Which one of the following ‘discontinuities’ separates the Earth’s crust from the mantle?
Mohorovicic
Understanding Earth's Layers and Discontinuities
The Earth is made up of several distinct layers. Scientists study these layers by observing how seismic waves travel through the Earth after earthquakes. When seismic waves encounter a boundary between layers with different properties (like density or composition), their speed and direction change. These boundaries are called discontinuities.
The question asks specifically about the discontinuity that separates the Earth's crust from the mantle. Let's look at the options provided, which are all names of known discontinuities within the Earth:
- Gutenberg Discontinuity: This discontinuity is located much deeper inside the Earth. It marks the boundary between the mantle and the outer core.
- Mohorovicic Discontinuity: Also known as the Moho, this is the boundary that separates the Earth's crust from the underlying mantle. It's identified by a sudden increase in the speed of seismic waves.
- Conrad Discontinuity: This discontinuity is sometimes found within the crust itself, separating the upper continental crust from the lower continental crust. It is not always present globally and doesn't separate the crust from the mantle.
- Repetti Discontinuity: This discontinuity is found within the mantle, separating the upper mantle from the lower mantle.
Based on the definitions, the discontinuity that separates the Earth's crust from the mantle is the Mohorovicic discontinuity.
Here is a summary of the major discontinuities:
| Discontinuity Name | Layers Separated | Location |
|---|---|---|
| Conrad | Upper Crust from Lower Crust (in some areas) | Within the Crust |
| Mohorovicic (Moho) | Crust from Mantle | Base of the Crust |
| Repetti | Upper Mantle from Lower Mantle | Within the Mantle |
| Gutenberg | Mantle from Outer Core | Base of the Mantle |
| Lehmann | Outer Core from Inner Core | Within the Core |
Therefore, the Mohorovicic discontinuity is the boundary between the crust and the mantle.
Revision Table: Key Earth Discontinuities
Let's quickly review the main points about the discontinuities to help remember them for exams.
- Moho: Crust & Mantle
- Gutenberg: Mantle & Outer Core
- Repetti: Upper Mantle & Lower Mantle
- Conrad: Upper Crust & Lower Crust (not everywhere)
Additional Information on Earth's Discontinuities
Discontinuities are essentially zones where seismic wave velocities change abruptly. These changes are interpreted by geophysicists as boundaries between layers with different physical and chemical properties. The Mohorovicic discontinuity, for instance, represents a significant change in rock composition and density from the less dense crustal rocks to the denser rocks of the upper mantle.
The depth of the Mohorovicic discontinuity varies. It is typically deeper under continents (around 30-70 km) than under oceans (around 5-10 km).
Studying these discontinuities helps scientists understand the internal structure, composition, and dynamics of our planet.
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