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TERRESTRIAL

 

    The first five planets nearest the sun are known as the terrestrial planets because of their Earthlike characteristics, especially their rocky composition. But each of the four terrestrial planets – Mercury, Venus, Earth, and Mars – has distinctive features that set it apart from its neighbors in the solar system.
Mercury, closest planet to the sun, orbits the sun in only 88 Earth days, but it revolves on its axis so slowly that one Mercurian day lasts 59 Earth days. It has a large iron core, suggesting that it lost most of its surface in an ancient collision. Mercury appears to be shrinking still as its iron core grows cold. Venus has the densest atmosphere of any planet in the solar system, composed largely of clouds of carbon dioxide some 40 miles thick. These clouds trap incoming sunlight, which heats the planet's surface to 864°F day and night. No trace of water has been found on Venus. It shines luminously, making it the third brightest object in our sky after the sun and the moon. Earth has a unique blue-and-white appearance because more than 70 percent of its surface is covered with water and its atmosphere is filled with clouds rich in oxygen. Both its optimum distance from the sun and the presence of water in three states-liquid, solid, and vapor-make Earth conducive to life. Earth's terrain varies more than that of any other planet, and its lifeforms thrive on land and in water. Mars’s iron-rich soil gives it a reddish glow. Half the size of Earth, Mars is known for the oversize features of its terrain. Its Valles Marineris canyon stretches some 2,500 miles, equivalent to the distance from Los Angeles to New York City. Its Olympus Mons volcano stands at least 15 miles high, more than twice the height of Mount Everest. Despite Mars's inhospitable atmosphere, the United States still hopes to send a manned spacecraft to this, Earth's closest neighbor. Ceres, a dwarf planet, resides in the asteroid belt that orbits the sun between Mars and Jupiter. About one-fourth the size of Earth's moon, Ceres is the largest object in that belt of solar system leftovers.

 


OUTER

  The planets that orbit the sun on the other side of the asteroid belt are called the outer planets. Four of them-Jupiter, Saturn, Uranus, and Neptune-are also known as gas giants. Jupiter, the innermost planet among the gas giants, is larger than all the other planets in our solar system combined – and then some. This giant planet was fittingly named for the premier Roman god. With at least 63 orbiting moons, Jupiter can almost be
likened to a sun at the center of its own miniature solar system. Much about the planet remains a mystery, however, because even powerful space-based telescopes cannot see its surface, which is obscured by perpetual cloud cover with streaks caused by raging storms. Saturn, the most distant planet visible to the naked eye, is noted for its brilliant rings-the remains of a torn-apart moon or asteroid – which shine more brightly than the planet itself. Composed mostly of hydrogen and helium gas, Saturn has such a low density that it would float like a cork if dropped into water. Uranus gets its blue-green glow from methane gas in its atmosphere and is composed almost entirely of hydrogen and helium. Its axis is uniquely tilted at 98°, possibly the result of a collision with an immense object. Its rings tilt sideways as well. Neptune, the smallest gas giant, has the most extreme weather of any planet in our solar system, with winds stronger than 1,200 miles an hour. The existence of Neptune was predicted mathematically before it was observed, since gravity from a large body seemed to be affecting the orbit of Uranus. Neptune takes 164.8 Earth years to orbit the sun. Pluto was counted as the ninth planetfrom the sun for 76 years, but it now ranks as the tenth (albeit dwarf) planet. Its orbit is oval shaped, so sometimes it crosses inside the orbit of Neptune and temporarily becomes the ninth planet from the sun again. Some astronomers consider Pluto and Charon, its largest moon, a double-planet system. Eris, a dwarf planet discovered in 2005, caused a scientific reconfiguration of the solar system. Cold, rocky, and extremely remote, Eris travels with its moon, Dysnomia, through the Kuiper belt.     3.6. EARTHQUAKES AND VOLCANOES Researchers believe that the Earth’s crust is cracked into huge pieces that fit together like a giant puzzle. The cracked sections, called tectonic plates, are supported by the oozing, soft rocks of the mantle beneath the Earth’s crust. The unstable borders between the plates are known as rings of fire. These areas are danger zones for both volcanoes and earthquakes. A volcano is an opening in the Earth’s crust which allows red-hot magma (molten rock) from the mantle to escape onto the surface of the Earth. An earthquake is a shaking of the ground caused by movements of rocks beneath the Earth’s surface.
Global Plate Tectonic Movement Earthquakes happen when vibrations are caused by the movement of rock along a fault, a fracture that exists in Earth’s crust. As the tectonic plates push against, pull away from, grind past, or dive under one another, fault zones are created. Sometimes tension builds up along a fault, and further movement can cause the release
of energy in the form of seismic waves, or vibrations in Earth’s crust. Those vibrations ripple violently through the crust, causing an earthquake. Faults in Earth’s crust can take different forms, depending on the kind of tectonic stress involved, the strength of the rock, the presence of groundwater along the fault plane, and the area of contact between the plates. Movement along faults can be fast or slow. Abrupt movement causes earthquakes; movement so slow as to be imperceptible is called fault creep. A severe earthquake can produce underground movements-forward and back, up and down, side to side and wavelike ripples. Seismographs around the world sense at least a million earthquake movements a year. People barely perceive most of these.
Like volcanic eruptions, most earthquakes happen along the edges of tectonic plates. California’s San Andreas Fault, for example, is a zone where the slow sideways movement of slabs has pushed rock formations some 350 miles from their sources. Major earthquakes tend to produce dangerous side effects such as landslides and tsunamis, adding greatly to the destruction and casualties.
       

