Interior of the Earth

The interior of the Earth is divided into different layers and in many ways such as based on chemical composition or mechanical properties. Chemically, Earth’s interior is divided into the crustmantle, and core. Mechanically, it can be divided into lithosphereasthenospheremesosphere, and barysphere.

Source of Information:

Most of our knowledge about the interior of the earth is largely based on indirect evidence. Yet, a part of the information is obtained through direct observations.

Direct Sources:

Mining,Deep ocean drilling, and volcanic eruptions are some examples of direct sources.

  • During the process of mining and drilling at different depths; rocks and minerals are extracted which gives information about the crust.
  • We also know through the mining activities and various deep drilling projects; that the temperature and density increase with the increasing depth.
  • Volcanic eruption suggests that there is at least such a layer below the earth’s surface which is in the liquid state.

But, direct sources are not very reliable because mining and drilling can be done only up to some depth.

Indirect Sources:

Indirect Sources are very important to know about the interior of the earth. Seismic waves, gravitation and falling meteors are some examples of indirect sources.

  • Meteors allow us to directly analyze the density, chemistry, and mineralogy of solid bodies having a similar composition to that of the Earth.
  • Gravitation force is greater near the poles and less at the equator. This is because of the distance from the center at the equator being greater than that at the poles.
  • The nature and properties of the composition of the earth’s interior may be successfully obtained on the basis of the study of various aspects of seismic waves mainly the velocity and travel part of these waves, while passing through a homogeneous solid body but these waves are reflected and refracted while passing through a body having heterogeneous composition and varying density zones.

If the earth would have been composed of homogeneous solid materials the seismic waves should have reached the core of the earth in a straight path but this is not the case in reality.

The recorded seismic waves denote the fact that these waves seldom follow straight paths rather they adopt curved and refracted paths. Thus, it becomes obvious that the earth is not composed of homogeneous materials rather there are variations of density inside the earth.

The systemic waves are reflected at the places of density changes. A regular change of density inside the earth causes a curved path to be followed by the seismic waves. Thus, the seismic waves become concave towards the earth’s surface.

S waves can not pass through liquid materials and P waves travel with the fastest speed through solid materials. P waves pass through liquid materials but their speed is slowed down.

After an in-depth study of seismic waves, Richard Dixon Oldham demonstrated that seismographs located at any distance within 105° from the epicenter, recorded the arrival of both P and S waves. However, the seismographs located beyond 145° from the epicenter, record the arrival of P waves, but no that of S waves. Thus, a zone between 105° and 145° from epicenter was identified as the shadow zone for both types of waves.

It appears from this observation that there is a core in liquid state which is located at the depth of more than 2900 km from the earth’s surface and surrounds the nucleus of the earth. Based on this finding, scientists have estimated that the iron and nickel of the core of the Earth may be in the liquid state.

Structure of the Earth:

Three zones of varying properties have been identified in the Earth-based on changes in the velocity of seismic waves while passing through the Earth.

Interior of the Earth

Crust:

  • The crust is the outermost brittle layer of the Earth, which consists of a broad mixture of rock types.
  • Nearly 1% of the Earth’s volume and 0.5% of Earth’s mass are made of the crust.
  • The thickness of the crust varies under the oceans and continents. Oceanic crust is thinner as compared to the continental crust.
  • The average thickness of the oceanic crust is 5 km, whereas that of the continental crust is around 30 km.
  • The continental crust is thicker in the areas of major mountain systems. It is as much as 70 km thick in the Himalayan region.
  • The materials of the continental crust tend to be less dense, lighter in color, thicker and considerably older than those of the oceanic crust.
  • Both types of the crust are made primarily of the silicates but the oceanic crust has somewhat more iron, magnesium, and calcium, whereas the continental crust has comparatively more aluminum, potassium, and sodium.
  • Major constituent elements of crust are Silica (Si) and Aluminum (Al) and thus, it is often termed as SIAL.
  • Based on the change in the velocity of seismic waves, the crust is divided into and because the velocity of P waves suddenly increases in the lower crust.(1) Upper Crust(2) Lower Crust
  • The discontinuity between the upper crust and lower crust is termed as the Conard Discontinuity.
  • The average density of the upper and lower crust is 2.8 and 3.0 respectively due to this, the average velocity of P waves in the upper crust is 6.1 km/s, while it becomes 6.9 km/s in the lower crust.
  • The difference in density is because of the pressure of the superincumbent load. The formation of the minerals of the upper crust was accomplished at a relatively lower pressure than the minerals of the lower crust.
  • Mechanically entire crust forms part of the lithosphere , that is an outer brittle layer of the earth.

The lithosphere is the relatively rigid outer zone of the Earth which includes the continental crust, the oceanic crust and the part of mantle laying above the softer asthenosphere. The lithosphere is divided into several large fragments called plates. The thickness of lithosphere is about 100 km and mostly composed of granite with an average density of 3.5 g/cm3.

  • There is a sudden increase in the velocity of seismic waves at the base of the lower crust, as the velocity of seismic waves is about 6.9 km/s at the lowest crust but it suddenly becomes 7.9 to 8.1 km/s. This trend of seismic waves denotes discontinuity between the boundaries of crust and mantle.
  • This discontinuity was discovered by Andrija Mohorovicic and thus it is called as ‘Mohorovicic Discontinuity’ or simply ‘Moho Discontinuity’.

