Earth’s Interior With Special Reference to Seismology

1. Earth’s Interior
With Special Reference to Seismology
Mithun Ray
Department of Geography
Malda College
(University of Gour Banga)
E-mail: mithun.ray147@gmail.com

2. What do you imagine about the nature
of the earth?
a solid ball !!
or
a hollow ball !!

3. Diameter of the Earth
Equatorial= 12,756 km (7926 mile)
Polar=12,714 km (7900 mile)
Radius of the Earth
Equatorial= 6378.1 km (3,963.2 mile)
Polar= 6356.8 km (3949.9 mile)
Average Radius
6371km (3958.8)

4. How do we know
what’s inside the Earth?
Deepest Mine In
the world
Mponeng Gold Mine located at Johannesburg, South
Africa is the deepest mine in the world with an operating
depth of 4 km

5. Kola Super deep Borehole
(World’s deepest drill)

6. Sources of Information about the Interior
Major Sources
The studies of the paths and characteristics of
earthquake waves travelling through the Earth, and
 The laboratory experiments conducted on the surface
minerals and rocks at high pressure and temperature
Other Sources
 The properties of surface rocks
 Earth's motions in the Solar System,
 Earth’s gravity and magnetic fields, and
The flow of heat from inside the Earth.

7. Seismology
Study of earthquakes and
Seismic Waves that move
through and around the
earth

8. Seismic Waves /Earthquake waves
Waves
Body
Wave
‘P’ wave ‘S’ Wave
Surface
Wave
Love
wave
Rayleigh
Wave

9. ‘P’ / Primary/ Pressure/
Compressional / Longitudinal Wave
 P waves travel faster than any other type of wave
 They can travel through fluid or solid materials.
 Material is compressed and stretched in the
horizontal direction and the wave (disturbance) also
travels in the horizontal direction.
Ordinary sound
waves in air are P
waves
 Least destructive

10. ‘S’ / Secondary / Shear / Transverse Wave
 ‘S’ waves travel more slowly than ‘P’ waves
 The material vibrates up and down i.e. perpendicular
to the direction of wave propagation
 They can only travel through solid materials.
However, they may propagate in liquids with high
viscosity

11. A Rayleigh wave
is a seismic surface wave causing the ground
to shake in an elliptical motion, with no
transverse, or perpendicular, motion.
A Love wave
is a surface wave having a horizontal motion
that is transverse (or perpendicular) to the
direction the wave is traveling.

12. BODY WAVE
SURFACE WAVE

13. Reflection involves a change in direction
of waves when they bounce off a barrier.
Refraction of waves involves a change in the
direction of waves as they pass from one
medium to another.

14. Shadow Zone of
‘P’ wave
The shadow zone of P-
waves appears as a band
around the earth between
103° and 142° away from
the epicentre.
This is because P-waves
are refracted when they
pass through the transition
between the
semisolid mantle and the
liquid outer core.
However, the
seismographs located
beyond 142° from the
epicentre, record the
arrival of P-waves, but not
that of S-waves. This gives
clues about the solid inner
core.

15. S-waves do not travel
through liquids.
The entire zone beyond
103° does not receive S-
waves, and hence this
zone is identified as
the shadow zone of S-
waves.
This observation led to the
discovery of
the liquid outer core.
Shadow Zone of
‘S’ Wave

16. Shadow Zone of P-waves and S-waves
a zone
between 1
03° and
142° from
epicentre
was
identified
as
the shado
w zone for
both the
types of
waves.

17. Internal structure of the
Earth

19. CRUST
 The Crust is the outermost layer of the earth on
which all living world exists
 This is a very thin layer. It is also a brittle layer. It
is ranging from 5 km under the oceans to 100 km
under the mountainous areas of continents. Usually,
it’s about 40 km thick under the flat continents.
 The crust is about 0.5 % of the earth’s total mass.
These rocks and minerals are made from just 8
elements. They are Oxygen (46.6%), Silicon (27.72%),
Aluminum (8.13%), Iron (5.00%), Calcium(3.63%),
Sodium (2.83%), Potassium (2.70%) and Magnesium
(2.09%).

20. Continental Crust (SIAL):
 Composed of Silica (Si) and Aluminum (Al)
 Average density is 2.7 gm/ cm3
 This layer is mainly composed of Granite
Oceanic crust (SIMA):
Composed of Silica (Si) and Magnesium (Ma)
 Average density is 3.0 gm/ cm3
 This layer is mainly composed of Basalt

21. Isopach = lines of equal thickness of the crust

22. MANTLE
 The mantle is the middle layer.
 This makes up the largest volume of the Earth’s
interior. It is almost 2900 kilometers in thickness.
 It is comprised of about 83% of the Earth’s total
volume.
 It has an average density of 5.5 g/cm3
 It is also divided into two distinct layers as upper
mantle and lower mantle.

23. Asthenosphere
 The upper portion of the mantle is called
asthenosphere. The word astheno means weak.
 It is considered to be extending upto 400 km.
 It is the main source of magma that finds its way
to the surface during volcanic eruptions.
 It is made up of peridotites.

