Soils may be formed in place from rock or formed in weathered rock and minerals that have been transported from where the original rock occurred. Rocks Consist of mixtures of minerals. Igneous Metamorphic Sedimentary Igneous rocks are formed from molten magma and contain primary minerals. Sedimentary rocks are formed by deposition and cementation of weathered products. Metamorphic rocks are formed from igneous or sedimentary rocks by high pressure and temperature. Weathering Physical disintegration Chemical decomposition Physical disintegration causes decrease in size without appreciably altering composition. Differential stresses due to heating and cooling or expansion of ice break the rock. Abrasion due to water containing sediment or wind carrying debris is another type of physical weathering. Chemical decomposition and synthesis alter chemical composition. Four types of chemical weathering reactions are: hydrolysis, hydration, acid dissolution and redox (particularly, Fe2+ / Fe3+).
Five Factors of Soil Formation Parent material Climate Organisms Topography Time Soils defined -dynamic natural bodies having properties derived from the combined effect of climate and biotic activities, as modified by topography, acting on parent material over time. Parent Material Geologic material in which a soil forms. Residual Transported Colluvial Alluvial Marine Lacustrine Glacial Eolian Residual Develops in place from the underlying rock. If soil is young, properties tend to reflect effect of parent material. For example, Igneous and metamorphic rock, if Siliceous (granite and granite gneiss) acid and sandy Ferromagnesian (basalt and diorite) nonacid and clayey Sedimentary, if Limestone sand or clay impurities lead to sandy or clayey soil Sandstone shallow if SiO2 cements but deep if CaCO3 cements particles together sedentary Agent gravity water water water ice wind
Shale clay minerals in shale give rise to a clayey soil Colluvial Consists of coarse and stony debris detached from rocks and carried downslope by gravity. Alluvial These deposits occur as alluvial fans, flood plains and deltas. Alluvial fan occurs at the discharge of an upland stream into a broader valley below. Coarse textured material. Flood plains are adjacent to streams and rivers. During floods, coarse sediment is deposited nearest the existing channel and fine sediment further away, resulting in a natural levee. Changes in the course of the stream result in a complex spatial pattern of alternating coarse and fine sediments throughout the flood plain. If there is a change in grade, the stream may cut through existing deposits, thereby forming terraces. Complex soilscape on a flood plain.
A delta occurs at the mouth of river and marks the downstream extent of a flood plain. Marine Sediments Unconsolidated marine sediments deposited by streams emptying into oceans may undergo uplift. Common along Atlantic and Gulf coasts. Vary from sandy to clayey. Glacial Deposits From a series of glaciations during the Pleistocene epoch. Each advancing ice sheet accumulated a great mass of unconsolidated material which was deposited as glacial drift when the glacier melted and retreated. Material directly deposition from the ice is called glacial till and occurs in formations called moraines. Streams originating in a glacier transported sediment away and produced outwash plains. Where regional topography impounded glacial melt, lakes formed and lacustrine deposits accumulated. Deltas of coarser materials occur in what was the inflow region whereas finer materials were deposited further away.
Example Mississippi River natural levee showing different soils at different positions on the levee.
Leaching of soluble materials like CaCO3 increases with increasing precipitation. Movement of clay in soil profile increase with increasing precipitation. Silicate clay and Al and Fe oxide formation increase with increasing temperature. Generally, high rainfall and...
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