Radioactive Decay: Definition and Diagram

Radioactive Decay of Unstable Isotopes 

Earth’s Structure 

The interior of the Earth comprises of various 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. 

Mechanically, the Earth’s layers are divided into lithosphere, asthenosphere, mesospheric mantle (it is part of the Earth’s mantle present below the lithosphere and the asthenosphere), outer core, and inner core. 

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

Earth’s Heat 

Earth is a warm planet. The enjoyable conditions on Earth’s surface are mainly due to the radioactive processes taking place at the center of the Earth. Radioactivity causes our planet (Earth) to act like a hot-water bottle. Half the amount of heat required for our survival is given out by the radioactive disintegrations that take place in the rocks that form the Earth’s crust. The other half of the heat released comes mainly from the slow cooling of the Earth, called secular cooling. This cooling has been happening since the formation of the Earth. 

The thermal energy developing from the ground has been expected to be around 46 trillion watts (46 TW or terawatts), including 2 TW from volcanic eruptions and earthquakes. In addition to earthquakes and volcanic eruptions, most of this geothermal flux is given out in a continuous way. This process is called the geothermal flow. 

Several billion years ago, Earth might have cooled a couple of hundred degrees. Earth keeps almost a constant temperature, due to production of constant heat in its interior. It produces constant heat by the process of radioactive decay. This process includes the disintegration of natural radioactive elements present inside the Earth such as uranium. For example: Uranium is a special kind of element because when it decays, it produces heat, and this heat keeps the Earth from cooling off totally. 

Several rocks present within the Earth’s crust and in its interior undergo the process of radioactive decay. This process generates subatomic particles that zip away, and afterward collide with surrounding material present inside the Earth. Their energy of motion is changed to heat. 

In the absence of radioactive decay, there would not be many volcanoes and earthquakes – and less formation of Earth’s huge mountain ranges. 

Scientists have found out that half of the Earth’s unusual heat that explodes on its surface by volcanic eruptions and drives the massive movements of the continents is due to radioactivity. 

There are four radioactive isotopes inside the Earth that account for about 50% of its interior heat. Like a slow cooker, these four radioactive isotopes constantly release heat within the planet keeping it on a light simmer. 

Nearly 50% of the heat given off by the Earth is produced by the radioactive decay of elements like uranium and thorium, and their decay products. This is the conclusion given by an international team of physicists that has used the KamLAND detector in Japan to calculate the flux of antineutrinos emitting from deep within the Earth. 

Geophysicists think that heat moves from the interior of the Earth into space at a rate of about 44 × 1012 W (TW). But so far it is not clear how much of this heat energy is primordial, i.e., left over since the formation of the Earth – and how much heat energy is produced by radioactive decay. 

The highly popular model of heating by radioactivity is based on the bulk silicate Earth (BSE) model, which believes that radioactive materials, like uranium and thorium, are found in the Earth’s lithosphere and mantle, but not within the Earth’s iron core. The BSE model also states that a large quantity of radioactive material can be expected by studying igneous rocks formed on Earth, and the structure of meteorites. 

The four radioactive isotopes are: 

1. Uranium-238 (238U) 
2. Uranium-235 (235U) 
3. Thorium-232 (232Th) 
4. Potassium-40 (40K) 

The large amount of the transfer of heat takes place at mid-oceanic ridges. Whereas the minimum amount of heat transfer is from the interiors of the continent. 

The Earth produces a large amount of heat from the decay of naturally occurring radioactive isotopes in its interior. At present, the main radioactive isotopes are ⁴⁰K, ²³⁵U, ²³⁸U, and ²³²Th. An extinct radioactive isotope ²⁶Al may have been an essential source of heat of the Earth in the early history. There are many other radioactive isotopes present on the Earth, but they play a very insignificant role in the production of heat, either because of their low abundance or their low heat producing capacity. 

