Age and Structure of The Earth

Age

It is concluded on the basis of radiometric dating that the Earth’s crust must have been formed 4.6 billion years ago. 

1.Denudation or Sedimentation-

• This method is based on the fact that deposition has been going on in the basins right from the origin of the Earth.

Age=Total Thickness of Sediment/Sediments deposited in one year. 

2.Salinity-

• In this method, it is presumed that the first formed seas did not have saline water. All the salts of the seas today have been acquired from continuous weathering and erosion of rocks. 

Age=Total total salts of oceans/Salt deposited in one year. 

3. Tree rings:

• Trees have their annual growth rings. Secondary growth rings of trees can be counted. Each pair of such rings represents one year. 

4.Fossils-

• Age of earth can be measured on basis of evolution of animals and plants and fossils of these animals and planets can help to know about age of the earth. 

5.Glacial varves: 

• Glacial varves are deposits generated due to combined action of river(s) and glacier(s) which can give us idea about age of the earth. 

6.Radioactive Methods

• The rate of decomposition of radioactive substances can give us an idea about the age the earth Usually, the ‘half life’ period is the time in which half of the material is consumed or disintegrated and half of the number of atoms remains out as original and gives idea about The Earth. 

There are different methods-

Uranium-Lead method. 

Thorium-Lead method. 

Potassium-Argon method. 

Rubidium-Strontium method. 

Radiocarbon Dating method etc.

Structure of Earth includes following parts-

Atmosphere. 

Hydrosphere. 

Biosphere. 

Lithosphere. 

In this section we will restrict ourselves only to Lithosphere and earths interior and other parts will be discussed later on separately. 

Lithosphere-

• Lithosphere is the rigid upper layer of the Earth.

• The term lithosphere is used for the outer solid shell crust beneath the oceans and mountains consisting of the rocky crust and upper part of mantle.

• Lithosphere is about 100 km thick.

Asthenosphere -

• It is the molten layer of the Earth under the lithosphere. 

• The radius of the Earth is 6,370 km. Till now No one has reached the Centre of the Earth, therefore it is difficult to collect samples from the Centre. 

Sources of information of Earths Interior-

There are broadly two types of methods. 

Direct methods-

• It includes mining and drilling of the Earth surface.

• Gold mines in South Africa are deep upto 3 - 4 km.

• We have reached to maximum upto 12km depth inside the earth and that is  at Kola, in Arctic Ocean. 

Indirect Methods/Sources-

It includes -

• Pressure. 

• Temperature. 

• Density of eart. 

• Gravity and magnetic fields etc.

Lets discuss each of These methods used in Detail-

 1.Earth’s Internal Temperature-

• There is rise in Temperature as we move towards centre or core of the earth(Known as Geothermal Gradient) and this is evedent from the hot volcanic eruptions from the earth.

• Geothermal gradient increases 20o C to 30o C per kilometer as we move downwards. 

2.Earth’s Internal Pressure-

• The pressure of Earth increase with depth.

• The pressure near the Centre is very high and it is assumed that it is 3 to 4 million times the pressure of atmosphere at sea level.

3.Density

• As we moves toward the Centre of the Earth Due to increase in pressure and presence of heavier materials towards the earth’s center, there is increase in the density . 

4.Gravity and Magnetic Survey-

• information about the distribution of mass of the material in the crust is assessed by Gravity anomalies. 

• There is difference in the readings of gravity at different places and These readings differ from the expected values and this difference is called as  gravity anomaly.

• The value of the  gravitation force (g) is different at  different latitudes on the surface.

• The value of the  gravitation force (g) is greater near the poles and The value of the  gravitation force (g) is  less at the equator.

• This is because the radius at the equator is more as comparison to the poles. 

5.Volcanoes

• They are also one of the important source of information of earth as they provide the detail about materials inside earth. 

6.Meteorites

• It include Study of meteorites,Study of surface of other planet materials etc.

7.Seismic waves/Earthquakes

• This is most widely used method to know about interior of the earth.

• Seismic waves are the vibration produced by an earthquake.

