Rocks weathering/erosion

Figure 10.1 is a generalized model of the rock cycle. Both the sediments and the continental crust are recycled the former by processes of weathering, erosion, transportation, deposition, burial, uplift, and re-erosion and the latter by resetting... 

Pyroclastic ejected by volcanic emption Radiolarians a protozoon having a sihca skeleton Sedimentary processes whereby rocks (minerals) are formed from materials derived from other rocks by weathering, erosion, and transportation... 

Processes that produce sedimentary rocks include weathering, erosion, transportation, deposition, and diagenesis. Elemental fractionation during weathering is discussed in detail by Taylor and McLennan (1985) (see also Chapters... 

Weathering, erosion, transport, and deposition take material from the continental rocks to the sediments in the oceans. If there was no sink for these sediments, the ocean should fill up in less than 100 Ma. On the other hand, if there was no source for rocks on the continents, they should be degraded to ocean level in less than 50 Ma (Holland, 1978, p. 146). Neither of these figures make sense, and the reason is that they do not take continental plate tectonics into account. Sediments are metamorphosed and returned from the oceans back to the continents either by overthrusting (continental plate/con-tinental plate convergence) or subduction (continental plate/ocean plate convergence). In the former case we are dealing mainly with sediments from the continental shelf, and in the latter case... 

The focal areas of this book have relatively minor relationships to the primary formation of the Earth s crust which has caused a certain distribution of the chemical elements. They mainly deal with products of the alteration of the crust in geologic processes. We can presently still observe the weathering of solid rocks, the erosion of mountain ridges, and the transport of eroded materials as suspended and dissolved constituents in river and rain water, in ice and wind. In-situ weathering forms soils, and soils are the basis of food production for human nutrition. Therefore, soUs need special protection against the impact of toxic substances (see Part I, Chapters 4 and 5). 

The first step for students is to recognise that sedimentary rocks can only form from rocks which already exist and which are exposed to weathering at the Earth s surface. Only then will students be able to come to terms with the idea that weathered rock fragments are transported away from the parent rock and deposited as sediments on the sea bed. The timescale involved in weathering, erosion and sedimentation presents additional problems when students are trying to understand rock formation. 

Denudation is the lowering of the elevation of the Earth s surface by chemical weathering, erosion, and mass wasting. A model of chemical weathering would account for the rate of removal of material from soil and underlying rock by various dissolution processes. A first step toward validating such a model is to develop some independent estimates of the chemical denudation rate. 

A prolonged weathering, erosion and disintegration of rock materials over the surface of Earth produce clays. The constituent minerals of clays are highly stable in the surface environment and resistant to chemical weathering. Clays may also contain a non-mineral part, e.g. amorphases and organic matter. 

These may contain any of the heavy minerals, the species and proportions present depending on the nature of the rocks from which the sediments were derived, and on the ability of the minerals to withstand the processes of weathering, erosion, transport, deposition, burial, diagenesis, uplift, folding, and other tectonic effects and subsequent history up to the present time. These processes can have a considerable influence on the relative proportions of the heavy minerals, and the most resistant species such as zircon, tourmaline, and rutile may be the only survivors. 

In spite of low copper contents, massive horizontal development renders porphyry deposits amenable to large-scale production methods. Porphyry deposits are associated with igneous activity and intmsion of molten rocks into cooler parts of the earth s cmst, often in connection with the formation of mountains. Erosion of mountainous areas exposes these deposits to weathering, and, under the right conditions, enables the formation of oxidized or secondary copper deposits. Copper mines in the United States are Usted in Table 2. 

Soil is a key component of the rock cycle because weathering and soil formation processes transform rock into more readily erodible material. Rates of soil formation may even limit the overall erosion rate of a landscape. Erosion processes are also a key linkage in the rock cycle... 

Pavich, M. J. (1986). Processes and rates of saprolite production and erosion on a foliated granitic rock of the Virginia Piedmont. In "Rates of Chemical Weathering of Rocks and Minerals" (S. M. Coleman and D. P. Dethier, eds), pp. 551-590. Academic Press, New York. 

The most stable minerals are often physically eroded before they have a chance to chemically decompose. Minerals that decompose contribute to the dissolved load in rivers, and their solid chemical-weathering products contribute to the secondary minerals in the solid load. The secondary minerals and the more stable primary minerals are the most important constituents of clastic sedimentary rocks. Consequently, the secondary minerals of one cycle of erosion are... 

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