lecture 2

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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 ∂t se dim ent rate of soil production transport = −K∇ 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 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 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 [...]...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
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