ORED_ELGANA
04-26-2008, 10:39 AM
Moho & Lehmann Discontinuities
The seismograph, which detects and records the movement of seismic waves, was invented in 1880. By the end of that decade seismic stations were in place all over the world. At the time, geophysicists believed Earth to be made up of a liquid core surrounded by a solid mantle, itself surrounded by a crust, all separated by abrupt density changes in the Earth called “discontinuities.”
Mohorovičić Discontinuity
who discover The Mohorovicic Discontinuity?
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.
http://www.qmraa.net/up/uploads/0334accd3e.jpg (http://www.qmraa.net/up/)
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 below shows its location.
http://www.qmraa.net/up/uploads/3eb42af8ac.gif (http://www.qmraa.net/up/)
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?
Mohorovičić interpreted the acceleration of seismic waves observed within Earth's outer ***** 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 plains.
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)
http://www.qmraa.net/up/uploads/d0342cf977.jpg (http://www.qmraa.net/up/)
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 below.
http://www.qmraa.net/up/uploads/4a5d60fde0.jpg (http://www.qmraa.net/up/)
Lehmann discontinuity
Dr. Inge Lehmann (1888-1993): Discoverer of the Earth's Inner Core
Born in Denmark in 1888, Lehmann was a pioneer among women and scientists. Her early education was at a progressive school where boys and girls were treated exactly alike.
http://www.qmraa.net/up/uploads/9218281f32.jpg (http://www.qmraa.net/up/)
This was a sharp contrast to the mathematical and scientific community she later encountered, about which she once protested to her nephew, Niles Groes, “You should know how many incompetent men I had to compete with—in vain.” Groes recalls, “I remember Inge one Sunday in her beloved garden…with a big table filled with cardboard oatmeal boxes. In the boxes were cardboard cards with information on earthquakes…all over the world. This was before computer processing was available, but the system was the same. With her cardboard cards and her oatmeal boxes, Inge registered the velocity of propagation of the earthquakes to all parts of the globe. By means of this information, she deduced new theories of the inner parts of the Earth.”
The seismograph, which detects and records the movement of seismic waves, was invented in 1880. By the end of that decade seismic stations were in place all over the world. At the time, geophysicists believed Earth to be made up of a liquid core surrounded by a solid mantle, itself surrounded by a crust, all separated by abrupt density changes in the Earth called “discontinuities.”
Mohorovičić Discontinuity
who discover The Mohorovicic Discontinuity?
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.
http://www.qmraa.net/up/uploads/0334accd3e.jpg (http://www.qmraa.net/up/)
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 below shows its location.
http://www.qmraa.net/up/uploads/3eb42af8ac.gif (http://www.qmraa.net/up/)
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?
Mohorovičić interpreted the acceleration of seismic waves observed within Earth's outer ***** 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 plains.
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)
http://www.qmraa.net/up/uploads/d0342cf977.jpg (http://www.qmraa.net/up/)
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 below.
http://www.qmraa.net/up/uploads/4a5d60fde0.jpg (http://www.qmraa.net/up/)
Lehmann discontinuity
Dr. Inge Lehmann (1888-1993): Discoverer of the Earth's Inner Core
Born in Denmark in 1888, Lehmann was a pioneer among women and scientists. Her early education was at a progressive school where boys and girls were treated exactly alike.
http://www.qmraa.net/up/uploads/9218281f32.jpg (http://www.qmraa.net/up/)
This was a sharp contrast to the mathematical and scientific community she later encountered, about which she once protested to her nephew, Niles Groes, “You should know how many incompetent men I had to compete with—in vain.” Groes recalls, “I remember Inge one Sunday in her beloved garden…with a big table filled with cardboard oatmeal boxes. In the boxes were cardboard cards with information on earthquakes…all over the world. This was before computer processing was available, but the system was the same. With her cardboard cards and her oatmeal boxes, Inge registered the velocity of propagation of the earthquakes to all parts of the globe. By means of this information, she deduced new theories of the inner parts of the Earth.”