Geography Day 2: THE EARTH

Our Earth

Earth is the third planet from the Sun.

It is the densest and the fifth largest planet of the solar system.

It is also the largest of the four inner planets of the solar system.

Earth was formed around 4.6 billion years ago.

Initially, the earth was a molten planet with a very high temperature.

It was heavily cratered and had no oceans or the continents.

The earth started cooling gradually.

The atmosphere along with the continents and the oceans evolved in the subsequent stages.

These conditions paved the way for emergence of life on earth.

Life appeared first in the oceans around one billion years after the earth was formed.

The biosphere and other physical processes on the earth significantly altered the atmosphere.

The formation of the Ozone layer and earth’s magnetic field blocked the harmful solar radiations.

This permitted the ocean confined life to safely move to the land.

Movements of the Earth 

Galactic movement: It is the movement of the earth around the centre of the galaxy.

Rotation: It is the movement of the earth around its own axis. The earth rotates on a tilted axis of 23.5°. The earth takes 23 hours 56 minutes and 4 seconds for one rotation. The rotation causes days and nights. The rotational velocity of the earth is maximum at the equator and is nearly zero at the poles.

Revolution: It is the movement of earth around the Sun. The Earth moves around the Sun in an elliptical orbit and takes 365 ¼ days to complete one revolution. The shortest and the longest distance between the Sun and the Moon are called perihelion and aphelion, respectively.

Tilt of the Earth’s axis

The axis of the earth is inclined to the ecliptic plane (orbital plane: plane along which the earth orbits around the Sun) at an angle of 66.5°, giving rise to different seasons and varying lengths of days and nights. 

 If the axis was perpendicular to the orbital plane, all parts of the globe would have equal days and nights at all the times of the year.

Shape of the Earth

Earth is not a perfect sphere, but an oblate spheroid or ellipsoid.

This is because it is flattened at the poles and slightly bulged at the equator.

The equatorial bulge is caused due to rotation of the earth.

This bulge causes the diameter at the equator to be 43 km larger than the pole-to-pole diameter.

Therefore the diameter at the poles and equator vary.

Co ordinates of the Earth

Latitudes

These are the imaginary lines running horizontally around the globe.

Lines joining points of the same latitude are called parallels, and they trace concentric circles on the surface of the Earth, parallel to the equator and to each other.

The latitude of a point on the Earth’s surface is the angle between the equatorial plane and a line that passes through that point and is normal to the surface of a reference ellipsoid which approximates the shape of the Earth.

Each degree of latitude is about 110 km apart.

Zero degrees (0°) latitude is the equator, the widest circumference of the globe. The equator divides the globe into the Northern and Southern hemispheres.

Latitude is measured from 0° to 90° north and 0° to 90° south. The latitude 90° north is the North Pole and 90° south is the South Pole.

Longitudes

These are the imaginary lines running vertically around the globe. They are also called meridians.

The longitude of a point on the Earth’s surface is the angle east or west between a fixed reference meridian called the prime or first meridian and the meridian of that point.

All meridians are halves of great ellipses (often improperly called great circles), which converge at the north and south poles.

Unlike latitude lines, longitude lines are not parallel. Meridians meet at the poles and are widest apart at the equator.

The meridian passing near the Royal Observatory, Greenwich (near London, UK) has been chosen as the international zero degrees longitude (0°) reference line, the Prime Meridian.

The 180 degree line is a single vertical line called the International Date Line, and it is directly opposite the Prime Meridian.

Places to the east are in the eastern hemisphere, and places to the west are in the western hemisphere.

The degrees of longitude run 180° east and 180° west from the prime meridian.

Geographic coordinates

Latitude and longitude lines form imaginary grids over the Earth’s surface. By combining longitude and latitude measurements, any location on earth can be determined.

The units of measurement for geographic coordinates are degrees (°), minutes (‘) and seconds (“).

Like a circle, the Earth has 360 degrees. Each degree is divided into 60 minutes, which in turn is divided into 60 seconds.

Latitude and longitude coordinates also include cardinal directions: north or south of the equator for latitude, and east or west of the prime meridian for longitude.

Hemisphere

A hemisphere is half the Earth’s surface. The four hemispheres are the Northern and Southern hemispheres, divided by the equator (0° latitude), and the Eastern and Western hemispheres, divided by the prime meridian (0° longitude) and the International Date Line (180° longitude).

Time zones

The Earth’s time zones are measured from the prime meridian (0o longitude).

