A climate of warm summers, cold winters, and plentiful rain helps make much of this European farmland very productive. Almost all of Europe sits on the massive Eurasian Plate. Africa Africa, the second-largest continent, covers an area more than three times that of the United States. From north to south, Africa stretches about 8, kilometers 5, miles.
It is connected to Asia by the Isthmus of Suez in Egypt. A series of falls and rapids along the southern part of the river makes navigation difficult. The Nile has played an important role in the history of Africa. In ancient Egyptian civilization , it was a source of life for food, water, and transportation. The top half of Africa is mostly dry, hot desert. The middle area has savannas, or flat, grassy plains.
This region is home to wild animals such as lions, giraffes, elephants, hyenas, cheetahs, and wildebeests. The central and southern areas of Africa are dominated by rainforests. Much of Africa is a high plateau surrounded by narrow strips of coastal lowlands. Hilly uplands and mountains rise in some areas of the interior. Glaciers on Mount Kilimanjaro in Tanzania sit just kilometers from the tropical jungles below.
Even though Kilimanjaro is not far from the Equator, snow covers its summit all year long. The rift valley actually starts in southwestern Asia. The Great Rift Valley is a site of major tectonic activity, where the continent of Africa is splitting into two.
Geologists have already named the two parts of the African Plate. The area of central-eastern Africa is important to scientists who study evolution and the earliest origins of humanity. This area is thought to be the place where hominids began to evolve. The entire continent of Africa sits on the African Plate. Asia Asia, the largest continent, stretches from the eastern Mediterranean Sea to the western Pacific Ocean. There are more than 40 countries in Asia. Some are among the most-populated countries in the world, including China, India, and Indonesia.
The continent of Asia includes many islands, some of them countries. The plateaus in Central Asia are largely unsuitable for farming and are thinly populated. The continent has a wide range of climate regions, from polar in the Siberian Arctic to tropical in equatorial Indonesia.
Southeast Asia, on the other hand, depends on the annual monsoons, which bring rain and make agriculture possible. Asia is the most mountainous of all the continents. More than 50 of the highest peaks in the world are in Asia. Mount Everest, which reaches more than 8, meters 29, feet high in the Himalaya range, is the highest point on Earth.
These mountains have become major destination spots for adventurous travelers. Plate tectonics continuously push the mountains higher. As the landmass of India pushes northward into the landmass of Eurasia, parts of the Himalayas rise at a rate of about 2. The land there lies more than meters 1, feet below sea level. Although the Eurasian Plate carries most of Asia, it is not the only one supporting major parts of the large continent. The Indian Plate supports the Indian peninsula, sometimes called the Indian subcontinent.
The Australian Plate carries some islands in Indonesia. Australia In addition to being the smallest continent, Australia is the flattest and the second-driest, after Antarctica. The continent is sometimes called Oceania , to include the thousands of tiny islands of the Central Pacific and South Pacific, most notably Melanesia, Micronesia, and Polynesia including the U.
However, the continent of Australia itself includes only the nation of Australia, the eastern portion of the island of New Guinea the nation of Papua New Guinea and the island nation of New Zealand. Australia covers just less than 8. Its population is about 31 million. It is the most sparsely populated continent, after Antarctica. Rainfall is light on the plateau, and not many people have settled there. The Great Dividing Range, a long mountain range, rises near the east coast and extends from the northern part of the territory of Queensland through the territories of New South Wales and Victoria.
Mainland Australia is known for the Outback , a desert area in the interior. This area is so dry, hot, and barren that few people live there.
In addition to the hot plateaus and deserts in mainland Australia, the continent also features lush equatorial rainforests on the island of New Guinea, tropical beaches, and high mountain peaks and glaciers in New Zealand. Biologists who study animals consider Australia a living laboratory. When the continent began to break away from Antarctica more than 60 million years ago, it carried a cargo of animals with it.
