Earth interior layers explained: Crust, mantle, and core

1. Internal Structure of Earth
Nipin K P
Kp.nipin@gmail.com

2. Internal structure of Earth
• Based on the inferences drawn from the seismic data, Earth’s layers can be
divided in to three concentric zones as (i) Crust, (ii) Mantle, and (iii) Core.
Crust
• The outermost thin shell of the solid earth (0 to 40 km).
• Continents and ocean basins are parts of the crust.
• All significant geologic processes are taking place in the crust. earthquakes
originate in it; metals, oil and other earth materials also occurs here.
• There are two layers in the crust - Sial and Sima.

4. Sial
• The continental masses are made of relatively light rock called granite (specific
gravity 2.7) and sediments that are still lighter.
• The term sial represents rocks containing silica and alumina.
Sima
• The oceanic part made of dark heavy rock called basalt, with a specific gravity of 3.
• The term sima represents rocks containing silica and magnesia.
• Under oceans only sima layer is found and the sial layer appears to be absent.
• The boundary between these two layers is named as Conrad discontinuity after
the name of its discoverer.

5. • The Seismic study reveals that the solid portion is not uniformly laid on sima but
floats in it like an iceberg.
• It has been observed that 98% of the upper portion of the crust is composed mainly
of eight elements: O, Si, Al, Fe, Ca, Na, K and Mg.
• Of these oxygen and silicon constitute together 74 % of the total composition. The
remaining six account for 24 %.

6. Mantle
• The mantle lies beneath the earth's crust and extends down up to 2900 km in
depth.
• It makes up more than 80% of the earth's total volume and 67% by mass.
• Mantle is a viscous plastic mass of composition similar to that of sima.
• It has an average density of 4.53 g/cc.
• The speed of earthquake waves is so high - very rigid and dense rock,
• Mantle consists of solid ultramafic rock made up of magnesium-iron silicate
minerals.

7. Based on the behaviour of seismic waves, the mantle is divided in to two major parts
– Upper mantle and Lower mantle. A third division called Transition zone also
present.
Upper Mantle:
• The upper mantle extends from the crust to a depth of about 650 km and includes
the asthenosphere.
It consists of a high-velocity zone from 0 to 50 km thick,
a middle zone of low velocity about 100 km thick,
and a progressively homogeneous but denser zone about 250 km thick, to a depth
of 400 km.

8. Upper Mantle (Cont’d):
• The upper zone is a brittle solid and, together with the earth's crust, constitutes a
strong outer layer termed the lithosphere.
• The important minerals making up this upper zone are olivine, pyroxene, and
garnet, with accessory spinel and amphibole. So it is made up of ultrabasic rocks
like peridotite.
• The low velocity zone (LVZ) is also known as the asthenosphere, or plastic layer.
• It (LVZ) has a very sharp boundary. This layer is characterized by partial melting,
dehydration, or a small amount of water in the mineral assemblage.

9. Transition zone:
• Below the asthenosphere from 400 to 650 km, is the Transition zone, grading into
the lower mantle.
• At a depth of about 400-km there is a sharp increase in P-wave velocity, which
indicates a sudden increase in rock density.
• This is due to a rearrangement of the atoms that are composing the rock into a
more closely packed arrangement. This kind of physical change in atomic structure
is called a phase transition.
• Olivine, which is considered to be the major mineral component of the mantle, is
capable of undergoing such a phase transition.

10. Lower Mantle:
• From a depth of 650–2890 km.
• Density range: 4.4 to 5.6 g/cc
• Below the transition zone another discontinuity occurs, yielding another sharp
increase in P-wave velocity.
• Within the lower mantle, deeper than 700 km, seismic wave velocity increases
rather smoothly.
• It is probably composed of silicon, magnesium, and oxygen with some amounts of
iron, calcium, and aluminium.

11. Core:
• Below the mantle down to the centre, starts from a depth of 2890 to 6371 km.
• The entire core is believed to contain iron with minor amounts of Nickel and cobalt in
a metallic form, highly compressed due to the weight of superimposed mass.
• The core-mantle boundary is marked by an abrupt reduction in the P-wave velocity as
well as by the disappearance of the S-waves.
• The core is also divided into two parts, the inner core and the outer core.

12. Outer Core:
• It extends from about 2890 km depth to 5150 km depth.
• The outer core was discovered when it was found that P-waves were bent inwards
thereby producing a “shadow zone” at the surface (104° to 143° zone).
• Since the S-wave do not pass through the outer core, it is concluded that it may be in
the liquid state.
• Density range: 9.9 – 12.2 g/cc

13. • This conductive layer combines with earth’s rotation to create a dynamo effect that
maintains a system of electrical currents, thereby, creating the earth’s magnetic
field.
Inner Core:
• It extends from about 5150 km to the Earth’s centre at 6378 km.
• It transmits P-waves at a higher velocity which indicates that it is in the solid state.
• The inner core is made of solid iron and nickel and is suspended in the molten
outer core. (Density range: 12.8 – 13.1 g/cc)
• It is believed to have solidified as a result of pressure-freezing which occurs to most
liquids under extreme pressure.

14. Discontinuities:
• Seismic studies indicate that within the earth, at certain points, there would be rapid
changes in the velocity of the earthquake waves. These surfaces of sudden
variations in wave velocity are referred to as discontinuity surfaces.
Conrad discontinuity: Sial and Sima
Mohorovicic discontinuity: Crust and Mantle
Repetti discontinuity: Upper mantle and Lower mantle
Gutenberg - Wiechert discontinuity: Mantle and Core