The time at 0° longitude is called ‘Co-ordinated Universal Time’ (UTC) or the ‘Greenwich Mean Time’ (GMT).

With the Greenwich meridian as the starting point, each 15° east and west marks a new time zone.

The 24 time zones extend east and west around the globe for 180° to the International Date Line.

When it is noon along the prime meridian, it is midnight along the International Date Line.

A difference of 15 degrees in the longitude means a difference of one hour between the GMT and the place, whose longitude we are measuring.

When traveling west, we subtract one hour when we enter a new time zone. Conversely, for every time zone boundary we cross while travelling east, we add one hour.

International Date Line: It is an imaginary line on the surface of the earth running from north to south pole located at 180° longitude (180° E and 180° W are the same meridian).

Although the International Date Line generally follows the 180° meridian (most of which lies in the Pacific Ocean), it does diverge in places.

Since 180° runs through several countries, it would divide those countries not simply into two different time zones, but into two different calendar days.

To avoid such unnecessary confusion, the date line dips and bends around countries to permit them to share the same time.

Immediately to the left of the International Date Line (Eastern Hemisphere), the date is always one day ahead of the date (or day) immediately to the right of the International Date Line in the Western Hemisphere.

How does the International Date Line work?

If a person flies from the United States to Japan on Tuesday morning. Since he is traveling west, the time advances slowly, but once he crosses the International Date Line, it is suddenly Wednesday and he loses a day.

On the reverse trip, if he flies from Japan to the United States on Friday morning, as he crosses the Pacific Ocean, the day gets later quickly as he crosses time zones moving eastward in an air-place. However, once he crosses the International Date Line, the day changes to Thursday and he gains a day.

Example:

If a plane departed at 4:00 pm on November 2nd from Tokyo, Japan (approximately 135 degrees E), and the flight takes 9 hours and 30 minutes, what time and date would the plane arrive in Los Angeles?

The starting and ending points of the trip are 105° or seven time zones apart.

The plane took off from Japan at 4:00 pm on November 2nd, in Los Angeles it was 11 pm of November 1st.

Adding 9 and a half hours to this, the landing time would be 8:30 am on November 2nd.

Though it seems quite odd, this means that passengers who leave Japan in the afternoon on some date and fly for 9 and half hours, will reach Los Angeles in the morning of the same date.

Earth’s position with respect to Moon

The orbit of the Moon is not circular, but elliptical.

Apogee and perigee both refer to the position of the Moon in its elliptical orbit with respect to Earth.

At one end of this ellipse it makes its closest approach to the Earth (356 410 km), known as perigee.

At the other end it reaches its greatest separation (406 697 km), called apogee.

In other words, Moon is at its apogee when it is the farthest and is at perigee when it is closest to the earth.

It is due to this variation in the distance between the Earth and the Moon, the Moon appears slightly bigger when at perigee and smaller at apogee.

Earth’s position with respect to Sun 

The Earth’s orbit around the Sun is not a circle, but is slightly elliptical. Therefore, the distance between the earth and the sun varies throughout the year.

At its nearest point on the ellipse, the Earth is 147,166,462 km from the Sun.

This point in the Earth’s orbit is known as perihelion and it occurs around January 3.

The Earth is farthest away from the Sun on around July 4, when it is 152,171,522 km from the Sun. This point in the earth’s orbit is called aphelion.

Equinox

As the Earth moves around its orbit, it reaches two points during the year (around 20 March and 22 September), where the tilt of its axis causes it to be straight relative to the Sun, i.e., the tilt of the Earth’s axis is inclined neither away from nor towards the Sun.

On 20th March it is called Spring or Vernal equinox; it will be the first day of the season of Spring, whereas on 22nd September it is called Autumn equinox when it is the first day of the season of Autumn.

The Sun is vertically overhead at the equator on these two days.

These two days are termed as equinoxes meaning ‘equal nights’ because on these two days all parts of the world have equal days and nights.

Solstice

The solstice occurs twice in each orbit of the earth around the sun.

After the March equinox the Sun appears to move north and is vertical overhead the Tropic of Cancer on about 21st June. This is known as June or Summer Solstice, when the northern hemisphere will have its longest day and shortest night.

This will be the first day of the season of Summer.

By about 22nd December, the Sun will be overhead at the Tropic of Capricorn. This is the Winter Solstice when the northern hemisphere will have its shortest day and longest night. This will be the first day of the season of Winter.Earth’s position with respect to Sun.

In the Southern Hemispheres, the solstices are reversed.

