1. Planet Earth

2. Basic Facts
The Earth is a medium-sized planet with a diameter of 13,000 km
It is one of the inner or terrestrial planets
It is composed primarily of heavy elements, such as iron, silicon, and oxygen
It has much less light elements, such as hydrogen and helium, than the outer planets
Earth's orbit around the Sun is nearly circular
The Earth is the only planet in our solar system that is neither too hot nor too cold
It is warm enough to support liquid water on its surface
It is “just right” to sustain life — at least life as we know it
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3. Some Properties of the Earth
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4. Earth's Interior (1)
The interior of the Earth is difficult to study even with today's amazing technology
Its composition and structure must be determined indirectly from observations made near or at the surface only
Earth’s skin is a layer only a few kilometers deep
The Earth is composed largely of metals and silicate rock
Most of this material is in a solid state, but some of it is hot enough to be molten
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5. Earth's Interior (2)
The structure of the interior of the Earth has been probed in great detail by measuring the transmission of seismic waves through it
Seismic waves are waves that spread through the interior of the Earth from earthquakes or explosion sites
Seismic waves travel through Earth rather like sound waves through a struck bell
A bell’s sound frequencies depend on what material the bell is made of and how it is constructed
Similarly, the way seismic waves travel through a planet can reveal some information about its interior
From seismic studies, scientists have learned that the Earth’s interior consists of several distinct layers with different compositions
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6. Seismic Waves in Earth's Interior
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7. Earth’s Internal Layers (1)
The Earth is divided into four main layers: crust, mantle, core, and inner core
The crust is the top layer, the part we know best
The crust under the oceans covers % of the surface, is typically about 6 km thick, and is composed of volcanic rocks called basalt
Basalts are produced by cooling volcanic lava and made primarily of silicon, oxygen, iron, aluminum, and magnesium
The continental crust covers 45% of the surface, is 20 to 70 km thick and is mainly composed of another class of volcanic rocks called granite
The whole crust makes up only about 0.3% of the Earth’s total mass
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8. Earth’s Internal Layers (2)
The mantle is the largest part of the solid Earth, stretching from the base of the crust down to a depth of 2,900 km
The mantle is more or less solid, but may deform and flow slowly due to the high pressures and temperatures found there
Below the mantle is Earth’s dense metallic core
It contains iron, and probably also nickel and sulfur, all compressed to a very high density
The core is 7,000 km in diameter
Its outer part is liquid
The inner core is 2,400 km in diameter and is probably solid
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9. Differentiation
Scientists believe that the Earth’s layered interior resulted from differentiation
This is the process by which gravity helps separate the interior of an initially molten planet into layers of different compositions and densities
When much of the planet is still molten, the heavier metals sink to the center to form a dense core, while the lightest elements float to the surface to form a crust
When the planet cools, this layered structure is preserved
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10. Earth’s Magnetic Field
Additional clues about the Earth's interior can be learned from its magnetic field
The Earth behaves in some ways as if a giant bar magnet were inside it
The magnet is roughly aligned with the rotational axis of the planet
The Earth’s magnetic field is generated by moving material in Earth’s liquid metallic core
The circulating liquid metal sets up an electric current, which in turn produces a magnetic field
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11. Earth’s Magnetosphere (1)
The Earth's magnetic field extends into surrounding space and traps small quantities of charged particles, such as electrons, that roam about the solar system
Within this region, called the magnetosphere, the Earth’s field dominates over the weak interplanetary magnetic field extending outward from the Sun
Most of the charged particles trapped in this region originate from the hot surface of the Sun, flowing out in a stream called the solar wind
This elongates the magnetosphere far beyond the Earth in the direction pointing away from the Sun
The Earth’s magnetosphere was discovered in 1958 by instruments on the first U.S. Earth satellite, Explorer 1
This satellite recorded the ions (charged particles) trapped in the inner part of the magnetosphere
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12. Earth’s Magnetosphere (2)
The regions of high-energy ions in the magnetosphere are often called the Van Allen Belts after the physicist who built the instrumentation for Explorer 1 and correctly interpreted its measurements
This region has a fairly complex structure
Animation
Cross-sectional view of Earth’s magnetosphere as revealed by spacecraft missions

