Earth one of the planets in the solar system, the third in distance from the sun and the fifth largest of the planets in diameter. The mean distance of the earth from the sun is 149,503,000 km (92,897,000 mi). It is the only planet known to support life, although some of the other planets have atmospheres and contain water. 49122vrb31ytk6x COMPOSITION The rocks of the lithosphere have an average density of 2.7 and are almost entirely made up of 11 elements, which together account for about 99.5 percent of its mass. The most abundant is oxygen (about 46.60 percent of the total), followed 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 two shells—the crust and upper mantle—that are divided into a dozen or so rigid tectonic plates (see Plate Tectonics). The crust itself is divided in two. The sialic or upper crust, of which the continents consist, is made up of igneous and sedimentary rocks whose average chemical composition is similar to that of granite and whose density is about 2.7. The simatic or lower crust, which forms the floors of the ocean basins, is made of darker, heavier igneous rocks such as gabbro and basalt, with an average density of about 3. rt122v9431yttk 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 lower mantle below by a zone of weakness known 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 earth’s 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 known as the asthenosphere, it is solid, and its density, increasing with depth, ranges from 3.3 to 6. The upper mantle is composed of iron and magnesium silicates, as typified by the mineral olivine. The lower part may consist of a mixture of oxides of magnesium, silicon, and iron. Seismological research has shown that the core has an outer shell about 2225 km (1380 mi) thick with an average density of 10. This shell is probably rigid, and studies show that its outer surface has depressions and peaks, the latter forming where warm material rises. In contrast, the inner core, which has a radius of about 1275 km (795 mi), is solid. Both core layers are thought to consist largely of iron, with a small percentage of nickel and other elements. Temperatures in the inner core may be as high as 6650°C (12,000°F), 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 wildlife habitat and recreational opportunities, prevent soil erosion and flooding, help provide clean air and water, 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 warming. By decreasing the amount of carbon dioxide in the atmosphere, forests may reduce the effects of global warming. Water Conservation Clean freshwater resources are essential for drinking, bathing, cooking, irrigation, industry, and for plant and animal survival. Unfortunately, the global supply of freshwater is distributed unevenly. Chronic water shortages exist in most of Africa and drought is common over much of the globe. The sources of most freshwater supplies, groundwater (water located below the soil surface) reservoirs and rivers, are under severe and increasing environmental stress because of overuse, water pollution, and ecosystem degradation. Over 95 percent of urban sewage in developing countries is discharged untreated into surface waters such as rivers and harbors. About 65 percent of the global freshwater supply is used in agriculture and 25 percent is used in industry. Freshwater conservation therefore requires a reduction in wasteful practices like inefficient irrigation, reforms in agriculture and industry, and strict pollution controls worldwide. In addition, water supplies can be increased through effective management of watersheds (areas that drain into one shared waterway). By restoring natural vegetation to forests or fields, communities can increase the storage and filtering capacity of these watersheds and minimize wasteful flooding and erosion. Restoration and protection of wetlands is crucial to water conservation. Like giant sponges, wetlands stabilize groundwater supplies by holding rainfall and discharging the water slowly, acting as natural flood-control reservoirs. Environmental Impact Statement (EIS), formal process used to predict how a development project or proposed legislation will affect such natural resources as water, air, land, and wildlife. The environmental impact statement was first introduced in 1969 in the United States as a requirement of the National Environmental Policy Act. Since then, an increasing number of countries have adopted the process, introducing legislation and establishing agencies with responsibility for its implementation. Environmental impact statements have mostly been applied to individual projects and have led to various offshoot techniques, such as health impact assessments, social impact assessments, cumulative effects assessments, and strategic environmental assessments (environmental assessments of proposed policies, programs, and plans). In some cases, social and economic impacts are assessed as part of the environmental impact statements. In other cases, they are considered separately. An EIS usually involves a sequence of steps: (1) screening to decide if a project requires assessment and to what level of detail; (2) preliminary