Convection in the Earth – Importance

Earth’s Structure 

The interior of the Earth comprises of several circular layers of which the crust, the mantle, the outer core, and the inner core are important because of their distinctive physical and chemical characteristics. 

The crust is a solid silicate, the mantle is in the form of viscous molten rock, the outer core is in the form of a viscous liquid, and the inner core is in the form of a dense solid. 

Chemically, Earth is divided into the crust, upper mantle, lower mantle, outer core, and inner core. 

Importance of Understanding Earth’s Interior Structure 

Understanding the structure of the Earth’s interior, i.e., crust, mantle, core, and various forces such as heat, seismic waves radiating from Earth are important to understand – 

• The development of the Earth’s surface, its existing shape and future. 
• geophysical occurrences like volcanoes, earthquakes, etc. 
• Earth’s magnetic field. 
• The internal structure of various objects in the solar system.  
• The development and present structure of the atmosphere. 
• For mineral study. 

Earth’s Surface 

• There are various geological activities that shape the Earth’s surface. 
• The forces that affect these activities come from both above and below the Earth’s surface. 
• Activities that are caused by forces from inside the Earth are called endogenous activities (Endo meaning “in”). 
• Exogenous activities (Exo meaning “out”) take place from forces on or above the Earth’s surface. 
• The major geological characteristics of the Earth’s surface such as mountains, plateaus, lakes mostly arise from endogenous activities such as folding, faulting that are caused by forces from the interior of the Earth. 

Geophysical activities like volcanoes, earthquakes etc. 

• The forces that cause disastrous incidents such as earthquakes and volcanic eruptions occur deep within the Earth’s surface. For example, earthquakes occur due to the movement of the tectonic plates and the energy needed for the movement of tectonic plates is provided by the conventional currents in the mantle. 
• Similarly, volcanoes happen through the openings and cracks created by the tectonic movements. 

Earth’s magnetic field 

• The temperature of the outer core varies from 4400°C in the outer core regions to 6000°C near the inner core region. Heat sources include energy given out by the compression of the core, energy given out at the inner core border as it grows (latent heat of crystallization), and radioactivity of elements for instance uranium, thorium, and potassium. 
• The variation in temperature, pressure, and composition within the outer core produce convection currents in the molten iron of the outer core as it cools, dense matter sinks whereas warm, less dense matter rises. This flow of liquid iron produces electric currents, which in turn create magnetic fields. 
• Charged metal particles going through these fields keep on creating electric currents of their own, and like this the cycle goes on. This self-maintaining loop is called the geo-dynamo. 
• The benefit of this magnetic field is that it protects the Earth from the Sun’s damaging solar wind. 
• The outer core layer is very important because without this layer, Earth will not have a magnetic field and without a magnetic field, Earth will not have life, ocean, and atmosphere on it. 

The interior structure of different solar system objects 

• The complete solar system was created from a single nebular cloud, and the process of the creation of every solar system object is supposed to be similar to that of the Earth. 

Evolution and current composition of the atmosphere 

• For life to prosper on the surface of the Earth, the atmosphere should have necessary elements such as oxygen for respiration, CO₂ and other greenhouse gases to control the temperature on the surface, ozone to protect life from harmful ultraviolet radiation and the proper atmospheric pressure. 
• All these components of the Earth’s atmosphere provide their presence to the volcanic eruptions that explain the Earth’s interior. 

Direct Sources of Information about Earth’s Interior 

• Mining 
• Drilling, for example: Deep drilling of ocean 
• Volcanic eruptions 

Deep mining of Earth and drilling gives the information of the nature of rocks deep down the surface. 

Volcanic eruptions help in giving the direct information of Earth’s interior. 

Indirect Source of Information about Earth’s Interior 

Increase in pressure and temperature with depth 

• The diameter of the Earth and gravitation help in assessing pressure deep inside the Earth. 
• Volcanic eruptions and presence of hot springs, geysers etc. gives information about an Earth’s interior which is extremely hot. 

Seismic waves 

One effective way scientists learn about Earth’s interior is by seeing the movement of evergy from the point of an earthquake, called seismic waves. Seismic waves move outward in all directions from where the ground break down at an earthquake. Seismograph stations calculate the energy emitted by these earthquakes. There are two waves that help to understand the interior of the Earth. The seismic waves calculated in mantle studies are known as body waves, because these waves move through the body of the Earth. The velocity of body waves changes with density, temperature, and rock type. 

