Earth's crust and mineral resources
The Earth's crust is the outermost layer of the planet, characterized by its lower density compared to the underlying mantle. It consists of two distinct types: continental and oceanic crust, which differ in thickness, density, composition, age, and formation processes. Continental crust, which makes up around 40% of the Earth's surface, is generally thicker and less dense, while oceanic crust, which covers about 60% of the surface, is thinner and denser. Both types of crust are crucial for the formation and storage of nonrenewable natural resources such as metallic ores and hydrocarbons.
Metallic ores, largely generated at oceanic crust locations, are predominantly extracted from the continental crust. Hydrocarbon deposits are mainly found in sedimentary basins on continental crust or nearby continental margins. The age and structure of these crustal regions significantly influence the distribution and abundance of mineral resources. As demand for these resources grows, technology plays a vital role in exploration efforts across the globe, although concerns about resource depletion loom large. Future resource frontiers include deep-sea mining, deeper oil drilling, and harnessing geothermal energy, reflecting ongoing efforts to tap into Earth's complex mineral wealth.
Earth's crust and mineral resources
The earth’s crust is the outer hard layer of the planet. The crust overlies the Earth’s mantle and is separated from it by the Mohorovičič discontinuity, or Moho. There are two great classes of crust on Earth, oceanic and continental, which differ in thickness, composition, density, age, mode of formation, and significance for mineral resources.
Background
The earth consists of a nested set of spheres of different composition and of decreasing density with distance from the center of the Earth. The crust is the outermost and lowest-density hard shell, significantly less dense (2.7 to 3.0 grams per cubic centimeter) than the underlying mantle (3.3 grams per cubic centimeter). The earth’s two distinct types of crust—continental and oceanic—differ in five fundamental aspects: thickness, density, composition, age, and mode of formation.
Continental and Oceanic Crust
Continental crust is generally found beneath the exposed parts of the Earth’s surface known as continents. In addition, continental crust is submerged and makes up the continental shelves and submerged continental platforms. Correspondingly, a larger proportion of the Earth’s surface is composed of continental crust (40 percent) than is exposed above sea level as continents (25 percent). Oceanic crust makes up the floor of the oceans; in rare cases it rises above sea level, such as in Iceland and Ethiopia. Our store of nonrenewable natural resources is produced and kept in the crust. hydrothermal systems associated with oceanic crust formation at mid-ocean ridges produce metal deposits. Nearly all economic ore deposits are extracted from the continental crust. Basins in the continental crust and along the continental margins are the principal sites for the formation and storage of oil and gas deposits.
Typical continental crust is about 40 kilometers thick, has a density of about 2.7 grams per cubic centimeter, and has a bulk composition similar to the volcanic rock andesite; it is about 60 percent silicon dioxide (SiO2). Continental crust as old as 4 billion years has been found, and 2.5 billion-year-old continental crust is common. The earth is about 4.5 billion years old, and continental crust from the Earth’s first 500 million years has not been preserved. This contrasts with the situation for Earth’s moon, where the lunar highlands preserve crust that formed shortly after the moon itself. Oceanic crust is about 6 kilometers thick, has a density of about 3.0 grams per cubic centimeter, and has a bulk composition similar to the volcanic rockbasalt (about 50 percent SiO2). Although ophiolites may be much older, the oldest in situ oceanic crust is about 170 million years old.
The large difference in age between oceanic and continental crust reflects the greater density of the former, which allows it to slide back into the mantle along subduction zones. In contrast, buoyant continental crust is difficult to subduct. The formation of oceanic and continental crusts is fundamentally different as well: Oceanic crust forms by seafloor spreading at mid-ocean ridges, whereas continental crust forms at island arcs lying above subduction zones (such as Japan or the Mariana Islands in the western Pacific). Although the area of oceanic crust is much larger than that of continental crust (60 percent versus 40 percent of the Earth’s surface), the volume of continental crust is much larger than that of oceanic crust (80 percent versus 20 percent).
Metal and Hydrocarbon Deposits
The two types of crust play different roles in the formation of nonrenewable natural resources such as metallic ores and hydrocarbons. Metallic ores are predominantly produced at divergent or convergent plate boundaries—that is, where oceanic crust is either produced or destroyed. Vast deposits of manganese and cobalt exist on the deep-sea floor in the form of manganese nodules. Hydrocarbon deposits form principally in basins on continental crust or beneath continental margins, at the boundary between oceanic and continental crust. The configuration of continents may also be important for controlling oil and gas deposits, because it can cause the formation of restricted basins where oxygen-poor waters allow organic matter to be preserved and buried. The relatively thin sedimentary sequences typically deposited on oceanic crust are not conducive to formation and preservation of hydrocarbon deposits.
The distribution of mineral and hydrocarbon resources is strongly controlled by the age of the crust and the sedimentary basins that these harbor. In spite of the fact that the oceanic crust is the principal factory for generating ore deposits, a minuscule proportion of these are presently exploited, largely for economic reasons. Because of its age and mode of formation, the continental crust acts as a warehouse for ore deposits produced over Earth’s history, especially those deposits produced at convergent plate boundaries. Particularly rich ores are preserved in crust produced in the first 2 billion years of Earth history, and those nations which have large tracts of such ancient crust (among them are Australia, Canada, Russia, and South Africa) are blessed with especially rich metal deposits.
Resource Frontiers
A wide range of mineral and hydrocarbon resources are sought on all continents except Antarctica. This search benefits increasingly from abundant technological resources, including satellite remote sensing, geophysical surveys, geochemical studies, and traditional field mapping, and from the tremendous increase in computing power available to process large and complex data sets. These nonrenewable resources are likely to be depleted in the future, leading to a rise in prices that will reward exploitation of “frontier” deposits. Resource frontiers pertaining to the Earth’s crust include mining and drilling for oil deeper below the continental surface, drilling for oil in deeper water offshore, the mining of deep-sea resources, and exploiting geothermal and hydrothermal resources for energy, including the tremendous heat energy stored in the deep continental crust and vented from hydrothermal sites along the midocean ridges.
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