Weathering and Sedimentary Rocks
Weathering
Physical (mechanical) and Chemical Weathering
Physical Weathering-mechanical breakdown of rocks-no change in composition
Expansion and Contraction
Thermal changes
Exfoliation
Biological Disintigration
Increase in surface area
Chemical Weathering
Oxidation-addition of oxygen to structure (rust)
Hydrolosis (adding of water to mineral structure)
Dissolution (acidic rain, groundwater)
formation of clay minerals
high capacity for holding water-expanisve clays
Role of physical weathering in chemical weathering-increase in surface area
Differential weathering due to mineral content
Bowen’s reaction series and resistance to weathering (Olivene most susceptible,
etc.)
QUARTZ is VERY resistant to chemical weathering
Soils-formed by the weathering of bedrock
weathering rates and soil formation depends on:
precipiation, temperature, vegetation
Tropical soils are highly leached due to intense weathering
Sedimentary Rocks
Sediment is the product of weathering
Two types of sedimentary rocks
DETRITAL (made from transportation of sediment) and
CHEMICAL(precipitation of material)
DETRITAL sedimentary rocks
Grain size
Grain Shape
Grain sorting
Grain composition
As the sediment matures (more time) it becomes rounder, smaller, better sorted and has a
higher % of quartz.
Main classification is based on grainsize: SHALE, SANDSTONE, CONGLOMERATE,
BRECCIA
CHEMICAL sedimentary rocks
Don’t follow same rules for size, shape, sorting as they typically form in place
Main rocks: SALT (NaCl), LIMESTONE (CaCO3), and CHERT (SiO2)
Also COAL (plant material) forms in oxygen poor swamps-matures through various
stages with increasing carbon content PEAT< LIGNITE<BITUMINOUS<ANTRACITE
(almost pure C)
Sedimentary Structures: tell us about environment of deposition
Ripple marks
Sole marks
cross beds
mudcracks
rain drop impressions