Major Discontinuities in Earth’s Interior
The internal structure of our planet is characterized by several distinct layers separated by transition zones known as discontinuities. These boundaries, discovered through seismic wave analysis, reveal critical information about Earth’s composition and physical properties. Each discontinuity represents an abrupt change in seismic wave behavior, allowing scientists to map Earth’s inaccessible interior. The following table consolidates detailed information about the five major discontinuities within Earth’s interior.
A. Conrad Discontinuity
| Characteristic | Description |
|---|
| Depth | 15-20 kilometers beneath continental crust |
| Location | Between upper and lower continental crust |
| Discovery | Named after seismologist Victor Conrad |
| Seismic Effect | Longitudinal seismic wave velocity increases abruptly from approximately 6 to 6.5 km/sec |
| Distribution | Only found in continental regions, absent in oceanic crust |
| Layers Separated | Upper continental crust (sial – silica-aluminum rich) and lower continental crust (sima – silica-magnesium rich) |
| Significance | Originally thought to represent chemical distinction between felsic rocks (granite) in upper crust and mafic rocks (basalt) in lower crust |
| Modern Interpretation | Possibly represents transition from amphibolite facies to granulite facies metamorphism |
| Prominence | Not as pronounced as the Mohorovičić discontinuity; absent in some continental regions |
B. Mohorovičić Discontinuity (Moho)
| Characteristic | Description |
|---|
| Depth | 35 km beneath continents, 8 km beneath oceanic crust |
| Location | Between crust and mantle |
| Discovery | Discovered by Andrija Mohorovičić in 1909 through earthquake seismogram analysis |
| Seismic Effect | P-wave velocity increases from 6 km/sec above to 8 km/sec below; distinct change in velocity of all seismic waves |
| Distribution | Global presence (largest named natural object on Earth) |
| Depth Variation | 5-10 km under oceans, up to 70 km under mountains |
| Layers Separated | Earth’s crust and underlying mantle |
| Special Cases | Beneath mid-ocean ridges, defines the lithosphere-asthenosphere boundary |
| Discovery Method | Mohorovičić observed two sets of P-waves and S-waves from shallow-focus earthquakes – direct path waves and those refracted by high-velocity medium |
| Thickness | Characterized by up to 500 km thickness |
C. Repetti Discontinuity
| Characteristic | Description |
|---|
| Depth | Approximately 660-700 kilometers deep |
| Location | Between upper and lower mantle |
| Seismic Effect | Seismic waves increase in speed when passing through this boundary |
| Layers Separated | Upper mantle (cooler, pasty consistency) and lower mantle (hotter, more liquefied) |
| Significance | Important boundary in mantle convection dynamics |
D. Gutenberg Discontinuity
| Characteristic | Description |
|---|
| Depth | Approximately 2,900 kilometers below Earth’s surface |
| Location | Between lower mantle and outer core |
| Also Known As | Wrichert-Gutenberg discontinuity or core-mantle boundary (CMB) |
| Discovery | Named after Beno Gutenberg; discovered in 1912-1914 |
| Seismic Effect | P-waves decrease in velocity; S-waves completely disappear (cannot transmit through liquids) |
| Layers Separated | Solid lower mantle and liquid outer core |
| Physical Properties | Outer core approximately 700°C hotter than overlying mantle; higher density due to greater iron percentage |
| Characteristics | Narrow, uneven zone with undulations up to 5-8 km wide |
| Dynamic Nature | Boundary slowly moves deeper as Earth’s interior heat dissipates and core gradually solidifies |
| Geophysical Significance | Affected by mantle convection (possible driving force of plate tectonics) and outer core currents (responsible for Earth’s magnetic field) |
E. Lehmann Discontinuity
| Characteristic | Description |
|---|
| Primary Location | At depths of about 200-220 km in the upper mantle |
| Secondary Usage | Alternatively refers to boundary between outer and inner core at 5,150 km depth |
| Discovery | Named after Inge Lehmann (1888-1993), Danish seismologist who also discovered Earth’s inner core |
| Seismic Effect (Upper Mantle) | Marked by abrupt increase in both compressive primary seismic waves and secondary shear waves |
| Layers Separated (Deep) | Liquid outer core and solid inner core |
| Distribution | Occurs twice as frequently beneath continents than under oceans; not globally continuous |
| Location in Mantle | Inside the asthenosphere, considered the lower end of the low-velocity zone |
| Significance | Important boundary for understanding Earth’s internal structure and dynamics |
Each of these discontinuities provides crucial information about Earth’s internal structure and composition, acting as key reference points for understanding our planet’s formation, current state, and ongoing dynamic processes. The seismic wave behavior at these boundaries continues to be studied by geologists and geophysicists to refine our knowledge of Earth’s inaccessible interior regions.
Geographical Phenomena
The Interior of the Earth
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