 



Volcanoes

Cross-section through a stratovolcano:   Volcanoes are produced when heat and pressure build up deep inside Earth. Material that makes up the mantle and the lower part of the crust reaches such high temperatures, the rock melts. This molten rock, called magma, collects in underground chambers where heat, pressure, and gases cause it to erupt to the surface.
1. Large magma chamber 9. Layers of lava emitted by the volcano
2. Bedrock 10. Throat
3. Conduit (pipe) 11. Parasitic cone
4. Base 12. Lava flow
5. Sill 13. Vent
6. Dike 14. Crater
7. Layers of ash emitted by the volcano 15. Ash cloud
8. Flank
Volcanoes most often appear at the edges of Earth’s crustal plates, the 16 separate continental and oceanic segments that move in relation to each other. The action between the two plates determines the type of volcano. Volcanoes also emerge in the middle of plates at hot spots, where the heat of interior molten rock burns through the crust. The Hawaiian Islands emerge from a hot spot that has moved over time with the Pacific plate.
Volcanoes spew not just lava but also poisonous gases, ash, cinders, and fragments of rock. This deadly combination of volcanic products, called pyroclastic flow, can produce more devastation than a lava flow.
Volcanoes take diverse shapes depending on the type of magma involved and the structure of the chamber and vents through which it erupts. Japan’s Mount Fuji is a composite cone, built of alternating layers of lava and ash. Hawaii has shield volcanoes, formed from thin, runny lava that travels far before it hardens. A magma chamber in the northern Rockies, site of an ancient collapsed volcano, fuels
Yellowstone’s hot springs, vents, and geysers-the world’s largest collection of these geothermal features. About 1,900 active volcanoes can be found on Earth. Volcanoes that have not erupted in a long time are called dormant; volcanoes that once erupted but will never erupt again are called extinct.
RING OF FIRE, EARTH’S VOLCANIC RIM, is home to more than half of the volcanoes on Earth. This configuration in the Pacific Ocean, an arc more than 24,000 miles long, follows the rim of the Pacific plate. In this zone of subduction, the Pacific plate dives under the edge of continental plates. As a result, rock melts and fuels volcanoes on the surface.
       

 

 

Types of Volcanoes

 

STRATOVOLCANOES are steep cones built of ash, rock, and lava spewed out during eruptions and deposited in layers.
SHIELD VOLCANOES are dome-shaped mountains built of lava flows, usually composed of basalt.
CALDERAS are large bowl-shaped depressions with in-facing rims, often collapsed volcanic cones.
SOMMAVOLCANOES are calderas now partly filled with newly formed central cones.
COMPLEX VOLCANOES reveal volcanic structures formed of multiple craters and summits.
       

 

 

WHAT IS A TSUNAMI?

 

A tsunami is a deadly series of seismic sea waves: ocean waves triggered primarily by the movement of the ocean floor during strong earthquakes. Volcanic eruptions in or near the ocean may also cause tsunamis. Tsunamis generally cause major damage to coastlines and can cause death.   The worst tsunami In history occurred after a 9.0 earthquake off the northwest coast of Sumatra, Indonesia, on December 26, 2004. The resulting oceanwide tsunami struck Thailand, Malaysia, Sri Lanka, India, the Maldives, and Africa. Nearly a quarter million people died as a result.
A tidal wave is an ocean wave of unusual strength caused by the same tidal forces that create the daily tidal ebb and flow. One such wave appears annually at China’s Hangzhou Bay.