13. What Comes to Your Mind upon Hearing “Rocks”?

14. Rocks (1)
Both basalt & granites are examples of igneous rock, which is any rock that has cooled from a molten state
All volcanically produced rock is igneous
There are two other kinds of rock
Sedimentary rocks are made of fragments of igneous rocks or the shells of living organisms deposited by wind or water and cemented without melting
Metamorphic rocks are produced when high temperature or pressure alters igneous or sedimentary rocks physically or chemically
These are commonly found on Earth, but not on other planets
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15. Rocks (2) A fourth kind of rock is primitive rock
Its formation is believed to date back to the formation of the planet
Primitive rock has largely escaped chemical modification by heating
Thus, it is thought to represent the original material out of which the planetary system was made
No primitive rock is left on the Earth because the planet was heated early in its history
Primitive rocks may be found in comets, asteroids, or small planetary satellites
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16. Geology & Plate Tectonics
Geology is the study of the Earth’s crust and the processes that have shaped it throughout history
Not until the middle of the 20th century, did geologists succeed in understanding how landforms are created
Plate tectonics is a theory that explains how slow motions within the Earth’s mantle move large segments of the crust, resulting in
a gradual drifting of the continents
the formation of mountains and other large-scale geological features
The Earth's crust and upper mantle are divided into about a dozen major plates that fit together like the pieces of a jigsaw puzzle
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17. Plate Tectonics (1)
These plates are capable of moving slowly relative to one another
In some places, such as the Atlantic Ocean, the plates are moving apart, and elsewhere they are being forced together
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18. Plate Tectonics (2)
The driving power behind the plates’ motion is provided by slow convection of the mantle
Convection is a process by which heat escapes from the interior through the upward flow of warmer material and the slow sinking of cooler material
As the plates move slowly, they bump into one another and cause dramatic changes in the Earth’s crust over time
Basically, four types of interactions between crustal plates are possible at their boundaries:
They can pull apart
One plate can burrow under another
The can slide alongside each other
They can jam together
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19. Rift Zones Plates pull apart from each other along rift zones
Most rift zones are in the oceans
An example is the Mid-Atlantic ridge, which is driven by upwelling currents in the mantle
A few rift zones are also found on land
The best known is central African rift, an area where the African continent is slowly breaking apart
Animation

20. Subduction Zones
When two plates come together, one plate is often forced down beneath another in what is called a subduction zone
Continental masses cannot be subducted but thinner oceanic plates can be “easily” pushed down into the upper mantle
A subduction zone is often marked by an ocean trench
Subducted plates forced down into regions of high temperature and pressure eventually melt several hundred kilometers below the surface
Animation