The two types of body waves are: P-waves or primary waves, and S-waves or secondary waves. P-waves, are also called pressure waves, that are developed by compressions.  

Primary waves (P-waves):  

These are fastest moving waves that move at about 6 to 7 km/sec (about 4 miles). Hence, they reach first at the seismometer.  

P-waves move deep within the Earth’s interior, and travel through both solid and liquid (through the whole Earth) mediums. P-waves travel straight. They expand and contract on their way. P-waves cause least damage of all the waves. 

As P-waves come across the liquid outer core, that is less rigid than the mantle, they slow down. This makes the them arrive late and further away than would be anticipated. This result in a P-wave shadow zone. Hence P-waves cannot be picked up at seismographs 104o to 140o from the earthquakes focus point. 

Secondary Waves (S-waves): 

The secondary waves are little bit slower (4-5 km/sec) than P- waves. They arrive at a given location after the P-waves. S-waves travel deep within Earth’s interior, but they only move through solids (crust and mantle). They move up and down, in a S-like motion, and are more damaging than P-waves. S-waves cannot move through liquid medium. 

By following seismic waves, scientists are able to study Earth’s interior. P-waves slow down at the mantle core border, so by this we know that the outer core is less rigid than the mantle. S-waves disappear at the mantle core border, so this show that the outer core is liquid. Other hints to Earth’s interior contains the fact that the Earth’s total density is greater than the density of crustal rocks, so the core must be made of something dense material, such as metal. Also, since Earth has a magnetic field, there must be metal present inside it. Iron and nickel are both magnetic in nature. Lastly, meteorites and Earth are evolved from the same nebular cloud. Hence, they are likely to have a same internal composition. 

When meteoroids fall to Earth, their outer layer is burnt during their fall due to severe friction and the inner core is visible. The heavy material structure of their cores proves that the structure of the inner core of the Earth is same. 

Seismic discontinuity 

Seismic discontinuities are the areas on Earth where seismic waves act very differently as compared to the surrounding regions due to a noticeable change in physical or chemical properties. 

The Mohorovicic (Moho) discontinuity 

Mohorovicic (Moho) discontinuity creates the boundary between the crust and the upper area of the mantle (asthenosphere) where there is a discontinuity in the seismic velocity. 

It occurs at an average depth of about 8 kilometers under ocean basins and 30 kilometers underneath continental surfaces. 

The basis of the Mohorovicic discontinuity (Moho) is thought to be a change in the chemical composition of rocks containing feldspar (above) to rocks that do not contain feldspars (below). 

Gutenberg Seismic Discontinuity / Core-Mantle Boundary 

The Gutenberg discontinuity is also known as the core-mantle boundary (CMB). At the CMB, S-waves, which cannot travel in liquid, suddenly vanish, and P-waves are strongly bent or refracted. This notifies seismologists that the solid and molten formation of the mantle has given way to the blazing liquid of the outer core. 

Lehman Siesmic Discontinuity / The Inner Core 

The transition zone between outer and inner core is Lehman Siesmic Discontinuity. The Lehmann discontinuity is an sudden increase of velocities of P-wave and S-wave at the depth of 220±30 km. It is discovered by Inge Lehmann, a seismologist. It is present below continents, but not usually below oceans. 

Repiti Discontinuity 

The transition zone between outer mantle and inner mantle is repiti discontinuity. 

Mantle 

• Mantle is made up of rock; it is hot and is present below the crust. It expands up to a depth of 2900 km below the crust. The mantle is divided into the upper and lower mantle. 
• Mantle mainly comprises of silicate rocks that are rich in iron and magnesium. Olivine, garnet, and pyroxene are the common silicates found in the mantle. The mantle is made up of constituent elements – 45% oxygen, 21% silicon, and 23% magnesium (OSM). • In the mantle, temperatures vary from around 200°C at the upper boundary with the crust to about 4,000°C at the core-mantle boundary. 
• Because of the difference in temperature, there is a circulation of convective material in the mantle (through solid, the elevated temperatures in the interior of the mantle cause the silicate material to be adequately ductile). 
• In the mantle, rocks move continuously up and down due to internal heat from the core area and form convective currents. Convection of the mantle is shown at the surface by the movement of tectonic plates. These currents cause rock plates to move and collide with each other that results in earthquakes. 
• Tectonic plates are formed by the combination of the upper mantle and crust. These plates move very slowly. The point where plates touch each other is called a fault. 
• The transfer of heat and material in the mantle helps to identify the landscape on the Earth. Activity in the mantle pushes plate tectonics to cause volcanoes, seafloor spreading, earthquakes, and mountain-building (orogeny). 

