Physical Geology Introduction


WEATHERING and EROSION

Changes at the Earthıs Surface



Physical - a reduction in grain size without an 
accompanying change in chemical composition




Chemical - chemical changes as Earth materials 
react at the surface in contact with a reactive 
atmosphere




Chemical Weathering



CO2 and Oxygen are two highly reactive 
components in the atmosphere



H2O + CO2 = H2CO3 [carbonic acid]



H2CO3= HCO3 + H+



pH is a measure of the concentration of the 
hydrogen ion - as pH goes up, the concentration of 
H+ goes down....pH=7 is neutral and pH = 6 is weak 
acid.


The worldıs oceans have a pH of about 8 - a weak 
base



Rainwater has a pH of about 6 - a weak acid
Some elements, like Fe can exist in more than one 
oxidation state....Fe metal, Fe2+ (has lost two     
electrons) and Fe3+ (has lost three electrons).



Some components (like Oxygen) are good 
oxidizing agents and remove electrons.



Other components produce a reducing 
environment and supply electrons



Clay Minerals	



Kaolinite - stable geologic environment ... all of the 
easily oxidized materials have been leached away.



Illite - like mica but some potassium has been 
leached away



Expandable clays - will accept water molecules into 
their structure




Organic matter (tissue, vegetation, etc.) 
breakdown rapidly if the environment is oxidizing.

To preserve organic matter (to eventually produce 
coal and hyrdocarbons) the environment must be 
reducing....an environment which does not have a 
strong oxidizing agent present



Some material, like calcium carbonate is soluble in 
a weak acid; others, like quartz are soluble in a 
base.



Stability of Earth Materials



Bowenıs Reaction Series is a good guide


High temperature minerals break down more 
rapidly than low temperature minerals


Thus, quartz is chemically stable at the Earthıs 
surface whereas olivine alters rapidly


With the exception of quartz, most common 
minerals react to form clay minerals




Physical Weathering


Changes in size but not in composition


Decrease in size is accompanied by an increase in 
surface area


Chemical reactions begin at surfaces - therefore, 
an increase in surface area will speed up chemical 
changes



Abrasion history - usually use quartz


Particle Sizes


> 2 mm : pebbles, boulders

1/16 to 2 mm : sand

1/256 to 1/512 mm : silt


< 1/512 mm : clay