Monday, 19 September 2011

preservation of fossils

fossils are remains of living organisms, they are the hard parts of whole or fragmented organisms.

Factors that affect fossilisation: 

  • Original composition- many fossils made of made of calcite or aragonite which can be alterd easily. Hard parts made of silica  may be preserved unaltered.
  • Energy levals - High energy produces lots of fragments; low energy produces more complete fossils.
  • Transport distance - fossils are fragmented during transport due to abrasion
  • Rapidity of burial  - faster means more chance of whole body preserved as scavengers dont eat them.
  • Amount of oxygen - Accelerates breakdown of the organism due to bacterial decay
  • size of sediment - Fine sediment preserves organisms better than coarse sediment. only poor-quality  fossils can be found in coarse sandstone or gravel 
  • Diagenesis - These are the changes within sediments after burial. The composition and acidity of the percolating groundwater is important, as it may dissolve or replace the fossil with another mineral.   

Types or preservation   

Replacement -   Replacement occurs when original material is dissolved atom by atom and substituted with another mineral.

Silicification -  This occurs when percolating groundwater rich in silica dioxide moves through the rock. Petrified wood is a common example. 

Pyritisation -  this is the replacement of original material by iron pyrites. took place when the enviroment was devoid of oxygen.

Carbonisation - This process occurs during burial as the overlying mass of rocks increases the pressure and temperature. this allows volatiles or gases within the organic material to be driven off .

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