Weathering and Sedimentary rock Lectures
Weathering of rocks
Two types
Mechanical: physically breaking rock into smaller pieces
Chemical: changes rocks chemical components into one or more new components
Mechanical weathering
Does not change the chemical constituents of the rock
increases surface area of the rock (allows for more rapid weathering)
Mechanisms of Mechanical Weathering
Frost wedging
Unloading or sheeting
Less dominant forms of mechanical weathering:
Thermal expansion
primarily occurs in desert regions
Biological activity
plant roots and animals expand fractures and cracks
Abrasion
caused by wind or water
Secondary crystal growth
Chemical Weathering
Complex processes that breaks down mineral structures
remember rocks are made of minerals
Primary agent is water
Three primary reactions for chemical weathering
dissolution, oxidation and hydrolysis
Dissolution
Water molecules are polar
Minerals with ionic bonds are “pulled apart” as the ions are attracted to the water. Creates solutions.
Hydrolysis
Water can behave as an acid
H2O à H+ + OH-
Hydrolysis is the process by which hydrogen ions replace other ions in the atomic structure of the mineral.
Most common way silicate minerals are weathered
Olivine: (Mg,Fe)2SiO4 +4H+ +4OH-
Product: Fe++ + Mg++ + 4OH- & Silicic Acid (H4SiO4)
Feldspars: KAlSi3O8 or NaAlSi3O8 or CaAl2SiO8
Products: K, Na and Ca cations
2HCO3
4 H4SiO4
Kaolinite (Clay)
Weathering of Feldspar:
Creates clay
Silica rich solution can redeposit silica and create chert deposits or used as cement for sedimentary rocks
Increasing the acidic nature of H2O
Carbonic acid (H2CO3):
Organic matter
Limestone
Atmosphere
Sulfuric acid:
Car exhaust
Volcanic eruptions
Oxidation
Transfer of an electron to an oxygen atom
Often affects Iron and Magnesium
Important in decomposition of mafic minerals
Examples?
Oxidation of iron creates Hemitite
4Fe + 3O2 à 2Fe2O3
Causes “rusted” rocks
Stability of Minerals
Minerals formed at high temperature and/or pressures are chemically unstable on the earth’s surface.
Based on Bowen’s reaction series which minerals will weather away the quickest on the surface?
Which will be the most stable?
Based on the chemical reactions we’ve discussed
Where would you expect chemical weathering to dominate?
Where would you expect mechanical weathering to dominate?
What will affect the rate of weathering?
How do rocks weather?
Weather from outside-in
Spheriodal weathering
Differential Weathering
What happens to all the product created by weathering?
Forms Regolith
Regolith: weathered rock particles
Regolith can form soil or lithify to create new rock
Soil
Combination of Water, Air, Regolith and Organic matter
Formation of soil is dependent on Climate.
Warm wet creates thick soil layer
Dry cold climates create thin layers
Excessive water can leach nutrients from soil
Soil Profile
Soil can be divided in to layers or horizons that become more defined with time
A vertical section through all horizons is known as a soil profile
Idealized soil profiles have 5 horizons:
Top to bottom: O, A, E, B,C and bedrock
3 basic soil types
Pedalfer:
Aluminum and Iron rich minerals
Areas of heavy rain
Pedocal:
Soils rich in calcium carbonate
Often found in dry climates
Laterites:
Deeply weathered soils (tropics)
Rich in iron and aluminum (little else)
Very red soils… infertile
What happens to all the product created by weathering?
Sedimentary rocks form from it…
Sandstone
Mudstone (shale)
Limestone
Conglomerate
Breccia
Coal
Sedimentary rocks
Why should we care about sedimentary rocks?
How are sedimentary rocks created
Lithification
Compaction and Cementation of Sediments
Precipitation
Water evaporates and leaves minerals behind
Types of Sedimentary rocks
Clastic (Lithification processes)
made of sediment particles
sandstone, mudstone (shale), conglomerate; breccia
Chemical
precipitated from a solution
limestone, opal, chert
Biochemical
compression and leaching
Clastic sedimentary rocks
Chemical maturity
Composition of the particles in the sediment
Mature: quartz rich --> resists weathering
sediment has been around for a long time
Immature: olivine rich--> weathers easy
sediment is very young
Physical maturity
shape of the particles in the sediment
sorting--> are all the particles the same size
poor vs. well sorted
rounding--> how round are the edges
angular; subangular; rounded
size--> smaller particle--> more mature
What does maturity tell us?
Environments of Sedimentary rocks: Look at the following pictures and think about what type of sedimentary rock would form in each of these environments
What type of sediment is present? What type of rock would form?
Textures of Clastic Sedimentary rocks:
Bedding
Laminar Bedding
Cross Beds—created by wind and water currents. Can tell direction of flow
Ripples
Mud Cracks
Raindrops
Chemical sedimentary rocks
Precipitation of supersaturated solutions
limestones (massive & travertine)
salt deposits
gypsum
jasper, chert, opal
Biochemical
coquina limestone
coal
Warm shallow Marine environment creates…
…Limestone precipitated from a solution
Limestone
resource for building and industry
forms in shallow marine environment
accumulation of shells cemented by Calcium Carbonate
precipitation of calcium carbonate
certain temperatures and pressures cause CaCO3 to precipitate
Other
Salt and gypsum--> often found in desert regions
Jasper, chert, opal--> desert regions and ocean deposits
Coal--> compression of plant debris in low oxygen environment
Lignite Coal—lowest grade (lots of plant matter still visible)
Bituminous coal: Medium grade…most common
Anthracite coal: high grade metamorphosed
Why are sedimentary rocks important?