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Mohorovičić Discontinuity - The Moho
What is the Mohorovičić Discontinuity?
The Mohorovicic Discontinuity, or "Moho", is the boundary
between the crust and the mantle. The red line in the drawing at right
shows its location.
In geology the word "discontinuity" is
used for a surface at which seismic waves change velocity.
One of these surfaces exists at an average depth of 8 kilometers
beneath the ocean basin and at an average depth of about 32 kilometers beneath
the continents. At this discontinuity, seismic waves accelerate.
This surface is known as the Mohorovicic Discontinuity or often simply referred to as the "Moho".
How Was the Moho Discovered?
The Mohorovicic Discontinuity was discovered in 1909 by Andrija Mohorovicic , a Croatian
seismologist. Mohorovicic realized that the velocity of a seismic
wave is related to the density of the material that it is moving through.
He interpreted the acceleration of seismic waves observed within Earth's
outer shell as a compositional change within the
earth. The acceleration must be caused by a higher density material being present at depth.
The lower density material immediately beneath the surface is now
commonly referred to as "Earth's crust". The higher density below the crust
became known as "Earth's mantle". Through careful density calculations Mohorovicic determined
that the basaltic oceanic crust and the granitic continental
crust are underlain by a material which has a density similar to an olivine-rich
rock such as peridotite.
How Deep is the Moho?
The Mohorovicic Discontinuity marks the lower limit of Earth's crust. As
stated above it occurs at an average depth of about 8 kilometers beneath the ocean
basins and 32 kilometers beneath continental surfaces.
Mohorovičić was able to use his discovery to study
thickness variations of the crust. He discovered that the
oceanic crust has a relatively uniform thickness while continental
crust is thickest under mountain ranges and thinner under
The map below illustrates the thickness of Earth's crust.
Note how the thickest areas (red and dark brown) are beneath some of Earth's
important mountain ranges such as: Andes (west side of South America), Rockies
(Western North America), Himalayas (north of India in South-central Asia) and
Urals (north-south trending between Europe and Asia)
Has Anyone Ever Seen the Moho?
No one has ever been deep enough into the earth to see the Moho and no wells have ever
been drilled deep enough to penetrate it. Drilling wells to
that depth is very expensive and very difficult because
of the extreme temperature and pressure conditions. The
deepest well that has been drilled to date was located on
the Kola Peninsula of the Soviet Union. It was drilled to
a depth of about 12 kilometers. Drilling to the Moho through
oceanic crust has also been unsuccessful.
There are a few rare locations where mantle material has been brought to the surface
by tectonic forces. At these locations, rock that used to be at the crust - mantle
boundary is present. A photo of rock from one of these locations is shown at right.
Contributor: Hobart King
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|Image of Earth's internal structure by USGS - Mohorovicic Discontinuity (red line) added by Geology.com
|Thickness of Earth's crust by USGS - since the Moho is at the base of the crust this map also shows depth to Moho.
|Ordovician ophiolite in Gros Morne National Park, Newfoundland.
Ancient mantle rock exposed at the surface.
(GNU Free Documentation License Image).