Antineutrinos are produced not only in the decay of uranium, thorium, and potassium isotopes but in a fission product in nuclear power reactors.  

Tracking the heat 

All models of the interior of the Earth depend on indirect evidence. For example, the bulk silicate Earth (BSE) model of the Earth’s interior, think that the mantle and crust comprise of only lithophiles (“rock-loving” elements) and the core comprises of only siderophiles (elements that “like to be with iron”). So, all the heat from radioactive decay generates from the crust and mantle – almost 8 terawatts from uranium 238 (238U), and another 8 terawatts from thorium 232 (232Th), and 4 terawatts from potassium 40 (40K). 

How hot is it inside the Earth?  

So far no one has directly come close to exploring Earth’s interior. Hence, all geophysicists do not agree on the hotness of the Earth’s core. But the seismic waves, i.e., rate of travel of waves from earthquakes can tell scientists a lot about the materials that make up the Earth. Seismic data also shows whether these materials are in liquid, solid or partially solid state. In the meantime, laboratory data also shows that temperatures and pressures of the interior of Earth’s materials should begin to melt. 

From this data, Earth’s core temperature is expected to be about 5,000 to 7,000ﹾC. That is as hot as the surface of the Sun, but very much cooler than the Sun’s interior. 

While the heat energy generated in Earth’s interior is massive. It is around 5,000 times less powerful than energy received by Earth from the Sun. The sun’s heat energy makes the weather and eventually causes erosion. So, Earth’s heat energy creates mountains whereas the Sun’s heat energy again breaks them down bit by bit. 

Seismic waves 

One effective way scientists learn about Earth’s interior is by seeing the movement of energy 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 waves are: Body waves (P-waves or primary waves, and S-waves or secondary waves) and surface waves (Love waves and Raleigh 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 travels through both, solids, and liquids (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 P-waves 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 also, but they only move through solids (crust and mantle). S- waves move up and down, in a S-like motion, and they are more damaging than P-waves. S-waves cannot move through liquid medium. 

Surface waves (L-Waves): 

The body waves interact with the surface rocks and produce new set of waves called surface waves or long waves or L-waves. These surface waves travel only along the surface. Due to long wavelength of surface waves, they are also called long period waves. These waves are low–frequency transverse waves (shear waves). 

They develop in the immediate vicinity of the epicentre and affect only the Earth’s surface and stop at smaller depth. 

Surface waves lose energy very slowly, with distance, than the body waves because surface waves move only across the surface whereas the body waves move in all directions. 

Particle movement of surface waves (amplitude) is greater than that of body waves, hence surface waves are highly damaging among the earthquake waves. 

Surface waves are slowest among the earthquake waves and hence record last on the seismograph. 

Love waves: 

Love waves are the fastest surface wave, and these waves move the ground from side-to-side. 

Rayleigh waves 

A Rayleigh wave rotates along the ground just like a wave moves across a lake or an ocean. Since it rolls, it moves the ground up and down and side-to-side in the direction of the wave. 

Most of the trembling and destruction from an earthquake is because of the Rayleigh wave. 

Fig. No.4: S wave and P wave 

Fig. No.5: Types of Seismic waves 

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 we know that 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, because Earth has a magnetic field, there must be metal present inside the Earth. 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. 

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. 
• It 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 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 high 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 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 a warmer region to a cooler region till the entire region 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 just 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 complete 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 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

• Earth is made up of various layers – crust, mantle, outer core, and inner core.Earth produces constant heat by the process of radioactive decay. This process includes the disintegration of natural radioactive elements present inside the Earth such as uranium.
• 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.
• The two types of waves are: Body waves (P-waves or primary waves, and S-waves or secondary waves) and surface waves (Love waves and Raleigh waves).
• Asthenosphere (astheno means weak) is the upper portion of the mantle. It is present just below the lithosphere ranging up to 80-200 km.
• Mantle maps show seismic velocities, showing patterns deep below Earth’s surface.