Interior of the Earth

An understanding of the earth’s interior is essential to follow the nature of changes going on

over the earth’s surface which are related to the deep laid internal forces operating from within

the earth. This understating of the earth’s interior is based mainly on indirect sources, because so

far it has not been possible to have access to the inner levels of the earth’s structure.

EVIDENCES ABOUT EARTH’S INTERIOR

Direct Sources

1. DEEP DRILLING IN SEARCH OF OIL.

2. VOLCANIC ERUPTIONS - The recurrent volcanic eruptions throwing out extremely

hot, molten material from the earth’s interior and the existence of hot springs,

geysers etc. point to an interior which is very hot. They serve as windows giving

information about the outer 200 kms of the earth’s interior.

Indirect Sources

1. EVIDENCE FROM THE METEORITES - The meteorites are solid bodies freely traveling in

space which accidentally come under the sphere of influences of the earth’s gravity

and as a result fall on earth (or collide with it). Their outer layer is burnt during their

fall due to extreme friction and the inner core is exposed. Earth’s average density of

5.5 grams per cubic centimeter (g/cm3) is similar to the average density of meteorites,

which supports the idea that planet Earth is itself composed of meteoritic material.

Furthermore, the heavy material composition of their cores confirms the similar

composition of the inner core of the earth, as both evolved from the same star

system in the remote past.

2. STUDY OF SEISMIC WAVES – One way scientists learn about Earth's interior is by looking

at seismic waves. Seismic waves travel outward in all directions from where the ground

breaks at an earthquake. By studying their path movement x-ray like pictures of the

earth’s interior can be obtained.

3. HIGH PRESSURE-TEMPERATURE LABORATORY EXPERIMENTS – The interior of the earth

can be determined by analyzing seismic wave propagation utilizing their large density

differences. However, this analysis requires information about the structures and

properties of Earth's layers, and consequently, the rocks (minerals) that constitute the

layers must be investigated. Currently the drilling depth is limited to only ~10 km, which

is extremely small compared to Earth's radius of ~6,400 km. Therefore, high-pressure,

high-temperature experiments that artificially reproduce the conditions of Earth's

interior are very useful in these investigations. In high-pressure, high-temperature

experiments, experimental samples are encapsulated in high-pressure cells covered by

heaters and a pressurizing medium.

ROLE OF SEISMIC WAVES IN EXPLORING THE EARTH’S

INTERIOR

The are many different seismic waves, but all of basically of three types:

Compressional or P (for primary)

Transverse or S (for secondary)

Surface or L

The first two wave types, P and S , are called body waves because they travel or propagate through the body of Earth. The surface waves travel along Earth's surface and their amplitude decreases with depth into Earth. The body waves give us information about the earth’s interior.

Compressional or P-Waves

P-waves are the first waves to arrive because they travel the fastest. They typically travel at speeds

between ~1 and ~14 km/sec. The slower values corresponds to a P-wave traveling in water, the higher

number represents the P-wave speed near the base of Earth's mantle. P-waves are sound waves whose

velocity depends on the elastic properties and density of a material.

As a P-wave passes the ground is vibrated in

the direction that the wave is propagating.

S-Waves

Secondary , or S waves, travel slower than P waves and are also called "shear" waves because they don't change the volume of the material through which they propagate, they shear it. S-waves are transverse waves because they vibrate the ground in a the direction "transverse", or perpendicular, to the direction that the wave is traveling.

As a transverse wave passes the ground

perpendicular to the direction that the

wave is propagating. S-waves are

transverse waves.

The S-wave speed, call it b, depends on the shear modulus and the density. Typical S-wave propagation speeds are on the order of 1 to 8 km/sec. The lower value corresponds to the wave speed in loose, unconsolidated sediment, the higher value is near the base of Earth's mantle. An important distinguishing characteristic of an S-wave is its inability to propagate through a fluid or a gas because a fluids and gasses cannot transmit a shear stress and S-waves are waves that shear the material.

