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Core-mantle boundary
An interface 2,900km below the Earth's surface separating the mantle and core.
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Inner core
The inner section of the core 5,155km deep to the Earth's centre at 6,371km, and consisting of solid iron alloy.
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Low-velocity zone
The asthenosphere underlying oceanic lithosphere in which seismic waves travel more slowly, probably because rock has partially melted.
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Lower mantle
The deepest section of the mantle, stretching from 670km down to the core-mantle boundary.
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Moho
The seismic-velocity discontinuity that defines the boundary between the Earth's crust and mantle.
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Outer core
The section of the core, between 2,900km and 5,150km deep, that consists of liquid iron alloy.
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P-wave shadow zone
A band between 103 and 143 degrees from an earthquake epicentre, as measured along the circumference of the Earth, inside which P-waves do not arrive at seismograph stations.
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Reflection
The bouncing off of a ray as it hits a boundary between two different materials.
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Refraction
The bending of a ray as it passes through a boundary between two different materials.
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S-wave shadow zone
A band between 103 and 180 degrees from the epicentre of an earthquake inside of which S-waves do not arrive at seismograph stations.
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Seismic ray
The changing position of an imaginary point on a wave front as the front moves through rock.
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Seismic-reflection profile
A cross-sectional view of the crust made by measuring the reflection of artificial seismic waves off boundaries between different layers of rock in the crust.
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Seismic tomography
Analysis by sophisticated computers of global seismic data in order to create a three-dimensional image of variations in seismic-wave velocities within the Earth.
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Seismic-velocity discontinuity
A boundary in the Earth at which seismic velocity changes abruptly.
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Transition zone
The middle portion of the mantle, from 400 to 670km deep, in which there are several jumps in seismic velocity.
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Upper mantle
The uppermost section of the mantle, reaching down to a depth of 400km.
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Velocity-versus-depth curve
A graph that shows the variation in the velocity of seismic waves with increasing depth in the Earth.
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Wave front
The boundary between the region through which a wave has passed and the region through which it has not yet passed.
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