 

 

Water

Oceans

 

  Oceans cover nearly three-fourths of Earth’s surface. They surround all the continents and give Earth its blue appearance when viewed from space. Although the oceans are composed of a contiguous body of water measuring some 139 million square miles, geographers divide it into four entities: largest to smallest, Pacific, Atlantic, Indian, and Arctic.
About 3.5 percent salt, ocean water also contains traces of all the chemical elements found on Earth. It enables life on the planet as part of the water cycle. The oceans also regulate global temperatures by absorbing heat in the summer and releasing it in winter. Currents, wind, density gradients, and Earth’s
rotation keep the ocean in motion. As Earth spins, wind and surface currents are deflected to the right in the Northern Hemisphere and to the left in the South Hemisphere. As a result, enormous gyres transport warm water from equatorial regions into the much colder polar regions. The ocean floor contains myriad physical features, starting with the continental shelf, the submerged extension of the continents. The shelf descends gradually before dropping off sharply in the continental slope and then softening again in the continental rise.
The deep-ocean floor, or abyssal plain, features hills and underwater volcanoes as well as the Mid-Ocean Ridge, a mountain chain more than 40,000 miles long. In the rift molten rock rises from Earth’s interior, forming new seafloor. Some areas of the ocean floor also have deep, narrow depressions called trenches. The deepest of all is the Challenger Deep in the Mariana Trench in the Pacific Ocean near Guam, more than 35,000 feet below the surface.
       

 

 

RIVERS

Rivers-large natural streams of flowing water-run through every continent. Wide rivers even flow beneath Antarctica’s massive ice sheet. Rivers are found in every kind of terrain. Some flow continuously with great force, some experience seasonal surges, and some dry up intermittently. South America’s Amazon River carries more water than any other river on Earth.
Rivers vary widely in length. A river may have a fairly short course, or it can span much of a continent. The Mississippi River bisects most of the United States, from its source in Minnesota to its delta in Louisiana. Rivers play a major role in the water cycle, discharging large amounts of fresh water into the oceans, where evaporation occurs.
Clouds form from the resulting water vapor and travel inland, creating precipitation and a supply of fresh water to rivers and streams. Since the beginning of human settlement, river valleys have been favored locations. They provide a reliable water supply for settlers and their crops and provide a means for moving people and goods. As industries developed, river waters were harnessed to power machinery. In flood, however, out-of-control rivers threaten lives and property.

 

 

HOW DO RIVERS FORM THEIR SHAPES?

 

 

From its source in a melting mountain glacier, a river flows swiftly downhill, cutting a narrow valley.    
Smaller streams, called tributaries, flow in from springs and lakes. Where the river tumbles over rocks and down steep bluffs, rapids and waterfalls occur.    
Farther downstream the terrain flattens out and the river flows more slowly, winding from side to side. A mature river meanders, creating large, lazy loops that sometimes curve so much, only a narrow neck of land separates either side of the loop.
The river gradually widens and forms a floodplain, and as it nears the ocean it may form a marsh. Shedding the heavy load of sand, silt, and clay it has been moving along, the river creates an expanse of flat, fertile land called a delta.

 

Lakes

 

Bodies of water surrounded by land, lakes are found on every continent and in every kind of environment, totaling in the millions throughout the world. Lakes vary in size, from small ones typically called ponds to bodies of water so large they are known as seas.
Lakes form in basins, depressions in the Earth’s surface, which are created in a number of ways. Many lakes, especially those found in the Northern Hemisphere, trace their origins to the work of glaciers at the height of the last ice age.
The glaciers ground out pits and hollows in the land they traveled over. When the ice retreated, the depressions filled with water. Glaciers also carved valleys and then dammed them with the deposits they left behind, forming lakes.
Shifts in Earth’s crust create depressions that may fill with water from rainfall or streams. When crustal movement occurs near the ocean, a part of that ocean may be cut off by an uplifted block of land. Volcanoes also participate in the formation of lakes.
The crater of an inactive volcano may fill with water, as will a caldera, the depression formed when a volcano blows its top and collapses. Meandering rivers, landslides, and the dam-building work of beavers also create lakes.
Artificial lakes are created as water-supply reservoirs or for recreational purposes.  
Lakes receive their water from rain, ice- and snowmelt, streams, and groundwater. Lakes can be open or closed. An open lake discharges water by an outlet such as a stream or by seepage. A closed lake has no such outlet but loses water from evaporation.
In some lakes evaporation greatly concentrates mineral content, creating salty bodies such as the Great Salt Lake in Utah, with waters saltier than the ocean.
The largest lake in the world is the Caspian Sea. Its salinity varies from one region to another. It was formed when tectonic movements created barricades that cut off and enclosed a portion of the ocean.
No water flows out of the Caspian Sea into any ocean. Located at the nexus between Asia and the Middle East, it covers about 170,000 square miles
Of the world’s next largest lakes, three belong to the Great Lakes system of North America – Lake Superior, Lake Huron, and Lake Michigan and two are found in Africa – Lake Victoria and Lake Tanganyika.
Lake Superior Lake Huron Lake Michigan Lake Victoria Lake Tanganyika
             

