Seismic Discontinuities of the Earth

At a Glance

• Conrad Within the crust, between sial and sima
• Mohorovicic (Moho) Crust and mantle
• Repetti Upper and lower mantle
• Gutenberg Mantle and core, about 2,900 km
• Lehmann Outer and inner core, about 5,150 km
• Exam link UPSC Prelims, Geography: Interior of the Earth and Magnetic Field

Where Does It Fit?

This is the fourth topic of the category on the interior of the Earth and its magnetic field. The previous topic described the three main layers, and this one names the boundaries between them. The next topic covers the dynamo theory and the magnetic field. Previous topic: Crust, Mantle and Core – Internal Structure of the Earth. Next topic: Dynamo Theory and the Magnetic Field of the Earth.

What Is a Seismic Discontinuity?

In short: a discontinuity is a layer boundary where seismic waves suddenly speed up, slow down or bend.

Seismic waves travel at a speed that depends on the density and rigidity of the material. Where the material changes sharply, the velocity of the waves changes abruptly, and part of the energy is reflected and refracted. A zone where this change is recorded is called a seismic discontinuity. The discontinuities were found by studying the travel time curves of earthquake waves, and they mark the boundaries between the major layers of the interior, described in Crust, Mantle and Core – Internal Structure of the Earth. Most are named after the scientists who identified them.

Which Are the Important Discontinuities?

What Happens at the Moho and the Gutenberg Discontinuity?

The Mohorovicic discontinuity, or Moho, was discovered in 1909 by the Croatian seismologist Andrija Mohorovicic, who noticed that seismic waves from a Balkan earthquake reached some stations earlier than expected, because the waves speed up on entering the denser mantle. It marks the base of the crust, and it is the boundary used to measure the thickness of the crust. Below the Moho, the velocity of P waves jumps to about 8 km per second.

The Gutenberg discontinuity lies at about 2,900 km, where the mantle ends and the liquid outer core begins. At this boundary the P wave velocity drops suddenly and S waves vanish, which produces the shadow zones described in Seismic Waves and Their Types. The density rises sharply from about 5.5 to about 10 g per cm cubed.

Why Is the Lehmann Discontinuity Important?

In 1936 the Danish seismologist Inge Lehmann found that some weak P waves recorded in the P wave shadow zone could only be explained if the Earth had a solid inner core inside the liquid outer core. The boundary between the two, at about 5,150 km, is called the Lehmann discontinuity. P waves speed up as they enter the solid inner core, which showed that the very centre of the Earth is solid, a conclusion that was later supported by further observations. As an analytical point, the discovery shows how an anomaly in data, not a new instrument, can change a scientific model, since the evidence was already in the seismograms awaiting a better explanation.

Is the Lithosphere Boundary a Discontinuity Too?

The base of the lithosphere, at about 100 km on average, is marked by a zone where seismic waves slow down, often called the low velocity zone, which coincides with the weak asthenosphere. It is a change in strength more than in composition, so it differs from the Moho, which is a change in composition. The Conrad discontinuity lies inside the crust, and the Repetti discontinuity lies inside the mantle, so neither marks a boundary between the three main layers. The order of the discontinuities from the surface downward, Conrad, Moho, Repetti, Gutenberg and Lehmann, is a frequent Prelims question, and the position of each helps to place the layers.

Together, the discontinuities complete the picture of the interior, which then connects with the origin of the magnetic field in the outer core, treated in Dynamo Theory and the Magnetic Field of the Earth.

Why Is This Topic Important?

• Factual Names and positions asked
• Structure Layer boundaries
• Seismology Evidence for the core

What Came Next?

The next topic describes the dynamo theory and the magnetic field of the Earth. Next topic: Dynamo Theory and the Magnetic Field of the Earth.

Key People and Terms

• Discontinuity Boundary with a sharp change in wave speed
• Moho Crust-mantle boundary
• Conrad Boundary inside the crust
• Gutenberg Mantle-core boundary
• Lehmann Outer-inner core boundary
• Low velocity zone Slow wave zone in the asthenosphere

Exam Corner

Mains Angle

Questions ask the layers and boundaries together. These points add depth.

• Boundaries Defined by wave velocity
• Evidence Shadow zones prove them
• Discoverers Link names to depths