Isolated from life on other continents, the animals developed into creatures unique to Australia, such as the koala, the platypus, and the Tasmanian devil.
The reef itself is 1, kilometers 1, miles of living coral communities. Most of Australia sits on the Australian Plate. Antarctica Antarctica is the windiest, driest, and iciest place on Earth.
Antarctica is larger than Europe or Australia, but unlike those continents, it has no permanent human population. People who work there are scientific researchers and support staff, such as pilots and cooks. The climate of Antarctica makes it impossible to support agriculture or a permanent civilization.
Temperatures in Antarctica, much lower than Arctic temperatures, plunge lower than degrees Celsius degrees Fahrenheit. Scientific bases and laboratories have been established in Antarctica for studies in fields that include geology , oceanography , and meteorology. Antarctica is also an ideal place for discovering meteorites, or stony objects that have impacted Earth from space. The dark meteorites, often made of metals like iron , stand out from the white landscape of most of the continent.
Antarctica is almost completely covered with ice, sometimes as thick as 3. Like all other continents, Antarctica has volcanic activity. The most active volcano is Mount Erebus, which is less than 1, kilometers miles from the South Pole. Antarctica does not have any countries. However, scientific groups from different countries inhabit the research stations.
A multinational treaty negotiated in and reviewed in states that research in Antarctica can only be used for peaceful purposes. Vostok Station, where the coldest temperature on Earth was recorded, is operated by Russia. All of Antarctica sits on the Antarctic Plate. Microcontinents In addition to the seven major continents, Earth is home to microcontinents, or pieces of land that are not geologically identified with a continent.
Major microcontinents include:. Also called cosmic dust or space dust. Gas molecules are in constant, random motion. Also called the Somali Peninsula. The last ice age peaked about 20, years ago.
Also called glacial age. Monsoon usually refers to the winds of the Indian Ocean and South Asia, which often bring heavy rains. Regions are the basic units of geography. Sea level is determined by measurements taken over a year cycle. Also called lithospheric plate. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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If a media asset is downloadable, a download button appears in the corner of the media viewer. Despite its promise, aquaculture is no better, since three pounds of wild fish are caught to feed every pound of farmed salmon sent to market—creating entirely new fisheries, which deplete hitherto unscathed wild fish populations, including krill, a critical corner-stone of the marine food web and essential to the survival of Antarctic species such as penguins.
Furthermore, farmed salmon become severely contaminated by pollutants in their feed chow; some European aquacultured salmon is so badly tainted that people have been advised to consume it only once every five months [for more on which seafood is safe to eat, see here. The truth is that the full consequences of modern fishing methods are brutal and far-reaching, and they were not really understood before the release of a seminal study published in , detailing how industrialized fisheries, in a manner akin to virulent pathogens, typically reduce the community of large fish by 80 percent within the first 15 years of exploitation.
Co-authors Boris Worm and Ransom Myers of Dalhousie University in Nova Scotia concluded that in the wake of decades of such onslaughts, only 10 percent of all large fish tuna, swordfish, marlin and groundfish cod, halibut, skate, and flounder are left anywhere in the ocean.
Their study was based on factors modern fisheries managers ignore: historical data; in this case, the catch reports from Japanese long-liners dating from the s, when the global tuna catch was less than , tons, compared with 3.
Apparently no one really remembers how many big fish used to inhabit the sea or how big they got. In many cases, the fish caught today are under such intense fishing pressure, they never even have the chance to reproduce.
Poor fishers do this largely to meet the demand of rich nations—to supply aquarium fish for the United States and live food fish for Hong Kong.
In this way, some coral reef species have been locally extinguished in the course of only one or two spawning events. By analyzing 10, historical restaurant menus from Boston to San Francisco, a project called the History of Marine Animal Populations , out of the University of Southern Denmark, finds that lobster was so abundant in the 19th century that middle-class Americans snubbed it as food for the poor. IT USED TO BE, in the heyday of wildlife filmmaking, that you could chum off the California coast for a few hours or a day or two and attract dozens of full-size eight-foot blue sharks, along with a gaggle of youngsters and the occasional, powerful foot mako or two.
