...g inland seas, lakes, rivers, and underground aters. The average depth of the oceans is 3794 m 12,447 ft, more than five times the average height of the continents. The mass of the oceans is approximately 1.35 quintillion 1.35 1018 metric tons, or about 14400 of the total mass of the earth.The rocks of the lithosphere have an average density of 2.7 and are almost entirely made up of 11 elements, hich together account for about 99.5 percent of its mass. The most abundant is oxygen about 46.60 percent of the total, folloed by silicon about 27.72 percent, aluminum 8.13 percent, iron 5.0 percent, calcium 3.63 percent, sodium 2.83 percent, potassium 2.59 percent, magnesium 2.09 percent and titanium, hydrogen, and phosphorus totaling less than 1 percent. In addition, 11 other elements are present in trace amounts of 0.1 to 0.02 percent. These elements, in order of abundance, are carbon, manganese, sulfur, barium, chlorine, chromium, fluorine, zirconium, nickel, strontium, and vanadium. The elements are present in the lithosphere almost entirely in the form of compounds rather than in their free state. These compounds exist almost entirely in the crystalline state, so they are, by definition, minerals.The lithosphere comprises to shellsthe crust and upper mantlethat are divided into a dozen or so rigid tectonic plates see Plate Tectonics. The crust itself is divided in to. The sialic or upper crust, of hich the continents consist, is made up of igneous and sedimentary rocks hose average chemical composition is similar to that of granite and hose density is about 2.7. The simatic or loer crust, hich forms the floors of the ocean basins, is made of darker, heavier igneous rocks such as gabbro and basalt, ith an average density of about 3.The lithosphere also includes the upper mantle. Rocks at these depths have a density of about 3.3. The upper mantle is separated from the crust above by a seismic discontinuity, called the Moho, and from the loer mantle belo by a zone of eakness knon as the asthenosphere. Shearing of the plastic, partially molten rocks of the asthenosphere, 100 km 60 mi thick, enables the continents to drift across the earths surface and oceans to open and close.The dense, heavy interior of the earth is divided into a thick shell, the mantle, surrounding an innermost sphere, the core. The mantle extends from the base of the crust to a depth of about 2900 km 1800 mi. Except for the zone knon as the asthenosphere, it is solid, and its density, increasing ith depth, ranges from 3.3 to 6. The upper mantle is composed of iron and magnesium silicates, as typified by the mineral olivine. The loer part may consist of a mixture of oxides of magnesium, silicon, and iron.Seismological research has shon that the core has an outer shell about 2225 km 1380 mi thick ith an average density of 10. This shell is probably rigid, and studies sho that its outer surface has depressions and peaks, the latter forming here arm material rises. In contrast, the inner core, hich has a radius of about 1275 km 795 mi, is solid. Both core layers are thought to consist largely of iron, ith a small percentage of nickel and other elements. Temperatures in the inner core may be as high as 6650C 12,000F, and the average density is estimated to be 13.Forest Conservation Forests provide many social, economic, and environmental benefits. In addition to timber and paper products, forests provide ildlife habitat and recreational opportunities, prevent soil erosion and flooding, help provide clean air and ater, and contain tremendous biodiversity. Forests are also an important defense against global climate change. Through the process of photosynthesis, forests produce life-giving oxygen and consume huge amounts of carbon dioxide, the atmospheric chemical most responsible for global arming. By decreasing the amount of carbon dioxide in the atmosphere, forests may reduce the effects of global arming.Hoever, huge areas of the richest forests in the orld have been cleared for ood fuel, timber products, agriculture, and livestock. These forests are rapidly disappearing. The tropical rainforests of the Brazilian Amazon River basin ere cut don at an estimated rate of 50,000 sq km 20,000 sq mi per year in the late 1980s. The countries ith the most tropical forests tend to be developing and overpopulated nations in the southern hemisphere. Due to poor economies, people resort to clearing the forest and planting crops in order to survive. hile there have been effective efforts to stop deforestation directly through boycotts of multinational corporations responsible for exploitative logging, the most effective conservation policies in these countries have been efforts to relieve poverty and expand access to education and health care.In the United States and Canada, forests are threatened by extensive logging, called clear-cutting, hich destroys plant and animal habitat and leaves the landscape bare and unproductive if not properly reforested. Small pockets of ancient forests from 200 to 1200 years old still exist but are threatened by logging interests. Until the 1990s, the U.S. Forest Service as directed by Congress to maximize the harvest of timber in order to provide jobs. In the late 1980s and early 1990s, hoever, environmentalists sued the government for violating the National Environmental Policy Act NEPA, and heavy logging as deemed nonsustainable. As a result, the timber harvest as reduced and foresters ere directed to follo a more sustainable policy called ecosystem management. This policy required foresters to focus on conserving natural habitats rather than maximizing tree harvest. Despite this change, many ancient forests remain unprotected.ater Conservation Clean freshater resources are essential for drinking, bathing, cooking, irrigation, industry, and for plant and animal survival. Unfortunately, the global supply of freshater is distributed unevenly. Chronic ater shortages exist in most of Africa and drought is common over much of the globe. The sources of most freshater supplies, groundater ater located belo the soil surface reservoirs and rivers, are under severe and increasing environmental stress because of overuse, ater pollution, and ecosystem degradation. Over 95 percent of urban seage in developing countries is discharged untreated into surface aters such as rivers and harbors.About 65 percent of the global freshater supply is used in agriculture and 25 percent is used in industry. Freshater conservation therefore requires a reduction in asteful practices like inefficient irrigation, reforms in agriculture and industry, and strict pollution controls orldide.In addition, ater supplies can be increased through effective management of atersheds areas that drain into one shared ateray. By restoring natural vegetation to forests or fields, communities can increase the storage and filtering capacity of these atersheds and minimize asteful flooding and erosion. Restoration and protection of et...
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