• This is produces due to breaking and friction of different layers of earth interior and these activity releases energy in form of vibrations known as seismic waves. 

Focus,Hypocentre and Epicentre

• Focus is the point where the energy of an earthquake is released and alternatively, it is called the hypocentre. 

Epicentre:

• The point on the surface of the earth which is located nearest to the focus, is called epicentre.  It is a located directly above the focus.

Seismic waves are of two types:

A. Body waves. 

B. Surface waves. 

A. Body waves 

• These waves are generated when release of energy at the focus  and they move in all directions travelling through the body of the earth. 

Body waves are further classified into two types-

Primary waves or P-waves-

• The waves travel from the earthquake focus through Earth and the first wave to arrive is P-wave.

• P-waves are also referred as longitudinal or compressional waves  because they travel through solid, liquid or gaseous materials as a succession of compression and expansions and therefore, .

• P-wave are similar to sound waves . These types of waves have short wavelength along with high frequency.

• They are also called as pull or push waves.

• They can travel through all three mediums like gaseous, liquid and solid materials. 

Secondary waves or S-waves  -

• S-wave travels through solid rock at a little more than half the velocity of P-waves.

• They are also called as Shear waves and they displaces material at right angles to their path of travel.

• They do not travel through liquids and gasses.

• They are Transverse or transitional waves and have short wavelength and high frequency.

• There velocity changes with change in density of solid materials.

B. Surface waves

• On seismograph These types of waves are reported at Last. 

• These waves are more destructive.

• The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane confined to the Earth’s surface, like waves on the ocean and causes Crust and Trough. 

These waves are further divided into Two parts-

Love wave: 

• Love waves shake the surface side-to-side. 

Rayleigh wave:

• They are complex surface waves in which ground vibrates in a rolling, elliptical motion that diminishes as the depth increases partly in the direction of propagation and partly at right angles. 

Shadow Zone-

• There are some specific areas where the earth quack waves are not reported and these areas are called as Shadow Zone.

• Both P and S-waves were reported when seismographs  were placed within 105° from the epicentre. 

• The seismographs that were placed or  located beyond 145° from epicentre, recorded the arrival of only the P-waves,and there was no arrival of  S-waves.

• Thus, a zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves.

• S-waves were not received beyond The entire zone from 105° .

• The shadow zone of P-wave is much smaller than that of the S-waves.

• The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre.

• The shadow zone of S-waves is very large and it covers approximately over 40 per cent of the earth surface. 

Measuring Earthquake-

There are two parameters that are measured for Earth quack I.e. magnitude and Intensity. 

The magnitude is related to the energy released during the Earthquake. 

The Intensity is related to damage caused by Earth Quack. 

Richter Scale-

• It is used to measure the magnitude of the E.Q.

• The magnitude is expressed in numbers, 0-10.

• 0 being lowest magnitude and 10 being highest.

Mercalli scale-

• It is used to measure the intensity of E.Q.

• It is named after Mercalli and he was an Italian seismologist.

• The range of intensity scale is from 1-12.

Earth is broadly divided into 3 Layers

1. Crust.

2. Mantle. 

3. Core. 

It is the outermost solid part of the earth. It is brittle in nature. 

It is further divided into two parts. 

Oceanic crust-

Oceanic crust is thinner and denser as compared to the continental crust. 

Composition -

• Basalt underlain by Gabbro is its major composition. 

• It has high seismic p-wave velocity as comparison to contenental crust. 

Continental Crust-

It is thicker and less denser than oceanic crust.

It has low seismic p-wave velocity as comparison to oceanic crust.

Composition-

• Granite,Schists,Gnesis and other plutonic rocks. 

Layer of Crust is further divided into Two parts-

SiAl-

• It has silicon and aluminum in abundance.

• It has more than 65% of silica content(known as Felsic rocks like granite etc.).

• It is also known as Upper continental crust and consists of all types of rocks exposed at the terrestrial surface.

SiMa-

• It is known as lower continental crust.

• It is denser than SiAl.

• Silica and Magnesium is its primary constituents.