assessment to identify key impacts, their magnitude, significance, and importance; (3) scoping to ensure the EIS focuses on key issues and to determine where more detailed information is needed; (4) implementing the main EIS study, which involves detailed investigations to predict impacts, assess their consequences, or both. After a project is completed a post audit is sometimes done to determine how close the EIS's predictions were to the actual impacts. A growing number of businesses commission independent audits that help set environmental performance targets, particularly regarding waste disposal and energy use. The term environmental audit is applied to the voluntary regulation of an organization's practices in relation to its environmental impact. Greenpeace, international environmental organization dedicated to preserving the earth's natural resources and its diverse plant and animal life. The organization campaigns against nuclear weapons testing, environmental pollution, and destructive practices in fishing, logging, and other industries. Greenpeace was founded in Vancouver, British Columbia, Canada, in 1971 by members of the Don't Make a Wave Committee, a small group opposed to nuclear weapons testing by the United States military in Alaska. The group renamed itself Greenpeace to reflect the broader goal of creating a green and peaceful world. Greenpeace won fame for its daring exploits calculated to attract media attention to environmental issues. Greenpeace members in rubber rafts have disrupted whaling expeditions by positioning themselves between the whales and hunters' harpoons. They used similar tactics in Newfoundland to protest the clubbing of baby harp seals, whose soft white fur is highly valued by clothing manufacturers. The organization is well known for scaling corporate skyscrapers and factory smokestacks to hang protest banners. Greenpeace's aggressive style has often led to conflicts with corporations, local authorities, and even national governments. In 1985 the Greenpeace ship Rainbow Warrior, on a voyage to protest French nuclear weapons testing in the Pacific, sank in a New Zealand port, and the crew photographer, Fernando Pereira, drowned. Investigations revealed that the ship had been deliberately sabotaged with explosives planted by undercover agents of the French military. The resulting scandal rocked the highest levels of the French government, leading to the resignation of Defense Minister Charles Hernu and the dismissal of Admiral Pierre Lacoste, director of the French Secret Service. During the 1990s Greenpeace has been troubled by internal disagreements over political strategy. Some members want to persist with a militant approach, emphasizing civil disobedience and physical confrontation. Other members, including the organization's leaders, are convinced that Greenpeace must work cooperatively with the companies and industries that have been its targets in the past. Greenpeace has about 3 million dues-paying members and more than 40 offices in 30 countries. Its international headquarters are in Amsterdam, Netherlands.
The earth is not a perfect sphere but is slightly oblate, or flattened at the poles. The diameter of the earth measured around the North Pole and the South Pole is about 42 km (26 mi) less than the diameter of the earth measured at the equator.
The earth consists of five parts: the first, the atmosphere, is gaseous; the second, the hydrosphere, is liquid; the third, fourth, and fifth, the lithosphere, mantle, and core, are largely solid. The atmosphere is the gaseous envelope that surrounds the solid body of the planet. Although it has a thickness of more than 1100 km (more than 700 mi), about half its mass is concentrated in the lower 5.6 km (3.5 mi). The lithosphere, consisting mainly of the cold, rigid, rocky crust of the earth, extends to depths of 100 km (60 mi). The hydrosphere is the layer of water that, in the form of the oceans, covers approximately 70.8 percent of the surface of the earth. The mantle and core are the heavy interior of the earth, making up most of the earth’s mass.
The hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. 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 1/4400 of the total mass of the earth.
However, huge areas of the richest forests in the world have been cleared for wood fuel, timber products, agriculture, and livestock. These forests are rapidly disappearing. The tropical rainforests of the Brazilian Amazon River basin were cut down at an estimated rate of 50,000 sq km (20,000 sq mi) per year in the late 1980s. The countries with 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. While 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, which 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 was directed by Congress to maximize the harvest of timber in order to provide jobs. In the late 1980s and early 1990s, however, environmentalists sued the government for violating the National Environmental Policy Act (NEPA), and heavy logging was deemed nonsustainable. As a result, the timber harvest was reduced and foresters were directed to follow 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.