21. Fault Zones
Crustal plates slide parallel to each other along much of their lengths
Boundaries so formed lead to the formation of cracks or faults
Along active fault zones, the motion of one plate relative to the other may amount to several centimeters per year
basically the same as the spreading rates along rifts
The creeping motions of the plates in fault zones build up stresses in the crust
The stresses are eventually released in sudden, violent slippages, a.k.a. earthquakes
The average motion of the plates is constant
The longer the interval between earthquakes, the greater the stress and the larger the energy released when the surface finally moves
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22. San Andreas Fault
It is on the boundary between the Pacific and North American plates
running from the Gulf of California to the Pacific Ocean northwest of San Francisco
The Pacific plate (west side) moves north carrying along Los Angeles, San Diego, and other parts of Southern California
In a few million years, LA will be an island off the coast of San Francisco
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23. More about San Andreas
The San Andreas Fault near Parkfield has slipped every 22 years during the past century
moving an average of about 1 m each time
In contrast, the average time interval between major earthquakes in the Los Angeles region is about 140 years
the average motion is about 7 m
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24. Mountain Building
When two continental masses are brought together by the motion of the crustal plates, they are forced against each other under great pressure
The surface buckles and folds, forcing some of the rock deep below the surface and others to raise to large heights (sometimes many kilometers!)
This is how mountain ranges form on Earth 
The Alps result from the interaction of the African plate with the European plate
We will see, however, that other mechanisms lead to the formation of mountains on other planets
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25. Volcanoes
Volcanoes mark the location where molten rock, called magma, rises from the upper mantle through the crust 
Volcanoes are formed numerously along oceanic rift zones where rising hot material pushes plates away from one another
Volcanic activity is also observed in subduction zones
In both cases, the volcanic activity brings to the surface large amount of materials from the upper mantle
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26. Earth’s Atmosphere It provides the air we breathe
The air of the atmosphere exerts a constant pressure (on the ground)
The atmospheric pressure at sea level is used to define the pressure unit called bar
Humans have existed mostly at sea level and are thus accustomed to such a pressure
The total mass of the atmosphere is ~5x1018 kg
Although this sounds like a lot, it constitutes only one millionth of the total mass of the Earth
Yet its composition is quite vital to us humans and other living creatures on the surface of this Earth
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27. Structure of Earth’s Atmosphere
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28. Troposphere Altitude range: Densest area of the atmosphere
Sea level - 9 miles
Densest area of the atmosphere
Most weather occurs and almost all aircraft fly in this region
Temperatures drop as elevation increases
Warm air, heated on the surface, rises and is replaced by descending currents of cooler air
The circulation generates clouds and other manifestations of weather
As one rises through the troposphere, one finds the temperature drops rapidly with increasing elevation
The temperature is near 50°C below freezing at the top of the troposphere
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29. Stratosphere Altitude range: Dry and less dense
miles
Dry and less dense
The air in this layer moves horizontally and does not move up and down within it
Temperatures here increase with elevation
Near the top of the stratosphere, one finds a layer of ozone (O3)
Ozone is a good absorber of ultraviolet light
It thus protects the surface from the sun's ultraviolet radiation and makes it possible for life to exist on the planet
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30. Mesosphere Altitude range:
miles ( km)
Temperatures fall as low as -93° Celsius in this region
Chemicals are in an excited state, as they absorb energy from the sun
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31. Ionosphere Altitude range: 62 - 124 miles
This region is characterized by the presence of plasma
Its boundaries vary according to solar activity
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32. Thermosphere Altitude range:
miles ( km)
Temperatures increase with altitude due to the sun's energy, reaching as high as 1,727 degrees Celsius
Auroras, caused by the sun's particles striking the earth's atmosphere, occur at this level
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33. Exosphere Altitude range:
miles ( km)
The region begins at the top to the thermosphere and continues until it merges with interplanetary gases, or space
The prime components, hydrogen and helium, are present at extremely low densities
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34. Weather and Climate All planets with atmospheres have weather
Weather is simply the name given to the circulation of air through the atmosphere
The driving force behind weather is derived primarily from the sunlight that heats the Earth's surface 
As the planet rotates, and orbits the Sun, the slower seasonal changes cause variations in the amount of heat received by the different parts of the planet
The heat then redistributes itself from warmer to cooler areas giving rise to various weather patterns
Climate is a term used to describe the evolution of weather through long periods of time (decades or centuries)
Changes in climate are typically difficult to detect over short periods of time
However, their accumulating effects can be sizeable and sometimes quite dramatic

35. Role of Carbon Dioxide (CO2)
Upon striking the Earth’s surface, sunlight
is absorbed by the ground
heats the surface layers
is re-emitted as infrared or heat radiation
The CO2 in our atmosphere is transparent to visible light
allowing sunlight to reach the ground
However, CO2 is opaque to infrared energy
acting as a blanket, trapping the heat in the atmosphere and impeding its flow back to space
Such trapping of infrared radiation near a planet’s surface is called the greenhouse effect

36. Greenhouse Effect
On average, as much heat reaches the surface from the atmospheric greenhouse effect as from direct sunlight
This explains why nighttime temperatures are only slightly lower than daytime temperatures
It is estimated that the greenhouse effect elevates the surface temperature by about 23°C on the average
Without this greenhouse effect, the average surface temperature would be well below freezing
The Earth would be locked in a global ice age
Life as we know it would not be possible on Earth
Animation
On the other hand, increasing amounts of CO2 in our atmosphere could raise its average temperature to a much higher value
and then endanger life on our planet

37. Global Warming
Modern society increasingly depends on energy extracted from burning fossil fuels, releasing CO2 into the atmosphere
The problem is exacerbated by ongoing destruction of tropical forests, which we depend on to extract CO2 and replenish our supply of oxygen (O2)
Atmospheric CO2 has increased by about 25% in the last 100 years
In less than 100 years, the CO2 level will likely reach twice the value it had before the industrial revolution
The consequences of such an increase for the Earth are complex and not completely known
The Earth’s surface and atmosphere are extremely complicated systems
Scientists study how they are affected by global warming using elaborate computer models
Their conclusions are not yet firm at this point

38. Why is there no clear evidence of craters on Earth?

39. Suggested Answer Geological activity!

40. Earth Craters
Evidence of fairly recent impacts can be found on our planet's surface
The best studied case took place on June 30, , near the Tunguska River in Siberia, Russia 
There was an explosion 8 km above the ground
The shock wave flattened more than a thousand square kilometers of forest
The blast wave spread around the world and was recorded by instruments designed to record changes in atmospheric pressure
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