The document summarizes the structure and composition of the Earth. It describes how the Earth can be divided into layers including the crust, mantle, outer core, and inner core based on their chemical and mechanical properties. It also discusses seismic waves that travel through the Earth's layers and how they have been used to discover the boundaries between the crust and mantle, outer and inner core. The speed and behavior of different seismic waves provided evidence that the outer core is liquid while the inner layers are solid.

1. interior of earth
nmadhavan@jnu.ac.in

2. Structure of the Earth
• The Earth has an outer silicate solid crust, a highly
viscous mantle, a liquid outer core.
• Mechanically, it can be divided into
– lithosphere,
– Asthenosphere,
– Mesosphere,
– outer core,
– and the inner core.
• Chemically, Earth can be divided into the
– Crust,
– upper mantle,
– lower mantle,
– outer core,
– and inner core

5. Seismic wave
• Seismic wave are waves of energy that travel through the
earth, and are a result of
– an earthquake,
– explosion, or a volcano that imparts low-frequency acoustic energy.
• Seismic wave fields are measured by a
– Seismograph,
– Geophone,
– hydrophone (in water),
– or accelerometer.
• The propagation velocity of the waves depends on density
and elasticity of the medium.
– Velocity tends to increase with depth, and ranges from approximately
2 to 8 km/s in the Earth's crust up to 13 km/s in the deep mantle.

6. Types of seismic waves
• Primary waves (P-waves) are compressional waves that are longitudinal in
nature.
– P waves are pressure waves that are the initial set of waves produced by an
earthquake.
– These waves can travel through any type of material, and can travel at nearly
twice the speed of S waves.
– In air, they take the form of sound waves, hence they travel at the speed of
sound.
– Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in
granite.
• Secondary waves (S-waves) are shear waves that are transverse in nature.
– These waves typically follow P waves during an earthquake and displace the
ground perpendicular to the direction of propagation.
– Depending on the propagational direction, the wave can take on different
surface characteristics; for example, in the case of horizontally polarized S
waves, the ground moves alternately to one side and then the other.
– S waves can travel only through solids, as fluids (liquids and gases) do not
support shear stresses.
– S waves are slower than P waves, and speeds are typically around 40% of that
of P waves in any given material.

7. P and S waves in Earth's mantle and core
When an earthquake occurs, seismographs near the epicenter are able
to record both P and S waves, but those at a greater distance no longer
detect the high frequencies of the first S wave. Since shear waves
cannot pass through liquids, this phenomenon was original evidence for
the now well-established observation that the Earth has a liquid outer
core.

8. Crust - Mantle Boundary
Velocity jump as the crust-mantle boundary, and often refer to it
as the Mohorovicic discontinuity.

9. S wave shadow zone
Core - Mantle Boundary discovered by Gutenberg

10. Inner Core - Outer Core Boundary
No S waves were received at seismographs stations from 104o to 140o from an earthquake;
the S wave shadow zone is caused by the outer core, which is liquid iron/nickel.

11. Lithosphere - Asthenosphere Boundary

12. The green line represents the temperature in the Earth as a function of depth.
The yellow line represents the temperature at which mantle rocks just begin to melt;
notice that pressure raises the melting temperature of mantle rocks.
Between about 100 and 250 km depth, the "geotherm" grazes the "mantle solidus;
" this is where the mantle is softest and may even be partially molten in some areas.

13. thickness of Earth's crust
the thickest areas (red and dark brown) are beneath some of Earth's important mountain
ranges such as: Andes (west side of South America), Rockies (Western North America),
Himalayas (north of India in South-central Asia) and Urals (north-south trending between
Europe and Asia)

16. Earth active zones with reference to seismic or volcanic

17. references
• http://en.wikipedia.org/wiki/Structure_of_the
_Earth
• http://en.wikipedia.org/wiki/Seismic_wave