Natural Sources To Study Earth’s Interior

There are two components of natural sources to probe the earth’s interior. They are:

Volcanicity
Seismology

Volcanicity

Vulcanicity is a process within which endogenic forces create a fracture or fault. These fractures or faults bring magmatic materials, gases, vapour or rock fragments to the surface. The ejected material or lava is very hot and is part of rocks in a melted form.

The appearance of melted material provided the idea that a permanent magma chamber exists in the lower part. But this idea is also refuted. Scientists have also proved that the increasing pressure in the interior increases the rocks’ melting point. Hence, there may not be a permanent magma chamber.

We know very well that food is cooked more quickly in a pressure cooker than in an open pot. In an open pot, the water gets boiled at a much lower temperature (1000 C) in comparison to a pressure cooker (1210 C). The vapour generated in the pressure cooker is not easily allowed to escape. It keeps on accumulating inside. Accumulation of vapour increases the pressure inside. An increase in pressure forces the water to boil at a much higher temperature. This experiment suggests that the increasing pressure is the cause of boiling water or melting rocks at a much higher temperature.

Scientists suggest that there is no permanent magma chamber. Magma is generated because of the removal of the overlying rock pressure. Overlaying rocks are removed by shifting of rock mass due to faulting. Once magma is generated, it becomes larger in volume.

Since there is no vacuum space inside, magma pushes the overlying rocks and breaks. The volcanism is the result, and it appears on the surface. Therefore, vulcanicity is also not much of great help in understanding the interior.

Seismology

Seismology is a branch of scientific knowledge to study earthquakes and their propagated waves. An earthquake is a sudden vibration or shake of the ground of an area due to the abrupt breaking of a part of a plate causing instability in the region. This leads to earthquake waves.

The earthquake waves are recorded by an instrument commonly known as a seismograph. It is important to mention here that seismology is the only source by which the entire earth could be probed. The probe provides authentic and complete information about all parts of the earth.

Earthquake gets their birth from a depth below the earth’s surface. This depth could be anything from a few meters to hundreds of km. According to the United States Geological Survey (USGS), for scientific purposes, the originating depth of an earthquake may vary from 0 to 700 km.

The point from where the earthquake originates is known as the focus. The shortest distance from the focus to the earth’s surface. It is a perpendicular distance exactly above from focus. It is referred to as epicentre. Epicentre (Fig) being the closest place on the earth’s surface, experiences the earthquake first. It is recorded later in distant places from the epicentre.

Principally, there are three types of seismic waves- Primary, Secondary and Surface waves. They are recorded one after the other by the instrument.

Primary Waves

Primary (P) waves are known as compressional waves. They are also termed push and pull waves (Fig). It is like sound waves that we hear. The sound generated from its source pushes the air available nearby, and it keeps on hitting the adjacent air molecules. In this way, the sound reaches our ears, and we hear.

Earthquake occurs due to plate movement and the breaking of the plate edge. Thus, generated pressure pressurizes the surrounding rocks. This pressure keeps moving further and is recorded by the seismographs installed at different places.

It behaves as if we are piercing a nail in the wall with the help of a hammer. With the force with which we hammer the nail, the nail goes inside, but with the same force, the hammer is returned towards us. This gives a back-and-forth or forward and backward movement of the quake waves.

Characteristics of Primary (P) waves:

It is the fastest wave; hence, it reaches first and gets recorded in the seismograph.
It travels in all three states of matter – solid, liquid and gas.
Its velocity is greater in solid, less in liquid and very slow in gas.
Its velocity increases if there is an increase in the density of the rocks and vice versa.
Once the state of matter changes from solid to viscous or liquid, its velocity decreases even if the density is greater.
The velocity of P waves varies from 5.5 km per second at or near the surface to 13.0 km per second in the deep interior.

Secondary Waves

Secondary (S) waves are known as transverse waves. These waves travel at the right angle to the direction of the wave propagation. These waves seem to be like light waves. These types of waves are just like waves on the calm water of a pond when we throw a pebble (Fig). Since these waves travel horizontally at the surface, they are more dangerous than P waves.

