Lesson 4 - Looking through Earth Inside

Hot Concept

From the previous chapter, we have discussed the concept of the plate tectonics theory and the pieces of evidence that support the explanations for the occurrence of volcanic eruptions, earthquakes, and the formation of mountain ranges. We also discussed the relationship of each geologic event to the different plate motions, explaining why such events take place especially along plate boundaries. However, the reason that plates move can be further explained by understanding the structure of Earth.

Getting into Earth’s interior is such a challenge, especially that Earth’s interior is a region of high temperature and high pressure. Moving deeply into Earth increases the pressure, and the temperature gets even higher that solid rocks melt. The molten rocks are then expelled through volcanic eruptions, which you have learned in the previous chapter.

By looking at models, performing experiments, and making detailed observations, scientists have concluded that Earth’s interior is a combination of solid and fluid layers. These layers can be observed using seismic waves.

Earth’s Interior

Earth is divided into three distinct layers. These layers are composed of the thin outside crust, a fluid region in the middle wherein Earth’s mass is concentrated, and a solid inner core (with an outer core made up of molten metals). Each layer has its characteristics and properties.

There are other ways to classify the layers of Earth. One is based on its material or chemical composition; the other is based on mechanical characteristics such as temperature and the ability of seismic waves to penetrate across Earth’s interior. Figure 2.1 shows the layers of Earth based on their composition and mechanical properties.

Andrija Mohorovičić was born on 23 January 1857 in Volosko, Western Croatia. He was one of the major meteorologists and seismologists of the 19^th and 20^th centuries. In 1892, he became the head of the meteorological observatory in Zagreb. He received a doctorate in Philosophy in 1893 and, in 1910, became an associate university professor. Mohorovičić then taught geophysics and astronomy at the university level and became a member of the Yugoslav Academy of Sciences and Arts. In 1909, Andrija studied the Pokuplje earthquake and the seismic P-waves associated with this event. He determined the boundary between the crust and the mantle, which is now named the Mohorovičić discontinuity (or simply, the Moho). Velocities of seismic waves were used to identify this boundary. Mohorovičić studied all continents and oceans and determined that this discontinuity is present at all locations. He also came up with a function used to calculate the increasing velocity of seismic waves with increasing depth. This function is now known as Mohorovičić’s law. He retired in 1921 and passed away 15 years later in 1936.

The Compositional Layers of Earth

The crust, mantle, and core are the three layers of Earth.

The outermost layer of Earth is the crust. It is the layer made up of the oceanic and continental crust, as what we have discussed in the previous chapter. Oceanic crust is a dense and thin layer that lies under the ocean basins. Continental crust, on the contrary, is thicker and lighter (less dense) than the oceanic crust. Due to its less dense composition, the continental crust “floats” over the oceanic crust. The oceanic crust consists primarily of basalt rocks, whereas the continental crust is made up of granite and andesite rocks.

The mantle is the layer beneath the crust. It is made up of solid rock and lies between the core and the crust. It extends to about 2 900 km thick and has a mass of 4.01 x 10^24 kg- contributing to 67% of the total mass of Earth. Moreover, the mantle is hot and dense due to the idea that temperature and pressure increases with depth. Studies made by scientists proved that the mantle is made up of solid rocks based on evidence in the seismic wave behavior as these waves travel along the mantle. They also associate the mantle’s material to be similar to that of an ultramafic rock called peridotite, which is made of iron and magnesium.

Furthermore, the mantle’s temperature is also hot due to the transfer of heat that takes place from the core. Heat flows outward through the processes of conduction and convection. When heat transfers through conduction, the heat coming from the core travels through the atoms by rapid collision with one another. So, conduction occurs in the mantle region as well. Convection on the other hand, is caused by heat currents that allow materials to flow and move freely Convection currents inside the mantle start when the materials near the core are heated and particles move fast. These changes cause a decrease in the density of these materials, allowing them to move up. As the materials move above the mantle, they cool down and start to sing back, replacing the hot materials that were heated by the core. This process of cooling and heating generates convection cells, which are responsible for the movement of the crust, similar to a floating material on water that is starting to boil.

The inner part of Earth is the core. The core is the first one to be formed during the early years of Earth. The core is made up of iron and nickel alloy, which is also the reason that Earth has a magnetic field. The core makes up 31% of Earth’s composition. Released by the core is due to radioactive decay inside it. The core is divided into inner and outer cores.