• In this portion amount os Silica decreases and Silica ranges from 40 to 50 %(Known as Mefic rock ,Example- Basalt).

• If we go futher deep the composition of Silica content further start decreasing I.e. around 40 % or less (Known as Ultamefic).

• The portion of the interior beyond the crust is called the mantle.

• Its upper surface is about 5 to 10 km below the oceanic crust and about 20 to 80 km below the continental crust.

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

• Mantle comprises 83% by volume of the Earth and 68% by mass.

• The mantle is also heterogeneous in nature, which is indicated by many discontinuities of lower order. 

Lithosphere and Astheno sphere-

Lithosphere-

• The crust and the uppermost part of the mantle are called lithosphere.

• Its thickness ranges from 10-200 km. 

Asthenosphere-

Lower part of the upper mantle over which the lithospheric plates slides is called Astheno sphere. 

Composition:-

• High velocity of seismic waves indicates more denser material and less silica content (which means Ultramefic composition). 

Also there is decrease in velocity of seismic waves below Lithosphere which indicates presence of some liquid layers and that we have discussed above is known as Asthenosphere. 

• It is the innermost part of The earth.

• It is also called as NiFe(Nickle and Ferrous ,means iron).

• The core mantle boundary is located at the depth of 2,900 km.

• Core is separated from the mantle by Gutenberg discontinuity.

• It is further divided into Two parts I.e. Inner core and Outer core.

• On the basis of seismic waves we can say that outer core is liquid and inner core is Solid.

• This portion is thickest potion of earth and have highest density. 

Various Discontinuities in The Earth

Conrad discontinuity-

• It is between upper and lower crust.This boundary is observed in various continental regions at a depth of 15 to 20 km, however it is not found in oceanic regions.

Mohorovicic or Moho discontinuity-

• It separates crust from the mantle. 

• Its depth is highest in pleatue followed by continets and than oceans. 

Repiti Discontinuity-

• It is between Outer mantle and Inner mantle.

Gutenberg discontinuity-

•  It separates mantle from the core. 

Lehmann-Bullen discontinuity-

•  It is between inner and outer core.

Some other important terms-

Cryosphere : Cryosphere  is the part of the Earth’s surface where water is in solid state which can be in the form of snow, ice and glacier. 

Milky Way Galaxy : It is one of the hundreds of millions of galaxies present in the Universe.

Stromatolites : They are the early microbial communities whose fossils have been first reported from Archean rocks. 

Important One liner on the topic Structure of the Earth :

1.What are the three main layers of the Earth?

Answer: Crust, Mantle, and Core.

2.What is the composition of the Earth's crust?

Answer: It is primarily made up of silicate rocks.

3.What is the composition of the Earth's mantle?

Answer: It is mainly composed of silicate minerals that contain magnesium and iron.

4.What is the thickness of the Earth's crust?

Answer: The thickness of the Earth's crust varies between 5 and 70 km.

5.What is the boundary between the Earth's crust and mantle called?

Answer: The Mohorovičić discontinuity or Moho.

6.What is the state of matter of the Earth's inner core?

Answer: The Earth's inner core is solid.

7.What is the state of matter of the Earth's outer core?

Answer: The Earth's outer core is liquid.

8.What is the composition of the Earth's core?

Answer: The Earth's core is primarily made up of iron and nickel.

9.What is the temperature at the center of the Earth?

Answer: The temperature at the center of the Earth is estimated to be around 6,000 degrees Celsius.

10.What causes the Earth's magnetic field?

Answer: The Earth's magnetic field is caused by the movement of molten iron in the outer core.

11.What is the name of the phenomenon where seismic waves bend as they pass through different layers of the Earth?

Answer: Refraction.

12.What is the name of the boundary between the Earth's mantle and core?

Answer: The Gutenberg discontinuity.

13.What is the name of the layer in the upper mantle where rocks become partially molten?

Answer: The asthenosphere.

14.What is the name of the process where tectonic plates collide and one plate is forced underneath the other?

Answer: Subduction.

15.What is the name of the layer of the Earth's atmosphere closest to the surface?

Answer: The troposphere.