Internal structure & composition of the earth

1. Layers / Divisions of the earth

2. 2

3. 3

4. Layers / Divisions of the earth
The Core, Mantle, and Crust constitute the three main layers of earth (Fig. 5.1).
Compositionally (chemically), the outer thin crust is mostly silicate (SiO2-based)
and mantle, the layer below is accompanied by metal oxides (such as MgO, FeO,
Al2O3, CaO, and Na2O) in mineral composition (Taylor and McLennan 1985) (see
also Fig. 4.3 ).
Mantle is the largest by volume, making up almost 87 % of the earth.
The base of the mantle to the center of the earth, is made up of iron (90 %),
nickel (5 %) with a possible admixture of carbon, silicon, oxygen, sulfur, and
hydrogen (comprising around 5 % by mass).
The core makes up 35 % of the total mass of the earth and is probably made of
almost pure iron, perhaps even in a single crystal form.
4

5. 5

6. 6

7. 7

8. Continental Crust (0–75 km)
This is the outer most layers and forms the
surface of the earth.
It is primarily composed of crystalline rocks with
low-density buoyant minerals that are largely
dominated by silicates (Quartz; SiO2) and
feldspars (metal-poor silicates).
As cold rock deforms slowly, this rigid and brittle
outer layer is also called the Lithosphere
(lithos meaning rocky or strong layer)
8

9. Oceanic Crust (0–10 km)
The majority of the earth’s crust was made through
volcanic activity.
The oceanic ridge system, a 40,000 km long network of
volcanoes, generates new oceanic crust
at the rate of 17 km3 per year, and covers the ocean
floor with Basalt, an igneous rock.
Hawaii and Iceland are two classic examples of such
accumulations.
9

10. Upper Mantle (10–400 km)
Solid fragments of the upper mantle have been
found in eroded mountain belts and volcanic
eruptions.
These include such minerals as Olivine [(Mg,
Fe)2SiO4], Pyroxene [(Mg, Fe)SiO3], and others
that crystallize at high temperatures.
The asthenosphere, part of the upper mantle
(Fig. 5.1), might well be partially molten.
10

11. Transition Region (400–650 km)
The transition region or Mesosphere (for middle mantle) is
sometimes also called the Fertile layer (Fig. 5.1).
It is the source of basaltic magma and complex aluminum-
bearing silicate minerals containing calcium, aluminum,
and garnet.
When cold, this layer is dense due to the presence of
garnet.
It is buoyant when hot as these minerals melt easily to
form basalt which rises through the upper layers as
Magma.
11

12. Lower Mantle (650–2890 km)
The lower mantle (Fig. 5.1) is probably composed of silicon,
magnesium, and oxygen with some amounts of iron, calcium,
and aluminum.
D”” Layer (2700–2890 km)
It is also called the D prime (D””) and is 200–300 km thick.
Although it is often identified as part of the lower mantle (Fig.
5.1), seismic data suggest that this layer might differ chemically
from the lower mantle.
12

13. Outer Core (2890–5150 km)
The outer core is hot and composed of electrically conducting liquid
made mainly of iron and nickel.
This conductive layer (Fig. 5.1) combines with earth’s rotation
to create a dynamo effect that maintains a system of electrical
currents, thereby, creating the earth’s magnetic field.
This layer is not as dense as pure molten iron, thus, suggesting the
presence of lighter elements also.
It is suspected that about 10 % of the layer is composed of sulfur and
oxygen as these elements are abundant in the cosmos and also
dissolve readily in molten iron.
13

14. Inner Core (5150–6378 km)
The inner core is made of solid iron and nickel and is
suspended in the molten outer core, unattached to the
mantle (Fig. 5.1).
It is believed to have solidified as a result of pressure-
freezing which occurs to most liquids under extreme
pressure.
14

15. 15

16. 16

17. 17

18. 18

19. 19

20. 20

21. 21

22. • The interior structure of the Earth is layered in
spherical shells.
• These layers can be defined by their chemical and
their rheological properties.
• Earth has an outer silicate solid crust, a
highly viscous mantle, a liquid outer core that is
much less viscous than the mantle, and a
solid inner core.
23

23. • The crust ranges from 5–70 kilometers (3.1–43.5 mi) in depth
and is the outermost layer.
• The thin parts are the oceanic crust, which underlie the ocean
basins (5–10 km) and are composed of dense (mafic) iron
magnesium silicate igneous rocks, like basalt.
• The thicker crust is continental crust, which is less dense and
composed of (felsic) sodium potassium aluminium silicate
rocks, like granite.
• The rocks of the crust fall into two major categories – sial and
sima (Suess,1831–1914). It is estimated that sima starts about
11 km below the Conrad discontinuity.
• The uppermost mantle together with the crust constitutes
the lithosphere. The crust-mantle boundary occurs as two
physically different events.
24