But the last time I tried this, only two baby blue sharks, all of four feet long, appeared after days of chumming. In the interval between , when cameramen were forced to work with safety divers to fend off more sharks than they knew what to do with, and , when we were obliged to film the baby sharks close-up with wide-angle lenses to make them look bigger, long-liners, trawlers, and drift netters came to the west coast.
Sharks are killed incidentally in large numbers by all three forms of industrial fishing, but they are also targeted by their own fishery, primarily for soup. At expensive eateries across Asia, middle-class diners slurp this pricey food, even as the World Conservation Union adds ever more shark species to its Red List of Threatened Species. Fishing fleets kill an estimated million sharks per year across the globe. In the Gulf of Mexico, the number of oceanic whitetip sharks has plunged 99 percent since the s, driving this once common pelagic species into virtual extinction.
A study of the North Atlantic found that overall shark populations have declined more than 50 percent since Sadly, sharks are slow breeders, with most delivering small litters some only twins after reaching a late sexual maturity some at 25 years old , after which they typically deliver litters at three-year intervals. The results of such slow reproduction make recovery from overfishing notoriously difficult. When porbeagle sharks were overfished by Europeans in the s, the species struggled for the next 30 years, finally achieving some semblance of health in the s, only to become the target of U.
The end of big fish in the sea is more than an aesthetic loss. Marine ecologist Mark Hixon of Oregon State University has published widely on coral reef ecosystems, and his work illustrates how biodiversity and community stability thrive in the presence of predators and competitors.
The removal of either or both destabilizes the remaining species. Hence big sharks, tuna, swordfish, and halibut are more than picturesque giants; they are keystone species that play greater roles in maintaining ecosystem function than seems obvious based on the size of their population. Hixon also argues that not all spawners are created equal, and that the most valuable members of fish populations are what he and his colleagues call the Big Old Fat Female Fish BOFFFs , who produce better-quality and -quantity eggs than younger females.
Yet fisheries managers continue to promote the targeting of older fish, followed by younger fish, until none can grow old. Hixon tells me that we need a Kuhnian paradigm shift in fisheries management. As on land, protecting places is the best way to preserve life. In , the World Conservation Union listed , protected areas on earth. But only 4, of these were protected marine areas, preserving less than 0. To reach parity, we need to add 23 times as many marine reserves and offshore national parks, or 10 times more total area—and perhaps even more, since the liquid medium of the ocean is more in-terconnected, and the fate of its disparate realms more intertwined than here.
For all at last return to the sea—to Oceanus , the ocean river. Sweden, for example, calculates that its populace of 8. Crematory emissions are a small but growing percentage of the total global mercury pollution, the vast majority of which enters the foodweb as a biologically active derivative of the inorganic mercury released by the smokestacks of the coal and chlorine industries.
Oxidized in the atmosphere and piggybacking on raindrops, this form of mercury eventually settles to the bottom of oceans and lakes, where it is converted to dangerous methylmercury by aquatic bacteria, which are eaten by plankton, which are eaten by fish, and bigger fish—with each subsequent meal bioaccumulating in higher levels until apex predators such as tuna and whales carry mercury levels as much as 1 million times higher than the waters around them.
As do we. The European Union warns pregnant women to limit their consumption of both tuna and swordfish because of brain damage to their unborn children, and the U. Food and Drug Administration warns pregnant women, lactating women, and young children not to eat swordfish, shark, tilefish, or king mackerel, though the powerful tuna lobby succeeded in keeping tuna off that list.
Yet the Bush administration, circumventing the Clean Air Act, has enabled coal-fired power plants to delay curtailing significant mercury emissions until For most of the residents of the bayou country of southern Louisiana, these are welcome winds; only a month has passed since Hurricane Katrina made landfall, and 11 days since Hurricane Rita, and these northerlies are cold and dry enough to dismantle any additional tropical storms from the top down.
