Gutenberg Discontinuity

Gutenberg Discontinuity Definition

The Gutenberg Discontinuity is situated inside the earth at a depth of about 2900 kilometres below the surface. The Gutenberg discontinuity separates the core and the mantle of the earth. Abundant, powerful forces reside below the earth's surface. These forces are responsible for triggering earthquakes, erupting lava through volcanoes, creating precious stones, and changing the landforms on the earth's surface from time to time, over the years. The structure of the earth has been a subject of study since ancient times. 


The Gutenberg discontinuity was named after Beno Gutenberg, who contributed several important facts and understanding of the earth's interior in 1913, which led to groundbreaking discoveries related to the inner layers of the earth.


Discontinuities of the Earth Layers

The interiors of the earth are made of different kinds of elements that differ from each other in physical and chemical properties like temperature, density, composition, etc. The interiors of the earth are divided into separate layers. The fundamental layers of the earth are the crust, mantle, and core. These layers are further divided into the upper and lower crust, upper and lower mantle, and the outer and inner core. These layers are separated from each other by transition zones. These transition zones are popularly known as Discontinuities.


The various discontinuities of the earth are as follows.

  1. Conrad Discontinuity: The Conrad Discontinuity separates the upper and lower crust.

  2. Mohorovicic Discontinuity: This is the transition zone between the crust and the mantle.

  3. Repetti Discontinuity: This separates the upper mantle from the lower mantle.

  4. Gutenberg Discontinuity: This is the transition zone between the lower mantle and the outer core.

  5. Lehmann Discontinuity: This is the layer separating the outer core from the inner core.


The Weichert Gutenberg Discontinuity

Beno Gutenberg, a seismologist who studied the inner layers of the earth, observed that at a certain depth of the earth's surface, primary waves of earthquake slowed down dramatically. The secondary waves were stopped altogether. Secondary waves usually transmit entirely through a solid material but cannot travel through liquids. Hence, he concluded that at a specific depth at which the secondary waves of earthquakes vanish, there might be some liquid layer present. Seismic waves abruptly change at this liquid layer. This layer was called the Gutenberg Discontinuity. The layer is also called the Weichert-Gutenberg discontinuity or the Oldham-Gutenberg discontinuity.


The Gutenberg discontinuity occurs deep within Earth's interior, at a depth of roughly 2,900 km (1,800 mi) when seismic waves (produced by earthquakes or explosions) travel through Earth abruptly change. Primary seismic waves (P waves) slow down while secondary seismic waves (S waves) vanish completely at this depth. S waves shear material and cannot pass through liquids, hence the unit above the discontinuity is thought to be solid, while the one below is thought to be liquid or molten. 


This dramatic difference defines the boundary between the lower mantle (which is solid) and the underlying outer core, two parts of the earth's interior (believed to be molten). The Weichert-Gutenberg discontinuity is another name for this phenomenon. The molten region of the outer core is expected to be roughly 700 degrees Celsius (1,292 degrees Fahrenheit) hotter than the mantle above it.


The discontinuity proved that below this layer, the interior of the earth must be liquid, and above this layer, the interior of the earth would be solid. In reality, the outer core, which is below the Gutenberg discontinuity, is liquid with a much higher density than the mantle. It contains high quantities of iron. Below the outer core lies the inner core with densely packed iron and nickel. Above the Gutenberg Discontinuity lies the lower mantle, which is solid in nature but has a lower density than the outer core.


It's also denser, which is likely due to higher iron content. The core-mantle boundary, or CMB, is a distinct barrier between the core and the mantle that was identified by the difference in seismic waves at this level. It's a narrow, irregular zone with undulations up to 5–8 kilometres (3-5 miles) broad. The heat-driven convection activity inside the upper mantle affects these undulations, which could be the driving factor behind plate tectonics—the movement of parts of Earth's fragile shell. The underlying eddies and currents within the outer core's iron-rich fluids, which are ultimately responsible for Earth's magnetic field, affect these undulations in the core-mantle border.


How Gutenberg Discontinuity was discovered?

Seismologists monitored and analysed the Earth's seismic waves in the early 1900s to better understand earthquakes and investigate the Earth's interior. The discovery of P (primary) and S (secondary) seismic waves by Richard Dixon Oldham in 1897 allowed scientists to explore the structure of the Earth's interior. Seismologists were able to identify distinct rock structures under the Earth's surface by monitoring differences in the velocity of the P and S waves produced by earthquakes.


For a long time, geologists and seismologists assumed that the Earth's centre included various layers. Oldham detected the existence of the Earth's core in 1906 and provided a rough estimate of its size, which turned out to be wrong. He also noticed a seismic "shadow zone" in which no P waves were detected.


