INTRODUCTION TO THE EARTH

The Earth is the third planet from the Sun in our Solar System. It is the planet we evolved on and the only planet in our Solar System that is

known to support life.

SIZEThe Earth is about 7,926 miles (12,756 km) in diameter. The Earth is the fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and Neptune).

Eratosthenes (276-194 BC) was a Greek scholar who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and Alexandria. He properly assumed that the Sun's rays are virtually parallel (since the Sun is so far away). Knowing the distance between the two locations, he calculated the circumference of the Earth to be 250,000 stadia. Exactly how long a stadia is is unknown, so his accuracy is uncertain, but he was very close. He also accurately measured the tilt of the Earth's axis and the distance to the sun and moon.

THE MOONThe Earth has one moon. The diameter of the moon is about one quarter of the diameter of the Earth.

The Earth and the moon. Photo taken by NASA's Galileo mission in 1990.

The moon may have once been a part of the Earth; it may have been broken off the Earth during a catastrophic collision of a huge body with the Earth billions of years

ago.

MASS, DENSITY, AND ESCAPE VELOCITYThe Earth's mass is about 5.98 x 1024 kg.

The Earth has an average density of 5520 kg/m3 (water has a density of 1027 kg/m3). Earth is the densest planet in our Solar System.

To escape the Earth's gravitational pull, an object must reach a velocity of 24,840 miles per hour (11,180 m/sec).

LENGTH OF A DAY AND YEAR ON EARTH

Each day on Earth takes 23.93 hours (that is, it takes the Earth 23.93 hours to rotate around its axis once - this is a sidereal day). Each year on Earth takes 365.26

Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once).

The Earth's rotation is slowing down very slightly over time, about one second every 10 years.

THE EARTH'S ORBIT

Earth rising over the moon. Photo taken by NASA's

Apollo 8 mission.

On average, the Earth orbits 93 million miles (149,600,000 km) from the Sun. This distance is defined as one Astronomical Unit (AU). The Earth is closest to the Sun (this is called perihelion) around January 2 each year (91.4 million miles = 147.1 million km); it is farthest away from the Sun (this is called aphelion) around July 2 each year (94.8 million miles = 152.6 million km).

Orbital EccentricityThe Earth has an orbit that is close to being circular; its orbital eccentricity is 0.017.

Planet-Sun Orbital Diagram

Label the aphelion (farthest point in

orbit) and perihelion (closest point in orbit)

of a planet in orbit.Answers

(Eccentricity is a measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; an eccentricity between 0 and 1 represents an

elliptical orbit.)

THE EARTH'S AXIS TILT AND THE SEASONSThe Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four seasons of the year: Summer, Spring, Winter and Autumn. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year. This affects the amount of sunlight each receives. For more information on the seasons, click here.

SPEEDAt the equator, the Earth's surface moves 40,000 kilometers in 24 hours. That is a speed of about 1040 miles/hr (1670 km/hr). This is calculated by dividing the circumference of the Earth at the equator (about 24,900 miles or 40,070 km) by the number of hours in a day (24). As you move toward either pole, this speed decreases to almost zero (since the circumference at the extreme latitudes approaches zero).

The Earth revolves around the Sun at a speed of about 30 km/sec. This compares with the Earth's rotational speed of approximately 0.5 km/sec (at middle latitudes - near the equator).

For more information on the speed of the Earth, click here.

TEMPERATURE ON EARTHThe temperature on Earth ranges from between -127°F to 136°F (-88°C to 58°C; 185 K to 311 K). The coldest recorded temperature was on the continent of Antarctica (Vostok in July, 1983). The

hottest recorded temperature was on the continent of Africa (Libya in September, 1922).

The greenhouse effect traps heat in our atmosphere. The atmosphere lets some infrared radiation escape into space; some is reflected back to the planet.

For more information on the greenhouse The size of the atmosphere in this

illustration is greatly exaggerated in order to show the greenhouse effect. The Earth's atmosphere is about 300 miles (480 km)

thick, but most of the Earth's atmosphere is within 10 miles (16 km) of the Earth's

surface.

effect, click here.

ATMOSPHEREThe Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases.

The atmosphere was formed by planetary degassing, a process in which gases like carbon dioxide, water vapor, sulphur dioxide and nitrogen were released from the interior of the Earth from volcanoes and other processes. Life forms on Earth have modified the composition of the atmosphere since their evolution.

For more information on the atmosphere, click here.

Earth Activities

Earth coloring page The Earth and the Moon coloring page

Find It! , a quiz on the Earth.

Label the Planet-Sun Orbital Diagram . Answers.

Continents Printout: Label the continents, the poles, and the equator.

Ocean Map: Label the Oceans

Interactive Earth Puzzle

Make a globe from papier-mâché

A simple-to-make earth pendant

Earth day crafts

Earth Cloze Printout : A fill-in-the-blanks activity on Earth. Answers

Write Ten Things About the Earth (plus one thing you would like to change about it).

How to write a report on a planet - plus a rubric.

Interior Structure of the Earth 

Layers of the EarthThe earth is divided into three main layers: Inner core, outer core, mantle and crust.

The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulphur (S). The inner core is under such extreme pressure that it remains solid.

Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner.

Earth Crust:

THE CRUST

1. Continental crust (25-40 km)

2. Oceanic crust (~6 km)

3. Mantle

1. Upper mantle (650 km)

2. Lower mantle (2235 km)

4. Core

1. Outer core: liquid (2270 km)

2. Inner core: solid (1216 km)

The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle , so it can fracture in earthquakes .

The shell of the earth, the crust, can be said to have two different thicknesses.

Under the oceans, it is relatively thin. It varies in thickness from 5 to 8 km. Under the land masses, it is relatively thick. The thickness of the continental crust varies from 10 to 65 km.

The eggshell analogy for the crust is not an exaggeration. It is paper thin compared with the radius of the earth which is approximately 6400 km.

