Weathering & Sediment

WEATHERING & WEATHERING & SEDIMENTSEDIMENT

PELAPUKAN & SEDIMENPELAPUKAN & SEDIMEN

What are the processes involved in the creation of sediments and sedimentary rocks?

Weathering - Process which acts at the earth's surface to decompose and

breakdown rocks.Erosion - The movement of weathered

material from the site of weathering. Primary agent is gravity, but gravity acts in concert

with running water.

Sedimentary Rocks• Processes– Weathering = sediments and soils– Erosion and transport– Deposition– Lithification and Diagensis

Processes

Transportation - Transporting medium usually water. More rarely wind or glacial ice. Deposition - Occurs when energy necessary to transport particles is no longer available. Deposition due to the gentle settling of mineral grains. Can also be result of chemical precipitation due to changing conditions. Lithification - Involves several steps. All taken together are termed Diagenesis.

Compaction - Squeezing out of water. Cementation - Precipitation of chemical

cement from trapped water and circulating water. Recrystallization - Growth of grains in

response to new equilibrium conditions

Weathering & Sediments

• Types of Weathering– Mechanical & Chemical

• Types of Sediment– Siliciclastic– Chemical/Biochemical– Carbonaceous

What factors influence rates of weathering and where on earth are these rates highest?

Weathering & Sediments• Controlling Factors• Source• Climate• Topography and latitude• Sea level• Tectonic Setting

Weathering Processes

• Mechanical - physical disintegration

• Processes– Thermal Weathering– Frost (ice) Wedging

Mechanical Weathering

Expansion and Contraction - the thermal heating and cooling of rocks causing expansion and contraction.Frost Action - Water freezes at night and expands because the solid occupies greater volume. Action wedges the rocks apart. Requires adequate supply of moisture; moisture must be able to enter rock or soil; and temperature must move back and forth over freezing point.Exfoliation - process in which curved plates of rock are stripped from a larger rock mass. Example Half Dome. Exact mechanism uncertain but probably due to unloading.Other types - Cracking of rocks by plant roots and burrowing animals.

Mechanical Weathering

• Processes• Exfoliation - removal of overburden pressureMechanical Weathering• Processes• Wetting & Drying• Salt• Plants

Chemical Weathering

• Chemical weathering - alteration of chemistry and or mineralogy of source rock

• most important agent of chemical weathering = water

– Atmospheric gasses

Chemical Weathering

Factors which effect the rate of chemical weathering are:

Particle size - Smaller the particle size the greater the surface area and hence the more rapid the weatheringCompositionClimate (See Figure)Type and amount of vegetation

Chemical Weathering• Processes• Hydrolysis - reaction between silicate minerals and acids

– H+– solution– Aluminum silicates = clays

Chemical Weathering

• Processes• Solution - soluble minerals exposed to meteoric water• Carbonic acid• karst

Chemical Weathering

• Oxidation - alteration of iron and manganese in silicate minerals• Iron and manganese oxides• Reduction - soluble ferrous iron (Fe2+)

Chemical Weathering of Rocks

formation of carbonic acidH2O + CO2 ------->> H2CO3

Acid then dissociates and the following happens 2KAlSi3O8 (feldspar)+ 2H+ + H2O ------->> Al2Si2O5(OH)4 (clay)+ 2K+ + 4SiO2            

Quartz - slow process and largely ineffective. Quartz remains quartz. Grains are rounded.Feldspar - weathers to clay with the cations Na, Ca, and K going into solution. Clays that can form include kaolinite (pure aluminum silicate), illite and montmorillonite. Factors which dictate clay formation are (a) climate; (b) time; (c) parent material.Muscovite - Same as aboveFerromagnesian minerals - weather to clay plus highly insoluble iron oxides, essentially varieties of limonite (rust).

Chemical and mechanical weathering enhance each other

• Mechanical - increases effective surface area• Chemical - differential weathering• Where is both types of weathering most intense?

Products of Weathering

• Source-rock residues - siliciclastics• Secondary minerals - mudrocks• Soluble constituents - evaporites, limestone, chert

Soils Soil - Surficial material that forms due to weathering. Includes an organic component. Many different soil types. Factors effecting their formation are:1) Climate2) Relief3) Bedrock material4) Time

1. O horizon - Organic layer 2. A horizon - Zone of leaching - Cations are leached from this horizon by strongly acid solutions generated in the O horizon3. B horizon - Zone of Accumulation - Cations leached out of the A horizon accumulate here. Horizon consists of clays, iron and aluminum oxides. Deposition due to neutralization of acid solutions.4. C horizon - Partially decomposed parent material. Lower most zone.

Soil Types

Pedalfer - Named for the abundance of Al and Fe in the B horizon. Occur in temperate, humid climates. Lie generally east of the Mississippi River, correspond with 63 cm/yr rainfall contour.Pedocal - Named for the accumulation of calcium carbonate in the B horizon. Characteristic of temperate, dry climates. Lie generally west of Miss River. Poorly developed A horizon, B horizon is caliche (calcium carbonate).Laterites - Tropical soils thought to represent the end products of weathering. Characterized by stark red color and abundance of iron and aluminum oxides and lesser clay minerals. Requires abundant rainfall.