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Planet Earth
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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Some Properties of the Earth
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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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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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Seismic Waves in Earth's Interior
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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
55% 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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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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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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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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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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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
What Comes to Your Mind
upon Hearing “Rocks”?

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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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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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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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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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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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
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
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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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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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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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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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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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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Structure of Earth’s Atmosphere
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Troposphere
Altitude range:
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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Stratosphere
Altitude range:
9 - 31 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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Mesosphere
Altitude range:
31 - 62 miles (50 - 100 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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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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Thermosphere
Altitude range:
124 - 310 miles (200 - 500 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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Exosphere
Altitude range:
310 - 434 miles (500 - 700 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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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
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
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
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
Craters
Why is there no clear evidence of craters on Earth?
Suggested Answer
Geological activity!
Earth Craters
Evidence of fairly recent
impacts can be found on
our planet's surface
The best studied case
took place on June 30,
1908, 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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