The total weight of the continental crust is less than 0.3% of the weight of the earth.

Variations in the crust thickness are compensated by the weight of the water and the differences in the specific gravities of the crust under the oceans (3.0 to 3.1) and under the continents(2.7 to 2.8).

If one thinks of the crust as virtually floating on the mantle, one is less likely to wonder why the earth does not wobble as it rotates about its axis.

The weight of the crust plus the mantle has a reasonably uniform distribution over the globe.

THE MOHO

The Moho, or the Mohorovicic Discontinuity, refers to a zone or a thin shell below the crust of the earth that varies in thickness from 1 to 3 km.

In seismology, the term "discontinuity" is used in its general sense. It refers to a change over a short distance of a material property. In this case, the "short distance" may be as long as 3 km, a trifle compared with the radius of the earth.

In that zone, the P-wave velocity has been observed to increase from approximately 6 to approximately 8 km/sec.

The Moho is considered to be the boundary between the crust and the mantle.

The increase in P-wave velocity is ascribed to change in composition of the medium. Rocks of the mantle are poorer in silicon but richer in iron and magnesium

THE MANTLE

The mantle can be thought of having three different layers. The separation is made because of different deformational properties in the mantle inferred from seismic wave measurements.

(1) The upper layer is stiff.

It is presumed that if the entire mantle had been as stiff, the outer shell of the earth would stay put. This stiff layer of the mantle and the overlying crust are referred to as the lithosphere. The lithosphere is approximately 80-km thick

(2) Beneath the lithosphere is a soft layer of mantle called the asthenosphere.

Its thickness is inferred to be several times that of the lithosphere.

One may think of this as a film of lubricant although film is not exactly the word for something so thick. It is assumed that the lithosphere, protruding (meaning: extending beyond) parts and all, can glide over the asthenosphere with little distortion of the lithosphere

(3) The mesosphere is the lowest layer of the mantle.

Considering the vagueness in defining the lower boundary of the asthenosphere it would be expected that the thickness and material properties of the mesosphere are not well known.

It is expected to have a stiffness somewhere between those of the lithosphere and the asthenosphere.

THE CORE

It is known that the pressure increases toward the center of the earth. So does the temperature. The liquid outer layer versus the solid inner layer is rationalized by recognizing that the melting point of the material increases (with pressure) at a faster rate than the temperature as the center of the earth is approached.

Minerals and Rock

Minerals are naturally occurring inorganic substances of more or less definite chemical composition, displaying more or less definite physical properties.

Rocks:

Geologist define rock as aggregates or mass composed of one or more commonly, several of minerals. There are few exceptions to this rule: not all rocks are composed of minerals-for example, coal.

Engineers (or contractor) define rock to be a ‘hard, durable material that can’t be excavated without blasting’. The definition is based on strength and durability.

Minerals are naturally occurring inorganic substances of more or less definite chemical composition, displaying more or less definite physical properties.

As the basic constituent of rock, minerals control much of rock behavior. Some minerals are very strong and resistant to deterioration and produce rock with similar properties, while others are much softer and produce weaker rock.

More than different 2000 minerals are present in the earth’s crust. They can be identified by their physical and chemical properties; by standard tests; or by examination under microscope.

1. Colour2. Streak

3. Hardness: Mohs scale of hardness

4. Cleavage

5. Fracture

6. Luster

Color:

1. Some minerals have characteristics color due to composition of the minerals and the arrangement of the constituent atoms: for example black color of magnetite, green of chlorite and brassy yellow of pyrite

2. Minerals like quartz and calcite have variable color

3. Color can’t be sole identification property

Streak:

1. Color of mineral in powder form is called streak2. Powder is obtained by crushing the mineral.

3. Color of the streak differs from color of mineral: for example the color of pyrite is brass yellow and its streak is dark green.

Cleavage:

1. The cleavage of the minerals is its capacity to split more readily in certain directions than in others, due to the arrangement of the atoms.

2. Minerals break with ease producing smooth surfaces is called perfect cleavage. It can be either good, distinct, indistinct and imperfect.

3. Some minerals such as mica have perfect cleavage in one direction. The feldspars, which is the most abundant of all minerals, have two cleavages.

Luster:

1. Appearance of mineral in ordinary light (that is the appearance due to reflected light). Luster may be metallic, glassy, earthy, pearly or silky

2. If the minerals looks metal as do galena and pyrite, its luster is said to be metallic. If the minerals looks glassy, like quartz, its luster is glassy.

Hardness:

1. The hardness of a mineral, as commonly determined on fresh material, is measured by its ability to resist scratching. If a mineral is scratched by a knife, it is softer than the knife. If it cannot be scratched by a knife, the two are equal hardness or the mineral is the harder.

2. In order to have a standard method of expressing hardness of minerals, a simple scale, known as the Mohs scale, has been universally adopted.

3. In sequence of increasing hardness from 1 to 10, the following minerals are used as standard of comparison:

4. Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase (feldspar), Quartz, Topaz, Corundum and Diamond

Other Characteristics:

Crystal Form: Internal atomic arrangement in definite geometric patterns is sometimes outwardly expressed in crystal form.

Specific Gravity is meant the weight of a substance compared with the weight of an equal volume of water. The specific gravity of quartz is 2.65. Some minerals are heavy than the others. The specific gravity of majority minerals range from 2.55 to 3.2.

Magnetism: A few minerals are attracted by a magnet. Of these minerals, magnetite, and pyrrhotite are the most common examples.

Rock Forming Minerals 

1. Feldpars:

Feldspar is the most abundant minerals. There are two types. Orthoclase feldspars contain potassim (KalSi3O8) and usually range from white to pink. Plagioclase feldspars contain sodium (NaAlSi3O8), calcium (CaAl2Si2O8) or both, and range from white to gray to black,. Feldspars have moderate hardness.

