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Earth Materials and Health : Earth Processes and Human Physiology
Except for radiant energy from the sun, the resources necessary for sustaining life are derived chiefly from the near-surface portions of the land, sea, and air. Intensive utilization of earth materials has enhanced the quality of human life, especially in the developed nations. However, natural background properties and earth processes such as volcanic eruptions, as well as human activities involving the extraction, refining, and manufacturing of mineral commodities, have led to unwanted side effects such as environmental degradation and health hazards. Among the latter are airborne dusts and gases, chemical pollutants in agricultural, industrial, and residential waters, and toxic chemical species in foodstuffs and manufactured products. Of course, at appropriate levels of ingestion and assimilation, most earth materials are necessary for life, but underdoses and overdoses have adverse effects on human health and aging.
Although the environmental concentration of a substance is important and relatively easy to measure, its specific chemical form (a function of the biogeochemical environment, complex species interactions, Eh, and pH) determines the substance’s reactivity and therefore its bioaccessibility. In the case of earth materials, specific mineralogical characteristics (e. g., mineralogy, grain size) must also be considered together with these
chemical factors when assessing bioaccessibility. Thus, a number of analytical easurements are required to accurately assess the bioavailability of a naturally occurring chemical and mineralogical species. For most, an optimal dose range enhances health, whereas too little (deficiency) or too much (toxicity) have adverse impacts. Because the bioavailabilities of a spectrum of earth materials present in the environment constitute critical variables that influence human health - particularly where regional and local ‘hotspots’ of earth material deficiency or toxicity occur - The bioavailabilities of earth materials must be quantified by collaborative, integrated geological and biomedical research. To understand the physiological responses of the human body to the ingestion and assimilation of earth materials, this chapter begins by briefly describing the dynamic.
geological processes responsible for the areal disposition of earth materials in the near-surface environment, with particular attention to soil characteristics. This is followed by a brief description of those aspects of human physiology that are - through their responses to bioaccessible nutrients and hazardous materials - directly responsive to the biogeochemical environment.
EARTH PROCESSES
The near-surface portions of the planet and their complex couplings with - and feedbacks from - the atmosphere, hydrosphere, and biosphere make up the interactive earth systems so crucial for life. In turn, these dynamic systems are a reflection of the origin and geological evolution of the earth in the context of solar system formation. The following brief review of earth’s deep-seated and surficial processes provides the physical context for the public health component of human interactions with the earth.
Planetary Architecture and Crustal Dynamics
The solid earth consists of a series of nested shells. The outermost thin skin, or crust, overlies a magnesium silicate-rich mantle, the largest mass of the planet. Beneath the mantle is the earth’s iron-nickel core. The terrestrial surface is unique among the planets of our solar system, possessing an atmosphere, global oceans, and both continents and ocean basins. Incoming sunlight powers oceanic-atmospheric circulation. Solar energy absorbance and transfer mechanisms are responsible for the terrestrial climate and its variations, as well as for cyclonic storms and coastal flooding. In the solid portions of the planet, the escape of buried heat through mantle flow has produced the earth’s crust, as well as energy and mineral deposits and all terrestrial substances necessary for life in the biosphere.
Although imperceptible to humans without geophysical monitoring, continuous differential vertical and horizontal motions characterize the earth’s crust. This remarkable mobility explains the growth and persistence of long-lived, high-standing continents and the relative youth of low-lying ocean basins, although the former are being planed down by erosion and the latter are being filled through sedimentary deposition. The earth’s surface is continuously being reworked, and a dynamic equilibrium has been established between competing agents of crustal erosion and deposition (external processes) versus crustal construction (an internal process). Crustal deformation, a consequence of mantle dynamics, is the ultimate cause of many geological hazards, including earthquakes and
tsunamis, volcanic eruptions, and landslides. In addition to the direct fatalities and injuries, natural catastrophes result in the displacement of surviving populations into unhealthy environments where communicable diseases can -and often do-spread widely.
Plate Tectonics - Origins of Continental and Ocean Crust
Scientists have studied the on-land geology of the earth for more than two centuries, and much is known concerning the diverse origins of the continental crust, its structure, and constituent rocks and minerals. Within the past 35 years, marine research has elucidated the bathymetry, structure, and physicochemical nature of the oceanic crust, and as a result we have a considerably improved appreciation of the manner in which various parts of the earth have evolved with time. A startling product of this work was the realization that, beneath the relatively stiff outer rind of the planet (the lithosphere), portions of the more ductile mantle (the asthenosphere) are slowly flowing. Both continental and oceanic crusts form only the uppermost, near-surface layers of great lithospheric plates; differential motions of these plates - plate tectonics - are coupled to the circulation of the underlying asthenosphere on which they rest. The eastern and western hemispheric continents are presently drifting apart across the Atlantic Ocean and have been doing so for more than 120 million to 190 million years. Locally, continental fragments came together in the past and others are presently colliding, especially around the Pacific Rim.