The Mechanical Layer of Earth

The compositional layer of Earth, as we have discussed previously, are the crust, the mantle and the core. Aside from these compositional layers, Earth can also divided into layers based on strength and rigidity. These are the mechanical layers of Earth. Each mechanical layer hi properties such as temperature, ability of seismic waves to penetrate its material composition, and its phase (i.e., solid or liquid). The five mechanical layers are the lithosphere, the asthenosphere the mesosphere, the outer core, and the inner core.

The lithosphere is composed of the crust and the upper mantle. It is solid, rigid, and brittle, extending to about 100 kilometers in depth. Being rigid and brittle, the lithosphere has the tendency to break under pressure and mechanical interaction, which is responsible for earthquakes. It is also divided into different tectonic plates that move above the next layer, which is asthenosphere.

The asthenosphere is a region of partially molten mantle materials that can flow. Even though it flows, this region or layer is not liquid. The asthenosphere is responsible for the motion of tectonic plates. It is approximately 200 kilometers in depth below the surface.

Beneath the asthenosphere lies a region of increased pressure where the flow ceases. This region is called the mesosphere. It is the lowest part of the mantle, next to the outer core.

The outer core is the external part of the core. This region is liquid in form and is composed mainly of an iron-nickel alloy. The temperature in this region is so high that it reaches to about 4 000-5 000°C. This region is also the one responsible for the magnetic field generation of Earth. Beneath the outer core is a solid inner core. Intense pressure and extremely high temperature do not affect the inner core due to its composition. The inner core is mainly made up of a very dense and heavy iron and nickel material, which does not melt easily at very high temperatures.

Boundaries or Seismic Discontinuities

When an earthquake occurs, the seismic waves generated move in all directions inside

Earth’s interior. The difference in time recorded by seismic stations between P- and S-waves determines the speed or velocity of each wave from the epicenter. The velocity of the seismic waves depends on the type and the property of material at which it moves. Seismic waves move faster in solids than in liquids. Also, the more compact the molecules are in the material, the faster these waves move.

Inside Earth, molten regions slow down the movement of P-waves, and hinder the transmission of S-waves due to their characteristics.

As the waves pass through the different layers, they are reflected, refracted, or even produce new wave phases that result from different seismic velocities. The quick change in seismic velocities in a boundary is called the seismic discontinuity.

These seismic discontinuities have given scientists the idea that there exists a boundary within the different layers of Earth. The Mohorovičić discontinuity is one of the boundaries that exhibit the sudden change in seismic velocity. The Mohorovičić discontinuity can be found in the boundary between the mantle and the crust. The Gutenberg and Lehmann discontinuities are two other boundaries in the core-mantle and inner core-outer core boundaries, respectively.

The Mohorovičić Discontinuity

The boundary between the crust and the mantle is known as the Mohorovičić discontinuity popularly known simply as Moho. It was named after the Croatian seismologist Andr Mohorovičić who discovered its existence in 1909, He found out that seismic waves abruptly change from a constant difference at a distance greater than 200 km from the epicenter. Readings from seismograph records show that the relationship between time and distance of earthquake arrival at different seismic stations within 200 km from the epicenter is proportional, showing a constant ratio between time and distance. However, at 200 km and beyond, he found that quick jump in reading was manifested in the seismogram, showing that it becomes faster is terms of its arrival time. This change was accounted through the idea of reflection and refraction of seismic waves after these waves encounter a different density. The change in density occurs because the mantle is greater in density than the crust. With this finding, he found out that ther is a boundary between the two layers of Earth’s interior.

It was also found that the Moho has an average depth of around 8 km under the ocean basin and around 32 km under the continental crust.

The Gutenberg Discontinuity

In 1913, the German-born American seismologist Beno Gutenberg found out that, at some depth, the P-wave slows down and hinders the movement of the S-wave. This discovery proves that S-waves cannot travel in liquid regions. Gutenberg also expressed that this resulted in the idea that the core is made up of molten materials because it does not allow S-waves to penetrate it. Because of his discovery, the boundary at which the mantle meets the core has been called the Gutenberg discontinuity.

The Lehmann Discontinuity

In the region between the inner and outer core exists a boundary known as the Lehmann discontinuity, named after a Danish seismologist and geophysicist Inge Lehmann. Her discovery is a breakthrough in the study of Earth’s interior. Prior to her discovery, seismologists believed that the core was made up of a single molten sphere. However, she explained that based on the analysis of seismic wave measurements, the core is made up of a molten region and a solid region. This is based on data showing a sudden increase in the P-wave and S-wave velocities a a depth of 220 km.