Weathering and Sedimentary Rocks



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 ................
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