THE EARTH

AN INTRODUCTION

Origin and Evolution of the Earth

Early Theories

1755 – Immanuel Kant -The Solar System is formed from the nebulous material.

1796 – Laplace gave Nebular Hypothesis. The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

1900 – Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets.

1929 – Tidal hypothesis was given by James Jeans and it was modified by Jeffrey and was called as Collision hypothesis.

Modern theories – Big Bang Theory

• Alternatively called the expanding universe hypothesis.
• As per this theory, in the beginning, all matter or substance forming this universe existed at one place as a tiny ball. This tiny ball had an extremely small volume, infinite density and temperature.
• At the Big Bang, this ball blasted fiercely and forcefully and started a substantial process of expansion which continues to this day.
• Now it is accepted that this event took place 13.7 billion years ago.

Evolution of the Earth

Evolution of the Earth

The age of Earth is approximately one-third of the age of the universe. Earth formed around 4.54 billion years ago by accretion from the solar nebula.

• • The first stage of the evolution of Lithosphere, Atmosphere, and Hydrosphere is marked by the loss of primordial atmosphere.
  • In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere.
  • Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis.

The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.

The Size and Shape of Earth

The Size and Shape of Earth

The Size of Earth

• • Earth’s circumference – At the equator, Earth’s circumference is 40,075.16 km. It is slightly smaller between the North and South poles at 40,008 km.
  • Earth’s diameter – At the poles is 12,713.5 km while it is 12,756.1 km at the equator.

The Shape of Earth

• Our planet is properly described as an oblate spheroid rather than a true sphere.

The Surface of the Earth

The Surface of the Earth

• • Its surface varies in elevation from the highest mountain peak, Mount Everest, at 8850 meters above sea level, to the deepest oceanic trench, the Mariana Trench of the Pacific Ocean, at 11,033 meters below sea level, a total difference in elevation of 19,883 meters.

Distance from Sun and Moon

• Distance between the Earth and the Sun is 150,000,000 kilometers
• Distance between the Earth and the Moon is 385,000 kilometers

THE GEOGRAPHIC GRID— LATITUDE AND LONGITUDE

THE GEOGRAPHIC GRID— LATITUDE AND LONGITUDE

Any understanding of the distribution of geographic features over Earth’s surface requires some system of accurate location. The simplest technique for achieving this is a grid system (Latitude & Longitude).

Latitude

• • Latitude is a description of location expressed as an angle north or south of the equator.
  • Latitude varies from 0° at the equator to 90° north at the North Pole and 90° south at the South Pole.
  • Seven latitudes are of particular significance in a general study of Earth.

1. Equator, 0°

2. Tropic of Cancer, 23.5° N

3. Tropic of Capricorn, 23.5° S

4. Arctic Circle, 66.5° N

5. Antarctic Circle, 66.5° S

6. North Pole, 90° N

7. South Pole, 90° S

Zones of Latitude

Regions on Earth are sometimes described as falling within general bands or zones of latitude. The following common terms associated with latitude are used throughout this book (note that there is some overlap between several of these terms):

• • Low latitude—generally between the equator and 30°N and S
  • Midlatitude—between about 30° and 60° N and S
  • High latitude—latitudes greater than about 60° N and S
  • Equatorial—within a few degrees of the equator
  • Subtropical—Around 25–30° N and S
  • Polar—within a few degrees of the North or South Pole

Each degree of latitude on the surface of Earth covers a north–south distance of about 111 kilometers (69 miles). The distance varies slightly with latitude because of the flattening of Earth at the poles.

Longitude

Longitude

Longitude is an angular description of east–west location, also measured in degrees, minutes, and seconds.

Prime meridian – 0° longitude

Reference line for east–west measurement.

At least 13 prime meridians were in use in the 1880s.

Now Internationally accepted Prime Meridian passes through the Royal Observatory at Greenwich, England.

International date line – 180th meridian. Its Zigzag.

These lines, called meridians, are not parallel to one another except where they cross the equator.

Earth Movements

Earth Movements

Earth’s Rotation on Its Axis: Earth rotates from west to east on its axis a complete rotation requiring 24 hours.

Earth’s Revolution around the Sun: Each revolution takes 365 days, 5 hours, 48 minutes, and 46 seconds, or 365.242199 days.

