Weathering, Erosion and Soils

Before we can consider sedimentary rocks, we have to look at weathering to see how raw material for sedimentary rocks is produces.

Weathering i s the breakdown of rocks exposed at the Earth's surface; erosion is the removal of the debris so produced that lowers the level of the Earth's surface.

Important part of the rock cycle, so appropriate to discuss here as weathering and erosion provides the material for sediments and ultimately sedimentary rocks.

Two types of weathering

1. Chemical weathering

Rocks exposed to the oxygen and water in the atmosphere, like other things (Buicks) will corrode. Several processes

A. A. Clay mineral formation high temperature silicates (all types) break down into into low temperature sheet silicate minerals called the clay minerals.

Example Feldspar KAlSi3O8 ---> kaolinite Al2Si2O5 (OH)4

These processes depend on temperature and on presence of other materials such as dissolved CO2 from atmosphere.

The layered structure of sheet silicates can hold molecular water and/or organic molecules. This vary important for soils, as we will see laters.

[ Digression on properties of clay minerals ]

B. Oxidation

C. Solution. Even silicates slightly soluble, but especially important for halite, gypsum and limestones

H2O + CO2 -----> H2(CO3)2

H2(CO3)2 + CaCO3 -----> Ca (HCO3)2

Accumulation of dissolved material in oceasns is why the sea is salty

Natural zones of weakness - fractures and joints. These form pathways for water and are enlarged by chemical action

Physical(=mechanical) weathering

Again natural fractures assist in the weathering process

Thermal expansion and contraction

Frost wedging alternate freezaing and thawing of water in fractures and cavities

Expansion and contraction of clays (in sed rocks) with wetting and drying

Biological action - tree roots, ruminants ( ), and now humans

Weathering Forms and Products

Spheroidal weathering - joints naturally break rock masss into angular blocks. Weathering then faster at corners and edges. Tends to round off the blocks. Exfoliation

Solution pits and runnels - In very soul;ubale rock, the sharp suface details are removed and takes on a smoother appearance. With more solution shallow pits are developed. If water can run , then get solution runnels, miniature valleys

Saprolite - deep weathering, almost complete breakdown to clay, typical of tropical climate but original features of rock still retained

Regolith - froaments of weathered rock accumulate on outcrop and are not washed away. Common in deserts where is litthe rainfall

Weathering and Climate

Types of weathering and speed of weathering influenced by temperature (higher T, faster chemical reaction) and presence of water. These controlled by climate:

o Warm, dry - low latitude deserts, Sahara, Sonoma

o Warm, wet - rain forests,

o Temperate/cold, dry - high latitude deserts eg Gobi

o Temperate/cold, wet - Boreal forests

Soils: the residue of weathering

Soil vs regolith. Soils is regolith plus decayed organicmatter

Soil profile

A horizon - organic matter, humus. Leaching zone

B horizon - clay minerals - hold water and nutrients

C horizon - regolith

Some soil types:

laterites (warm wet climates),

pedocals (dry climates)

pedalfers (temperate, wet climate)