Rock cycle

Metamorphic rock melts, crystallizes Under heat and pressure, igneous rock deforms and recrystallizes 

A part of the crust crucial for the existence of humans and most other nonaquatic life forms is the thin layer of weathered rock, partially decayed organic matter, air spaces, and water composing soU that supports plant life. Were Earth the size of a geography classroom globe, the average thickness of the soil layer on it would be only about the size of a human ceU The top layer of soil that is most productive of plants is topsoil, which is often only a few centimeters thick in many locations or even nonexistent where poor cultivation practices and adverse climatic conditions have led to its loss by wind and water erosion. The conservation of soil and enhancanent of soil productivity are key aspects of sustainability. Soil is discussed in detail in Chapter 10. 

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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... 

Kempe, S. (1979b). Carbon in the rock cycle. In "The Global Carbon Cycle" (B. Bolin, E. T. Degens, S. Kempe and P. Ketner, eds), pp. 343-377. Wiley, New York. 

Fig. 1.6. The rock cycle. Rocks are weathered to form sediment, which is then buried. After deep burial, the rocks undergo metamorphosis or melting, or both. Later they are deformed and uplifted into mountain chains, only to be weathered again and recycled. Some injection of rock from the upper mantle is irreversible, that is, non-cyclic. (Adapted from M.J. Pidwimy, www.geog.ouc.bc.ca/phys.geog/)...

The global rock cycle. Source-. After Bice, D. Exploring the Dynamics of Earth Systems ... 

Modeling Earth s Rock Cycle. http //www.carieton.edu/departments/geol/DaveSTELLA/ Rock%20Cycle/rock cycle.htm. 

Using the rock cycle as an example, we can compute the turnover time of marine sediments with respect to river input of solid particles from (1) the mass of solids in the marine sediment reservoir (1.0 x 10 g) and (2) the annual rate of river input of particles (1.4 X lO g/y). This yields a turnover time of (1.0 x 10 " g)/(14 x lO g/y) = 71 X lo y. On a global basis, riverine input is the major source of solids buried in marine sediments lesser inputs are contributed by atmospheric feUout, glacial ice debris, hydrothermal processes, and in situ production, primarily by marine plankton. As shown in Figure 1.2, sediments are removed from the ocean by deep burial into the seafloor. The resulting sedimentary rock is either uplifted onto land or subducted into the mantle so the ocean basins never fill up with sediment. As discussed in Chapter 21, if all of the fractional residence times of a substance are known, the sum of their reciprocals provides an estimate of the residence time (Equation 21.17). 

From the perspective of the global rock cycle (Figure 1.2), volcanic activity is the ultimate source of minerals comprising the crust. The crust is 27.7% by mass silicon and 46.6% oxygen, so it is not surprising that silicates are the dominant mineral type. Weathering of these minerals generates siliclastic particles. These are also referred to as detrital silicates. 

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

A second mechanism by which CO2 is regenerated as part of the crustal rock cycle is thought to occur imder high pressures and temperatures such as found in subduction zones and under thick sedimentary prisms in the continental rise. This decarbonation... 

Linkages between the global carbon and crustal rock cycles. 

As the rock cycle continues, the calcium silicate minerals are eventually uplifted onto land where they imdergo chemical weathering. This reaction involves acid hydrolysis driven by carbonic acid. The latter is derived from the dissolution of the magmatic CO2 in rainwater ... 

We cannot disagree with Stanley (2005) and Stanley et al. (2005) that our planet is a system of integrated components driven by the Earth s internal heat and external energy from the Sun. It is evident that geospheric processes like plate tectonics, volcanism, and the rock cycle are linked to the hydrosphere, atmosphere, climate system, and biosphere, and their interactions form the global processes that we observe. 

To begin the discussion, we will present briefly a view of the modern carbon cycle, with emphasis on processes, fluxes, reservoirs, and the "CO2 problem". In Chapter 4 we introduced this "problem" here it is developed further. We will then investigate the rock cycle and the sedimentary cycles of those elements most intimately involved with carbon. Weathering processes and source minerals, basalt-seawater reactions, and present-day sinks and oceanic balances of Ca, Mg, and C will be emphasized. The modern cycles of organic carbon, phosphorus, nitrogen, sulfur, and strontium are presented, and in Chapter 10 linked to those of Ca, Mg, and inorganic C. In conclusion in Chapter 10, aspects of the historical geochemistry of the carbon cycle are discussed, and tied to the evolution of Earth s surface environment. 

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... 

Figure 10.1. A generalized diagram for the steady-state rock cycle. Sediments, S, and continental crystalline crust, C, masses are in units of metric tons. Ss, Cs, Sc, Cc, and M are fluxes in units of 109 tons y-l due to erosion of sediments, metamorphism, erosion of crystalline rocks, recycling of crystalline rocks (resetting of ages during tectogenesis), and cycling of oceanic crust, respectively. Total sedimentation rate is 9 x 109 tons y-l. (After Gregor, 1988.)...

Mackenzie F.T. and Pigott J.P. (1981) Tectonic controls of Phanerozoic sedimentary rock cycling. J. Geol. Soc. London 138, 183-196. 

Today, basin-scale mass transfer of some materials (e.g., helium, water, and petroleum) is unquestioned (e.g.. Hunt, 1996). OAer materials (e.g., titanium and the REEs) are sufficiently mobile to appear within authigenic precipitates, but are likely to be immobile on the scale of a hand specimen. Mobilities of the major elements that make up sandstones and shales (silicon, aluminum, calcium, sodium, potassium) remain controversial. Conflicting notions about processes in rock suites across the wide range of burial conditions and alteration show that fundamental questions remain unanswered about the nature of the volumetrically significant processes within a major segment of the rock cycle. It is very likely that something is wrong, or at least inadequate. 

A transition between the completion of weathering and the advent of reverse weathering must occur in the crust in order to maintain the acid balance within the rock cycle (e.g., Urey, 1956 Krauskopf, 1979) and to maintain levels of atmospheric and oceanic CO2 within observed secular ranges (e.g., Berner et al, 1983 Berner, 1991). Early work on reverse weathering investigated the degree to which early marine diagenesis... 

Certainly, CO2 evolved during late diagenesis must ultimately return to the atmosphere/ocean. It also seems clear that transport of major components such as silicon and potassium between sandstones and shales at a scale of a few meters is required and can perhaps be accomplished by diffusion (Thyne et ai, 2001). New data, especially for shales, must be obtained before simultaneous quantitative balances can be proposed for the reactions in Table 1. The speciation of aluminum in pore fluids, the initial and final quantities of the reactants and products in both sandstones and shales, and the precise volumes of sandstones and shales in the sequences in question are key data needed to ascertain the scale of mobihty for the major elements in late diagenesis. Our abihty to answer basic questions about the rock cycle falls short, in large part, for lack of information about the major mineral components of shale, the most common type of sedimentary rock. 

The rock reservoirs on the modem Earth show a very narrow range of Mn/Fe ratios, ranging only from 0.016 to 0.019, which demonstrates how similar the two elements are in the normal terrestrial rock cycle. Carbonaceous chondrites, which are meteorites that presumably represent the primordial composition of the Earth as a whole, are enriched in iron relative to manganese compared to the Earth s cmst and mantle. This difference reflects the concentration of metallic iron, but not of manganese, in the Earth s core. Another variation in composition from the normal cmstal value of 0.017 is seen in Archean cmst, which averages 0.023, a value that is higher than any common igneous rocks. 

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