Seismic Wave Propagation

The fact that the waves travel at speeds which depend on the material properties (elastic moduli and density) allows us to use seismic wave observations to investigate the interior structure of the planet. We can look at the travel times, or the travel times and the amplitudes of waves to infer the existence of features within the planet, and this is a active area of seismological research. To understand how we "see" into Earth using vibrations, we must study how waves interact with the rocks that make up Earth.Several types of interaction between waves and the subsurface geology (i.e. the rocks) are commonly observable on seismograms

Refraction Reflection

As a wave travels through Earth, they are refracted.

When waves reach a boundary between different rock types, part of the energy is

transmitted across the boundary. The transmitted wave travels in a different direction which

depends on the ratio of velocities of the two rock types. Part of the energy is also reflected

backwards into the region with Rock Type 1, but I haven't shown that on this diagram.

Refraction has an important affect on waves that travel through Earth. In general, the seismic velocity in Earth increases with depth (there are some important exceptions to this trend) and refraction of waves causes the path followed by body waves to curve upward.

The overall increase in seismic wave speed with depth into Earth

produces an upward curvature to rays that pass through the mantle. A

notable exception is caused by the decrease in velocity from the

mantle to the core. This speed decrease bends waves backwards and

creates a "P-wave Shadow Zone" between about 100° and 140° distance

(1° = 111.19 km).

The second wave interaction with variations in rock type is reflection. A seismic reflection occurs when a wave impinges on a change in rock type (which usually is accompanied by a change in seismic wave speed). Part of the energy carried by the incident wave is transmitted through the material (that's the refracted wave described above) and part is reflected back into the medium that contained the incident wave.

When a wave encounters a change in material properties (seismic

velocities and or density) its energy is split into reflected and

refracted waves.

The amplitude of the reflection depends strongly on the angle that the incidence wave makes with the boundary and the contrast in material properties across the boundary. For some angles all the energy can be returned into the medium containing the incident wave.

The actual interaction between a seismic wave and a contrast in rock properties is more complicated because an incident P wave generates transmitted and reflected P- and S-waves and so five waves are involved. Likewise, when an S-wave interacts with a boundary in rock properties, it too generates reflected and refracted P- and S-waves.

Internal Structure of the Earth

The earth is divided into three layers i.e., crust, mantle and core.

Crust

CRUST is the outer thin layer with a total thickness of around 100 km. it forms 0.5 per cent of

the earth’s volume. The outer covering of the crust is of sedimentary material and below that be

crystalline, igneous and metamorphic rocks which are acidic in nature.

The crust is of two types i.e., continental and oceanic. (see class notes for further elaboration)

Mantle

80% of the earth’s volume is contained within the mantle. Mantle is described as a solid rocky layer,

and the most common rock is peridotite. Peridotite – ultra basic rock, consisting largely of

olivine, hence its predominantly dark green colour (olivine – silicate of magnesium mg2SiO4 to

silicates of iron Fe2SiO4). The crust increases its temperature with depth, but this trend does not

continue downward into the mantle. This means mantle has an effective method to transmit heat

outward i.e., some form of convection. Material in this zone exhibit plastic behaviour, i.e., when

the material encounters short lived stresses, such as seismic waves, the material behaves like an

elastic solid. However, in response to long term stresses, this same rocky material will flow. So S

waves can penetrate through mantle, yet, this layer is not able to store elastic energy like a brittle

solid and is thus incapable of generating earthquakes.

The mantle can be further divided into 3 parts – outer mantle, aesthenosphere, and inner mantle. (see

class notes for further elaboration)

Core

CORE lies between 2900 km to 6400 km below the earth’s surface and accounts for 83 per cent

of the earth’s volume. The central core has the heaviest mineral of highest density. It is

composed of nickel and iron (ferrous)and is, therefore, called “nife”, while a zone of mixed

heavy metals + silicates separated the core from outer layers. (see class notes for further elaboration)