Mid-ocean ridges represent the near-surface expression of hot, slowly ascending mantle currents with velocities on the order of a few centimeters per year. Whether this upwelling is due to part of a convection cell that returns asthenosphere to shallower levels after it has been dragged to depths by a lithospheric plate sinking elsewhere, or is a consequence of deeply buried thermal anomalies that heat and buoy up the asthenosphere, is not known, but both processes probably occur to varying degrees. Approaching the seafloor, the rising mantle undergoes decompression and partial melting to generate basaltic liquid. The magma within the upwelling asthenosphere is less dense and thus even more buoyant. It rises toward the interface with seawater and solidifies to form the oceanic crust, capping the stiffer, less buoyant mantle. The mid-oceanic ridges -
divergent plate boundaries? are spreading centers where the cooling lithospheric plates that overlie the ductily flowing mantle currents are transported at right angles away from the ridge.
As it moves away from the ridge axis, the cooling oceanic lithosphere gradually thickens at the expense of the upper part of the asthenosphere. Heat is continuously lost, so the lithosphere-asthenosphere boundary (solid, rigid mantle above; incipiently molten, ductile mantle below), which is very close to the sea bottom beneath the oceanic ridge, descends to greater water depths away from the spreading center because its overall density increases. Unlike light continental lithosphere floating on a denser mantle, the oceanic lithosphere has a slightly greater density than the asthenosphere below, and so the oceanic plate will sink back into the deep mantle where geometrically possible.
An oceanic plate moves away from the ridge axis until it reaches a convergent plate boundary. Here, one slab must return to the mantle to conserve volume - the process of subduction. A bathymetric low, or trench, marks the region where bending of the down-going oceanic slab is greatest. It is difficult for continental crust-capped lithosphere to sink because it is less dense than the mantle below; however, due to the descent of oceanic lithosphere, the dragging of a segment of continental crust into and down the inclined subduction zone occasionally takes place.
Production of new oceanic crust along submarine ridge systems results from this plastic flow of the deep earth, as does addition to- And deformation of - the continental crust in the vicinity of seismically and volcanically active continental margins and island arcs. Oceanic ridges are sited over upwelling mantle columns, whereas along subduction zones, lithospheric plates are descending beneath active continental margins and island arcs. In contrast to submarine ridges, however, continents are also typified by mountain belts that display evidence of great crustal shortening and thickening (e. g., the Appalachians, Himalayas, and Alps). These compressional mountains contain great tracts of preexisting layered rocks, now contorted into fault-bounded blocks of folded rock. Such collisional mountain belts mark the sites of present or ancient plate boundaries.
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Earth Materials and Health : Earth Processes and Human Physiology
Except for radiant energy from the sun, the resources necessary for sustaining life are derived chiefly from the near-surface portions of the land, sea, and air. Intensive utilization of earth materials has enhanced the quality of human life, especially in the developed nations. However, natural background properties and earth processes such as volcanic eruptions, as well as human activities involving the extraction, refining, and manufacturing of mineral commodities, have led to unwanted side effects such as environmental degradation and health hazards. Among the latter are airborne dusts and gases, chemical pollutants in agricultural, industrial, and residential waters, and toxic chemical species in foodstuffs and manufactured products. Of course, at appropriate levels of ingestion and assimilation, most earth materials are necessary for life, but underdoses and overdoses have adverse effects on human health and aging.
Although the environmental concentration of a substance is important and relatively easy to measure, its specific chemical form (a function of the biogeochemical environment, complex species interactions, Eh, and pH) determines the substance’s reactivity and therefore its bioaccessibility. In the case of earth materials, specific mineralogical characteristics (e. g., mineralogy, grain size) must also be considered together with these
chemical factors when assessing bioaccessibility. Thus, a number of analytical easurements are required to accurately assess the bioavailability of a naturally occurring chemical and mineralogical species. For most, an optimal dose range enhances health, whereas too little (deficiency) or too much (toxicity) have adverse impacts. Because the bioavailabilities of a spectrum of earth materials present in the environment constitute critical variables that influence human health - particularly where regional and local ‘hotspots’ of earth material deficiency or toxicity occur - The bioavailabilities of earth materials must be quantified by collaborative, integrated geological and biomedical research. To understand the physiological responses of the human body to the ingestion and assimilation of earth materials, this chapter begins by briefly describing the dynamic.
geological processes responsible for the areal disposition of earth materials in the near-surface environment, with particular attention to soil characteristics. This is followed by a brief description of those aspects of human physiology that are - through their responses to bioaccessible nutrients and hazardous materials - directly responsive to the biogeochemical environment.
EARTH PROCESSES
The near-surface portions of the planet and their complex couplings with - and feedbacks from - the atmosphere, hydrosphere, and biosphere make up the interactive earth systems so crucial for life. In turn, these dynamic systems are a reflection of the origin and geological evolution of the earth in the context of solar system formation. The following brief review of earth’s deep-seated and surficial processes provides the physical context for the public health component of human interactions with the earth.
