Earth

Basic Facts

• Earth formed approximately 4.6 billion years ago, as explained by theories of Solar System formation.
• It is the third planet from the Sun and the fifth-largest planet in the Solar System.
• It is an inner, terrestrial, solid planet.

Goldilocks or Habitable Zone

The Goldilocks zone is the region around a star where conditions may permit liquid water and therefore potentially support life.

Earth supports life because of a combination of factors, including:

• abundant liquid water;
• a suitable atmosphere containing gases such as nitrogen, oxygen, and carbon dioxide; and
• a moderate global average surface temperature of about 15-16 °C.

Shape of Earth

Earth’s shape is described as an oblate spheroid, or more precisely a geoid. It is slightly wider at the equator and flatter at the poles because rotation and the resulting centrifugal effect produce an equatorial bulge.

Geoid Shape and Its Effects

Earth has an approximately geoid, or oblate-spheroid, shape: it bulges slightly at the equator and is flattened at the poles.

• Earth’s radius is greatest at the equator and decreases towards the poles.
• Because gravitational acceleration varies inversely with distance from Earth’s centre, it is relatively lower near the equator and higher near the poles.

Structure of Earth

Earth can be studied in terms of its solid internal layers and gaseous atmospheric layers.

Solid Layers

These layers are distinguished chiefly by mineral composition and density.

Gaseous Layers of the Atmosphere

These layers are distinguished mainly by temperature variation:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
5. Exosphere

Spheres of Earth

• Lithosphere: the solid part of Earth.
• Atmosphere: the gaseous envelope surrounding Earth.
• Hydrosphere: Earth’s liquid or water component.
• Biosphere: the zone in which life exists through the interaction of the lithosphere, atmosphere, and hydrosphere.

Fields Generated by Earth

Earth generates several fields:

• Magnetic field: associated primarily with the movement of iron-rich material in the outer core.
• Electric field: associated with charged particles and ions.
• Gravitational field: produced by Earth’s mass.

Rotation of Earth

Rotation is Earth’s movement around its own axis.

• Earth takes approximately 24 hours, or one Earth day, to complete a rotation.
• It rotates from west to east.

Axis of Rotation

The axis of rotation is the imaginary line passing through the North Pole, Earth’s centre, and the South Pole, around which Earth rotates.

Earth’s axis is tilted:

• approximately 23.5° from the perpendicular to the orbital plane; or
• approximately 66.5° from the orbital plane itself.

The tilt is not fixed. It varies between approximately 22.1° and 24.5°.

Milankovitch Cycle: Variation in Axial Tilt

The long-term variation in Earth’s axial tilt between about 22.1° and 24.5° is one component of the Milankovitch cycles. This variation, known as obliquity, repeats over a period of roughly 41,000 years.

Effects of Rotation

1. Day and Night

Rotation produces day and night. The imaginary circle separating the illuminated half of Earth from the dark half is called the circle of illumination.

Because Earth is axially tilted, different places experience variations in the length of day and night over the course of a year.

2. Earth’s Shape

Rotation generates centrifugal force, contributing to Earth’s characteristic oblate-spheroid or geoid shape: a slight equatorial bulge and polar flattening.

3. Coriolis Force

Earth’s west-to-east rotation produces an apparent deflective effect called the Coriolis force.

Its effect differs by hemisphere:

• moving objects are deflected to the right in the Northern Hemisphere;
• moving objects are deflected to the left in the Southern Hemisphere.

Revolution of Earth

Revolution is Earth’s movement around the Sun in a fixed, elliptical orbit.

• Earth takes approximately 365.25 days to complete one revolution.
• The additional quarter-day accumulates to approximately one full day every four years.
• This extra day, 29 February, is called a leap day; a year containing it is a leap year.
• In the Gregorian calendar, years divisible by 4 are generally leap years. Century years are exceptions unless they are also divisible by 400.

Effects of Revolution

1. Earth’s Position Relative to the Sun

Because Earth’s orbit is elliptical, its distance from the Sun varies through the year.

The notes emphasise that this distance variation helps keep Earth’s annual temperature range moderate. However, Earth’s axial tilt-not its changing distance from the Sun-is the primary cause of the seasons.

2. Creation of Seasons

Earth’s revolution around the Sun, combined with its axial tilt, produces four seasons:

1. Spring
2. Summer
3. Autumn or fall
4. Winter

The seasonal cycle is marked by the apparent overhead position of the Sun on particular dates.