25. CORE
 The core is found about 2,900 kilometers (1,802
miles) below Earth’s surface, and has a radius of about
3,485 kilometers (2,165 miles).
 The core is made of two layers: the outer core,
which borders the mantle, and the inner core.
 The outer core is in liquid state while the inner core
is in solid state.
 The core is made up of very heavy material
mostly constituted by nickel and iron. It is
sometimes referred to as the NiFe layer.

28. বসু ও মাইতি,২০১৪

29. Discontinuity
All layers of earth’s interior are
separated from each other through a
transition zone. These transition
zones are called discontinuities

31. Conrad Discontinuity: Transition zone between
SIAL and SIMA.
Mohorovicic Discontinuity: Transition zone
between the Crust and Mantle.
Repiti Discontinuity: Transition zone between
Outer mantle and Inner mantle.
Gutenberg Discontinuity: Transition zone
between Mantle and Core.
Lehman Discontinuity: Transition zone between
Outer core and Inner core.

32. Conrad Discontinuity:
The transition zone between thee upper and lower part of the
lithosphere, is called as Conrad discontinuity. The name come
from the Austrian geophysicist Vector Conrad . Up to the
middle 20thcentury the upper crust in the continental region
was seen to consist of felsic rock such as granite and the lower
one consist of more magnesium rich mafic rocks such as
basalt. Therefore, the seismologists of that time considered
that Conrad discontinuity should correspond to a sharply
defined contact between the chemically distinct layers
of SIAL and SIMA. In passing through the Conrad
discontinuity the velocity of longitudinal seismic waves
increases abruptly from approximately 6to6.5km/sec.

33. Mohorovicic Discontinuity:
The transition zone between the crust and mantle is called as
mohorovicic discontinuity. The mohorovicic discontinuity
was discovered by Andrija Mohorovicic in the year of 1909.
The Moho lies at the depth of 35km beneath the continents
and 8km beneath the oceanic crust. The Moho separates both
the continental crust and the oceanic crust from underlying
mantle. The Moho lies almost entirely within the lithosphere,
only beneath the Mid Oceanic Ridge does it define lithosphere
and asthenosphere boundary. Immediate above the Moho
velocity of the P wave is 6km/sec and just below the Moho it
becomes 8km/sec. Moho is characterised by up to 500km
thick.

34. Gutenberg Discontinuity:
The mantle –core transition zone is called Gutenberg
discontinuity. In the year of 1912 Weichert Gutenberg
was discovered this discontinuity at the depth of 2900km
beneath the earth surface. In this zone the velocity of
seismic waves changes suddenly. The velocity of P wave
decreases and S wave completely disappear at this depth.
S wave shear material and cannot transmit through
liquid. So, it is believed that the part of above the
discontinuity is solid and part of beneath then
discontinuity is liquid or molten form. This molten
section is thought to be 700°c, hotter than the overlying
mantle. It is also denser, probably due to a greater
percentage of iron.

35. Repiti Discontinuity
it is the transition zone between Outer mantle and Inner
mantle.
Lehmann discontinuity
It is the transition zone between outer and inner core.
The Lehmann discontinuity is an abrupt increase of P-
wave and S-wave velocities at the depth of 220±30 km,
discovered by seismologist Inge Lehmann.It appears
beneath continents, but not usually beneath oceans, and does
not readily appear in globally averaged studies. Several
explanations have been proposed: a lower limit to the
pliable asthenosphere, a phase transition,[and most
plausibly, depth variation in the shear wave anisotropy.

37. Seismic Waves and
the Interior of the Earth
 Reflection causes P and S waves to rebound whereas
refraction makes waves move in different directions.
 The variations in the direction of these waves are
inferred with the help of their record on seismograph.
Change in densities greatly varies the wave velocity. By
observing the changes in velocity, the density of the
earth as a whole can be estimated. By the observing the
changes in direction of the waves (emergence of shadow
zones), different layers can be identified.

38.  For both kinds of waves, the speed at which the wave
travels also depends on the properties of the material
through which it is traveling.
 Scientists are able to learn about Earth’s internal
structure by measuring the arrival of seismic waves at
stations around the world.
 For example, we know that Earth’s outer core is
liquid because s-waves are not able to pass through it;
when an earthquake occurs there is a “shadow zone”
on the opposite side of the earth where no s-waves
arrive.
 Similarly, we know that the earth has a solid inner
core because some p-waves are reflected off the
boundary between the inner core and the outer core.

39.  By measuring the time it takes for seismic waves
to travel along many different paths through the
earth, we can figure out the velocity structure of the
earth.
 Abrupt changes in velocity with depth correspond
to boundaries between different layers of the Earth
composed of different materials.
The structure of Earth’s deep interior cannot be
studied directly. But geologists use seismic
(earthquake) waves to determine the depths of layers
of molten and semi-molten material within Earth.
Geologists are now using these records to establish
the structure of Earth’s interior.

40. This Power Point Presentation (PPT) has been
prepared especially for Undergraduate Students. The
materials (Maps, Diagrams and Images) used in this
presentation were collected and compiled mainly
from variours academic blogs, research papers,
books etc. The author has designed this PPT
according to his convenience to deliver the lecture.

41. THANK YOU