Mantle:

  • The portion of the Earth’s interior beyond the crust is called the mantle.
  • The mantle having a mean density of 4.6 g/cm3 , extends for a depth of 2900 km inside the earth. Nearly 84% of the Earth’s volume and 67% of the Earth’s mass is occupied by the mantle.
  • Based on changes in the velocity of seismic waves and density, the mantle is divided into the upper and lower mantle.
  • (1) Upper Mantle from Moho discontinuity to the depth of 1000 km. from 1000 km to 2900 km.(2) Lower Mantle
  • The discontinuity between the upper mantle and the lower mantle is known as Repetti Discontinuity.
  • The velocity of seismic waves relatively slows down in the uppermost zone of the upper Mantle for a depth 100 to 200 Km. This zone is called Low-velocity zone.
  • The mantle is composed of silicate rocks that are rich in Iron and Magnesium relative to the overlying crust.
  • The major constituent elements of the mantle are Silica (Si) and Magnesium (Ma) and hence it is also termed as SIMA.
  • Mechanically the upper portion of the mantle is called asthenosphere.

The asthenosphere is the zone of Earth’s mantle which lies beneath the relatively rigid lithosphere between 50 and 300 km, below the surface and representing a mechanical boundary between more rigid regions above and below. It is approximately commensurate with that zone of the mantle which transmits seismic waves at low velocity.

The asthenosphere represents the location in the mantle where the melting point is most closely approached and the transition between a rigid lithosphere and a viscous asthenosphere is gradational.

The asthenosphere is composed of hot semi-molten and therefore deformable rock may be plate or density-driven or both. The asthenosphere is the main source of magma and it is the layer over which the lithospheric plates/continental plates move (plate tectonics).

  • The portion of the mantle which is just below the lithosphere and asthenosphere, but above the core is called Mesosphere.
  • The Mantle-Core boundary is determined by the Weichert – Gutenberg Discontinuity, at the depth of 2900 km. There is a pronounced change of density from 5.5 g/cm3 to 10.0 g/cm3 along the Gutenberg discontinuity. This sudden change in density is indicated by the sudden increase in the velocity of P waves (13.6 km/s).

Core:

  • The core is the central part of the earth, extends from the lower boundary of the mantle at the depth of 2900 km to the center of the earth (up to 6371 km).
  • The core constitutes nearly 16% of earth’s volume and 32% of earth’s mass.
  • It is divided into two zones and , the dividing line being at the depth of 5150 km.(1) Outer Core(2) Inner Core
  • The discontinuity between the upper and the lower core is called as Lehmann Discontinuity.
  • S waves disappear in the outer core, this means that the outer core should be in the molten state.
  • The inner core extends from the depth of 5150 km to the center of the earth.
  • This innermost portion of the earth is in solid-state, the density of which is 13.3 to 13.6. P waves travel through this zone with a speed of 11.23 km/s.
  • The core is made up of very heavy metallic materials mostly constituted by Nickel (Ni) and Iron (Fe), hence it is also called as NIFE.
  • The presence of iron (Ferrum) indicates the magnetic property of the earth’s interior. This property also indicates the rigidity of the earth.
  • Barysphere is sometimes used to refer to the core of the earth.
  • The inner core is solid, the outer core is liquid and the mantle is solid and plastic. This is because of the relative melting points of the different layers and the increase in temperature and pressure as the depth increases.
  • The upper mantle is both hot and under relatively little pressure, the rock in the upper mantle has a relatively low viscosity. In contrast, the lower mantle is under tremendous pressure and therefore has a higher viscosity than the upper mantle.
  • The metallic nickel, iron in the outer core is liquid because of high temperature, despite the high pressure. As the pressure increases, the nickel and iron in the inner core become solid because the melting point of iron increases dramatically at these high pressure.

Temperature, Pressure, and Density of the Earth’s Interior:

Temperature:

The recurrent volcanic eruptions throwing out extremely hot, molten material from the earth’s interior and the existence of hot springs, geyser, etc. point to a very hot interior.

The temperature increase from the surface of the earth, downward at the rate of 2 °C to 3 °C for 100 meters. The rate of increase in temperature is not uniform throughout. It is faster in some places and slower at other places.

The following facts may be presented about the thermal condition of the earth’s interior:

  • The asthenosphere is partially molten. The temperature is around 1100 °C at the depth of 100 km, which is nearer to the initial melting point.
  • The temperature at the mantle-core boundary, standing at the depth of 2900 km is about 3700 °C.
  • The temperature at the junction of outer molten Core and inner solid core standing at the depth of 5,100 km is 4,300 °C.

It, thus, appears that there is a decreasing trend in the increasing temperature rate inside the earth.

Pressure:

It was believed that the very high density of the core was because of the heavy pressure of overlying rocks. It is a common principle that pressure increases the density of rocks.

Since the weight and pressure of rocks increases with increasing depth and hence the density of rocks also increases with increasing depth.

Thus, it is proved that the very high density of the core of the Earth is due to the very high pressure prevailing there because of the superincumbent load.

But, this inference is proved wrong on the ground that there is a critical limit in each rock on which the density of that cannot be increased in spite of increasing pressure therein.

It may be, thus, forwarded that very high density of the core of the earth is not because of the very high pressure prevailing there.

If the high density of the core of the Earth is not because of the high pressure of overlying rocks then the core must be composed of intrinsically heavy metallic materials of high density.

Various experiments have revealed that the core of the Earth is made of the mixture of iron and nickel. This inference is also validated based on the geocentric magnetic field.

Density:

Due to the increase in pressure and the presence of heavier metallic material towards the center of the earth, the average density of the layers gets on increasing from crust.

The satellite studies have revealed the following results about the density of various parts of the earth:

  • average density of the earth’s surface= 2.6 to 3.3 g/cm3
  • the mantle heaving the average density of 4.6 g/cm3
  • average density of the core= 11 g/cm3 and
  • the average density of the earth= 5.51 g/cm3.