8- Weathering and Erosion

Common reactions involved in chemical weathering include the following:

(1) Dissolution (2) Hydrolysis (3) Oxidation (4) Hydration (5) Chemical weathering produced by organisms

Carbonic acid causes Dissolution and Hydrolysis

-Carbonic acid (H2CO3) forms when CO2 and H2O molecules combine in rainwater. -Carbonic acid ionizes to form hydrogen ions (H+) and bicarbonate ions (HCO3 - ). -Dissolution: Carbonic acid also reacts with calcite in limestone to produce bicarbonate and calcium ions. -Hydrolysis: Bicarbonate ions react with feldspar, Weathering it to kaolinite clay and silicate and releasing bicarbonate and potassium ions.

Chemical and physical weathering help and reinforce each other (positive feedback).

-The faster the chemical decay (chemical weathering), the weaker the pieces and the more susceptible to breakage into smaller pieces (physical weathering). -The smaller the pieces, the greater the surface area available for chemical weathering.

Dissolution of limestones by groundwater:

-enhanced by the atmospheric CO2 contained in rainwater (carbonic acid) -As CO2-rich water moves down to the water table, through the unsaturated zone to the saturated zone, it creates openings as it dissolves carbonate minerals.

relative stability of minerals at Earth's surface

Fastest->slowest weathering least->most stable: Halite-dissolution Calcite-dissolution Olivine-dissolution followed by oxidation Ca-plagioclase-hydrolysis Pyroxene-Dissolution followed by Oxidation Amphibole-Dissolution followed by Oxidation Na-plagioclase-hydrolysis Biotite-Dissolution followed by Oxidation Orthoclase (potassium feldspar)-hydrolysis Muscovite-hydrolysis Clay (various types)-hydration Quartz Gibbsite (aluminum hydroxide) Hematite (iron oxide)

hydrolysis reaction

Feldspar (2KAlSi3O8) + carbonic acid (2H2CO3) + water (H2O)--> Kaolinite (clay) (Al2Si2O5(OH)4)

Sugar, Coffee beans, Salt, Black pepper, etc....

The smaller the pieces, the greater the surface area available for chemical weathering.

dissolution example

Weathered limestone blocks and columns of 2500-year-old Greek ruins at Segesta, Italy, show pitted , etched surfaces caused by chemical solution.

Why weathering is important?.

Weathering produces: 1- all the clays of the world 2- all the soils of the world 3- the dissolved substances that are carried by rivers to the ocean.

In general, the degree of chemical weathering is greatest...

in warm and wet climates

oxidation process for olivine:

where olivine in the presence of carbonic acid is converted to dissolved iron, bicarbonate, and silicic acid: Fe2SiO4+ 4H2CO3 —> 2Fe(II) + 4HCO3 - + H4SiO4 olivine + (carbonic acid) —> dissolved iron + dissolved carbonate + dissolved silicic acid where olivine in the presence of oxygen, the dissolved iron is then quickly converted to hematite: 2Fe(II) + 4HCO3- + ½ O2 + 2H2O —-> Fe2(III)O3 + 4H2CO3 dissolved iron + bicarbonate + oxygen + water—-> hematite + carbonic acid

Karst topography Is characterized by...

1) Sinkholes: surface depressions formed by the collapse of caves or other underground void spaces 2) Springs: places where water flows naturally from the ground (from spaces in the bedrock). 3) Disappearing streams: streams that terminate abruptly by seeping into the ground.

Physical (mechanical) weathering:

1- Natural zones of weakness (Joints, fractures....etc. 2- Exfoliation and spheroidal weathering 3- Root wedging 4- Frost wedging 5- Salt wedging 6- Differential thermal expansion of minerals creates stress in rocks e.g. Deserts 7-Other forces: rivers, glaciers, waves, burrowing creatures (from earthworms to gophers, and HUMANS (in the past century, perhaps the most energetic agents of physical weathering on the planet.

Chemical effects of chemical weathering on silicates:

1- it leaches, dissolves away, cations and silica 2-it hydrates, or adds water to, the minerals 3- makes the solutions less acidic

Factors that control the weathering of rocks are:

1-The properties of the parent rock 2-The climate 3-The presence or absence of soil 4-The length of exposure

Karst topography is most strongly advanced in regions with:

1-high-rainfall climate, with abundant vegetation 2-extensively jointed limestone formations 3-appreciable hydraulic gradient.

Rivers:

= The energy of the river directly affects what is being moved = Rivers in flood stage have greater energy, can move larger pieces with greater force. Therefore the potential for mechanical weathering is greater = Moving sand and silt acts like sandpaper on the larger rocks in the river bed and on each other. = Rocks hitting other rocks can break, making smaller pieces which can then become part of the suspended load.