2. Quartz is also very common ingredient in many kinds of rock. It is silicate (SiO2), and usually has a translucent to milky white color. The luster is vitreous. Quartz is harder than most minerals (hardness 7), and thus is very resistant to weathering. Chert is a type of quartz sometimes found in sedimentary rocks. It can cause problem when used as concrete aggregate.

3. Mica: Translucent thin sheets or flakes. There are two common varieties. Muscovite is potassium aluminium silicate of colorless or silvery tint, pearly luster and especially one very perfect cleavage which permits the mineral to be split into thin elastic sheets that when bent spring back to shape. Biotite, the other common variety, is a complex silicate of potassium, magnesium and iron and aluminum.

Mica: Biotite and muscovite are similar in physical properties. Both are soft, 2.5-3, with one perfect cleavage. The sheets of mica have very low coefficient of friction, which can produce shear failure in certain rocks, such as schist.

4. Ferromagnesian minerals: A class of minerals, all of which contain both iron and magnesium. This class includes pyroxene, amphibole, hornblende and olivine. These minerals are dark color and a moderate hardness.

5. Calcite: A mineral made of calcium carbonate (CaCO3). It is usually white, pink or gray. It is soluable in water, and thus can be transported by ground water into cracks in rock where it precipitate out of solution. It also can precipitate in soil, becoming a cementing agent. Calcite is much softer then quartz or feldspar. The hardness is 3. Have vigorous reaction to hydrochloric acid.

6. Dolomite: Similar to calcite with magnesium added. Less vigorous reaction to dilute hydrochloric acid.

7. Iron Oxides: Another class of minerals, all of which contain iron (FeO3). The most common iron oxides are hematite, Fe2O3 ; hydrous iron oxide that are often called limonite and magnetite. Although less common, these minerals give a distinctive rusty color to some rocks and soils and can act as cementing agents. The compact varieties have a hardness of 5.5-6, but earthy form are soft. The luster is sub-metallic.

8. Gypsum: A soft minerals often occuring as a precipitate in sedimentary rocks. It is colorless to white and has economic value when found in thick deposits. For example, it is used to make drywall. Gypsum is water soluble and thus can dissolve under the action of ground water, which can lead to other problems.

A look At The Inner Earth

Studying the earth's interior can help us to understand earthquakes, volcanoes, plate tectonics and more about

the inner earths natural processes.

In general the earth's interior has been sorted by Gravity. Heavier elements like iron tend to sink toward the center or core of the earth. Lighter materials, the silicates, oxygen compounds and water have risen to become part of the crust. This action has created distinct layers within the earth and is still in process today.

The Inner Earth is composed of three main parts; the crust, the mantle, and the core as shown in the diagram of the earth's interior below:

The Crust - silicate rocks, primarily granite and basalto Oceanic Crust - mostly basalt

o Continental Crust - igneous, metamorphic, and sedimentary rocks

The Mantle - iron and magnesium rich silicate rocks

o Upper Mantle

o Lower mantle

The Core - iron nickel alloy

o Outer core

o Inner core

The CrustThe crust of the earth is very slowly growing thicker. Volcanic activity is continually adding mass to the crust. Though the crust is solid it is made up of about 12 plates. They are called Tectonic Plates. These plates are in constant motion. The movement is caused by convection currents in the mantle. The movement is very slow, averaging about 2 inches a year. This is about like the growth of your fingernails.

The crust is about 0.5 % of the earth’s total mass.

The crust is made up of silicate rocks such as granite and basalt.

The continental crust is much thicker than the oceanic crust as shown here in the diagram of the earth's crust.

Oceanic crust - The crust under the oceans is about 10 km thick and is generally made up of rock rich in iron and magnesium. These are primarily basalt formed by volcanic action at the mid ocean ridges. The oceanic crust is denser than continental crust.

Continental crust (continental cratons) - Where there are continents the crust is about 30 to 50 km thick. It is made up of igneous, metamorphic, and sedimentary rocks. The continental crust is less dense than the oceanic crust. When the continental crust collides with oceanic crust through plate movement the continental crust rides over the top of the oceanic crust while the oceanic crust is pushed back down towards the mantle.

Earth's Interior - The MantleMuch less is known about the mantle than the crust. The crust we can see, measure, dig and drill. The mantle is different. We have little direct contact with the inner earth. We can tell some things about the mantle by studying volcanoes and what comes out of them.

Much of what we believe is true about the earth's interior comes from studying Seismology. Seismology began as the study of earthquakes and the seismic waves they produced. These waves travel through the earth and move at different speeds in different materials. By studying these waves and how they move through the inner earth we can learn about the its structure.

The upper mantle

Is made up of rocks rich in magnesium and iron, and poor in silica; mostly peridotites. It is about 400 km thick and is much denser than the crust.

It comprise 10% of the earth’s mass

The Lower mantle It is more dense and contains a greater amount of iron than the upper mantle It is about 1900 km thick

It makes up 41% of the earth’s mass

Earth's Interior - The core The core is composed primarily of a nickel-iron alloy. There is an outer core that is liquid and an inner core that is solid.

Outer core o Is about 2100 km thick.

o It makes up about 30 % of the Earth's total mass.

Inner core

o Is about 1300 km thick.

o It makes up about 2 % of the Earth's total mass.

Some points to remember in studying the earth’s interior: There are distinct layers to the earth’s interior. Heavier elements tend to sink to the core

Lighter elements rise toward the surface

Much more is known about the crust than the mantle and core.

What we believe is true about the mantle and core comes from studying seismology.

Introduction

Three centuries ago, the English scientist Isaac Newton calculated, from his studies of planets and the force of gravity, that the average density of the Earth is twice that of surface rocks and therefore that the Earth's interior must be composed of much denser material. Our knowledge of what's inside the Earth has improved immensely since Newton's time, but his estimate of the density remains essentially unchanged. Our current information comes from studies of the paths and characteristics of earthquake waves travelling through the Earth, as well as from laboratory experiments on surface minerals and rocks at high pressure and temperature. Other important data on the Earth's interior come from geological observation of surface rocks and studies of the Earth's motions in the Solar System, its gravity and magnetic fields, and the flow of heat from inside the Earth.

