PHYSICAL GEOGRAPHY
LECTURE 1
● INTERIOR STRUCTURE OF THE EARTH
● SEISMIC WAVES
Internal Structure of Earth
Structure of Earth is divided into four major components:
Crust, Mantle, Outer core, inner core. Each layer has its
unique chemical composition, physical state, and can
impact life on Earthʼs surface. Large size, non-uniform
structure, high density and temperature make Earthʼs
structure very complex.
LAYERS OF EARTH
LAYERS OF EARTH
LAYERS OF EARTH
LAYERS OF EARTH
LAYERS OF EARTH
LAYERS OF EARTH
LAYERS OF EARTH
DISCONTINUITIES
● All the layers within the Earth are separated from each
other through a Transition zone. These transition
zones are called discontinuities.
● They are marked by clear cut variations in density of
material, velocity of seismic waves, temperature and
pressure conditions.
There are five discontinuities inside the earth.
● Conrad Discontinuity: Transition zone between SIAL
and SIMA.
● Mohorovicic Discontinuity: Transition zone between
the Crust and Mantle.
● Repitti Discontinuity: Transition zone between Upper
mantle and Lower mantle.
There are five discontinuities inside the earth.
● Gutenberg Discontinuity: Transition zone between
Mantle and Core.
● Lehmann Discontinuity: Transition zone between Outer
core and Inner core.
Sources of information about Earthʼs Interior
Direct Sources Indirect Sources
● Surface rock
● Volcanos ● Seismic Waves & Seismicity
● Mining Projects ● Meteorites
● Drilling Projects ● Gravitation
● Deep Ocean Drilling Project ● Magnetic field
● Integrated Ocean Drilling
Project
SEISMIC WAVES
● Body waves (P & S Waves) – Body waves are generated due to release of
energy at the focus and these waves travel in all directions through the
interior of the earth.
● Surface waves (L & R waves) – The body waves interact with the surface
rocks and generate new sets of waves called surface waves. These waves
move along the surface. These waves are more destructive, causing the
displacement of rocks thereby collapsing the structure
PRIMARY WAVES
● P-waves or Primary waves or Compressional waves – P-waves travel faster,
about 6 km per sec in the upper crust and are first to arrive at the surface. These
waves are similar to sound waves as they travel through gas, liquid as well as
solid materials. P-waves vibrate parallel to the direction of the wave. These
waves exert pressure on the material in the direction of propagation
thereby, creating a density difference in the material leading to stretching
and compression of the material.
SECONDARY WAVES
● S-waves or Secondary waves or Shear waves – S-waves arrive at the
surface with some time lag, and are slower (about 3.5 km per sec in
the upper crust). S-waves only travel through solid materials. This
characteristic feature of S-waves helps in understanding the
structure of the interior of the earth. These are tranS-waves vibrate
perpendicular to the wave direction in the vertical plane. They
create crests and troughs in the material through which they travel.
Shadow Zone
● Seismographs are located at far off places to record the seismic
waves. However, there are certain areas where the waves are not
reported. Such a zone is called a “shadow zone”. For each
earthquake, there exists an altogether different shadow zone.
● When a seismograph is located at any distance within 105° from
the epicentre, it records both P-waves and S-waves.
Shadow Zone
● When a seismograph is located beyond 145° from the epicentre, it
records only P-waves. A zone between 105° and 145° from the
epicentre was identified as the shadow zone for both types of
seismic waves.
● The shadow zone of S-waves is much larger than that of P-waves
and is over 40% of the earthʼs surface. The shadow zone of
P-waves appears as a band around the earth between 105°
-145° away from the epicentre.
Shadow Zone
Love waves:
● This kind of surface waves cause horizontal shifting of the earth
during an earthquake. They are much slower than body waves but
are faster than Rayleigh Waves. Confined to the surface of the
crust, Love waves produce entirely horizontal motion.
Rayleigh waves:
● These waves follow an elliptical motion. A Rayleigh wave rolls
along the ground just like a wave rolls across a lake or an ocean.
Because it rolls, it moves the ground up and down and side-to-side in
the same direction that the wave is moving. Most of the shaking felt
from an earthquake is due to the Rayleigh wave, which can be
much larger than the other waves.
QUESTION CORNER
● Give an account of the interior structure of the Earth and
comment on the shadow zones of seismic waves.
● Seismic waves play a major role in solving the mysteries
of the Earthʼs interior. Explain.
● What are seismic waves? Give a detailed account of
Seismic waves.