• An equinox occurs when the Sun’s direct rays fall on the equator.
• A solstice occurs when the Sun’s direct rays reach one of the tropics.

Equinoxes

On an equinox, the Sun’s rays fall directly on the equator and neither pole is tilted towards the Sun. Day and night are approximately equal in duration across Earth.

Spring or Vernal Equinox

• Occurs around 21 March.
• Marks the transition from winter towards summer in the Northern Hemisphere.
• Temperatures generally begin to rise in the Northern Hemisphere.

Autumn or Fall Equinox

• Occurs around 23 September.
• Marks the transition from summer towards winter in the Northern Hemisphere.
• Temperatures generally begin to fall in the Northern Hemisphere; this is the sense in which it is called the fall equinox in the notes.

Solstices

Summer Solstice

The Northern Hemisphere’s summer solstice occurs around 21 June, when the Sun’s rays fall directly over the Tropic of Cancer (23.5° N).

Conditions in the Northern Hemisphere

• It is summer.
• Temperatures are relatively high.
• Days are longer and nights are shorter.
• Places between the Arctic Circle and the North Pole experience up to 24 hours of daylight.

Conditions in the Southern Hemisphere

• It is winter.
• Temperatures are relatively low.
• Days are shorter and nights are longer.
• Places between the Antarctic Circle and the South Pole experience up to 24 hours of darkness.

Winter Solstice

The Northern Hemisphere’s winter solstice occurs around 22 December, when the Sun’s rays fall directly over the Tropic of Capricorn (23.5° S).

Conditions in the Northern Hemisphere

• It is winter.
• Temperatures are relatively low.
• Nights are longer and days are shorter.
• Places between the Arctic Circle and the North Pole experience up to 24 hours of darkness.

Conditions in the Southern Hemisphere

• It is summer.
• Temperatures are relatively high.
• Days are longer and nights are shorter.
• Places between the Antarctic Circle and the South Pole experience up to 24 hours of daylight.

Seasonal Summary

Apparent Movement of the Sun

Because of Earth’s revolution and axial tilt, the Sun appears to migrate between the Tropic of Cancer and the Tropic of Capricorn during the year.

• Dakshinayan (southward movement): From about 21 June to 22 December, the overhead Sun appears to move southward from 23.5° N, across the equator, to 23.5° S.
• Uttarayan (northward movement): From about 22 December to 21 June, it appears to move northward from 23.5° S, across the equator, to 23.5° N.

Latitude and Longitude

Latitudes and longitudes form an imaginary coordinate grid used to identify the exact location of a place on Earth.

• Latitudes run horizontally, or east-west.
• Longitudes run vertically, connecting the poles.

Latitudes

Latitudes are imaginary circles that measure the angular distance of a place north or south of the equator.

• They are parallel to one another.
• They are conventionally counted from 0° at the equator to 90° at each pole.
• The equator is the latitude with the largest radius.
• Their circumference decreases towards the poles, where each becomes a point.

Equator

The equator (0°) is the largest circle of latitude. It passes through Earth’s centre in an east-west plane and lies at an equal distance from both poles.

It divides Earth into:

• the Northern Hemisphere, north of the equator; and
• the Southern Hemisphere, south of the equator.

Uses of Latitude

Latitudes can be used to estimate north-south distance, provided the locations are compared along a meridian.

Earth’s circumference is approximately 40,000 km, so:

[ 1^\circ \text{ of latitude} \approx \frac{40,000}{360} \approx 111 \text{ km} ]

For example, India’s approximate latitudinal extent from 8° N to 38° N is 30°. This gives an estimated north-south distance of 30 × 111 approximately 3,330 km; its measured extent is about 3,214 km.

Important Latitudes

• The tropics are the farthest latitudes at which the Sun can appear directly overhead.
• The polar circles mark the approximate limits within which at least one full day of continuous daylight or darkness occurs each year.
• These important latitudes help divide Earth into heat zones.

Heat or Temperature Zones

Earth has three broad heat zones based on the angle and intensity of incoming sunlight.

1. Tropical or Torrid Zone

• Extends from the Tropic of Cancer to the Tropic of Capricorn.
• Receives the most direct sunlight and is Earth’s hottest broad heat zone.
• The midday Sun is overhead at least once a year at every latitude in this zone.
• It is overhead once annually at each tropic and twice annually at locations between the tropics.