The planet Earth is made up of three main shells: the very thin, brittle crust, the mantle, and the core; the mantle and core are each divided into two parts. All parts are drawn to scale on the cover of this publication, and a table at the end lists the thicknesses of the parts. Although the core and mantle are about equal in thickness, the core actually forms only 15 percent of the Earth's volume, whereas the mantle occupies 84 percent. The crust makes up the remaining 1 percent. Our knowledge of the layering and chemical composition of the Earth is steadily being improved by earth scientists doing laboratory experiments on rocks at high pressure and analyzing earthquake records on computers.

The Crust

Because the crust is accessible to us, its geology has been extensively studied, and therefore much more information is known about its structure and composition than about the structure and composition of the mantle and core. Within the crust, intricate patterns are created when rocks are redistributed and deposited in layers through the geologic processes of eruption and intrusion of lava, erosion, and consolidation of rock particles, and solidification and recrystallization of porous rock.

Figure 1. The oceanic crust at the island of Hawaii is about 5 kilometers thick. The thickness of the continental crust under eastern California ranges from 25 kilometers under the Great Valley to 60 kilometers under the Sierra Nevada.

By the large-scale process of plate tectonics, about twelve plates, which contain combinations of continents and ocean basins, have moved around on the Earth's surface through much of geologic time. The edges of the plates are marked by concentrations of earthquakes and volcanoes. Collisions of plates can produce mountains like the Himalayas, the tallest range in the world. The plates include the crust and part of the upper mantle, and they move over a hot, yielding upper mantle zone at very slow rates of a few centimeters per year, slower than the rate at which fingernails grow. The crust is much thinner under the oceans than under continents (see figure above).

The boundary between the crust and mantle is called the Mohorovicic discontinuity (or Moho); it is named in honor of the man who discovered it, the Croatian scientist Andrija Mohorovicic. No one has ever seen this boundary, but it can be detected by a sharp increase downward in the speed of earthquake waves there. The explanation for the increase at the Moho is presumed to be a change in rock types. Drill holes to penetrate the Moho have been proposed, and a Soviet hole on the Kola Peninsula has been drilled to a depth of 12 kilometers, but drilling expense increases enormously with depth, and Moho penetration is not likely very soon.

The Mantle

Our knowledge of the upper mantle, including the tectonic plates, is derived from analyses of earthquake waves (see figure for paths); heat flow, magnetic, and gravity studies; and

laboratory experiments on rocks and minerals. Between 100 and 200 kilometers below the Earth's surface, the temperature of the rock is near the melting point; molten rock erupted by some volcanoes originates in this region of the mantle. This zone of extremely yielding rock has a slightly lower velocity of earthquake waves and is presumed to be the layer on which the tectonic plates ride. Below this low-velocity zone is a transition zone in the upper mantle; it contains two discontinuities caused by changes from less dense to more dense minerals. The chemical composition and crystal forms of these minerals have been identified by laboratory experiments at high pressure and temperature. The lower mantle, below the transition zone, is made up of relatively simple iron and magnesium silicate minerals, which change gradually with depth to very dense forms. Going from mantle to core, there is a marked decrease (about 30 percent) in earthquake wave velocity and a marked increase (about 30 percent) in density.

The Core

Figure 2. Cross section of the whole Earth, showing the complexity of paths of earthquake waves. The paths curve because the different rock types found at different depths change the speed at which the waves travel. Solid lines marked P are compressional waves; dashed lines marked S are shear waves. S waves do not travel through the core but may be converted to compressional waves (marked K) on entering the core (PKP, SKS). Waves may be reflected at the surface (PP, PPP, SS).

The core was the first internal structural element to be identified. It was discovered in 1906 by R.D. Oldham, from his study of earthquake records, and it helped to explain Newton's calculation of the Earth's density. The outer core is presumed to be liquid because it does not transmit shear (S) waves and because the velocity of compressional (P) waves that pass through it is sharply reduced. The inner core is considered to be solid because of the behavior of P and S waves passing through it.

Cross section of the whole Earth, showing the complexity of paths of earthquake waves. The paths curve because the different rock types found at different depths change the speed at which the waves travel. Solid lines marked P are compressional waves; dashed lines marked S are shear waves. S waves do not travel through the core but may be converted to compressional waves (marked K) on entering the core (PKP, SKS). Waves may be reflected at the surface (PP, PPP, SS).

Data from earthquake waves, rotations and inertia of the whole Earth, magnetic-field dynamo theory, and laboratory experiments on melting and alloying of iron all contribute to the identification of the composition of the inner and outer core. The core is presumed to be composed principally of iron, with about 10 percent alloy of oxygen or sulfur or nickel, or perhaps some combination of these three

elements.

This table of depths, densities, and composition is derived mostly from information in a textbook by Don L. Anderson (see Suggested Reading). Scientists are continuing to refine the chemical and mineral composition of the Earth's interior by laboratory experiments, by using pressures 2 million times the pressure of the atmosphere at the surface and temperatures as high as 20000C.

PLATE TECTONICS

Continental Drift

Alfred Wegener first proposes Continental Drift in his book published in 1915. Suggests that 200 million years ago there existed one large supercontinent which he called Pangaea (All Land)(Figure). This was not really a new idea, but Wegener offered several lines of evidence in support of his proposal.

1. Fit of the Continents - Noted the similarity in the coastlines of North and South America and Europe and Africa. Today the fit is done at the continental shelf and it is nearly a perfect match.

2. Fossil Similarities - Mesosaurus, (Figure) reptile similar to modern alligator which lived in shallow waters of South America and Africa.

3. Rock Similarities

a. Rocks of same age juxtaposed across ocean basins. (Figure)

b. Termination of mountain chains. (Figure)

4. Paleoclimatic Evidence

a. Glacial deposits at equator

b. Coral reefs in Antarctica

Idea was rejected by North American geologists because Wegener couldn't come up with a mechanism for continental drift. Suggested tidal forces, but physicists showed this to be impossible. Wegener dies in 1930 and his idea dies with him.

