Mohorovičić Discontinuity

The Moho is the fundamental boundary separating the Earth's crust from the underlying mantle. Seismic waves accelerate sharply once they pass the Moho.

From a chemical perspective, the moho represents a sharp compositional change, where the Crust meets the upper Mantle.

Chemical contrast

Above Moho

• Chemistry: Rich in Silica, Aluminum, Sodium, Potassium
• Mineralogy: Dominated by Feldspar (low density) and Quartz.
• Density: 2.7 - 3.0 g/cm3
• Rock Type: Granite/Gneiss (Continental) or Basalt/Gabbro (Oceanic)

Below Moho

• Chemistry: Rich in Magnesium, Iron
• Mineralogy: Dominated by Olivine and Pyroxenes
• Density: 3.3 g/cm3
• Rock Type: Peridotite

Seismic Jump

Because Olivine is much stiffer and denser than Feldspar, seismic waves jump from traveling at 6-7 km/s in the crust to >8 km/s in the mantle. This velocitiy jump defines the Moho's depth.

Depth Variations

The depth is not uniform. It mirrors the topography of the surface, but in reverse due to Isostasy.

1. Under Oceans: The Moho is shallow (5-10 km)
2. Under Continents: The Moho is deep (35 km)
3. Under Mountain Ranges: The Moho is the deepest. (70 km)

The Petrologic vs. Seismic Moho

For a deeper geochemical understanding, it is important to note that the "Seismic Moho" (where waves speed up) and the "Petrologic Moho" (where the rock type actually changes) usually match, but not always.

• The Serpentinite Problem: In some oceanic settings, seawater seeps deep into the mantle and turns the upper Peridotite into Serpentinite. Serpentinite is chemically "mantle" but has a low density like "crust." This can hide the true Moho from seismic sensors.
• Cumulate Zones: At the bottom of the crust, crystals settling out of magma chambers can create a transition zone of "intermixed" rocks (Dunites and Pyroxenites) rather than a sharp line, making the Moho a "zone" rather than a boundary.

Question and Answers

What does the "Moho" (Mohorovičić discontinuity) physically represent?::The boundary between the Earth's Crust and the Mantle.

What is the primary physical change observed at the Moho that led to its discovery?::A sharp increase in seismic wave velocity (specifically P-waves).

Describe the chemical contrast across the Moho in terms of dominant elements.
?
Above (Crust): High Si, Al, Na, K (Felsic/Intermediate).
Below (Mantle): High Mg, Fe (Ultramafic).

In terms of Rock Type, the Moho is the boundary between which two general groups?:::Crustal rocks (Granite/Gabbro) and Mantle rocks (Peridotite).

How does the depth of the Moho compare under Oceans vs. Continents?
?
Oceans: Shallow (5–10 km).
Continents: Deep (35–70 km).

Is the Moho a boundary between solid and liquid?::No, both the lower crust and the uppermost mantle are solid rock.