Earth’s Rotation

• • Earth rotates along its axis from west to east.
  • It takes approximately 24 hrs to complete on rotation.
  • Days and nights occur due to rotation of the earth.
  • The circle that divides the day from night on the globe is called the circle of illumination.
  • Earth rotates on a tilted axis. Earth’s rotational axis makes an angle of 23.5° with the normal i.e. it makes an angle of 66.5° with the orbital plane. Orbital plane is the plane of earth’s orbit around the Sun.
  • The South and North Magnetic Poles are the wandering points on the Earth’s Northern and Southern Hemisphere where the geomagnetic field lines are directed vertically upwards
  • Temperature falls with increasing latitude because of the spherical (Geoid) shape of the earth and the position of the sun. So the energy received per unit area decreases from equator to poles and also Equator receives direct sunlight while Poles receive slant or oblique rays of the Sun.

TIME ZONES

• To standardize time, so that we could all agree on the time no matter where we were, we developed different TIME ZONES.
• To make time zones, we divided the Earth up into 24 different sections, with each one 15 degrees apart. This makes it so that every time zone is a one hour difference.
• As you travel East each time zone decreases by one hour
• As you travel West each time zone increases by one hour.

The second motion of the earth around the sun in its orbit is called revolution. It takes 365¼ days (one year) to revolve around the sun. Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.

The second motion of the earth around the sun in its orbit is called revolution. It takes 365¼ days (one year) to revolve around the sun. Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.

Revolution

Revolution develop the seasons of the year , which are determined by reference to both the solstices and the equinoxes.

• Two Solstices occur annually, around June 21 and December 21.
• Two Equinoxes occurs twice each year: around 20 March and 23 September.

Solstice

Summer Solstice – 21 June

• On this day Tropic of Cancer in North Hemisphere receives vertical rays of Sun.
• North hemisphere observes longest day and South Hemisphere has longest night on Summer Solstice.
• At this time the North Pole experiences continuous day while South Pole observes continuous night.
• During period of Summer solstice to Autumnal Equinox, the North Hemisphere is inclined towards the Sun and this is the reason why the North Hemisphere observes summer in this period. Since South Hemisphere is inclined away from the sun at this period of time the South Hemisphere observes winter.

Solstice

Winter Solstice – 22 Dec

• Earth is in equivalent opposite position of its orbit as compared to June 21.
• On this day the North Pole is away from view of Sun and South Pole is inclined towards the Sun.
• At this time the Tropic of Capricorn receives vertical rays from the sun.
• South Hemisphere has longest day and North Hemisphere has longest night. At this time North Pole observes continuous night and South Pole observes continuous day.

Equinox

An equinox is commonly regarded as the instant of time when the plane of Earth’s equator passes through the center of the Sun.The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditional harvest festivals are celebrated on the date of the equinoxes.

Vernal Equinox – March 21 Equinoxes occur midway between the Solstices. During equinoxes Earth’s Polar axis is at 90° with the line drawn from Sun. In other words the equator receives vertical rays from Sun. The days and nights all over the Earth are equal on Equinoxes.

Autumnal Equinox – Sept 23 Autumnal equinox occurs when the Earth is in equivalent opposite position of its orbit as compared to Vernal Equinox.

Other Points

• Between periods of Vernal Equinox (March 21) to Summer Solstice (June 21). the North Hemisphere has spring and South Hemisphere has autumn season.
• Between Summer Solstice (June 21) to Autumnal Equinox (Sept 23) the North Hemisphere has summer season and South Hemisphere has winter season.
• During Autumnal Equinox to Winter Solstice (Dec 22) the North Hemisphere has autumn season and South Hemisphere has spring season.
• During Winter Solstice to Vernal Equinox, North Hemisphere has winter season and South Hemisphere has summer season.

Solar Eclipse

It is caused when the moon Revolving around the Earth comes in between the Earth and the Sun, thus making a part or whole of the sun invisible from a particular part of the Earth thus, the eclipse can be partial or complete. It occurs during the day.

Types of Solar Eclipses

There are 4 different types of solar eclipses. How much of the Sun’s disk is eclipsed, the eclipse magnitude, depends on which part of the Moon’s shadow falls on Earth.

1. Partial solar eclipses occur when the Moon only partially obscures the Sun’s disk and casts only its penumbra on Earth.

2. Annular solar eclipses take place when the Moon’s disk is not big enough to cover the entire disk of the Sun, and the Sun’s outer edges remain visible to form a ring of fire in the sky. An annular eclipse of the Sun takes place when the Moon is near apogee, and the Moon’s antumbra falls on Earth.