Magnetism and Paleomagnetism

Earth is a bar magnet with a magnetic north and south. At poles a compass needle dips vertically. Downward at the north pole, upward at the south pole and horizontal at the equator. Magnetic poles do not correspond with geographic poles. Variation is termed the magnetic declination. It is 16 degrees east in California. However, it has been found that even though the magnetic and geographic poles do not correspond today when the location of the magnetic north pole is averaged over a 5,000 year period it does correspond with geographic north. Magnetic pole moves as much as 25 km per year.

Causes of Earth's Magnetism

First thought to be the result of a permanently magnetized core. However, it has been shown that when any substance is heated above 500 degrees C it looses its permanent magnetism.

Earth is a Dynamo - Outer core is a fluid consisting largely of iron, so it is an excellent conductor. Electromagnetic currents are generated and amplified by motion within the liquid caused by convection. Rotation of the Earth unifies the random convective movements generating the magnetic field.

Paleomagnetism

In the 1950's scientists discover how to measure paleomagnetism (magnetism frozen in the rock at the time it formed). With this knowledge scientists could tell the direction and latitude of geomagnetic pole at the time the rock formed. Europeans were the first to extensively study paleomagnetic pole locations and found that by 500 MY ago magnetic north was located near Hawaii. At first it was assumed the poles were free to wander (Apparent Polar Wandering). North American geologists attempted similar studies largely to disprove the Europeans and found that 500 my ago North American rocks showed the magnetic north pole to be in the East Pacific, 3000 miles to the west of the European magnetic north at that time. (Figure)

At the same time a series of bathymetric surveys of the ocean basins revealed a system of ridges and trenches with high heat flow over the ridges. H. Hess (1962) rushes to print with the idea of Sea Floor Spreading. Postulates convection cells beneath ocean basins to drive the spreading.(Figure)

Fred Vines supports Hess with his explanation of symmetrical magnetic stripes on either side of the Atlantic Mid-ocean ridge (Figure).

Plate Tectonics

Theory of Plate Tectonics is born. Plate - is a rigid piece of lithosphere floating on a partially plastic substrate (asthenosphere).

Seven Major Plates (See Figure in your Text)

1. Pacific 2. North American

3. South American

4. African

5. Eurasian

6. Antarctic

7. Indo-Australian

Types of Plate Boundaries (Figure)

A. Divergent (Figure)- Spreading Center - Constructive Margin. Characterized by ocean ridges and sea floor spreading.

B. Convergent - (Figure) Characterized by trenches and island arcs

1. Ocean - Ocean (Japanese Islands) 2. Ocean - Continental (Cascade Mountains)

3. Continent - Continent (Himalayas)

C. Transform - Plates moving past one another along strike- slip faults. (Figure)

Additional Evidence in Support of Plate Tectonics

Distribution of earthquakes along plate margins Location of earthquake foci along steeply-dipping subduction zones

Age dating sediments on either side of the ridge indicates the sediments get progressively older away from the mid-ocean ridge axis

Thickness of sediments also increases away from ridge

Driving Force (Figure)

1. Convection Cells 2. Hot Spots

The Earth's AtmosphereComposition                         Layers

 

Atmosphere...

The Earth is surrounded by a blanket of air called the atmosphere. The atmosphere is made up of various gases that act as a protective shield for the Earth and allow life to exist. Without it, we would be burned by the intense heat of the sun during the day or frozen by the very low temperatures at night.

Composition...

More than three quarters of the atmosphere is made up of nitrogen and most of the rest is oxygen.  However it is the remaining 1%, a mixture of carbon dioxide, water vapour and ozone, that not only produces important weather features such as cloud and rain, but also has considerable influence on the overall climate of the Earth,through mechanisms such as the greenhouse effect and global warming.  (Stay with us

and we'll learn more about the changing climate in a later section!)

Pressure...

The atmosphere consists of five layers, held around the planet by the force of gravity. As you move upwards through the layers, atmospheric pressure decreases rapidly with height and the air temperature also changes. It is these, more complicated, changes in temperature which are used to divide the atmosphere into the layers described below.

Exosphere

  Above a height of about 500km is the exosphere, a layer where the atmosphere merges into space. Satellites are stationed in this area, 500km to 1000km from Earth.

Thermosphere

  The thermosphere is the fourth layer in the atmosphere, between 80km and 110km above the Earth. Space shuttles fly in this area and it is also where the aurora lights are found. Auroras are wispy curtains of light caused when the sun strikes gases in the atmosphere above the Poles.

Mesosphere

  Beyond the stratosphere the air is very thin and cold. This area is known as the mesosphere, and is found between 50km and 80km above the Earth’s surface.

Stratosphere

  The stratosphere is the second layer of air above the Earth’s surface and extends to a height of 50km. It is here that we find the ozone layer. The ozone layer absorbs much of the sun’s harmful radiation that would otherwise be dangerous to plant and animal life.

Troposphere

The troposphere is the layer closest to the Earth, approximately 11km high. Weather occurs only in the troposphere because it is this layer that contains most of the water vapour. Weather is the way water changes in the air, and so without water there would be no clouds, rain, snow or other weather features.

The troposphere is an unstable layer where the air is constantly moving. As a result, aircraft flying through the troposphere may have a very bumpy ride – what we know as turbulence. You may have experienced this when flying before! Because of this turbulence, most jet airlines fly higher above the Earth in the stratosphere. Here the air is more still and clear as they can fly above the clouds.

Although the atmosphere extends to a height of 1000km, it is nevertheless still very important for life on the surface of the Earth.  This is because of something known as air pressure, which we will learn more about on the next page. 

What Is the Difference Between Weather and Climate?

