Crust volcanism

However, most of the sedimentary mass is of detrital origin. Nutrient elements (Si, P, V, N, and trace metals) are removed in the ocean as biological debris to sediments. The main sink of substances is by hydrothermal reactions (volcanic activity) at locations of seafloor spreading and circulation through the ocean crust. Volcanic activities in the oceans are extensive, and produce submarine lava flows which are unstable in seawater. The high temperature (200-400 °C) is not only important for basalt-seawater reactions but also triggers circulation. Subduction (see Chapters 2.2.1.1 and 2.6.4.3) transfers seawater and its constituents back to the magma. 

The outer shell of the earth, consisting of the upper mantle and the crust (Figure I4. lO), is formed of a number of rigid plates. These plates are 20 in number and are shown in Figure 14.1 I. Of these, six or seven are major plates, as can be seen in the map. The edges of these plates define their boundaries and the arrows indicate the direction of their movement. These plates contain the continents, oceans and mountains. They almost float on the partially molten rock and metal of the mantle. The outer shell, known as the lithosphere, is about 70 to 1,50 km thick. It has already moved great distances below the etirth s surface, ever since the earth was formed and is believed to be in slow and continuous motion all the time. The plates slide on the molten mantle and move about lO to 100 mm a year in the direction shown by the arrows. The movement of plates is believed to be the cause of continental drifts, the formation of ocean basins and mountains and also the consequent earthquakes and volcanic eruptions. 

The sediment reservoir (1) represents all phosphorus in particulate form on the Earth s crust that is (1) not in the upper 60 cm of the soil and (2) not mineable. This includes unconsolidated marine and fresh water sediments and all sedimentary, metamorphic and volcanic rocks. The reason for this choice of compartmentalization has already been discussed. In particulate form, P is not readily available for utilization by plants. The upper 60 cm of the soil system represents the portion of the particulate P that can be transported relatively quickly to other reservoirs or solubilized by biological uptake. The sediment reservoir, on the other hand, represents the particulate P that is transported primarily on geologic time scales. 

Volcanic activity has a significant effect on the mobilization of metals, particularly the more volatile ones, e.g., Pb, Cd, As, and FFg. Effects of volcanism are qualitatively different from those of the weathering and other near-surface mobilization processes mentioned above, in that volcanism transports materials from much deeper in the crust and may inject elements into the atmospheric reservoir. 

In contrast to the southern volcanic zone, Parinacota volcano lies on very thick continental crust (> 70 km) in the central volcanic zone of Chile. Bourdon et al. (2000a) showed that young Parinacota lavas encompass a wide range of U-Th disequilibria. excesses were attributed to fluid addition to the mantle wedge but °Th-excesses in lavas from the same volcano are more difficult to explain. The lavas with °Th-excesses also have low ( °Th/ Th) (< 0.6) characteristic of lower continental crust characterized by low Th/U and in their preferred model. Bourdon et al. (2000a) attributed the °Th-excesses to contamination by partial melts, formed in the presence of residual garnet, of old lower crustal materials. 

Scientists believe that the sulfur in Venus atmosphere came from volcanic eruptions. Earth has experienced its fair share of volcanic eruptions, too. However, the sulfur from early eruptions on Earth was incorporated into solid sulfur compounds. Indeed, sulfur is an important element found in many of the compounds that make up Earths crust. 

Sidney Fox and Kaoru Harada, in particular, used simulation experiments to show how volcanism may have been involved in the synthesis of prebiotic molecules. They heated a stream of gas (CH4, NH3 and H2O) to about 1,123 K (using a silicate contact) after cooling, they could detect glycine, alanine, p-alanine and aspartic acid (among others). This experiment was intended to simulate exhalation from the earth s crust, as in volcanoes (Fox and Harada, 1961 Harada and Fox, 1964). 

As is true today, most phosphate in the primordial crust must have been sequestered in nearly insoluble calcium phosphates and carbonates or in basalts, and only dissolved monomeric phosphate was produced by weathering. [201] However, the volatile polyphosphate P4O10 is known to be a component of volcanic gases. [205] This material originates from the polymerization of phosphate minerals in mag-... 

Figure 12. Depth profile of Li isotopic composition (a) and concentration (b) in drilled oceanic crust at ODP Sites 504B (open symbols) and 896A (filled symbols) off Costa Rica (Chan et al. 2002a). The transition zone exhibits mixing between hydrothermal fluids and seawater. Average oxygen isotopic (5 0) composition of bulk samples decreases with depth upper volcanic zone = +7.8, lower volcanic zone = +6.4, transition zone = +5.4, sheeted dikes = +4.3. However, despite many sheeted dike samples having 5 Li less than unaltered MORB, there is no simple relationship between concentration and Li isotopes.

Earth s crust is a source of particles produced as a consequence of weathering and volcanic activity. Weathering of continental rocks generates terrigenous particles that are carried into ocean via rivers, glaciers, and winds. As shown in Table 13.2, the most abundant mineral types are quartz, plagioclase, and clay minerals. The most abimdant... 

Weathered fragments of continental crust comprise the bulk of marine sediments. These particles are primarily detrital silicates, with clay minerals being the most abmidant mineral type. Clay minerals are transported into the ocean by river runoff, winds, and ice rafting. Some are authigenic, being produced on and in the seafloor as a consequence of volcanic activity, diagenesis and metagenesis. 

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. 

As shown in Figure 19.6, vent fields have been confirmed (or inferred) across depths ranging from 200 to 4300 m, with the most lying between 2200 and 2800 m. Very shallow systems are associated with volcanism that has built oceanic crust above sea level, such as near Iceland, the Hawaiian Islands, and the Azores. 

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