<P> The top of the mantle is defined by a sudden increase in seismic velocity , which was first noted by Andrija Mohorovičić in 1909 ; this boundary is now referred to as the Mohorovičić discontinuity or `` Moho '' . The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere , an irregular layer with a maximum thickness of perhaps 200 km ( 120 mi ) . Below the lithosphere the upper mantle becomes notably more plastic . In some regions below the lithosphere , the seismic shear velocity is reduced ; this so - called low - velocity zone ( LVZ ) extends down to a depth of several hundred km . Inge Lehmann discovered a seismic discontinuity at about 220 km ( 140 mi ) depth ; although this discontinuity has been found in other studies , it is not known whether the discontinuity is ubiquitous . The transition zone is an area of great complexity ; it physically separates the upper and lower mantle . Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous . The D '' layer at the core -- mantle boundary separates the mantle from the core . In 2015 , research using gravitational data from GRACE satellites and the long wavelength nonhydrostatic geoid indicated viscosity increases by a factor of ten to 150 about 1,000 kilometres ( 620 mi ) below earth 's surface ; separate research also indicates sinking tectonic plates stall at this depth , leading Robert van der Hilst to speculate `` In term 's of structure and dynamics , 1,000 kilometers could be more important '' ( than the currently accepted 660 km depth upper -- lower division ) . The lower mantle also contains some discontinueous zones , called `` thermochemical piles '' which have been interpreted as either thermally differentiated , upwellings bringing warmer material towards the surface , or as chemically differentiated material . A principal source of the heat that drives plate tectonics is the radioactive decay of uranium , thorium , and potassium in Earth 's crust and mantle . </P>
<P> The top of the mantle is defined by a sudden increase in seismic velocity , which was first noted by Andrija Mohorovičić in 1909 ; this boundary is now referred to as the Mohorovičić discontinuity or `` Moho '' . The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere , an irregular layer with a maximum thickness of perhaps 200 km ( 120 mi ) . Below the lithosphere the upper mantle becomes notably more plastic . In some regions below the lithosphere , the seismic shear velocity is reduced ; this so - called low - velocity zone ( LVZ ) extends down to a depth of several hundred km . Inge Lehmann discovered a seismic discontinuity at about 220 km ( 140 mi ) depth ; although this discontinuity has been found in other studies , it is not known whether the discontinuity is ubiquitous . The transition zone is an area of great complexity ; it physically separates the upper and lower mantle . Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous . The D '' layer at the core -- mantle boundary separates the mantle from the core . In 2015 , research using gravitational data from GRACE satellites and the long wavelength nonhydrostatic geoid indicated viscosity increases by a factor of ten to 150 about 1,000 kilometres ( 620 mi ) below earth 's surface ; separate research also indicates sinking tectonic plates stall at this depth , leading Robert van der Hilst to speculate `` In term 's of structure and dynamics , 1,000 kilometers could be more important '' ( than the currently accepted 660 km depth upper -- lower division ) . The lower mantle also contains some discontinueous zones , called `` thermochemical piles '' which have been interpreted as either thermally differentiated , upwellings bringing warmer material towards the surface , or as chemically differentiated material . A principal source of the heat that drives plate tectonics is the radioactive decay of uranium , thorium , and potassium in Earth 's crust and mantle . </P>
which part of the earth is composed of the crust and upper mantle
which part of the earth is composed of the crust and upper mantle
the lithosphere
the lithosphere