2. Temperate Zones

• The northern temperate zone lies between the Tropic of Cancer and Arctic Circle.
• The southern temperate zone lies between the Tropic of Capricorn and Antarctic Circle.
• These regions receive sunlight throughout the year, but the Sun is never directly overhead.
• They generally have moderate temperatures and experience distinct seasons.

3. Polar or Frigid Zones

• Lie between each polar circle and its corresponding pole.
• Experience periods of 24-hour daylight and 24-hour darkness.
• Receive highly slanting solar rays and form the coldest broad heat zones.

Longitudes

Longitudes are imaginary semicircles, or meridians, running from pole to pole. They measure angular distance east or west of the Prime Meridian.

• A meridian and its opposite antimeridian together form a great circle.
• The Prime Meridian and the 180° meridian divide Earth into the Eastern and Western Hemispheres.
• There are 180 degrees of longitude east and 180 degrees west of the Prime Meridian.
• Meridians are farthest apart at the equator and converge at the poles.

Prime Meridian and GMT

The Prime Meridian (0°) was adopted internationally in 1884. It passes through the Royal Observatory at Greenwich, London.

The time at Greenwich became the reference known as Greenwich Mean Time (GMT):

• places east of Greenwich are ahead in time; and
• places west of Greenwich are behind in time.

International Date Line

The International Date Line (IDL) broadly follows the 180° meridian, where the time difference across the global longitude system reaches one calendar day.

• It bends around countries and island groups to avoid dividing them between two dates.
• It passes through the Bering Strait region.

Indian Standard Meridian and IST

India’s standard meridian is 82.5° E. It passes near Mirzapur in Uttar Pradesh and through Madhya Pradesh, Chhattisgarh, Odisha, and Andhra Pradesh.

Indian Standard Time is five hours and thirty minutes ahead of GMT:

[ \text{IST} = \text{GMT} + 5\text{ h }30\text{ min} ]

Calculating Time with Longitude

Earth rotates through 360° in 24 hours:

[ 360^\circ = 1,440 \text{ minutes}; \qquad 1^\circ = 4 \text{ minutes}; \qquad 15^\circ = 1 \text{ hour} ]

To compare local solar times:

• add four minutes per degree when moving east; and
• subtract four minutes per degree when moving west.

Crossing the International Date Line

Crossing the IDL changes the calendar date:

• when travelling westward, advance the date by one day;
• when travelling eastward, move the date back by one day.

This adjustment reconciles the approximately 24-hour difference between places immediately on opposite sides of the line.

Time Zones and Standard Time

Sir Sandford Fleming proposed a worldwide system of time zones in 1879.

• In principle, Earth’s 24-hour rotation divides it into 24 zones of 15° longitude each.
• Political boundaries and the need for a common national time mean actual time-zone boundaries rarely follow meridians exactly.
• Countries may select a central meridian for one national standard time or adopt multiple time zones.
• India uses 82.5° E rather than the ideal 15° increments of 75° E or 90° E because it passes closer to the country’s centre.
• During British rule, Bombay and Calcutta used local time standards based approximately on 75° E and 90° E respectively.
• Geographically large countries use several zones; the notes cite Russia, the United States, and Canada.

Overview of Theories on the Formation of Continents and Oceans

The notes trace the development of ideas through:

1. Continental Drift Theory - Alfred Wegener
2. Convection Current Theory - Arthur Holmes
3. Seafloor Spreading Theory - Harry Hess
4. Palaeomagnetic studies and radiometric dating
5. Plate Tectonic Theory - associated with McKenzie, Parker, and W. J. Morgan

Continental Drift Theory

German geographer Alfred Wegener presented Continental Drift Theory in 1915 as an early systematic explanation for the formation and distribution of continents and oceans.

Main Propositions

• Earth’s continents were once united in a supercontinent called Pangaea.
• Pangaea was surrounded by a vast ocean called Panthalassa.
• Around 220 million years ago, Pangaea began splitting into:
  • Laurasia in the north; and
  • Gondwanaland in the south.
• The water body between them was the Tethys Sea.
• These landmasses continued moving and ultimately formed the present continents and oceans.

Wegener’s Proposed Directions and Forces of Drift

Equatorward Drift

Wegener attributed movement towards the equator to a combination of:

• centrifugal force generated by Earth’s rotation;
• pole-fleeing force, which he believed drove continents away from the poles; and
• buoyant force, arising from continents floating on denser underlying material.

