---
title: "Mohorovičić Discontinuity | Earth Systems Science"
description: "Mohorovičić Discontinuity, or Moho, is the boundary between Earth's crust and mantle, identified by seismic wave speed changes in Earth Systems Science."
canonical: "https://fiveable.me/earth-systems-science/key-terms/mohorovicic-discontinuity"
type: "key-term"
subject: "Earth Systems Science"
unit: "Unit 2"
---

# Mohorovičić Discontinuity | Earth Systems Science

## Definition

The Mohorovičić Discontinuity, or Moho, is the boundary between Earth’s crust and mantle. In Earth Systems Science, you identify it by the sudden change in seismic wave speeds.

## What It Is

The Mohorovičić Discontinuity, usually called the Moho, is the boundary where Earth’s crust ends and the mantle begins. In Earth Systems Science, it is not a line you can see at the surface, it is a change in how rocks behave and how seismic waves move through them.

What makes the Moho useful is that it marks a big jump in composition and density. The crust is made of comparatively lighter rocks, while the mantle is made of denser ultramafic material. Because of that difference, seismic waves speed up when they cross from crust into mantle. Scientists use that speed change to map the boundary even though no drill has ever reached it.

The depth of the Moho is not the same everywhere. Under oceanic crust, it is usually much shallower, often around 5 to 10 kilometers down. Under continental crust, it can be much deeper, commonly 20 to 90 kilometers, especially beneath mountain belts where the crust is thickened.

That variation matters because Earth’s outer shell is not a single uniform layer. Oceanic crust is thinner and denser, while continental crust is thicker and more varied. So when you see a diagram of Earth’s internal structure, the Moho is the transition point that separates those two crustal settings from the mantle below.

The Moho was first identified through seismology, not by direct sampling. A strong earthquake sends out waves that travel through different layers at different speeds, and the pattern of their arrivals can reveal hidden boundaries. That makes the Moho a great example of how Earth Systems Science uses indirect evidence to study something hidden deep underground.

It also connects to plate tectonics. Plates are made of crust plus the uppermost rigid mantle, so the Moho is not the same thing as a plate boundary. Instead, it helps define the base of the crustal part of a plate, which is useful when you are comparing oceanic and continental lithosphere or tracing how tectonic settings change with depth.

## Why It Matters

The Mohorovičić Discontinuity matters because it is one of the main clues scientists use to reconstruct Earth’s internal structure. In Earth Systems Science, you are often asked to explain how we know what is below the surface, and the Moho is a clear example of evidence from seismic waves rather than direct observation.

It also gives you a way to connect composition, density, and tectonic behavior. When you know that the crust is less dense than the mantle, the speed change at the Moho makes sense. That same density contrast helps explain why crust can float on the mantle and why continental crust can be thicker than oceanic crust.

The Moho also shows up in bigger processes like mountain building, rifting, and volcanism. A thickened crust can push the Moho deeper, while new oceanic crust forms with a much shallower boundary. So if you are interpreting an Earth cross-section, a seismic profile, or a plate tectonics question, the Moho gives you a reference point for what is happening beneath the surface.

## Connections

### Crust

The Moho is the lower boundary of the crust, so you need to know what crust is before the boundary makes sense. Oceanic and continental crust differ in thickness and density, which is why the Moho sits at very different depths in different places. When you compare a thin ocean basin to a thick continent, the Moho is the line that separates those surface rocks from the mantle below.

### Mantle

The Moho marks the start of the mantle, so it is really a transition into a new material layer. The mantle is denser than the crust, and that density change is part of what seismic waves detect. If you are tracing how Earth’s internal layers are arranged, the Moho helps you see where crust ends and mantle behavior begins.

### Seismic Waves

Seismic waves are how scientists find the Moho. When those waves cross from crust into mantle, they change speed, which creates a recognizable signal in seismograms. In class, this often shows up as reading arrival times or interpreting a graph of wave behavior through Earth’s layers.

### [Geophysical Imaging](/earth-systems-science/key-terms/geophysical-imaging)

The Moho is a classic target for geophysical imaging because it is underground and cannot be seen directly. Methods like seismic reflection and refraction let scientists build a picture of the boundary from wave patterns. If you are studying how Earth scientists map hidden structures, the Moho is one of the best examples of that technique.

## On the AP Exam

A quiz question or diagram item may show an Earth cross-section and ask you to label where the crust ends and the mantle begins. You might also be asked to explain why seismic waves speed up at the Moho or compare oceanic and continental crust thickness. In a lab or data analysis task, you could interpret arrival times on a seismogram and identify the depth change that signals the boundary. A strong answer usually connects the wave behavior to the density change, not just the name of the layer.

## Mohorovičić Discontinuity vs Crust

The crust is the entire outer rock layer of Earth, while the Mohorovičić Discontinuity is the boundary at its base. The crust includes everything above the Moho, so the Moho is not a layer itself. If a question asks where the Moho is, think of it as the bottom edge of the crust and the top of the mantle.

## Key Takeaways

- The Mohorovičić Discontinuity, or Moho, is the boundary between Earth’s crust and mantle.
- Scientists identify the Moho by looking for a sudden increase in seismic wave speed.
- The boundary is much shallower beneath oceanic crust than beneath continental crust.
- The Moho is a major clue for understanding Earth’s internal structure and plate tectonics.
- It is a boundary, not a separate layer, so it marks a transition in rock composition and density.

## FAQs

### What is the Mohorovičić Discontinuity in Earth Systems Science?

It is the boundary between the crust and the mantle. Earth scientists locate it by using seismic waves, which move faster once they enter the denser mantle. That makes the Moho a hidden but very useful marker in Earth’s internal structure.

### Why does the Moho matter for plate tectonics?

The Moho helps you see where the crust ends inside a tectonic plate. Since oceanic and continental crust have different thicknesses, the depth of the Moho changes from place to place. That gives clues about tectonic setting, crust formation, and mountain building.

### How do scientists find the Moho if they cannot drill to it?

They use seismic waves from earthquakes or controlled sources. When the waves cross from crust into mantle, their speed changes and that shows up in the data. Seismograms and other geophysical methods let scientists estimate the depth of the boundary.

### Is the Mohorovičić Discontinuity the same thing as a plate boundary?

No. A plate boundary is where two tectonic plates meet, while the Moho is the boundary between crust and mantle. A plate can contain both crust and upper mantle, so the two terms describe different kinds of boundaries.

## Related Study Guides

- [2.1 Internal structure of the Earth](/earth-systems-science/unit-2/internal-structure-earth/study-guide/mUgBmporYaHuQc0D)

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