P1246990946APMSD - PowerPoint PPT Presentation

Title: P1246990946APMSD


1
Radioactivity In The Earth's Interior

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.
Primordial Heat According to the 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.

• History
• End 18th century Count Von Rumford - deducted
  convection.
• 1881 Osmond Fisher - suggested that the earths
  interior might convect.
• 1896 Henri Becquerel - discovered uranium salts
  emit rays.
• 1903 Pierre and Marie Curie - certain rocks
  poured out constant amounts of energy without
  diminishing size or changing in any detectable
  way.
• R.J. Strut - 50-60 times more radioactivity in
  crust than required to maintain the earths
  temperature.
• 1906 R.D Oldham - deduced the Earth had a core.
• 1909 Andrija Mohorovicic - discovered the
  boundary between the crust and the layer
  immediately below.
• 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.

• 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

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 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
Conduction Conduction is the most 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 1and 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 heat
via geothermal flux at a rate of about 4.4x1013J
per year. Plate tectonics 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 accounts for approximately 9, and
radiogenic heat lost directly from the
continental crust is about 17.

Most geological activity, barring 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
electromagnetic 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), cooled 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
mantle
PressureThe 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.
Heat Loss of heat from the earth drives
convection within 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.

• Blah blah effects transform
• Cause transform faults
• Cause strike-slip faults
• Earthquake zones

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!
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.