Westward Drift

Wegener attributed westward movement to the Moon’s gravitational pull and tidal drag acting against Earth’s west-to-east rotation. He proposed that this caused different parts of the landmass to move at different rates and break into the present continents.

These proposed driving forces were later shown to be inadequate, although the central idea that continents move was foundational.

Evidence Supporting Continental Drift

1. Jigsaw Fit

• The east coast of South America broadly fits the west coast of Africa.
• The margins around the Gulf of Mexico and Hudson Bay were also compared with parts of western Europe.

2. Geological Similarities

• Rock composition, structure, and age in the Appalachian Mountains of North America correspond with mountain belts in Ireland, Wales, and western Europe.

3. Tectonic Fit

• When the continents are reconstructed, the Caledonian mountain belts of northeastern North America, Greenland, and Scandinavia align as a continuous chain.

4. Permo-Carboniferous Glacial or Tillite Deposits

• Similar ancient glacial deposits occur in Antarctica and in present-day tropical or subtropical parts of South America, Africa, India, and Australia.
• Their distribution suggests these continents were once joined near the South Pole and later drifted apart.

5. Fossil Evidence

• Matching fossils of land organisms occur on opposite sides of the Atlantic.
• The ocean is too wide for these organisms to have crossed naturally, supporting the idea that the continents were once connected.

Criticism of Continental Drift Theory

• It did not explain why drift began when it did.
• It did not adequately explain the proposed equatorward and westward directions.
• Continents cannot simply plough through oceanic crust as Wegener envisaged.
• Lunar gravitational pull is too weak to drive continental movement.
• The theory did not explain major ocean-floor features such as mid-ocean ridges, trenches, and volcanic arcs.

Despite these limitations, Continental Drift Theory was a pioneering idea that laid the foundation for later theories.

Seafloor Spreading Theory

Harry Hess proposed Seafloor Spreading Theory in the early 1960s. It developed beyond continental drift by using evidence from:

1. Arthur Holmes’s convection-current concept;
2. palaeomagnetic studies; and
3. radiometric dating of ocean-floor rocks.

Convection Current Theory

Arthur Holmes proposed that heat within the mantle generates convection currents:

• Hot material rises through weak zones; this is the rising limb of a convection current.
• Near the cooler surface, it spreads laterally and solidifies.
• Farther away, cooler, denser material descends through weak zones; this is the falling limb.
• At depth, the material heats again and continues the circulation.

Hess applied this mechanism to the ocean basins. He proposed that new crust forms where mantle material rises at the centre of an ocean, spreads outward, and eventually returns to the mantle at its margins.

Palaeomagnetic Evidence

Palaeomagnetism is the study of records of Earth’s past magnetic field preserved in rocks.

• Basalt formed by volcanic activity contains iron-bearing minerals.
• As lava cools, these minerals align with the direction of Earth’s magnetic field at that time.
• Earth’s magnetic polarity has reversed repeatedly through geological history.
• Alternating, symmetrical bands of normal and reversed magnetic polarity occur on both sides of mid-ocean ridges.
• This symmetry shows that new oceanic crust formed repeatedly at the ridge and moved outward in both directions.

Dating Ocean-Floor Rocks

Radiometric dating shows that:

• rocks are youngest at the mid-ocean ridge;
• their age increases progressively towards continental margins; and
• even the oldest oceanic crust is much younger than Earth, indicating that old seafloor is recycled into the mantle.

Where oceanic crust descends, it forms a subduction zone and deep ocean trench. At spreading centres, rising and solidifying magma forms mid-ocean ridges. Volcanic material may also emerge above sea level to form volcanic islands and island chains, or archipelagos.

Significance of Seafloor Spreading

Seafloor Spreading Theory addressed major weaknesses in Continental Drift Theory by:

• identifying internal Earth processes capable of moving crust;
• explaining mid-ocean ridges and trenches; and
• using magnetic patterns and the age of seafloor rocks as supporting evidence.

Notebook page 74 contains a summary diagram of seafloor spreading; its substantive labels and relationships are incorporated above.

Plate Tectonic Theory

Plate Tectonic Theory was developed through work by scientists including Dan McKenzie, Robert Parker, and W. J. Morgan. It provides a unified explanation for much of Earth’s structural and geological dynamism and incorporates continental drift, seafloor spreading, and mantle convection.

Lithosphere

The lithosphere consists of the crust and the rigid uppermost mantle.

• It is a solid outer layer with an average thickness of roughly 100 km, though its thickness varies.
• It includes both continental and oceanic regions.