Convection in mantle: 

Heat flows in two various ways inside the Earth: 

Conduction: Heat is transferred through quick collisions of atoms, which can only take place if the material is in a solid state. Heat transfers from warmer places to cooler places till all the place has the same temperature. The mantle is hot mainly due to the conduction of heat from the core. 

Convection: If a material is able to move, convection currents form even if it moves very slowly. Earth’s mantle is thought to be comprised of olivine-rich rock. The temperature of the rock changes at different depths. The temperature is lowest immediately below the crust and it rises with depth. The highest temperatures are seen where the mantle material is in connection with the heat-producing core. This continuous rise of temperature with depth is known as the geothermal gradient. Different rock behaviors depend on the geothermal gradient, and these different rock behaviors are used to split the mantle into two different zones. Cool and brittle rocks are present in the upper mantle, whereas hot and soft (not molten) rocks are present in the lower mantle. Brittle rock in the upper mantle can break under stress and produce earthquakes. But soft rocks in the lower mantle flow when exposed to forces instead of breaking. The lower limit of brittle behavior of rock is the border between the upper and lower mantle. 

Asthenosphere 

Asthenosphere (astheno means weak) is the upper portion of the mantle. It is present right below the lithosphere ranging up to 80-200 km. 

Density of asthenosphere is higher than that of the crust. It is ductile, and mechanically weak. These characteristics of the asthenosphere help in the movement of plate tectonic and isostatic modifications (the elevated part at one part of the crust area is balanced by a depressed part at another crust area).  

Asthenosphere is the main source of magma that reaches to the surface during volcanic eruptions. 

Models of Mantle Convection 

For exmple, in case of soup bowl, hot soup from the bottom of the bowl to the top by convection. Some geologists think that even the process of Earth’s convection works in the same manner. That is — hot rock from the bottom of the mantle moves all the way to the top of the mantle before it gets cool and fall again. This entire process is callled as whole-mantle convection. Other group of geologists think that the upper mantle and lower mantle are too various groups to convect as one. They point to slabs of lithosphere that are falling back into the mantle, some of which seem to settle on the boundary between the upper and lower mantle, rather than falling straight through. They also noted chemical changes in the magma originating in various areas of the mantle. The changes are not regular with the entire mantle being well agitated. They say that double-layered convection is a well fit with the observations.  However other geologists say that there may be some spots where convection moves from the bottom of the mantle to the top of the mantle, and some group of geologist say that it does not move. 

Mantle Maps 

Innovative technology has allowed geologists and seismologists to produce mantle maps. Most mantle maps show seismic velocities, showing patterns deep below the Earth’s surface. Geoscientists hope that modern mantle maps can plot the body waves of as many as 6,000 earthquakes with magnitudes of at least 5.5. These mantle maps may be able to detect early slabs of subducted material and the accurate position and movement of tectonic plates. Many geologists think that mantle maps may even give proof for mantle plumes and their structure. 

Summary

1. Earth is made up of various layers- crust, mantle , outer core and inner core.
2. Crust is the thinnest outermost layer of the Earth.
3. Mantle is mainly made up of silicate rocks that are rich in iron and magnesium. In the mantle, rocks move continuously up and down due to internal heat from the core area and form convective currents.
4. Outer core is present in liquid state at the 5000ﹾC temperature.
5. The diameter of the Earth and gravitation helps in assessing pressure deep inside the Earth.
6. The inner core is the hottest layer of the Earth with a temperature of 7000ﹾC.
7. Seismic discontinuities are the areas on Earth where seismic waves act very differently as compared to the surrounding regions due to a noticeable change in physical or chemical properties.
8. Asthenosphere (astheno means weak) is the upper portion of the mantle. It is present right below the lithosphere ranging up to 80-200 km.
9. Mantle maps show seismic velocities that show patterns deep below the Earth’s surface.