24. • Earth's mantle extends to a depth of
2,890 km, making it the thickest layer of Earth.
The mantle is divided into upper and lower
mantle.
• The upper and lower mantle are separated by
the transition zone.
• The lowest part of the mantle next to
the core-mantle boundary is known as the D″
(pronounced dee-double-prime) layer.
25

25. The pressure at the bottom of the mantle is ≈140 GPa .
The mantle is composed of silicate rocks that are rich in
iron and magnesium relative to the overlying crust.
Although solid, the high temperatures within the
mantle cause the silicate material to be
sufficiently ductile that it can flow on very long
timescales.
Convection of the mantle is expressed at the surface
through the motions of tectonic plates.
26

26. The average density of Earth is 5,515 kg/m3.
Because the average density of surface material is only
around 3,000 kg/m3, we must conclude that denser
materials exist within Earth's core.
Seismic measurements show that the core is divided
into two parts, a "solid" inner core with a radius of
≈1,220 km and a liquid outer core extending beyond it
to a radius of ≈3,400 km.
The densities are between 9,900 and 12,200 kg/m3 in
the outer core and 12,600–13,000 kg/m3 in the inner
core.
27

27. The inner core was discovered in 1936 by Inge
Lehmann and is generally believed to be
composed primarily of iron and some nickel.
It is not necessarily a solid, but, because it is
able to deflect seismic waves, it must behave as
a solid in some fashion.
Experimental evidence has at times been critical
of crystal models of the core
28

28. 5 discontinuities
• Conrad – between outer & inner crust
• Mohorovicic - between crust & Mantle
• Repetiti – between outer and inner mantle
• wiechert gutenberg– Between mantle & core
• Lehmann – Between outer & inner core.
29

29. 30

30. 31

31. 32

32. 33

33. 34

34. Conrad discontinuity
• The Conrad discontinuity corresponds to the sub-
horizontal boundary in continental crust at which
the seismic wave velocity increases in a
discontinuous way.
• This boundary is observed in various continental
regions at a depth of 15 to 20 km, however it is
not found in oceanic regions.
• The Conrad discontinuity (named after
the seismologist Victor Conrad) is considered to
be the border between the upper continental
crust and the lower one. It is not as pronounced
as the Mohorovičić discontinuity, and absent in
some continental regions.
35

35. • Up to the middle 20th Century the upper crust in
continental regions was seen to consist of felsic
rocks such as granite (sial, for silica-aluminium),
and the lower one to consist of more
magnesium-rich mafic rocks like basalt (sima, for
silica-magnesium).
• Therefore, the seismologists of that time
considered that the Conrad discontinuity should
correspond to a sharply defined contact between
the chemically distinct two layers, sial and sima
36

36. 37

37. Low-velocity zone
• The low-velocity zone (LVZ) occurs close to the
boundary between the lithosphere and
the asthenosphere in the upper mantle.
• It is characterized by unusually low seismic shear
wave velocity compared to the surrounding depth
intervals. This range of depths also corresponds to
anomalously high electrical conductivity.
• It is present between about 80 and 300 km depth. This
appears to be universally present for S waves, but may
be absent in certain regions for P waves.
• A second low-velocity zone (not generally referred to
as the LVZ, but as ULVZ) has been detected in a thin
≈50 km layer at the core-mantle boundary.
• These LVZs may have important implications for plate
tectonics and the origin of the Earth's crust.
38

38. 39

39. Mohorovičić discontinuity
• The Mohorovičić discontinuity usually referred to as
the Moho, is the boundary between the Earth's crust and
the mantle.
• Named after the pioneering Croatian seismologist Andrija
Mohorovičić, the Moho separates both the oceanic
crust and continental crust from underlying mantle.
• The Moho lies almost entirely within the lithosphere; only
beneath mid-ocean ridges does it define the lithosphere–
asthenosphere boundary.
• The Mohorovičić discontinuity was first identified in 1909
by Mohorovičić, when he observed
that seismograms from shallow-focus earthquakes had two
sets of P-waves and S-waves, one that followed a direct
path near the Earth's surface and the other refracted by a
high-velocity medium.
40