Characteristics of Secondary (S) waves:

Its velocity is less in comparison to Primary (P) waves; hence, it is recorded after P waves in the seismograph.
It travels only through the solid state of matter.
Its velocity increases if there is an increase in the density of the rocks and vice versa.
Once the state of matter changes from solid to viscous, its velocity is reduced. But when the rocks are melted, it disappears completely.
The velocity of S waves varies from 3.25 km per second at or near the surface to 7.0 km per second in the interior.

Surface Waves or L Waves

Surface waves or Longitudinal (L) waves travel through the earth’s surface. Surface waves are manifestations of the P and S waves which finally reach the surface from the interior. These waves travel in the same way as if you are giving a jerk to the wet towel before spreading on a rope.

They are categorized into two waves: Love (named after A E H Love) waves and Rayleigh (named after J W S Rayleigh). Love waves (Fig) travel in horizontal and perpendicular wave propagation directions. The particle motion in Rayleigh waves (Fig) is in elliptical motion, generally retrograde in the vertical plane and parallel to the direction of wave propagation.

Characteristics of Surface (L) waves:

Its velocity is less in comparison to Primary (P) and Secondary (S) waves. Hence, it is recorded after P and S waves on the seismograph.
It travels only through the solid state of matter.
The velocity of L waves varies from 2.0 to 4.4 km per second, while the velocity of R waves is slightly lesser (from 2.0 to 4.2 km per second).
The velocity of L and R waves depends on the waves’ frequency and penetration in the earth’s upper layer.

Interpretation of Propagating Different Waves

As mentioned before, all seismic waves start propagating with the occurrence of earthquakes simultaneously. But they are recorded at different times on the seismograph. Its reason is the different velocities of different waves. The faster wave reaches quickly, but the slower wave reaches after a time lag. Look at Fig given below. It is showing the concept written above.

In the initial stage, the strain and stress operating on the rock produces very-very faint noise and is probably experienced by some sensitive animals whose body or nose is near to the surface. They get frustrated and behave very differently. After this P wave arrives as it is the fastest. Because of slower velocity, S waves take a little longer to reach the same place.

The difference between the arrival of P and S is known as time lag (Fig). The duration of time lag depends on the distance between the epicentre of the earthquake and the referred place or measuring station. The nearer place would observe a smaller time lag, whereas the far distant places would observe a bigger time lag. At the epicentre, it is probably difficult to identify the time lag because of less gap, or practically speaking, no time lag between P and S waves.

Recorded Earthquake on Seismograph Source

Seismology and Constitution of the Earth’s Interior

The waves generated during earthquake occurrence radiate in all directions from the focus. The radiated waves are not passing in straight lines but follow curved paths. The curvature of the paths is due to changing density from the earth’s surface to the core.

Due to the refraction of the waves, S waves are not found beyond an angular distance of 105° from the earthquake’s epicentre. In the same way, P waves are not traceable from 105° to 140° from the epicentre. These are known as shadow zones (Fig).

Propagation of P and S Waves in the Interior Source

But more important about the interior of the earth is revealed by the nature of the propagation of waves particularly P and S. From the surface of the earth towards the interior, both waves P and S are propagating with increasing velocity. Approximately at an average depth of 40 km, there is an increase in the velocity of both waves. It suggests that there is a sudden increase in the density of rocks at that depth.

At a depth of around 100 to 250 km, the velocity of both waves starts declining, and after around 700 km, velocity again becomes greater. Decreasing velocity in this belt indicates that the matter of rocks is semi-solid. Because of this, the velocity of both waves declines. This low-velocity zone is known as the asthenosphere. It is also named a transition zone.

Further beyond, the velocity of both waves increases continuously until a depth of 2900km. Increasing velocity shows that the density is higher and the state of the rocks is solid. From 2890 km to 2900 km, the rocks are again almost in a plastic state, i.e., neither solid nor liquid. This narrow belt is called as D layer, which is a condition of a transition zone (Fig). Beyond the depth of 2900 km, there is no trace of S waves, and the velocity of P waves declines very drastically.

Remember the characteristics of both waves; it suggests that the rocks at this depth are melted, and S waves do not travel in liquid. Reduction in the velocity of P waves is also due to the changing state of the matter or rocks. At around 5150 km depth, the velocity of the P waves increases. This is proof that the rocks become solid again.

Assimilation of Study of Earth’s Interior Source

Natural Sources to Study Earth’s Interior

Volcanicity