35
field of the Earth and heat the bottom of the mantle. Energy required by currents in the mantle is
provided by the ancient (primordial) heat of the
iron core transmitted via layer D”. Heat transport
in the core is very effective because the velocity
of the currents in the low viscosity molten iron is
high (Horváth and Dombrádi 2008). The internal
heat reservoir of the Earth will be able to supply
energy for several billions of years, i.e. until the
Sun would become a red giant and our planet
would be destroyed.
At the bottom of the mantle a low viscosity
thermal boundary layer is formed from which
two types of upwelling start. One type forms on
opposite sides of the Earth resulting in two enormous swells called African and Pacific superswells (Fig. 2.16). The figure shows that these
upwellings become flattened and extended like
mushrooms just below the phase boundary at the
depth of 660 km because this phase change
impedes the flow. Thin plumes, however, released
from the superswells melt the lithosphere and
reach the surface. These are called secondary
mantle plumes. Similar thin mantle plumes may
form around the bottom edge of the superplumes
feeding the superswells but separate from them
and these thin plumes could also reach the surface via the lithosphere from zone D. These are
called primary mantle plume.
Primary and secondary mantle plumes trigger
active volcanism on the surface: forming basalt
plateaus if the plume becomes wide before crossing the lithosphere (plume head) or forming a
series of volcanic cones if the stem of the plume
is bent under the lithosphere. The majority of the
currently active volcanoes on the Earth, however,
can be found along plate boundaries in subduction zones.
Along the accretionary boundaries of divergent lithospheric plates the upwelling of the
material of the upper mantle is passive, i.e. the
plates—as mentioned before—are moved by the
force of slab pull. Heavier (with greater density)
oceanic lithospheric plates descend down to the
lower boundary of the transition zone at the depth
of 660 km relatively fast (max. 10 cm/year). The
subducting slab moves horizontally along this
boundary and then descends again much slower
in the much more viscous lower mantle. Due to
this slowdown in the speed of descend the material of the subducting slab is congested becoming
wider (Fig. 2.16). The final boundary of this
descend of the slab is layer D” (at the depth of
2850–3000 km) where mixing with the material
of the layer the largest cycle of material flow on
Earth starts all over again. The operation of the
slab pull force and the passive movement of the
lithospheric plates in detail is still highly controversial and scientists still has some work to do in
order to fully understand the process.
This model gives clues to the interpretation of
global geological events in the past. The intensity
of plate tectonics was variable in the geological
past depending on the amount of heat of the
Earth: the further we go back in the past the more
intense were the currents in the mantle and the
higher was the number of hot spots and higher
amount of lava flowed onto the surface than in the
Holocene. Thinner lithospheric plates moved at
higher velocities causing more intense subductions and orogeny. The production and release of
CO 2 and other greenhouse gases was also more
intense contributing to the warming of the climate in certain geological ages. However, the
higher amount of dust and aerosols released into
the atmosphere facilitated cooling. These surface
and atmospheric effects are less intense today but
their role is none less significant. The problem for
the society and the biosphere is that such frequently catastrophic level effects occur very random in time and success in their prediction is
very limited.
2.6.2 Global Climate Simulation
Models
Global climate simulation models become
increasingly accurate with the increasing performance of computers and experience of scientists.
This also means that dynamic atmospheric models are now based not only on atmospheric
parameters but atmosphere–ocean, atmosphere–
continent interactions are also considered. Since
the operation of the entire climate system is simulated anthropogenic influence on climate cannot
2.6 Earth Models, “World Models”
field of the Earth and heat the bottom of the mantle. Energy required by currents in the mantle is
provided by the ancient (primordial) heat of the
iron core transmitted via layer D”. Heat transport
in the core is very effective because the velocity
of the currents in the low viscosity molten iron is
high (Horváth and Dombrádi 2008). The internal
heat reservoir of the Earth will be able to supply
energy for several billions of years, i.e. until the
Sun would become a red giant and our planet
would be destroyed.
At the bottom of the mantle a low viscosity
thermal boundary layer is formed from which
two types of upwelling start. One type forms on
opposite sides of the Earth resulting in two enormous swells called African and Pacific superswells (Fig. 2.16). The figure shows that these
upwellings become flattened and extended like
mushrooms just below the phase boundary at the
depth of 660 km because this phase change
impedes the flow. Thin plumes, however, released
from the superswells melt the lithosphere and
reach the surface. These are called secondary
mantle plumes. Similar thin mantle plumes may
form around the bottom edge of the superplumes
feeding the superswells but separate from them
and these thin plumes could also reach the surface via the lithosphere from zone D. These are
called primary mantle plume.
Primary and secondary mantle plumes trigger
active volcanism on the surface: forming basalt
plateaus if the plume becomes wide before crossing the lithosphere (plume head) or forming a
series of volcanic cones if the stem of the plume
is bent under the lithosphere. The majority of the
currently active volcanoes on the Earth, however,
can be found along plate boundaries in subduction zones.
Along the accretionary boundaries of divergent lithospheric plates the upwelling of the
material of the upper mantle is passive, i.e. the
plates—as mentioned before—are moved by the
force of slab pull. Heavier (with greater density)
oceanic lithospheric plates descend down to the
lower boundary of the transition zone at the depth
of 660 km relatively fast (max. 10 cm/year). The
subducting slab moves horizontally along this
boundary and then descends again much slower
in the much more viscous lower mantle. Due to
this slowdown in the speed of descend the material of the subducting slab is congested becoming
wider (Fig. 2.16). The final boundary of this
descend of the slab is layer D” (at the depth of
2850–3000 km) where mixing with the material
of the layer the largest cycle of material flow on
Earth starts all over again. The operation of the
slab pull force and the passive movement of the
lithospheric plates in detail is still highly controversial and scientists still has some work to do in
order to fully understand the process.
This model gives clues to the interpretation of
global geological events in the past. The intensity
of plate tectonics was variable in the geological
past depending on the amount of heat of the
Earth: the further we go back in the past the more
intense were the currents in the mantle and the
higher was the number of hot spots and higher
amount of lava flowed onto the surface than in the
Holocene. Thinner lithospheric plates moved at
higher velocities causing more intense subductions and orogeny. The production and release of
CO 2 and other greenhouse gases was also more
intense contributing to the warming of the climate in certain geological ages. However, the
higher amount of dust and aerosols released into
the atmosphere facilitated cooling. These surface
and atmospheric effects are less intense today but
their role is none less significant. The problem for
the society and the biosphere is that such frequently catastrophic level effects occur very random in time and success in their prediction is
very limited.
2.6.2 Global Climate Simulation
Models
Global climate simulation models become
increasingly accurate with the increasing performance of computers and experience of scientists.
This also means that dynamic atmospheric models are now based not only on atmospheric
parameters but atmosphere–ocean, atmosphere–
continent interactions are also considered. Since
the operation of the entire climate system is simulated anthropogenic influence on climate cannot
2.6 Earth Models, “World Models”