solid material. A geologist reads and interprets the reading of a ‘‘seismogram’’, just
as a doctor is able to interpret our heartbeat when reading a cardiogram.
When comparing the seismic sound wave velocity transiting through a rock
formation in relation to the density differences encountered, a seismologist is able
to infer the composition of the material. Three major discontinuities have been
found inside our Earth: (1) Core, (2) Mantle and (3) more rigid Lithosphere-crust
(Fig. 2.3).
The Core
A 2,900 km deep ‘‘discontinuity’’, also called the Gutenberg discontinuity, at the
border of the inner mantle and the outer edge of the core, is made up essentially of
a mixture of iron and magnesium silicates. This discontinuity marks the separation
between the Earth’s mantle and its core. Earth’s core is composed mainly of iron
and lesser amounts of nickel (3–6 %), and is revealed by a sharp drop in the
P-wave velocity from about 12 km/s to about 8 km/s which then increases again
up to 10–11 km/second at the inner core boundary (5,100 km depth), also called
the Lehman discontinuity after the Danish seismologist, Inge Lehman. The inner
core, at 5100–6130 km depth, is solid and essentially made up of nickel and iron.
Its density is around 9–13 g/cm
3 (Fig. 2.3). This solid inner core was formed
during the accumulation of heavy metal material that sank from the outer core,
which is less dense and is still in a molten state. It was Descartes, in 1644, who first
proposed that Earth must have a liquid core or center.
The outer core consists of liquid metallic material made up of siderophile (iron
loving) components. The siderophile elements such as the platinum group of
elements (rhenium, osmium, iridium, palladium and ruthenium) are likely to be
associated with the nickel-iron core. They are also called High Strength Elements
(HSE) because they prefer to be associated with metals rather than silicates. Hence
it is likely that they are more prevalent in the liquid outer-core than in the Earth’s
mantle.
Convection currents may have allowed material from the outer-core to be
mixed into the lower mantle, and will later enhance the distribution of the platinum
group elements towards the upper mantle and then enable them to appear in
erupted lava on the Earth’s surface. However, this hypothesis assumes that the
platinum group elements were differentiated during the early formation of the solid
inner-core to the liquid outer-core. Furthermore, we do not know about the real
composition of the liquid outer-core and the crystalline inner core. In addition, if
elements have fractionated from the inner to the outer-core, this must have happened at the very beginning of Earth’s formation due to what is observed by the
long-lasting half-life of the platinum radioisotope (
190 Pt). In fact, these chemical
hypotheses are in conflict with geophysical theories based on Earth’s heat flow
history, which is used in order to explain the evolution of the inner-outer core
The Solid Earth’s Interior
29
as a doctor is able to interpret our heartbeat when reading a cardiogram.
When comparing the seismic sound wave velocity transiting through a rock
formation in relation to the density differences encountered, a seismologist is able
to infer the composition of the material. Three major discontinuities have been
found inside our Earth: (1) Core, (2) Mantle and (3) more rigid Lithosphere-crust
(Fig. 2.3).
The Core
A 2,900 km deep ‘‘discontinuity’’, also called the Gutenberg discontinuity, at the
border of the inner mantle and the outer edge of the core, is made up essentially of
a mixture of iron and magnesium silicates. This discontinuity marks the separation
between the Earth’s mantle and its core. Earth’s core is composed mainly of iron
and lesser amounts of nickel (3–6 %), and is revealed by a sharp drop in the
P-wave velocity from about 12 km/s to about 8 km/s which then increases again
up to 10–11 km/second at the inner core boundary (5,100 km depth), also called
the Lehman discontinuity after the Danish seismologist, Inge Lehman. The inner
core, at 5100–6130 km depth, is solid and essentially made up of nickel and iron.
Its density is around 9–13 g/cm
3 (Fig. 2.3). This solid inner core was formed
during the accumulation of heavy metal material that sank from the outer core,
which is less dense and is still in a molten state. It was Descartes, in 1644, who first
proposed that Earth must have a liquid core or center.
The outer core consists of liquid metallic material made up of siderophile (iron
loving) components. The siderophile elements such as the platinum group of
elements (rhenium, osmium, iridium, palladium and ruthenium) are likely to be
associated with the nickel-iron core. They are also called High Strength Elements
(HSE) because they prefer to be associated with metals rather than silicates. Hence
it is likely that they are more prevalent in the liquid outer-core than in the Earth’s
mantle.
Convection currents may have allowed material from the outer-core to be
mixed into the lower mantle, and will later enhance the distribution of the platinum
group elements towards the upper mantle and then enable them to appear in
erupted lava on the Earth’s surface. However, this hypothesis assumes that the
platinum group elements were differentiated during the early formation of the solid
inner-core to the liquid outer-core. Furthermore, we do not know about the real
composition of the liquid outer-core and the crystalline inner core. In addition, if
elements have fractionated from the inner to the outer-core, this must have happened at the very beginning of Earth’s formation due to what is observed by the
long-lasting half-life of the platinum radioisotope (
190 Pt). In fact, these chemical
hypotheses are in conflict with geophysical theories based on Earth’s heat flow
history, which is used in order to explain the evolution of the inner-outer core
The Solid Earth’s Interior
29