It’s a sweltering midsummer day. “It must be global warming,” mutters someone. But is it the Earth’s changing climate that has made the day so warm? Or, is it just the weather that is so unbearable?

Weather is the mix of events that happen each day in our atmosphere including temperature, rainfall and humidity. Weather is not the same everywhere. Perhaps it is hot, dry and sunny today where you live, but in other parts of the world it is cloudy, raining or even snowing. Everyday, weather events are recorded and predicted by meteorologists worldwide.

Climate in your place on the globe controls the weather where you live. Climate is the average weather pattern in a place over many years. So, the climate of Antarctica is quite different than the climate of a tropical island. Hot summer days are quite typical of climates in many regions of the world, even without the affects of global warming.

Climates are changing because our Earth is warming, according to the research of scientists. Does this contribute to a warm summer day? It may, however global climate change is actually much more complicated than

that because a change in the temperature can cause changes in other weather elements such as clouds or precipitation.

limate and Weather

Some Definitions of Climate and WeatherWhat is the difference between weather and climate? It is a matter of time scale. Weather is the day-to-day stuff: on Thursday it might be “good,” on Saturday it might be “bad.” The climate cannot be harsh on Thursday and mild on Saturday, because it is not measured in terms of days, but in terms of many years. The weather can change in an hour, say, when a cold front moves in, bringing Arctic air on a Christmas day, suddenly making it is freezing cold. Or, on a summer day, the barometer falls announcing the arrival of a low-pressure center and a rainstorm that is not far behind. In contrast, when we say “the climate has changed,” we make a statement about a long-term shift from cold to warm, or from dry to wet, or vice versa.

Weather is typically described by a weather map, showing lines of equal pressure (called isobars), boundaries between air masses (called fronts), and direction of motion. Climate is described in terms of overall temperature distributions, air pressure distributions, precipitation and cloud cover, and by seasons. Also, climate is described in terms of its variability between years (called the "interannual variability").

A well-known climate change was the arrival of the “Little Ice Age” in Europe, sometime in the 14th century. Winters became more severe and many summers were cool and wet in northwestern Europe. In the centuries following, the Thames River froze regularly, and the ice cover provided a place for a winter fair for the people of London. In Holland, the canals froze over, and the Dutch painter Pieter Bruegel (1520-69) made his famous pictures showing ice skaters within winter landscapes. In the second half of the 19th century the climate warmed again, and the "Little Ice Age" ended.

Weather and Weather MapsA commonly seen weather map in mid-latitudes describes a large cyclonic eddy, which forms when a tongue of warm, moist tropical air invades cold, dry polar air masses. The center of the cyclone is a region of low pressure (indicated by the letter L). There are two fronts: a warm front and a cold front. An observer on the ground sees first the warm front, as the cyclone moves north - eastward. Air pressure drops, temperature rises with the arrival of the tropical air, and so does humidity. Cloudiness increases and a

 

Winter Landscape with Skaters and Bird Trap.

BRUEGEL, Pieter the Elder (b. cca. 1525, Breughel, d.

1569, Bruxelles). This is the most copied and imitated painting of the artist. The

moral content of the subject: life is full with dangers both for the young skaters and the

birds.

An idealized model of cyclonic eddy circulation in North America. On weather

maps, warm fronts are depicted by a red solid line with semi-circles that point towards the cooler air. Cold fronts are depicted by a solid blue line with triangles that point towards the cooler air. Cyclones are an area of low pressure (marked by the L)

around which airflows counterclockwise on the

northern hemisphere. The warm front is pushing

northward and a cold front is pulling southward.

rainstorm may follow. The reason for rain along fronts is that warm air surrounded by cold air must rise, and as it rises, it cools and loses its ability to hold water. The condensation of the water vapor within the rising air releases heat, so that the air warms and keeps rising till condensation stops. A few days later the cold front arrives, restoring cooler, drier weather. Cyclones are separated by high pressure centers (marked by an H on weather maps). Air moves clockwise around these H centers (anticyclonic circulation), on the northern hemisphere. The reason that the air moves in eddies, rather than in straight paths, because of the rotation of the Earth.

Climate and Regional Pattern MapsClimate is typically described by the regional patterns of seasonal temperature and precipitation over 30 years. Average annual temperature, average rainfall, average cloud cover, and average depth of frost penetration are all typical climate-related statistics. The main expression of climate on land is in the vegetation and the soil type produced in each climatic region. The idea to classify climate by temperature, rainfall and vegetation was introduced by Wladimir Köppen in 1900. This classification of climates was later modified by Rudolph Geiger and is now generally accepted as follows:

Humid Tropics (A): Known for their high temperatures year-round and for their large amounts of perennial precipitation. These regions are found near the equator.

Arid Climates (B): Characterized by little precipitation and huge daily temperature range.

Humid Middle Latitudes Climates (C): Dominated by land/water differences. The eastern seaboard of the U.S. would be an example of this class, with cool winters and mild summers.

Continental Climate (D): Found in the interior regions of land masses of exceptional size (i.e. Omaha, in the middle of the U.S.A.). Total precipitation is not very high in amount, and seasonal temperatures vary greatly.

Cold Polar Climates (E) Areas covered by permanent ice and tundra. Here, average temperatures reach above freezing only about one third of the year.

As the picture illustrates, the ocean has a huge influence on the climate. Climates can be classified as "marine" or "continental" depending on distance to the ocean. Both the highest and the lowest temperatures appear in the interior, far from the moderating influence of the ocean. Note how the lines of climate zones tend to run parallel to the coast, especially in western North and South America, or from interior to exterior of the continent, as in Australia. Precipitation is highest where warm marine air has access to coastal mountain ranges, as for example in the coastal regions of Oregon and Washington.

Consequences of Rotationfor Weather

Advancing cold fronts often bring rain.

A world map of Köppen climate types. See text for descriptions. Smaller case

letters refer to subclassifications that are not

discussed here.

