The Lehmann discontinuity is marked by an abrupt increase in seismic wave velocity. It occurs at depths of about 200 km (137 mi) from the surface and was named in honor of Inge Lehmann (1888-1993), who discovered it.
Lehmann was a Danish seismologist and geophysicist. She is the one who also discovered the Earth’s inner core.
In geology, discontinuities are surfaces marked by abrupt jumps in seismic velocities. Changes in composition, stress, fluids, elasticity, density, etc., usually cause these changes.
Also, the Lehmann discontinuity may mean the discontinuity separating the outer and inner core. However, use in this sense nowadays isn’t common.
Besides this discontinuity, others are Mohorovicic, Repetti, Gutenberg, and Conrad.
Where is it located?
The Lehmann discontinuity occurs in the upper mantle. It lies inside the asthenosphere and is considered the lower end of the low-velocity zone at depths of about 220 km (137 mi).
An asthenosphere is a ductile or pliable, weaker layer over which the lithosphere flows. Its uppermost section is marked with a zone of low seismic velocities, a high attenuation of seismic energy, and high electric conductivity. This zone is the makes the low-velocity zone or LVZ.
However, considering the inner and outer core, the Lehmann discontinuity lies between the outer and inner core at depths of about 5,150 (3,200 mi).
What are the characteristics of Lehmann discontinuity?
The Lehmann discontinuity has an increase or jump of both the compressive primary seismic wave and secondary shear waves. Therefore, velocities at this boundary are higher than in the LVZ zone above it.
It was first observed in Europe and later in North America. Observations have been made in other places, too.
However, the Lehmann discontinuity boundary doesn’t occur everywhere at depths around 220 km (137 mi). The reason for this is unclear.
Furthermore, it occurs twice as much beneath continents than under oceans. Also, it has the largest amplitudes under continents.
Lastly, this is not globally observed. It doesn’t appear in some areas, or its detection is difficult.
What causes the increase in seismic waves?
Knowledge of the Lehmann discontinuity has been with us for almost 60 years. However, what causes it remains a debatable issue.
This discontinuity appears thermally controlled. Also, it shows a change in anisotropic to a more isotropic state in the asthenosphere.
Anisotropic occurs when the direction of a measurement affects properties. In contrast, direction doesn’t affect properties in isotropic materials, i.e., they will be the same in all directions.
Also, the lithosphere is more anisotropic and a change to isotropic with depth in the upper mantle. Thus, this makes it a possible reason for the discontinuity.
Another proposal is a change from creep to diffusion creep deformation. This change has been seen in olivine under comparable pressure. However, it not occurring globally limits the theory.
Contrary to what some scientists thought, a change in pyroxene and silica change is unlikely to be the cause of the positive jump in seismic viscosity at this boundary.
Yes, a transition of pyroxene may occur at pressures corresponding to these depths. However, it would occur everywhere at this depth if that was the case. This makes it impossible to say why it doesn’t happen everywhere.
On the other hand, the silica phase occurs at 526 °C. If it occurs at a depth of 200 km (137 mi), then the temperature of the asthenosphere would be low. We know that the asthenosphere is at about 1300-1500 °C.
What is its significance?
Studies to understand this boundary are still ongoing. However, one important aspect of the Lehmann discontinuity is that it helps us understand the upper mantle better, including how it flows and its composition.