40. • The Mohorovičić discontinuity is 5 to 10
kilometres (3–6 mi) below the ocean floor,
and 20 to 90 kilometres (10–60 mi), with
an average of 35 kilometres (22 mi), beneath
typical continental crusts
• Below Indian peninsula 35 km
• Below western Ghats 50 km
• Below Himalayas 80 km
41

41. 42

42. Repetti discontinuity
• Repetti discontinuity is a postulated boundary
layer between two layers of the lower mantle.
It is defined by an increase in seismic
velocities with depth.
• It was named after William C. Repetti, an
American geophysicist who investigated this
boundary layer with seismological methods as
part of his doctoral thesis completed at St.
Louis University in 1930.
43

43. • This is not a zero-order boundary, but a gradual
transition. However, seismic studies with short-
period data suggest that the transition region has a
relatively small thickness (≈ 10 km).
• The cause of the Repetti discontinuity is so far
unclear and its occurrence has so far only been
proved by relatively few observations.
• These studies were mostly related to subduction
zones and showed a seismic discontinuity at very
different depths (between 900 and 1080 km).
44

44. • According to the average depth of its
occurrence, it is sometimes referred to in the
literature as a 920 km discontinuity.
45

45. Gutenberg Discontinuity
• The Gutenberg discontinuity occurs within Earth's
interior at a depth of about 2,900 km (1,800 mi)
below the surface, where there is an abrupt change
in the seismic waves (generated by earthquakes or
explosions) that travel through Earth.
• At this depth, primary seismic waves (P waves)
decrease in velocity while secondary seismic waves (S
waves) disappear completely. S waves shear material,
and cannot transmit through liquids, so it is believed
that the unit above the discontinuity is solid, while
the unit below is in a liquid, or molten, form.
• This distinct change marks the boundary between
two sections of the earth's interior, known as the
lower mantle (which is considered solid) and the
underlying outer core (believed to be molten). 46

46. • The molten section of the outer core is thought to be
about 700°C (1,292°F) hotter than the overlying
mantle. It is also denser, probably due to a greater
percentage of iron.
• This distinct boundary between the core and the
mantle, which was discovered by the change in seismic
waves at this depth, is often referred to as the core-
mantle boundary, or the CMB. It is a narrow, uneven
zone, and contains undulations that may be up to 5-8
km (3-5 mi) wide.
• These undulations are affected by the heat-driven
convection activity within the overlying mantle, which
may be the driving force of plate tectonics-motion of
sections of Earth's brittle exterior. These undulations in
the core-mantle boundary are also affected by the
underlying eddies and currents within the outer core's
iron-rich fluids, which are ultimately responsible for
Earth's magnetic field.
47

47. • The boundary between the core and the mantle
does not remain constant. As the heat of the
earth's interior is constantly but slowly
dissipated, the molten core within Earth gradually
solidifies and shrinks, causing the core mantle
boundary to slowly move deeper and deeper
within Earth's core.
• The Gutenberg discontinuity was named after
Beno Gutenberg (1889-1960) a seismologist who
made several important contributions to the
study and understanding of the Earth's interior.
• It has also been referred to as the Oldham-
Gutenberg discontinuity, or the Weichhert-
Gutenberg discontinuity
48

48. • The core–mantle boundary of the Earth lies between the
planet's silicate mantle and its liquid iron-nickel outer core.
• This boundary is located at approximately 2891 km
(1796 mi) depth beneath the Earth's surface. The boundary
is observed via the discontinuity in seismic wave velocities
at that depth.
• This discontinuity is due to the differences between the
acoustic impedances of the solid mantle and the molten
outer core. P-wave velocities are much slower in the outer
core than in the deep mantle while S-waves do not exist at
all in the liquid portion of the core.
• Recent evidence suggests a distinct boundary layer directly
above the CMB possibly made of a novel phase of the
basic perovskite mineralogy of the deep mantle
named post-perovskite.
49

49. Lehmann discontinuity
Inge Lehmann ForMemRS (13 May 1888 – 21 February 1993) was a
Danish seismologist and geophysicist. In 1936, she discovered that
the Earth has a solid inner core inside a molten outer core.
Before that, seismologists believed Earth's core to be a single molten sphere,
being unable, however, to explain careful measurements of seismic
waves from earthquakes, which were inconsistent with this idea.
Lehmann analysed the seismic wave measurements and concluded that Earth
must have a solid inner core and a molten outer core to produce seismic
waves that matched the measurements.
Other seismologists tested and then accepted Lehmann's explanation.
Lehmann was also the longest-lived woman scientist, having lived for over
104 years It appears beneath continents, but not usually beneath oceans,and
does not readily appear in globally averaged studies.
50