The Earth is a spinning globe where a point at the equator is travelling at around 1100 km/hour, but a point at the poles is not moved by the rotation. This fact means that projectiles moving across the Earth's surface are subject to Coriolis forces that cause apparent deflection of the motion.

Coriolis ForcesThe following diagram illustrates the effect of Coriolis forces in the Northern and Southern hemispheres.

The Coriolis force deflects to the right in the Northern hemisphere and to the left

in the Southern hemisphere when viewed along the line of motion.

Solar Heating and Coriolis Forces

Since winds are just molecules of air, they are also subject to Coriolis forces. Winds are basically driven by Solar heating. As the adjacent (highly idealized) image indicates, Solar heating on the Earth has the effect of producing three major convection zones in each hemisphere.

If solar heating were the only thing influencing the weather, we would then expect the prevailing winds along the Earth's surface to either be from the North or the South, depending on the latitude. However, the Coriolis force deflects these wind flows to the right in the Northern hemisphere and to the left in the Southern hemisphere. This produces the prevailing surface winds illustrated in the adjacent

figure.

For example, between 30 degrees and 60 degrees North latitude the solar convection pattern would produce a prevailing surface wind from the South. However, the Coriolis force deflects this flow to the right and the prevailing winds at these latitudes are more from the West and Southwest. They are called the prevailing Westerlies.

Realistic Weather PatternsThe adjacent animation shows GOES-8 weather satellite images over a 72-hour period from Dec. 29, 1996, through Jan. 1, 1997. This is a geosynchrous satellite, which means that it orbits the Earth with the same period as the Earth's rotation and therefore appears to be essentially motionless over a fixed position on the Earth's surface. For GOES-8 this fixed position looks down on North and South America.

In these composite images red indicates visible light (reflected sunlight), green indicates the 11 micron IR channel (thermal emission), and blue indicates the 3.9 micron channel (thermal + sunlight). At night the images are blue and green. The three periods of daylight in this 72 hour sequence are clearly visible as red-orange regions moving from East to West (right to left). In the IR channels, the natural intensity pattern has been inverted: warmer is darker, so that cool cloudtops stand out brightly.

One can see clearly the pronounced cloud flows associated with the strong westerlies at mid-latitudes in each hemisphere. (This is taken in Northern hemisphere Winter, so the heavier cloud cover in that hemisphere is not surprising.) Less obvious are the easterly

trade winds and the polar easterlies, though one can see vestiges of each if one looks carefully. Also apparent are the swirling motions associated with frontal systems. These are particularly pronounced at the boundaries between the mid-latitude westerly and polar wind flows in each hemisphere.

Cyclones & AnticyclonesThe swirling motions evident in the preceding animations are consequences of frontal systems anchored to high and low pressure systems, which are also called anticyclones and cyclones, respectively. The wind flow around high pressure (anticyclonic) systems is clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere. The corresponding flow around low pressure (cyclonic) systems is counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere. This is a consequence of the Coriolis force, as illustrated for the Northern hemisphere in the following figure.

Low pressure systems (left) and high pressure systems (right) in the Northern hemisphere

Magnetic Field of the EarthThe Earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth. The problem with that picture is that the Curie temperature of iron is about 770 C . The Earth's core is hotter than that and therefore not magnetic. So how did the Earth get its magnetic field?

Magnetic fields surround electric currents, so we surmise that circulating electic currents in the Earth's molten metalic core are the origin of the magnetic field. A current loop gives a field similar to that of the earth. The magnetic field magnitude measured at the surface of the Earth is about half a Gauss and dips toward the Earth in the northern hemisphere. The magnitude varies over the surface of the Earth in the range 0.3 to 0.6 Gauss.

The Earth's magnetic field is attributed to a dynamo effect of circulating electric current, but it is not constant in direction. Rock specimens of different age in similar locations have different directions of permanent magnetization. Evidence for 171 magnetic field reversals during the past 71 million years has been reported.

Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field. Mariner 2 found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth. Venus's rotation period of 243 Earth days is just too slow to produce the dynamo effect.

Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions for the aurora phenomena near the poles.

The north pole of a compass needle is a magnetic north pole. It is attracted to the geographic North Pole, which is a magnetic south pole (opposite magnetic poles attract).

THE AURORA

Northern and Southern Lights

Illustrated

The aurora -- the northern and southern lights -- is a product of magnetic fields of the Earth and Sun and of the solar wind. The Sun's gases are in constant motion. The Sun rotates differentially with a period of 25 days at the equator (a velocity of 2 kilometers per second), but longer -- up to 30 days -- closer to the rotation poles. As a result the solar gases "shear," or slide, against themselves. The outer third of the Sun is also in a state of convection, in which hot gases rise, radiate their heat at the surface (which shines at temperature of 5780 Kelvin), and thereby cool and descend. As a result of convection, the solar surface (the photosphere) takes on a finely granulated appearance.

The interior solar gases are ionized, electrons stripped from atoms. Motion of an ionized gas will produce a magnetic field. Solar rotation and convection thus combine to produce a dynamo that generates a solar magnetic field, which because of the differential rotation becomes concentrated into thick magnetic "ropes." The magnetic fields float upward, and where the "ropes" loop outward through the solar surface they inhibit the convection and thereby cool the local gases. The result is a pair of sunspots.

The field loops are unstable and are in a continuous state of formation, connection with each other, and collapse. The steady release of magnetic energy heats an outer solar corona (visible during eclipses of the Sun) to a temperature of two million or so degrees Kelvin. The magnetic fields in turn confine the corona into vast arc-like structures. Where the loops do not hold the corona back, it expands to create the solar wind, a flow of atomic particles (protons and electrons, basically ionized hydrogen) that impacts the planets and causes the gas tails of comets to be pushed away from the Sun.