Glaciers:

A glacier is nothing more than a frozen river still moving. It might only move an inch or two per year, but it still moves and this ice will erode the ground and rock below it faster and more aggressively than if it was just water. Its because glaciers also pick up and move the rocks that they run over and this gravel (chunks of rocks) can carve mountains down and cut valleys miles deep.

1-Dissolution:

A small proportion of carbon dioxide gas molecules (CO2) in air dissolves in rain droplets to form carbonic acid molecules (H2CO3). Formation of carbonic acid: Carbon dioxide (CO2) + water (H2O)= carbonic acid (H2CO3) Breakdown of calcite by carbonic acid into two water-soluble ions: CaCO3 + H2CO3 = Ca 2+ + 2HCO3- (Calcite + carbonic acid = Calcium ion + bicarbonate ion)

acid rock drainage (ARD)

A special type of oxidation takes place in areas where the rocks have elevated levels of sulfide minerals, especially pyrite (FeS2). Pyrite reacts with water and oxygen to form sulfuric acid, as follows: 2FeS2 + 7O2 +2H2O —-> 2Fe2+ H2SO4+ 2H pyrite + oxygen + water —-> iron ions + sulfuric acid + hydrogen ions -the runoff from these areas is called ARD

Why do some rocks weather more rapidly than others?

All rocks weather, but the manner and rate of their weathering vary.

Weathering:

General process by which rocks are broken down at Earth's surface.

2- Hydrolysis

Hydrolysis reactions break down not only feldspars, but many other silicate minerals as well, amphiboles, pyroxenes, micas, and olivines.

5- Chemical weathering produced by organisms

Organisms play a major role in the chemical-weathering process. Certain types of microbes obtain their life energy from the chemical bonds in minerals.

oxidation:

Oxidation reactions in rocks transform iron-bearing minerals into a rusty-brown mixtures of various iron-oxide and iron-hydroxide minerals, such as hematite (Fe2O3), goethite (α-FeO(OH), and limonite (FeO(OH)·nH2O. The oxidation, or rusting, of iron, is a familiar process example of oxidation. Reactions such as these made the surface of Mars red.

Chemical Weathering of Silicates

Quartz: very stable Feldspars: form clay minerals Mafic minerals: decompose to oxides

(1)Dissolution

Some minerals dissolve completely, and their components go into solution.

Talus, an apron-shaped pile of fragmental rock, has accumulated at the base of these cliffs near Mt. Snowdon, Wales

Talus, an apron-shaped pile of fragmental rock, has accumulated at the base of these cliffs near Mt. Snowdon, Wales

Karst Topography-China

The South China Karst is so impressive and is such an important part of China that it is World Heritage listed. It covers half a million square kilometers (193,000 square miles) of southern and central China.

4-Hydration:

The absorption of water into the crystal structure of minerals, causes some minerals, such as certain types of clay, to expand.

rain + carbon dioxide in atmosphere = carbonic acid

The amount of CO2 in the air is enough to make only very weak carbonic acid, but there is typically much more CO2 in the soil, so water that percolates through the soil can become significantly more acidic.

Sinkholes:

a small, steep depression in the land surface above the cavernous limestone formation. In some places, dissolution may thin the roof of a limestone cave so much that it collapses suddenly. Sinkholes contribute to a distinctive form of topography known as Karst topography.

3- Oxidation

another very important chemical weathering process. The oxidation of the iron in a ferromagnesian silicate minerals (e.g. pyroxene, amphibole, olivine, biotite) starts with the dissolution of the iron. *Fe(II) =Fe+2 = ferrous iron=soluble, Fe(III)=Fe+3 = ferric iron=not soluble.

(chemical weathering) The kinds of changes that take place are highly specific to the mineral and the environmental conditions.

example: quartz, are virtually unaffected by chemical weathering, while others, like feldspar, are easily altered

Chemical Weathering:

results from chemical changes to minerals that become unstable when they are exposed to surface conditions.

Erosion:

set of processes that loosen and move soil and rock downhill or downwind.

Physical (mechanical) weathering :

takes place when solid rock becomes fragmented by physical processes that do not change its chemical composition.

Chemical weathering:

takes place when the minerals in a rock are chemically altered or dissolved

The important characteristics of surface conditions that lead to chemical weathering are...

the presence of water (in the air and on the ground surface), the abundance of oxygen, and the presence of carbon dioxide, which produces weak carbonic acid when combined with water. That process is fundamental to most chemical weathering