lecture 2

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


Soil Age and Geomorphic Surfaces

•Soil age is dictated by age of “geomorphic surface”
–- Erosional
–Constructional
• Geological maps
•Soil = f (age of geomorphic surface, …)


On-Going Erosional
Geomorphic Surfaces:
VERTICAL DISTANCE

Age ~ Residence Time
LANDSURFACE

net diffusion
in

soil

= soil (kg or cm)/soil
input or loss (kg or cm)

≈ soil / soil production
rate

net diffusion
out

soil production from
rock or sediment

bedrock/sediment
SOIL/ROCK INTERFACE

HORIZONTAL DISTANCE

∂h
∂φ
ρ s = −ρ r
− ρ s∇qs
123
∂t 1 2 ∂t
3 se dim ent
rate
of
soil
production

transport =
−K∇ 2 z

FROG HOLLOW, AUSTRALIA


EROSIONAL GEOMORPHIC SURFACES

Soil Thickness ~ Hillslope Curvature

Australia Hillslope Soil Residence Times: Heimsath et al. (2002)
Soil Depth (cm)
25
37
40
50
65

Erosion Rate (m Ma-1)
49.08
49.42
25.99
11.79
11.05

Residence Time (ka)
5094
7487
15391
42409
58824

SOME GEOMORPHIC SURFACES NO LONGER EXPERIENCE
SIGNIFICANT EROSION OR DEPOSTION

Soil Age = Elapsed Time Since Erosion/Deposition Stopped

Terrace ages range from
~ 102 to > 1.5 Ma


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


Geomophic Surfaces and Soil Age

•Earth’s surface is very dynamic (on geological time scale)
•Much, or most, of earth’s 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


Relative Geological Time Scale
Era

Period

Quaternary

Cenozoic

Mesozoic

Paleozoic
Precambrian

Tertiary
Cretaceous
Jurassic
Triassic
Permian

Pennsylvanian

Mississippian
Devonian
Silurian
Ordovician
Cambrian

Epoch

Initial Age
(106 yr B.P.)

Biotic
Events

Geologic Events

Holocene

0.01

world-wide interglacial

Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene

2
5
24
37
58
66
144
208
245
286
320

extinction of large mammals, spread of modern
humans
early Homo
earliest homonid fossils
expansion of grasslands
primitive horses and camels
early primates
extinction of dinosaurs, expansion of mammals

360
408
438
505
570
~3,800
~4,600

early flowering plants
first dinosaurs, early birds and mammals
coal-forming swamps diminish
coal-forming swamps abundant

first amphibians and reptiles
first forests
early land plants
invertibrates dominant, first fish
expansive diversification of multi-celled life
origin of life
formation of earth

world-wide glaciation
beginning of antarctic ice caps
Himalayas begin to form
begins with meteor impact
formation of Rockies
break-up of Pangaea
warm conditions, low seasonality

oldest known rocks (~3,960)


The Significance of
Humans in Relative
Time


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


Soils and the Recognition of
the Immensity of Geological
Time

•Jame Hutton and his paradox of the
soil


Hutton’s 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


Geological Unconformity
that Contributed to
Huttons Recognition of
Geological Time
Hutton: If the succession of worlds
is established in the system of
nature…the 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 sea…The
mind seemed to grow giddy by looking so far back into the abyss of
time.