In general, the summer solstice is when the sun appears highest in the sky and the day is at its longest.

The winter solstice is when there is least amount of sunlight and the day is shortest.

When summer occurs in a hemisphere, it is due to that hemisphere receiving more direct rays of the Sun than the opposite hemisphere where it is winter. In winter, the sun’s energy hits the earth at oblique angles and is thus less concentrated.

EVOLUTION OF THE EARTH

The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium.

Over the years, the earth has transformed into a beautiful planet with life sustaining gases and water. This transformation is taking place since the earth was born 4.6 billion years ago.

Evolution of Lithosphere, Atmosphere and Hydrosphere

The earth during its early stages was in a volatile state.

Due to gradual increase in density, the temperature inside the earth has increased.

The material inside the earth gradually started getting separated into different layers depending upon the densities.

Heavier materials like the iron sank towards the centre of the earth and lighter ones moved towards the surface. Three distinct layers formed mainly the Crust, the Mantle and the Core.

The evolution of atmosphere may be divided into three stages.

1. In the first stage there was loss of the primordial atmosphere.
2. The hot interior of the earth contributed to the evolution of the atmosphere by means of degassing (release of water vapour and other gases) in the second stage.
3. Finally in the third stage, the atmosphere was modified by the biotic community through the process of photosynthesis.

Continuous volcanic eruptions contributed water vapour and gases to the atmosphere, the water vapour thus released started getting condensed as the earth began to cool.

The carbon dioxide dissolved in the rain water causing further decrease in temperature resulting in more condensation and more rains.

The rain water got collected in the depressions to give rise to oceans. The process of photosynthesis got evolved around 2500-3000 million years before the present, which progressively contributed oxygen to the atmosphere.

Geological Time Scale

The Geological Time is a system of chronological measurement that relates stratigraphy to time and is used to describe timing and relationships between events that have occurred throughout earth’s history.

The vast expanse of geological time has been separated into eons, eras, periods and epochs.

Eons	Era	Period	Epochs	Years before present in million years	Major events
	13700- 5000 million years			1370012000 5000	Big BangOrigin of Universe Origin of Sun
HadeanArchean Proterozoic	Azoic or the Precambrian Era			4800-38003800-2500 2500-570	Formation of oceans and continentsBlue green algae: Unicellular bacteria Soft bodied arthropods
	Palaeozoic570-245 million	CambrianOrdovician Silurian Devonian Carboniferous Permian		570-505505-438 438-408 408-360 360-286 286-245	No terrestrial life: Marine invertrebratesFirst fish Plants on land Amphibians First reptiles; coal beds Reptiles dominate
	Mesozoic245-65 million	TriassicJurassic		245-208208-144	Frogs and turtlesAge of dinosaurs
		Cretaceous		144-65	Extinction of dinosaurs
	Cenozoic65million years to present	Tertiary	PalaeoceneEocene Oligocene Miocene Pliocene Pleistocene Holocene	65-5758-37 37-24 24-5 5-2 2 million-10000 years 10000 to present	Small mammals(rats)Rabbits and hare Arthropod ape Flowering plants and trees Early humans Homo sapiens Modern man

Interior of the Earth

Dynamic changes on the surface of earth are related to the deep laid internal forces operating from within the earth.

Therefore, it is imperative to understand the interior of the earth.

Our knowledge about the interior of the earth comes from direct and indirect sources.

Direct sources

Surface rocks or the rocks which we obtain from the mining areas are the most easily available references.

Deep Ocean Drilling Projects and the deepest drill at Kola, Russia which is 12 km deep has contributed to some extent in understanding the interior of the earth.

Volcanic eruptions form another source of obtaining direct information. The molten material obtained from the eruption sites is analysed at the laboratories to know the composition.

Indirect sources

Most of our knowledge about the interior of earth is based on inferences drawn from indirect sources.

The indirect sources include gravitation, temperature and pressure studies, density studies, magnetic field and seismic activity.

The information obtained from the seismic activity is the most important of all the sources.

Seismic Studies

Seismic studies also called Seismology is the study of earthquake waves or seismic waves. These waves have two properties which make them useful in revealing the earth’s internal structure.

• Seismic waves are reflected when they strike the interface between two different materials having different elastic properties and density.
• Seismic waves bend or refract when they enter a medium in which they travel at a different speed, and the angle of refraction depends on the difference in wave velocities between the two medium.

These properties of reflection and refraction of the seismic waves and their travel time have been used in the study of interior of the earth.