It started roughly 150 degrees away from the epicentre and produced a "bullseye" on the other side of the Earth from the earthquake. Andrija Mohorovicic (1857-1936) identified the Earth's mantle, a unique layer immediately beneath the crust, in 1909. Beno Gutenberg (1889-1960), a German-American geologist, verified the existence of the core in 1914. He demonstrated that the P wave shadow zone was caused by a molten core's reflection and refraction of P waves, whereas the S wave shadow zone was caused by total absorption of S wave energy at the core, as shear waves cannot flow through liquids. Gutenberg estimated a highly precise depth of 1,800 miles (2880 km) for the core-mantle boundary after discovering that the shadow zone began 105 degrees from the epicentre. Scientists, on the other hand, were sceptical of Gutenberg's estimate until stronger seismologic data became available.


The core-mantle barrier, now known as the Gutenberg discontinuity in honour of Gutenberg, is assumed to be where the core's liquid iron and nickel meets the lower mantle's solid rock.


Shrinkage of the Core and Its Effect on the Gutenberg Discontinuity

The discontinuity between the mantle and the core is approximately 1800 miles below the earth's surface. However, this doesn't remain constant. The core of the planet experiences intense heat. This heat is perpetually and gradually dissipated, which forces the molten core to solidify and shrink slowly. The sinking of the core causes the Gutenberg discontinuity to sink deeper and deeper into the earth's surface gradually.

So, this is all about Mohorovicic discontinuity and Gutenberg Discontinuity. These discontinuities help in the study of the various interior layers of the earth. They give indirect information about the planet and assist in seismological studies.


Did You Know?

The discontinuities of the earth help in seismological studies. Seismology is the scientific study of earthquakes. The transition boundaries or the discontinuities of the earth give us information about the velocity of different earthquake waves in different layers of the earth, which helps us assess the nature of the layers of the earth.

FAQs (Frequently Asked Questions)

1. What is meant by Gutenberg discontinuity?

Gutenberg discontinuity refers to the transition zone between the mantle and the core. This discontinuity was identified by Weichert Gutenberg in 1912 at a depth of 2900 kilometres beneath the earth's surface. The velocity of seismic waves changes abruptly in this zone. At this depth, the velocity of the P wave diminishes, and the S wave vanishes completely.


The S wave shears the material and is unable to pass through the liquid. As a result, it's thought that the section of the discontinuity above it is solid, while the part below it is liquid or molten. This molten region is estimated to be 700°C hotter than the mantle above it. It's also denser, which is likely due to higher iron content.


It is a narrow, uneven zone with undulations up to 5-8 kilometres broad. The heat-driven convection activity inside the underlying mantle affects this undulation. The underlying eddies and current inside the outer core iron-rich fluids, which are ultimately responsible for the earth’s magnetic field, also affect these undulations. 


The mantle core boundary does not remain constant, it should be noted. The molten core within the earth hardens and shrinks as the heat of the planet's interior slowly dissipates, causing the core-mantle boundary to steadily shift deeper and deeper within the earth's core.

2. What is the depth of the Gutenberg discontinuity, and why is it there?

At a depth of around 1,800 miles below the surface, the Gutenberg discontinuity can be found. The velocity of P- and S- seismic waves changes abruptly at this depth as they travel.


P-waves slow down when they approach 1,800 miles, whereas S waves vanish completely. A liquid environment does not sustain S-waves (also known as 'shear waves').


This discontinuity is thought to mark the border between the lower mantle (solid) and the outer core due to the vanishing act of S-waves (believed to be molten).

The outer core's molten region is predicted to be around 1,292 degrees Fahrenheit hotter than the mantle above it. It's also denser, which is likely due to higher iron content.


The border, also known as the core-mantle boundary (CMB), is a thin, uneven zone with undulations that can be up to 3-5 miles broad. The heat-driven convection activity inside the upper mantle affects these undulations, which could be the driving factor behind plate tectonics—the movement of parts of Earth's fragile shell. The underlying eddies and currents within the outer core's iron-rich fluids, which are ultimately responsible for Earth's magnetic field, affect these undulations in the core-mantle border.


The boundary does not stay the same. The molten core within Earth progressively hardens and shrinks as the heat of the earth's interior is slowly dissipated, causing the core-mantle border to shift deeper and deeper within Earth's core.

3. What are the discontinuities of the Earth Layers?

The interiors of the earth are made up of several components with varying physical and chemical qualities such as temperature, density, composition, and so on. The earth's interior is separated into distinct layers. The crust, mantle, and core are the three basic layers of the earth. The upper and lower crust, upper and lower mantle, and outer and inner core are the different strata. Transition zones divide these strata from one another. Discontinuities are the common name for these transition zones.


The earth's many discontinuities are as follows:

  • The Conrad Discontinuity is the boundary between the upper and lower crusts.

  • The transition zone between the crust and the mantle is known as the Mohorovicic Discontinuity.

  • Repetti Discontinuity: This is the line that separates the upper and lower mantles.

  • The Gutenberg Discontinuity is the zone where the lower mantle meets the outer core.

  • The Lehmann Discontinuity is the layer that separates the outer and inner cores.

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