The boundary between the Earth's crust and mantle is known by which of the following names?

Introduction / Context:
The interior of the Earth is divided into layers such as crust, mantle, outer core, and inner core. Seismology has identified several boundaries between these layers, known as discontinuities, where the speed of seismic waves changes abruptly. Questions in physical geography frequently ask you to match these discontinuities with the boundaries they represent. This question focuses on the boundary between the Earth's crust and mantle.

Given Data / Assumptions:

• The options are Moho discontinuity, Lehmann discontinuity, Conrad discontinuity, and Gutenberg discontinuity.
• The question refers specifically to the boundary between crust and mantle.
• We assume standard naming conventions used in seismology and geology.

Concept / Approach:
The boundary between the crust and the mantle is known as the Mohorovicic discontinuity, commonly shortened to Moho. It is identified by an abrupt increase in the velocity of seismic waves, indicating a change in rock composition and properties. The Gutenberg discontinuity marks the boundary between the mantle and the outer core, while the Lehmann discontinuity separates the outer core from the inner core. The Conrad discontinuity is a boundary within the crust between upper and lower crustal layers in some regions.

Step-by-Step Solution:
1. Recall that the crust mantle boundary is named after the Croatian seismologist Andrija Mohorovicic.2. This boundary is therefore called the Mohorovicic or Moho discontinuity.3. The Moho is characterised by a sharp change in seismic wave velocities, reflecting a transition from crustal rocks to denser mantle rocks.4. The Gutenberg discontinuity, by contrast, lies deeper and marks the boundary between the mantle and the liquid outer core.5. The Lehmann discontinuity is even deeper, marking the boundary between the liquid outer core and the solid inner core.6. The Conrad discontinuity is an internal crustal boundary, not the main crust mantle contact.7. Therefore, the correct name for the crust mantle boundary is the Moho discontinuity.

Verification / Alternative check:
Diagrams of the Earth's internal structure in geography and geology textbooks clearly label the boundary between crust and mantle as the Moho. These diagrams show other discontinuities at deeper levels with the names Gutenberg and Lehmann. Explanations of seismic wave behaviour describe how waves refract or change speed at the Moho due to the density change between crustal and mantle rocks. This confirms that Moho discontinuity is the correct term for the crust mantle boundary.

Why Other Options Are Wrong:
Option B, Lehmann discontinuity, is incorrect here because it refers to the boundary between Earth's outer and inner core. Option C, Conrad discontinuity, is a secondary boundary within the crust itself. Option D, Gutenberg discontinuity, marks the boundary between the mantle and the outer core, not between crust and mantle.

Common Pitfalls:
Students sometimes confuse Gutenberg and Moho because both are named after scientists and involve changes in seismic velocities. Another pitfall is to forget the depth associated with each boundary. A helpful memory aid is that Moho corresponds to the first major boundary below the crust, while Gutenberg and Lehmann are much deeper, associated with the core. Keeping a simple mental diagram of the Earth's layers and their discontinuities can help you avoid mixing them up.

Final Answer:
The boundary between the Earth's crust and mantle is called the Moho discontinuity.