This once was one of the most prolific bodies of water on earth, a place where the outflow from the Mississippi River introduced freshwater nutrients into a deepwater environment. An average of 10 tons of mercury comes down the Mississippi every year, with close to another ton added by the offshore drilling industry. Dead zones occur wherever oceanic oxygen is depleted below the level necessary to sustain marine life, a result of eutrophication , or the release of excess nutrients into the sea, usually from agricultural fertilizers.
Fifty years ago no one imagined that the Green Revolution would prove so lethal to the world ocean. But now we know that chemical fertilizers cause plants to bloom in the sea as miraculously as they do on land, with deadly consequence.
The Gulf of Mexico suffers the downstream effects of the mighty Mississippi, which drains 41 percent of the contiguous United States, including all the intensively farmed breadbasket.
This outflow delivers enough nitrogen to stimulate explosions of plankton and microalgae, some of which form the red tides that produce major fish kills and dolphin or manatee die-offs. At even higher densities, as these plankton die en masse and settle to the bottom, they fuel a bloom of bacterial decomposers, which consume all the available oxygen in the water. The resulting condition, known as hypoxia, strikes the Gulf whenever oxygen levels fall below two milligrams per liter—an annual summertime event in the warming waters of the Gulf since the s.
Those creatures that can swim or walk away fast enough may survive. Three months ago, as the newly appointed executive director of Louisiana Universities Marine Consortium LUMCON , she took the helm of this 75,square-foot complex of laboratories, teaching facilities, apartments, offices, and seagoing vessels. So far her tenure has been largely spent digging out of the mud, repairing the wind damage, and casting an eye to the weather. Only the estuarine wetlands all around seem untouched, lovely, given that hurricanes are a familiar part of their evolutionary world.
Rabalais is weary. She still has a two-hour drive ahead of her to Baton Rouge, where she teaches at Louisiana State University— though I suspect she would rather board Pelican for a couple of days and leave her worries behind. A Texan by birth and schooling, she has been diving these waters since it was a fun thing to do; nowadays, it requires a certain courage.
A week earlier, while diving in zero visibility on a research station 26 miles offshore, Rabalais encountered an alligator at the surface blown out to sea by one or both of the hurricanes. Oddly, it acts like a living thing: growing in spring, thriving in summer, decaying in fall, gaining in size almost every year. Core sediment samples and computer hindcasting pinpoint its birth date to the aftermath of World War II, when a surplus of nitrogen destined for TNT was redeployed as agricultural fertilizer.
The six young men and women work efficiently, hurrying back to the mess deck between workstations, where the satellite TV plays back-to-back college football games. But for a first-time visitor to the northern Gulf of Mexico, this is far too fascinating a world, in a futuristic kind of way, to ignore.
The horizon in all directions is dotted with what from a distance look like small mangrove islands. Only these are oil and liquid natural gas rigs, with all their attendant satellites. At any given time, at least 50 structures punctuate the horizon, and often more than When we draw close, they prove enormous. Servicing them are countless powerful and speedy crew boats, most bigger and faster than Pelican, along with a constant fleet of helicopters in flight between rigs.
Even more strange is the lack of visible sea life. But here there is only emptiness and the occasional bobbing flight of a laughing gull. And this is only one of many dead zones. Robert Diaz, a hypoxia expert from the Virginia Institute of Marine Science, calculates the global number is doubling every decade. Furthermore, he suggests that at least in some areas hypoxia is rapidly becoming a greater threat to fish stocks than overfishing, since it disperses them off their feeding, spawning, and maturation grounds.
Why are salt marshes considered part of the intertidal zone? Much of the Mississippi River has been contained by levees, dikes, and other flood-control structures. Download advertisement. Add this document to collection s. You can add this document to your study collection s Sign in Available only to authorized users. Description optional.
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