicity-seeking fakers”
What research is being done to find a way to predict when and where an Earthquake will
occur ?
1) Short-Range Predictions
2) Long-Range Predictions
Short-Range Predictions - monitor possible precursors to an earthquake
1) swarms of microearthquakes prior to a Earthquake
2) Preseismic Uplift / Subsidence (Geodetic Level Surveys)
3) Tilting of Land
4) Geophysical observations (other than seismic)
5) Radon Gas
6) Changes in water tables
7) Seismic Activity
Short-Range Predictions - monitor possible precursors to an earthquake
1) swarms of microearthquakes prior to a Earthquake
problem - microearthquakes do not always precede a quake
2) Preseismic Uplift / Subsidence (Geodetic Level Surveys)
4) Geomagnetic observations
- resistivity
- magnetic
-
low frequency sounds
5) Radon Gas
6) Changes in water table
7) Seismic Activity (this is really a warning system)
Long-Range Predictions - work on premise that Earthquakes are repetitive.
1) Seismic Gaps
2) Paleoseismology (Old Earthquakes)
3) Fault Creep
4) Mapping of existing faults and scarps
1) Seismic Gaps - Parkfield, California
2) Paleoseismology (Old Earthquakes) - Gurvan Bulag thrust fault disrupting
alluvial fans in Mongolia
3) Fault Creep (using lasers / strain meters)
4) Mapping of existing faults and scarps
The problem with making predictions
1) Disruptive to society
2) Costly if not correct - Iben Browning - predicted a major Earthquake would occur in
New Madrid on Dec 2 or 3, 1990
- 100’s of schools were closed for a week
- industry all but unproductive for weeks before
- adults were terrified which affected their children
- public may no longer believe legitimate predictions
positive - temporary preparedness for earthquake
3) Makes public immune to actual risk if too many are false