Soils and Geologic Time

1
Soils and Geologic Time

• soil f (time, )
• Soil formation can be a slow process
• Not always observable on human time scale
• Need appreciation of geologic time scale

2
Soil Age and Geomorphic Surfaces

• Soil age is dictated by age of geomorphic
  surface
• - Erosional
• Constructional
• Geological maps
• Soil f (age of geomorphic surface, )

3
On-Going Erosional Geomorphic Surfaces Age
Residence Time soil (kg or cm)/soil input or
loss (kg or cm) soil / soil production rate
4
FROG HOLLOW, AUSTRALIA
5
EROSIONAL GEOMORPHIC SURFACES Soil Thickness
Hillslope Curvature
6
SOME GEOMORPHIC SURFACES NO LONGER EXPERIENCE
SIGNIFICANT EROSION OR DEPOSTION Soil Age
Elapsed Time Since Erosion/Deposition Stopped
Terrace ages range from 102 to gt 1.5 Ma
7
Relative Geological Time

• One of great intellectual developments of last
  millenium
• 4 Eras of time related to major biological events
  or changes
• - Precambrian
• Neary 4 billion years of time
• Evolution of bacteria and simple forms of life
  that still dominate our planet
• Paleozoic
• Cambrian explosion of life
• Evolution of land plants
• Ended with large extinction
• Mesozoic
• Age of dinasaurs
• Ended with extinction
• Cenozoic
• Age of mammals
• Tertiary vs. Quaternary
• Now experiencing great extinction event

8
Geomophic Surfaces and Soil Age

• Earths surface is very dynamic (on geological
  time scale)
• Much, or most, of earths surface has been
  altered inQuaternary period, and much of earth
  surface is Holocene in age
• Glaciation
• Loess deposition
• Fluvial deposition
• Erosion
• Pedology is therefore greatly concerned with
  Pleistocene and Holocene epochs

9
Relative Geological Time Scale
10
The Significance of Humans in Relative Time
11
Numerical Geological Time

• Truly developed in 20th century with advances in
  chemistry and the devolopment of radioactive
  clocks
• Variety of clocks continues to grow and is now
  especially useful in dating geomorphic surfaces

12
Soils and the Recognition of the Immensity of
Geological Time

• Jame Hutton and his paradox of the soil

13
Huttons Paradox

• Background of Hutton
• Viewed as originator of modern geology
• Yet he a unlikely candidate conventional
  Christian, gentleman farmer
• The Paradox
• World is adapted to to the purpose of man, which
  must include soils
• Hutton realized soil formation requires
  destruction of rocks, lowering of land surface,
  and ultimate loss of land fertilty
• How can a well balanced earth have both soil
  and denudation? Hutton recognized that
  regenerative forces of uplift and volanism are
  required
• The slowness of this process, combined with rocks
  in every stage of the cycle invked enormous
  magnitudes of time

14
Geological Unconformity that Contributed to
Huttons Recognition of Geological Time

• Hutton If the succession of worlds is
  established in the system of naturethe result of
  our present enquiry is that we find no vestige of
  a beginning, no prospect of an end.

John Playfair We felt ourselves carried back to
the time when the schitus on which we stodd was
yeat at the bottom of the seaThe mind seemed to
grow giddy by looking so far back into the abyss
of time.