Types of Crust

Asthenosphere

The asthenosphere is the weak, plastic part of the upper mantle beneath the lithosphere.

• In the notes, it is placed approximately 100-350 km below the surface.
• High temperature and pressure allow its material to deform slowly and, in places, partially melt.
• The rigid lithosphere moves over this mechanically weaker layer.

Lithospheric Plates

• The lithosphere is broken into fragments of varying size called tectonic or lithospheric plates.
• Earth is commonly described as having seven major plates and numerous minor plates.
• Plates move mainly horizontally, driven by interacting internal processes including mantle convection, slab pull, and ridge push.
• A plate may be continental, oceanic, or a combination of both.

Plate movement produces three main types of boundary:

1. divergent or constructive;
2. convergent or destructive; and
3. transform or conservative.

1. Divergent Boundaries: Constructive Margins

At a divergent boundary, two plates move away from one another, opening a crack or depression. Magma rises and solidifies to create new crust, so the margin is called constructive.

The notes associate divergence with:

• mid-ocean ridges;
• rift valleys and block mountains; and
• volcanic plateaus.

Mid-Ocean Ridges

Oceanic divergence forms long, continuous submarine mountain chains as rising mantle material solidifies on either side of the boundary. The Mid-Atlantic Ridge is an example.

Rift Valleys and Block Mountains

Continental divergence stretches and fractures the crust. Where blocks subside along faults, a long linear depression called a rift valley forms; raised blocks on either side form block mountains.

Examples include:

• the Narmada Rift Valley and the Vindhya-Satpura block mountains; and
• the Great African Rift Valley, where the Nubian and Somali parts of the African Plate are separating.

If East African rifting continues over geological time, the region may split and a new ocean basin may eventually form.

Volcanic Plateaus

Local crustal divergence may create long fissures. Repeated lava eruptions through these openings build layers of basalt, eventually forming a broad, elevated, flat-topped volcanic plateau. The Deccan Traps of India are an example.

2. Convergent Boundaries: Destructive Margins

At a convergent boundary, two plates move towards each other. Crust may be consumed through subduction or intensely compressed, so the margin is termed destructive.

There are three main types:

1. continent-ocean convergence;
2. ocean-ocean convergence; and
3. continent-continent convergence.

Continent-Ocean Convergence

When a continental plate collides with an oceanic plate:

• the denser oceanic plate sinks beneath the continental plate, forming a subduction zone;
• a deep ocean trench forms near the subduction boundary;
• earthquakes occur along the inclined Wadati-Benioff zone;
• descending oceanic crust contributes to melting and magma generation, producing volcanism; and
• compression raises fold mountains along the continental margin.

The Andes and parts of the Rockies are examples of mountain systems associated with continent-ocean convergence around the Pacific margins.

Ocean-Ocean Convergence

When two oceanic plates converge:

• the older or denser plate generally subducts beneath the other;
• a trench forms at the subduction zone;
• earthquakes occur along the Wadati-Benioff zone; and
• magma produces a curved chain of volcanic islands called an island arc.

Examples include the Aleutian Islands, Japan, the Philippines, and Indonesia.

Continent-Continent Convergence

Continental crust is relatively buoyant, so neither plate readily subducts when two continents collide.

• Ocean trenches and volcanic arcs are generally absent.
• Compression, folding, and crustal thickening create very high fold mountains.
• Earthquakes are common, including many shallow-focus earthquakes.
• The line along which the continental masses join is called a suture zone.

Examples include:

• the Himalayas, formed by collision of the Indian and Eurasian plates;
• the Indus-Tsangpo Suture Zone between those plates;
• the Alps, associated with convergence of the African and Eurasian plates; and
• the Zagros Mountains, associated with convergence of the Arabian and Eurasian plates.

The notes also cite older interior fold belts such as the Urals, Aravallis, and Appalachians as products of past continental convergence.

3. Transform Boundaries: Conservative Margins

At a transform boundary, also called a transform fault or strike-slip boundary, two plates slide horizontally past one another.

• Crust is neither created nor destroyed, so it is called a conservative margin.
• Friction and sudden rupture release seismic energy and cause earthquakes.
• Subduction, trenches, and boundary-related volcanism are generally absent.

Examples include:

• the San Andreas Fault between the Pacific and North American plates; and
• the Dead Sea Transform, associated with the Arabian and African plates.

Plate-Boundary Summary