Comets are "dirty snowballs," made of ices into which are embedded rocks and dust. When a comet nears the Sun, the ices sublime away to gas, which is ionized by sunlight and pushed away from the Sun by the solar wind to create the blue self-luminous gas tail. The released dust is pushed away by the pressure of sunlight, which is reflected from the tiny grains to create the white dust tail. That comet tails point away from the Sun was the first indication of the existence of the solar wind. (J. B. Kaler)

The Earth has a magnetic field that is produced by the rotation and convection of its deep, liquid iron core (which occupies about half the terrestrial radius and about a third of the Earth's mass). When the wind hits the Earth, it flattens our magnetic field on the sunward side, and stretches it out on the opposing side into a long magnetotail. Particles trapped by the Earth's field also form donut-shaped protective radiation belts that encompass us. Where the particles leak into the Earth's atmosphere around our magnetic poles, they excite the atoms of the Earth's upper air,

causing it to glow as the northern lights in a ring around the magnetic axis (which is tilted by about 10 degrees relative to the rotation axis). The ring's radius of about 20 degrees carries it across Alaska and northern Canada, where aurorae are common sights.

As the speeding solar wind hits the Earth's magnetic field, it creates a shock wave, compresses the forward side of the field, and stretches the far side into a long magnetotail. The field traps particles into the donut-shaped Van Allen radiation belts, which then protect the Earth against the wind. The interaction of the wind and the Earth's field generate two rings of electrical current that flow around the magnetic poles (which are offset relative to the rotation axis) and that in turn create the aurora. (From Stars, J. B. Kaler, Scientific American Library, Freeman, NY, 1992.)

When large-scale unstable magnetic loops reconnect and collapse, great balls of coronal gas are released from the Sun at high speed. If such a coronal mass ejection hits the Earth (the journey taking a couple days), it greatly disturbs our magnetic field and causes the auroral ring to expand to lower latitudes, allowing much of the more southerly population to see one nature's great displays. The northern lights are mirrored by the southern lights that center on the south magnetic pole, and expand northward during impacts of coronal mass ejections.

Imaging theEarth

As we launch our exploration of the rest of the Solar System, it is useful to recall what the Earth looks like using the various imaging techniques that we may wish to apply to distant planets and moons.

The Earth in Visible LightWe have seen a number of images of Earth in visible light, but mostly at large scale from a great distance. Here are two images from space of smaller objects on the Earth that we will be interested in looking for on other planets and moons: a canyon system, and an active volcano.

The Grand Canyon from space (Source) Mt. Etna from space (Source)

We shall find canyon systems and active volcanoes on other objects in the Solar System to are not too different from these images. Here is a set of links to satellite imaging of the Earth.

The Earth at NightThere is one aspect of the Earth's appearance that we do not expect to be repeated in the near future for other objects in the Solar System: at night the artificial light associated with human civilization is very visible from space. The following image shows the appearance of the United States at night as observed from a composite of many satellite passes.

The USA at night

The major source of light is from cities, but by looking carefully you can even see things like lights scattered in the Gulf of Mexico south of Louisiana that are from oil platforms (Ref). The photograph is from Defense Meteorological Satellite Program (DMSP) images.

Imaging in Ways other than Visible LightBecause our eyes are sensitive to visible light, our prejudice is to view things at those wavelengths. However, we now have instruments at our disposal that permit observations in many wasy other than the visible light region of the electromagnetic spectrum. These often offer considerable advantage; for example radar cuts through the ever-present thick cloud cover to give us images the surface of Venus that we could not obtain at visible wavelengths.

Radar Imaging of the Earth's Surface

The adjacent images show a comparison of the Mt. Everest region (border of Nepal and Tibet). The top image was taken through thick cloud cover with synthetic aperture radar on the space shuttle Endeavor; The bottom figure is an optical image of the same region taken from Endeavor (Ref). One can see many of the same features in the two photographs (the photographs were taken at different times of the year, so they have different snow covers).

The curving and branching features are glaciers. The radar technique used is sensitive to characteristics of the glacier surfaces such as the ice roughness and water content. Thus the glaciers show a variety of colors in the radar image but are a rather featureless gray or white in the optical photograph.

Radar (upper) and visual (lower) images of Mount Everest

Infrared and more Exotic Imaging

We have seen in the preceding sections examples of imaging the Earth in the infrared, ultraviolet, and X-ray regions of the spectrum. Here we show additional examples of IR images, and a more exotic technique combining magnetic and gravitational data that can even locate objects beneath the surface of the planet.

San Francisco Bay imaged in IR from space

Fossil crater imaged with representation of gravity and

magnetic field data

GOES-8 IR satellite image of water vapor in Earth's

atmosphere

The left figure shows the San Francisco Bay area imaged from space in the infrared (IR). Click on the image for a larger version revealing quite fine details such as bridges and highways (Source).

The middle figure shows a composite of local gravity and magnetic field variation data to image a 112-mile wide relic meteor crater in Yucatan that presently lies below several hundred meters of sedimentary rock. This crater, called Chicxulub, is famous because it is the leading candidate for the site of the asteroid impact that is thought to have killed the dinosaurs 65 million years ago in the K-T extinction (Ref).

The right image shows a GOES-8 weather satellite image in the 6.7 micron IR channel that is sensitive to the distribution of water vapor in Earth's atmosphere (Ref). The imager on this satellite records radiation emitted by water vapor in the upper troposphere. Regions with high concentrations of water vapor are bright, while dark spots signal lower water vapor concentrations.

Surface Temperature Maps

Infrared radiation is basically radiant heat. Therefore, IR detected from satellites can be used to determine the temperature of objects. The following image shows a color-coded map constructed from a composite of satellite data and surface observations giving surface temperatures on the Earth (Ref).

Surface temperatures on Earth for January 30, 1997

Here is a link to the current temperature map (updated on a 6 hour cycle), and here is a movie (2MB MPEG---slow download) of the temperature varation over the past week.

Sea Surface Temperature Maps

Similar methods as described above may be used to construct color coded maps of surface seawater temperatures. Here is an example (Ref):

Sea surface temperatures on Earth for January 29, 1997