 

Ice

 

Ice, which is water in its frozen, solid state, will form when the water temperature reaches 32°F. In cold weather, ice appears on rivers, lakes, and the ocean.
  In areas where it is perpetually cold, ice becomes part of the landscape as features such as glaciers, ice caps, and ice sheets.
Glaciers form in locations where snow accumulates faster than it can melt, usually in mountainous areas. Over time the snow becomes compressed and recrystallizes into ice. When the ice reaches a solid mass of a certain thickness it can begin to move, or flow, under its own weight. Most glaciers move very slowly, perhaps only a few inches a year, but under some circumstances they can advance quickly.
An ice cap is a thick layer of ice and snow that has formed a permanent crust over areas of land. Such formations are found primarily in polar regions.
Sometimes the term ice cap is used interchangeably with ice sheet, although an ice sheet usually is larger. Ice sheets that cover continents are known as continental glaciers. The Antarctic ice sheet, for example, is a continental glacier. It covers almost 90 percent of the continent of Antarctica and contains about 85 percent of the world’s ice and about two-thirds of the world's fresh water.  
The area of the Antarctic ice sheet is shrinking at what many scientists consider an alarming rate for the future of the planet-as is also the case with the Greenland ice sheet.
             

 

WHAT IS AN ICEBERG?

 

An iceberg forms when a large chunk of ice calves, or breaks off, from a glacier and falls into the sea. The word comes from the Dutch ijsberg, ice hill. Icebergs are formed of fresh water, not salt water.
The water in icebergs is so pure that in some places chunks of iceberg are removed and melted, and the resulting water is used in cooking and brewing. In the Northern Hemisphere, most icebergs originate from glaciers on Greenland, and they often drift southward into the North Atlantic Ocean. In the Southern Hemisphere, glaciers frequently calve from glaciers in Antarctica. As little as one-tenth of a glacier is visible above the water – a phenomenon that inspired the familiar phrase “That’s just the tip of the iceberg”.  
Sharp ice on the hidden parts of an iceberg poses a threat to ships such as the historic cruise liner Titanic, whose hull was pierced by underwater ice in 1912, resulting in the swift sinking of the ship and the tragic death of more than I ,500 people.
     

 

 

Light

Solar energy results from nuclear fusion in the sun’s core, which creates an enormous, continuous flow of radiant energy that travels throughout the solar system. The energy that reaches us from all the stars twinkling in our galaxy cannot compare with what we receive from the sun, the only significant source of energy for Earth’s atmosphere.

 

WHAT MAKES A RAINBOW?

The multicolored arc of a rainbow is produced by sunlight striking raindrops beneath a rain cloud. Light refracts – bends – when it passes through drops of water. Each color of light refracts at a different angle: Violet bends more than blue, which bends more than green, and so on, with red refracting the least.
If sunlight enters a raindrop at just the proper angle, it refracts, and its many colors spread into a visible array. Sunlight refracted through millions of raindrops forms a rainbow. On a primary rainbow, red is the outside color and violet the inside color.
Occasionally a secondary rain bow appears slightly higher in the sky, and in it the colors of the rainbow are reversed (opposite). A rainbow’s position in the sky depends on the sun's altitude above the horizon – the lower the sun, the higher the rainbow appears.

 

WHAT MAKES THE NORTHERN LIGHTS?

Named for the Roman goddess of dawn, an aurora is a colorful nightly light display in or near the Arctic and Antarctic Circles. In the north the display is called aurora borealis – northern lights. In the south, it is aurora australis – southern lights.
Gaseous elements in the sun split into electrically charged particles. The sun’s surface continuously sheds these particles, some of which flow toward Earth as solar wind. Those particles that penetrate the Earth’s magnetic field enter the upper atmosphere and bombard its gases. The resulting collisions produce energy visible as arcs, streaks, or curtains of colored light. With particularly intense solar wind, activity increases and the lights are seen from far away.  

 


Date: 2015-04-20; view: 918


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