Planetary Architecture and Crustal Dynamics
The solid earth consists of a series of nested shells. The outermost thin skin, or crust, overlies a magnesium silicate-rich mantle, the largest mass of the planet. Beneath the mantle is the earth’s iron-nickel core. The terrestrial surface is unique among the planets of our solar system, possessing an atmosphere, global oceans, and both continents and ocean basins. Incoming sunlight powers oceanic-atmospheric circulation. Solar energy absorbance and transfer mechanisms are responsible for the terrestrial climate and its variations, as well as for cyclonic storms and coastal flooding. In the solid portions of the planet, the escape of buried heat through mantle flow has produced the earth’s crust, as well as energy and mineral deposits and all terrestrial substances necessary for life in the biosphere.
Although imperceptible to humans without geophysical monitoring, continuous differential vertical and horizontal motions characterize the earth’s crust. This remarkable mobility explains the growth and persistence of long-lived, high-standing continents and the relative youth of low-lying ocean basins, although the former are being planed down by erosion and the latter are being filled through sedimentary deposition. The earth’s surface is continuously being reworked, and a dynamic equilibrium has been established between competing agents of crustal erosion and deposition (external processes) versus crustal construction (an internal process). Crustal deformation, a consequence of mantle dynamics, is the ultimate cause of many geological hazards, including earthquakes and
tsunamis, volcanic eruptions, and landslides. In addition to the direct fatalities and injuries, natural catastrophes result in the displacement of surviving populations into unhealthy environments where communicable diseases can -and often do-spread widely.
Plate Tectonics - Origins of Continental and Ocean Crust
Scientists have studied the on-land geology of the earth for more than two centuries, and much is known concerning the diverse origins of the continental crust, its structure, and constituent rocks and minerals. Within the past 35 years, marine research has elucidated the bathymetry, structure, and physicochemical nature of the oceanic crust, and as a result we have a considerably improved appreciation of the manner in which various parts of the earth have evolved with time. A startling product of this work was the realization that, beneath the relatively stiff outer rind of the planet (the lithosphere), portions of the more ductile mantle (the asthenosphere) are slowly flowing. Both continental and oceanic crusts form only the uppermost, near-surface layers of great lithospheric plates; differential motions of these plates - plate tectonics - are coupled to the circulation of the underlying asthenosphere on which they rest. The eastern and western hemispheric continents are presently drifting apart across the Atlantic Ocean and have been doing so for more than 120 million to 190 million years. Locally, continental fragments came together in the past and others are presently colliding, especially around the Pacific Rim.
Mid-ocean ridges represent the near-surface expression of hot, slowly ascending mantle currents with velocities on the order of a few centimeters per year. Whether this upwelling is due to part of a convection cell that returns asthenosphere to shallower levels after it has been dragged to depths by a lithospheric plate sinking elsewhere, or is a consequence of deeply buried thermal anomalies that heat and buoy up the asthenosphere, is not known, but both processes probably occur to varying degrees. Approaching the seafloor, the rising mantle undergoes decompression and partial melting to generate basaltic liquid. The magma within the upwelling asthenosphere is less dense and thus even more buoyant. It rises toward the interface with seawater and solidifies to form the oceanic crust, capping the stiffer, less buoyant mantle. The mid-oceanic ridges -
divergent plate boundaries? are spreading centers where the cooling lithospheric plates that overlie the ductily flowing mantle currents are transported at right angles away from the ridge.
As it moves away from the ridge axis, the cooling oceanic lithosphere gradually thickens at the expense of the upper part of the asthenosphere. Heat is continuously lost, so the lithosphere-asthenosphere boundary (solid, rigid mantle above; incipiently molten, ductile mantle below), which is very close to the sea bottom beneath the oceanic ridge, descends to greater water depths away from the spreading center because its overall density increases. Unlike light continental lithosphere floating on a denser mantle, the oceanic lithosphere has a slightly greater density than the asthenosphere below, and so the oceanic plate will sink back into the deep mantle where geometrically possible.
An oceanic plate moves away from the ridge axis until it reaches a convergent plate boundary. Here, one slab must return to the mantle to conserve volume - the process of subduction. A bathymetric low, or trench, marks the region where bending of the down-going oceanic slab is greatest. It is difficult for continental crust-capped lithosphere to sink because it is less dense than the mantle below; however, due to the descent of oceanic lithosphere, the dragging of a segment of continental crust into and down the inclined subduction zone occasionally takes place.
Production of new oceanic crust along submarine ridge systems results from this plastic flow of the deep earth, as does addition to- And deformation of - the continental crust in the vicinity of seismically and volcanically active continental margins and island arcs. Oceanic ridges are sited over upwelling mantle columns, whereas along subduction zones, lithospheric plates are descending beneath active continental margins and island arcs. In contrast to submarine ridges, however, continents are also typified by mountain belts that display evidence of great crustal shortening and thickening (e. g., the Appalachians, Himalayas, and Alps). These compressional mountains contain great tracts of preexisting layered rocks, now contorted into fault-bounded blocks of folded rock. Such collisional mountain belts mark the sites of present or ancient plate boundaries.
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