Geoscience Reference
In-Depth Information
Fig. 13.13
Vertisol's cracks
on the surface (Source:
Antonio Jordán, Spain)
years, Epoch for the range 10
7
-10
6
years, and Age for 10
6
years and less except
Holocene that started “only” 11.7 × 10
3
years before present.
We have seen that soil evolution was infl uenced by many factors, and among
them one or two were usually dominant. In the US Soil Taxonomy, the climate took
the dominant role in evolution of Alfi sols, Aridisols, Gelisols, Mollisols and Oxisols
and at least partly of Spodosols and Ultisols. We would like to mention here that
soils were earlier classifi ed using only two top taxons, climatic and aclimatic. This
oversimplifi ed scheme was abandoned many decades ago. Based on other soil-
forming factors, organisms played an important role upon soil evolution in Alfi sols,
Mollisols, and Spodosols. Parent material and topography also had dominance in
many orders.
Long before the shapes of present-day continents even existed, the climate, veg-
etation, and topography were all substantially and continually changing. Since they
are also soil-forming factors, the soils originating in previous geological eras
differed from our present-day soils. These old “granny” soils or paleosols are usu-
ally buried under younger sediments, or they are fossils within sedimentary
sequences. Some remnants of old soils are hidden from the changing climate and
vegetation below overhanging rock. Soils probably existed and changed in a very
primitive form starting already 3,400 myr (3.4 billion years) BP, judging according
to the existence of fi rst sedimentary rocks and some fossil biochemical marks. The
surface parts of igneous rocks were exposed to the infl uence of the atmosphere and
they weathered. This material, let us say a “primitive soil,” was eroded by rains and
transported to its fi nal destination - an ocean or a lake. There, muddy waters offered
building material of new rocks by sedimentation of sand or silt and clay. Finally, the
new, solid sedimentary rock was created by compression and cementation. Even
though the origin of the “primitive soil” precedes the evolution of sedimentary
rocks, we consider the fi rst appearance of sedimentary rocks as a well-working
