14_ErosionofContinentalSurfaces

14_ErosionofContinentalSurfaces - Soils erode: ESM 203:...

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1 ESM 203: Erosion of Continental Surfaces Fall 2007 Soils erode: Weathering has severely weakened the rock material as the soil i s produced Soils are granular, mobile, and subject to gravity on hillslopes They can be moved by several processes They enter rivers and are transported long distances downstream The conditions that mobilize soils (erosion) can be strongly affected by management They can also be natural (beyond human control) Fate of soil • The – existence – amount (depth) – physical, chemical, and biological) condition • of soil profiles are subject to changes in – environmental conditions – land management (including socioeconomic conditions --- rapacity, poverty, disease, efuel policy, etc) Soil profiles are differentiated into horizons that have various functions (root space, water and nutrient storage) that sustain the biosphere,including us A p : Decaying plant matter A o : Mineral horizon with some organic matter A 1 : Leached – most organic and clay and dissolved material removed B: Accumulation of clays, oxides, and solutes leached from upper horizons C: Unconsolidated, earthy, disturbed but little or no bioturbation D: Parent material with little or no weathering Chemicals are migrating through the soil as solids (e.g. C&N-rich humus), and solutes (e.g. nitrates, cations) That migration and storage processes can be altered to the detriment of soil functions (see ESM 202, while we retreat to the safety of physical processes)
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2 Profile of soil and weathering bedrock Soil depth is the product of the mass balance of soil formation and removal over time ( T ) Primary rock Soil Erosion (E) Weathering (W) z W and E are expressed as rates per unit area (kg m -2 yr -1 ) [ ] dt E W ? 1 z T 0 b ? = -- [ ] dt T 0 means “history” Mass balance of soil formation over time A ‘wave’ of erosion chases the ‘wave’ of decomposition into the bedrock Reflect on a three cases: (1) Craton in wet tropical climate (Amazon basin, W Kenya, Australia): – Stable craton, low gradients – thick forest vegetation (erosion rate (E) low) – high T, P, primary production, rate of weathering (W). ‘ – Long time. – Result is old, deep soil, leached of nutrients by high rainfall and acid production from vegetation.
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3 Mass balance of soil formation over time A ‘wave’ of erosion chases the ‘wave’ of decomposition into the bedrock Reflect on a three cases: (2) Mountain ranges (Utah Canyonlands, Sierra Nevada, So. Alps of NZ): – active tectonics, steep landscape (+/-) thin vegetation, high erosion rate (E) – weathering (W) slow (dry or
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14_ErosionofContinentalSurfaces - Soils erode: ESM 203:...

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