View
213
Download
0
Tags:
Embed Size (px)
Citation preview
12 July 2005 AST 2010: Chapter 7 2
Basic FactsThe Earth is a medium-sized planet with a diameter of 13,000 kmIt is one of the inner or terrestrial planets
It is composed primarily of heavy elements, such as iron, silicon, and oxygenIt has much less light elements, such as hydrogen and helium, than the outer planets
Earth's orbit around the Sun is nearly circularThe 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 surfaceIt is “just right” to sustain life — at least life as we know it
12 July 2005 AST 2010: Chapter 7 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 deepThe 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
12 July 2005 AST 2010: Chapter 7 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 sitesSeismic 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 constructedSimilarly, 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
12 July 2005 AST 2010: Chapter 7 7
Earth’s Internal Layers (1) The Earth is divided into four main layers: crust, mantle, core, and inner coreThe 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
12 July 2005 AST 2010: Chapter 7 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 coreIt contains iron, and probably also nickel and sulfur, all compressed to a very high density The core is 7,000 km in diameterIts outer part is liquid The inner core is 2,400 km in diameter and is probably solid
12 July 2005 AST 2010: Chapter 7 9
DifferentiationScientists 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 densitiesWhen 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 crustWhen the planet cools, this layered structure is preserved
12 July 2005 AST 2010: Chapter 7 10
Earth’s Magnetic FieldAdditional clues about the Earth's interior can be learned from its magnetic fieldThe 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
12 July 2005 AST 2010: Chapter 7 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 systemWithin this region, called the magnetosphere, the Earth’s field dominates over the weak interplanetary magnetic field extending outward from the SunMost 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
12AST 2010: Chapter 712 July 2005
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 measurementsThis region has a fairly complex structureAnimation
Cross-sectional view of Earth’s magnetosphere Cross-sectional view of Earth’s magnetosphere as revealed by spacecraft missionsas revealed by spacecraft missions
13AST 2010: Chapter 712 July 2005
What Comes to Your Mind upon Hearing “Rocks”?
12 July 2005 AST 2010: Chapter 7 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 igneousThere 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 meltingMetamorphic rocks are produced when high temperature or pressure alters igneous or sedimentary rocks physically or chemicallyThese are commonly found on Earth, but not on other planets
12 July 2005 AST 2010: Chapter 7 15
Rocks (2)A fourth kind of rock is primitive rock
Its formation is believed to date back to the formation of the planetPrimitive rock has largely escaped chemical modification by heatingThus, it is thought to represent the original material out of which the planetary system was madeNo primitive rock is left on the Earth because the planet was heated early in its historyPrimitive rocks may be found in comets, asteroids, or small planetary satellites
12 July 2005 AST 2010: Chapter 7 16
Geology & Plate TectonicsGeology is the study of the Earth’s crust and the processes that have shaped it throughout historyNot 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 continentsthe 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
12 July 2005 AST 2010: Chapter 7 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
12 July 2005 AST 2010: Chapter 7 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 timeBasically, four types of interactions between crustal plates are possible at their boundaries:
They can pull apartOne plate can burrow under anotherThe can slide alongside each otherThey can jam together
19AST 2010: Chapter 712 July 2005
Rift ZonesPlates 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
20AST 2010: Chapter 712 July 2005
Subduction ZonesWhen 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 mantleA 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 surfaceAnimation
12 July 2005 AST 2010: Chapter 7 21
Fault ZonesCrustal plates slide parallel to each other along much of their lengthsBoundaries so formed lead to the formation of cracks or faultsAlong 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 riftsThe creeping motions of the plates in fault zones build up stresses in the crustThe stresses are eventually released in sudden, violent slippages, a.k.a. earthquakesThe 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
12 July 2005 AST 2010: Chapter 7 22
San Andreas FaultIt 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 CaliforniaIn a few million years, LA will be an island off the coast of San Francisco
12 July 2005 AST 2010: Chapter 7 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
12 July 2005 AST 2010: Chapter 7 24
Mountain BuildingWhen 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
12 July 2005 AST 2010: Chapter 7 25
VolcanoesVolcanoes 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 anotherVolcanic activity is also observed in subduction zonesIn both cases, the volcanic activity brings to the surface large amount of materials from the upper mantle
12 July 2005 AST 2010: Chapter 7 26
Earth’s AtmosphereIt provides the air we breatheThe 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 barHumans have existed mostly at sea level and are thus accustomed to such a pressureThe 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 EarthYet its composition is quite vital to us humans and other living creatures on the surface of this Earth
12 July 2005 AST 2010: Chapter 7 28
Troposphere Altitude range:
Sea level - 9 miles
Densest area of the atmosphereMost weather occurs and almost all aircraft fly in this regionTemperatures 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 weatherAs 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
12 July 2005 AST 2010: Chapter 7 29
StratosphereAltitude range:
9 - 31 miles
Dry and less denseThe air in this layer moves horizontally and does not move up and down within itTemperatures here increase with elevationNear the top of the stratosphere, one finds a layer of ozone (O3)
Ozone is a good absorber of ultraviolet lightIt thus protects the surface from the sun's ultraviolet radiation and makes it possible for life to exist on the planet
12 July 2005 AST 2010: Chapter 7 30
Mesosphere
Altitude range: 31 - 62 miles (50 - 100 km)
Temperatures fall as low as -93° Celsius in this regionChemicals are in an excited state, as they absorb energy from the sun
12 July 2005 AST 2010: Chapter 7 31
Ionosphere Altitude range: 62 - 124 milesThis region is characterized by the presence of plasmaIts boundaries vary according to solar activity
12 July 2005 AST 2010: Chapter 7 32
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 CelsiusAuroras, caused by the sun's particles striking the earth's atmosphere, occur at this level
12 July 2005 AST 2010: Chapter 7 33
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
Weather and ClimateAll 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 planetThe 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 groundheats the surface layersis 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 energyacting 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
36AST 2010: Chapter 712 July 2005
Greenhouse EffectOn 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 averageWithout this greenhouse effect, the average surface temperature would be well below freezing
The Earth would be locked in a global ice ageLife 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 WarmingModern 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 systemsScientists study how they are affected by global warming using elaborate computer models Their conclusions are not yet firm at this point
12 July 2005 AST 2010: Chapter 7 40
Earth CratersEvidence 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 groundThe shock wave flattened more than a thousand square kilometers of forestThe blast wave spread around the world and was recorded by instruments designed to record changes in atmospheric pressure