Soils and Geologic Time soil f time Soil

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: Age ~ Residence Time = soil (kg or cm)/soil input or loss (kg or cm) ≈ soil / soil production rate

FROG HOLLOW, AUSTRALIA

EROSIONAL GEOMORPHIC SURFACES Soil Thickness ~ Hillslope Curvature

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 in. Quaternary 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

The Significance of Humans in Relative Time

Numerical Geological Time • Truly developed in 20 th 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.