Seismic waves can be measured by a seismograph after their travel through the earth.

Three types of waves have been recorded:

• P-waves or Primary waves: These are longitudinal in nature. They depend upon density and compressibility of the medium for their propagation. They travel through both solids and liquids.
• S-waves or Secondary waves: These are transverse in nature. They depend upon density and rigidity of the medium for their propagation, hence they cannot pass through liquids. This characteristic of S-waves has helped the scientists to understand the structure of interior of earth.
• L-waves or Surface waves: These are confined to earth’s surface and hence are not important for the study of earth’s interior.

In order to study the earth’s interior, these seismic waves are recorded at various places on earth’s surface. From an analysis of arrival times of these waves at various places, the known relationship between wave speeds and paths can be used to discover how the speed of seismic waves varies with depth inside the earth. If wave speed at any given depth is known, this can provide physical parameters such as density, rigidity and compressibility for various layers within the earth.

An analysis of the seismic waves has revealed that P and S waves behave differently in their travel through earth’s interior.

Velocity of P and S waves increases up to 2900 km which marks the beginning of outer core, there after S-waves disappear as they cannot pass through liquid, thus providing evidence of a liquid core and the P-waves slow down confirming the change of medium from solid to liquid.

Velocity of P-waves increases in the inner core, signifying that the medium has changed from liquid to solid.

A further analysis of P-wave shadow zone (where P-waves have been refracted) and S-wave shadow zone (where S-waves have been blocked) provides further data for mapping earth’s interior.

STRUCTURE OF THE EARTH

A vertical cross section of the earth’s interior shows the following zones or layers:

• The Crust
• The Mantle and
• The Core

The Crust

It is the outermost layer of the earth and is brittle in nature.

The thickness of the crust varies under the oceans and the continents. The oceanic crust is thinner in comparison to the continental crust.

The average thickness of oceanic crust is 5 km whereas that of the continental crust is around 30 km.

The continental crust is thicker (up to 70 km) in the areas of mountain ranges.

The continental crust is basically granitic (SIAL-Silicon and Alumininum).

It has two layers, upper layer is Felsic (Feldspar+Silica) and lower layer is Mafic (Magnesium+Iron).

The continental crust is less dense as compared to the oceanic crust.

The oceanic crust is mainly basaltic (SIMA- Silicon and Magnesium).

The average density of the upper crust is 2.8 g/cm3  and that of lower crust is 3.0 g/cm3.

A layer called Mohorovicic Discontinuity separates the crust from the upper mantle.

The Mantle

It extends from Mohorovicic discontinuity to a depth of 2900 km.

The upper portion of the mantle is called asthenosphere.

It extends from 100 km to 400 km and is plastic in nature.

It is a zone of low velocity for seismic waves.

It is the main source of magma which finds its way to the surface of earth during volcanic eruptions.

The mantle is divided into upper and lower mantle which are separated by the Repetti discontinuity. Mantle is believed to be composed of silicate minerals rich in iron and magnesium.

Mantle accounts for 68% of the total mass of earth and 83% of the total volume of earth.

Mantle has a density of 3.4 g/cm3.

The Core

This is the innermost and the densest layer of the earth.

It is divided into inner and outer core.

The outer core is separated from the mantle by the Weichert-Gutenberg discontinuity.

The outer core must be liquid in nature as S-waves deflect from here.

It extends up-to a depth of 5150 km.

The outer and inner core is separated by the Lehman discontinuity. 

The inner core is solid in nature and extends up-to 6371 km. The inner core is solid because of excessive pressure and heavy metals (mainly nickel and iron).

The density of the core at the centre is believed to be 13 g/cm3.

ELEMENTS, MINERALS AND ROCKS

The earth is composed of various kinds of elements.

What are elements?

Elements are substances that are made entirely from one type of atom.

An element is the simplest form of matter in that it cannot be further broken down by any chemical means.

Examples for elements: Iron (Fe), Gold (Au) , Hydrogen (H), Carbon (C), Oxygen (O), Helium (He), etc.

Various elements combine to form compounds.

Example: Water (H2O), Carbon dioxide (CO2), etc.

The following are the elements which constitute the earth when taken as a whole.

Elements	Percentage
1.Iron2.Oxygen 3.Silicon 4.Magnesium 5.Nickel 6. Sulphur 7.Calcium 8.Aluminium 9.Others	3530 15 13 2.4 1.9 1.1 1.1 1.0

About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium and the rest is constituted by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements.