History

1
December 2004 Amy StevensCora McKennaLouise Cau
lfieldZhiming Wang

• History
• End of 18th century Count Von Rumford - deducted
  convection.
  
• 1881 Osmond Fisher - the earths interior might
  convect.
  
• 1896 Henri Becquerel - discovered uranium salts
  emit rays.
  
• 1903 Pierre and Marie Curie - certain types of
  rocks poured out constant amounts of energy
  without diminishing size or changing.
  
• Early 19th century R.J. Strut - 50-60 times more
  radioactivity in crust than required to maintain
  the earths temperature.
  
• 1906 seismologist R.D Oldham - deduced the Earth
  had a core.
  
• 1909 Andrija Mohorovicic - discovered the
  boundary between the crust and the layer
  immediately below using seismology.
  
• Ernest Rutherford and Frederick Soddy - immense
  resources of energy bound up in these small
  amounts of matter, the radioactive decay of these
  reserves could account for most of the Earths
  warmth.
• This warmth within the Earth explains the source
  of the convection of Mohorovicics mantle layer.

Heat Production Primordial Heat According to t
he cold accretion model of the formation of the
planets, colliding bodies in a primordial cloud
of dust and gas coalesced by self-gravitation.
The gravitational collapse released energy that
heated up the Earth. The differentiation of a
denser core and lighter mantle from an initially
homogeneous fluid must have released further
gravitational energy in the form of heat. The
dissipation of the Earths initial heat still has
an important effect on internal temperatures. It
is thought that 20 - 50 of the present day
heat-flow is caused by the remains of the
primordial heat leaking out into space.

• Long-Lived Radioactive Isotopes
• The highest concentration is in the rocks and
  minerals of the Earths crust, while the
  concentrations in the mantle and core materials
  are low. However, continual generation of heat
  by radioactivity in the deep interior, though
  small, still influences initial temperatures .
• When a radioactive isotope decays, it emits
  energetic particles and ?-rays. The two
  particles that are important in radioactive heat
  production are ? and ?-particles.
• In order to be a significant source of heat, a
  radioactive isotope must have
  
• A half-life comparable to the age of the Earth
• The energy of its decay must be fully converted
  to heat
  
• The isotope must be sufficiently abundant
• The main isotopes that fulfill these conditions
  are

Short-Lived Radioactive Isotopes
There was probably a variety of short
lived-isotopes, most importantly Aluminium-26,
but also Chlorine-36, Iodine-129, Iron-60, and
Plutonium-244. These have half-lives so short
that they have long since vanished below the
limits of detectability.
It is impossible to make an accurate estimate of
the Earths present day rate of radioactive heat
production because the total abundances of
heat-producing radioactive elements are poorly
known. They can be measured precisely in any
single sample taken, but because the crust is so
variable in composition it is difficult to
estimate a reliable global average. A further
complication is that we know virtually nothing
about radioactive heat production in the earths
core.
Heat Transport Conduction Conduction is the mo
st important form of heat transport at crustal
and mantle lithospheric depths, where thermal
conduction and specific heat capacity are
dominant factors, as the material is rigid and
material flow is impossible. Thermal conductivity
of rocks is very small (between 1 and 3Wm-1k-1
for sedimentary). Thermal conductivity of
crystalline rocks of depths of even a few
kilometres is controlled by the intrinsic
properties of crystals in the rock. General
equation of heat conduction H dQ/dt -kAdT/dx
where kthermal conductivity of the material and
the temperature gradient is TH-TC/L. However,
at greater depths, thermal convection represents
the dominant mode of heat transport as the
temperature gradient becomes large.
Effects The interior of the Earth is losing hea
t via geothermal flux at a rate of about
4.4x1013J per year. Plate tectonics is the
mechanism by which the mantle sheds heat.
Conversely, mantle plumes / hot spots are the
way the core sheds heat. In terms of total heat
loss from the Earth at present, plate activity
contributes about 74, hot spots account for
approximately 9, and radiogenic heat lost
directly from the continental crust is about
17.

Most geological activity, excluding the laying
down of sediment occurs primarily at plate
boundaries. Plate tectonics explains important
features of the Earths surface and major
geologic events. Many of the Earth's natural
resources of energy, minerals, and soil are
concentrated near past or present plate
boundaries. The utilization of these readily
available resources has sustained human
civilizations, both now and in the past.
Radiation Radiation is the transfer of heat by el
ectromagnetic waves. At low temperatures, nearly
all the energy is carried by infrared waves and
as the temperature rises, the wavelengths shift
to shorter values. General equation of radiation
from a surface HAe?T4 , where ? is the
Stefan-Boltzman constant. Radiation is most
important in the core of the earth, where the
temperature is the highest.
The effects at Divergent (constructive) and
Convergent (destructive) plate boundaries
Convection The mantle is heated from within (by
the non-uniform distribution of radioactive
elements), heated from below (although this is
minor, at least for the mantle as a whole),
cooled from above and cools off with time. All of
these effects drive convective motions.
Important factors affecting convection in the ma
ntle
Heat Loss of heat from the earth drives
convection within the mantle. The heat is
transported through the mantle by convection
because differences in heat cause different
densities which generate sufficient stress to
deform the weak mantle material.
Pressure The increase in pressure with depth
means that viscosity, thermal conducitiviy, and
expansivity change, making it harder for material
to convect. The system responds by increasing the
dimensions of the thermal instabilities in order
to maintain buoyancy and to overcome viscous
resistance.

• The effects at transform plate boundaries
• Cause transform faults
• Cause strike-slip faults
• Earthquake zones

Viscosity In large scale vigorously convecting
systems, the variations of all physical
parameters relevant to dynamical systems are
important, but of these the enormous variations
of viscosity will be dominant. Kinetic viscosity
decreases very rapidly with temperature and
increases strongly with the proportion of silica
(Si02). Increasing pressure has exactly the
opposite effect on the viscosity of materials as
does the increasing temperature.
Oklo - Gabon, West Africa. A nuclear breeder
reactor formed by natural means. Occurred 1.7
eons ago when 235U made up 3 of natural uranium,
allowing just normal water to moderate it to
undergo fission. Its many reactors operated for a
period of around 1,000,000 years!
http//www.mantleplumes.org/convectionBowels of
the earth, ElderPhysics for geologists, Richard
E. Chapmanhttp//www.earth.rochester.edu/fehnlab/
ees21s/lect6.htmlhttp//www.uic.com.au/nip78.html
Fundamentals of Geophysics, William LowrieThe
Oxford Companion to the Earth, Hancock and Skinner