Physical Geology Introduction


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 

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 

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

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 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 

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 

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