3. Total solar eclipses happen when the Moon completely covers the Sun, and it can only take place when the Moon is near perigee, the point of the Moon’s orbit closest to Earth. You can only see a total solar eclipse if you’re in the path where the Moon’s casts its darkest shadow, the umbra.

4. Hybrid Solar Eclipses, also known as annular-total eclipses, are the rarest type. They occur when the same eclipse changes from an annular to a total solar eclipse, and/or vice versa, along the eclipse’s path.

Lunar eclipse

A lunar eclipse occurs when the Moon passes directly behind Earth and into its shadow. This can occur only when the Sun, Earth, and Moon are exactly or very closely aligned (in syzygy), with Earth between the other two. A lunar eclipse can occur only on the night of a full moon. The type and length of a lunar eclipse depend on the Moon’s proximity to either node of its orbit.

Earth’s Interior

THE EARTH INTERIOR AND ITS MATERIAL

The internal structure of the Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.

The internal structure of the Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.

Crust The crust is the outermost layer of the earth making up 0.5-1.0 per cent of the earth’s volume and less than 1 percent of Earth’s mass.

Depth – The thickness of the crust varies in the range of range of 5-30 km in case of the oceanic crust and as 50-70 km in case of the continental crust.

Lithosphere – crust + uppermost section of the mantle, It is an irregular layer with a max. thickness 200 km.

Mohorovicic (Moho) discontinuity forms the boundary between crust and asthenosphere.

Density

• • Density increases with depth, and the average density is about 2.7 g/cm3 (average density of the earth is 5.51 g/cm³).
  • The crust made up of heavier rocks having a density of 3 g/cm3.
  • The kind of rock seen in the oceanic crust is basalt.

Temperature

• Temperature of the crust increases with depth, reaching values typically in the range from about 200 °C to 400 °C at the boundary with the underlying mantle.
• The temperature increases by as much as 30 °C for every kilometre in the upper part of the crust.

Composition –

• • Continents are composed of lighter silicates — silica + aluminium (also called sial) like granite.
  • Oceans have the heavier silicates — silica + magnesium (also called sima)silicate igneous rocks like basalt.

Most Abundant Material of the Earth’s Crust rocks

By Element Approximate % by weight

1. Oxygen (O) 46.6

2. Silicon (Si) 27.7

3. Aluminium (Al) 8.1

4. Iron (Fe) 5.0

5. Calcium (Ca) 3.6

6. Sodium (Na) 2.8

7. Potassium (K) 2.6

8. Magnesium (Mg) 1.5

By Oxides Appro. % by weight

Silica 59.1%

Alumina 15.2%

Iron 6.8%

Lime 5.1%

Soda 3.7%

Magnesia 3.5%

Potash 3.1%

Water 1.3%

Mantle • It forms about 83 per cent of the earth’s volume and holds 67% of the earth’s mass.

Depth –

• It extends from Moho’s discontinuity to a depth of 2,900 km.

Density

• • The density of the upper mantle varies between 2.9 g/cm3 and 3.3 g/cm3
  • The density ranges from 3.3 g/cm3 to 5.7 g/cm3 in the lower mantle.

Temperature

• • In the mantle, temperatures range from approximately 200 °C at the upper boundary with the crust to approximately 4,000 °C at the core-mantle boundary.
  • Because of the temperature difference, there is a convective material circulation in the mantle (although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile).
  • Convection of the mantle is expressed at the surface through the motions of tectonic plates.
  • High-pressure conditions ought to inhibit seismicity in the mantle. However, in subduction zones, earthquakes are observed down to 670 km (420 mi).

Constituent elements – The mantle is made up of 45% oxygen, 21% silicon, and 23% magnesium (OSM).

Asthenosphere –

• • This is the upper portion of the mantle.
  • The word astheno means weak.
  • It is considered to be extending up to 400 km.
  • It is the main source of magma..
  • It has a density higher than the crust.

Mohorovicic (Moho) discontinuity forms the boundary

between crust and asthenosphere.

Core

Earth’s Layers – Seismic Discontinuities

• Mohorovicic Discontinuity (Moho) – separates the crust from the mantle, its average depth being about 35 km.
• A soft asthenosphere (highly viscous, mechanically weak and ductile). It’s a part of mantle.
• Gutenberg Discontinuity – lies between the mantle and the outer core. Below 2900 km from earth’s surface.

2 Earth’s Crust Chemical Composition

3 Earth’s Chemical Composition