Part
I
Planetary
perspective
I
want
to
know
how
God
created
this
world.
I
am
not
interested
in
this
or
that
phenomenon,
in
the
spectrum
of
this
or
that
element.
I
want
to
know
his
thoughts,
the
rest
are
details.
Albert
Einstein
Overview
Earth
is
part
of
the
solar
system
and
it
cannot
be
completely
understood
in
isolation.
The
chem-
istry
of
meteorites
and
the
Sun
provide
con-
straints
on
the
composition
of
the
planets.
The
properties
of
the
planets
provide
ideas
for
and
tests
of
theories
of
planetary
formation
and
evolution.
The
Earth
is
often
assumed
to
have
been
formed
by
the
slow
accumu
l
ation
of
p lan-
etesimals
-
small
cold
bodies
present
in
early
solar
system
history
.
In
particular,
types
of
stony
meteorite
called
chondrites
have
been
adopted
as
the
probable
primary
material
accreted
by
the
Ea
rth
.
This
material,
however,
has
to
be
exten-
sively
processed
before
it
is
suitable.
Study
of
the
Moon,
Mars
and
meterorites
d e
monstrates
that
melting
and
basaltic
volcan-
ism
is
ubiquitous,
even
on
very
small
bodies
.
Planets
form
hot,
or
become
hot
,
and
begin
to
differentiate
at
a
very
early
stage
in
their
evolu-
tion,
probably
during
accretion.
Although
prim-
itive
objects
have
survived
in
space
for
the
age
of
the
solar
system,
there
is
no
evidence
for
the
survival
of
primitive
material
once
it
has
been
in
a
planet.
One
would
hardly
expect
large
portions
of
the
Earth
to
have
escaped
this
planetary
differ-
entiation.
and
to
be
'primordial'
and
undegassed
.
The
present
internal
structure
of
the
Earth
was
main
ly
established
4.57
billion
years
ago.
This
is
not
a
centra
l
dogma
of
current
geochemical
models
but
the
use
of
high-precision
short-lived
isotope
data
promises
to
change
this
.
A
large
amount
of
gravitational
energy
is
released
as
particles
fall
onto
an
accreting
Earth,
enough
to
raise
the
temperature
by
tens
of
thou-
sands
of
degrees
and
to
evaporate
the
Earth
back
into
space
as
fast
as
it
forms.
Melting
and
vaporization
are
likely
once
the
proto-Earth
has
achieved
a
given
size.
The
mechanism
of
accre-
tion
and
its
time
scale
determine
the
fraction
of
the
heat
that
is
retained
,
and
therefore
the
tem-
perature
and
heat
content
of
the
growing
Earth.
The
'initial'
temperature
of
the
Earth
was
high.
A
rapidly
growing
planet
retains
more
of
the
gravi-
tational
energy
of
accretion,
particularly
if
there
are
large
impacts.
The
ma
g
ma-ocean
concept
was
developed
to
explain
the
petrology
and
geochemistry
of
the
Moon
.
It
proved
fruitful
to
apply
this
to
the
Earth,
2
PLANETARY
PERSPECTIVE
taking
into
account
the
petrological
differences
required
by
the
higher
pressures
on
the
Eart
h.
We
now
know
that
plate
tectonics,
at
least
the
recycling
kind,
is
unique
to
Earth,
perhaps
because
of
its
size
or
water
content
.
TI1e
thick-
nes
s
and
average
temperature
of
the
lithosphere
and
the
role
of
phase
changes
in
basalt
are
impor-
tant
.
Any
theory
of
plate
tectonics
must
explain
why
the
other
terrestrial
planets
do
not
behave
like
Earth.
(Reminder:
key
words
are
embedded
in
the
text
,
with
the
type
face
of
the
preceding
sentence.
These
words
and
phrases
can
be
entered
into
search
engines
to
obtain
background
material,
definition
s
and
references.)
Part
II
Ear
t h:
the
dynamic
planet
This
is
the
fourth
time
that
I
have
taken
part
in
a public
discussion
of
this
theory.
In
each
previous
one
a
distinguished
biologist
or
geologist
has
presented
the
case
for
drift,
and
has
been
followed
by
equally
distinguished
ones
who
have
pointed
out
facts
that
it
would
render
more
difficult
to
explain
...
The
present
impasse
suggests
that
some
important
factor
has
been
overlooked.
Sir
Harold
Jeffreys,
I
9
5 I
Overview
Plate
tectonics
on
Earth
,
at
present,
consists
of
about
a
dozen
large
semi-coherent
entities
-
called
plates
-
of
irregular
shape
and
size
that
move
over
the
surface,
separated
by
boundaries
that
meet
at
triple
junctions.
There
are
also
many
broad
zones
of
deformation.
Plate
tectonics
is
often
regarded
as
sim-
ply
the
surface,
or
the
most
impor-
tant,
manifestation
of
thermal
con-
vection
in
the
mantle
[this
phrase,
and
phrases
in
the
same
typeface,
is
a
Googlet;
see
Preface
or
type
it
into
a
search
engine].
In
this
view
the
plates
are
driven
by
thermal
and
density
variations
in
the
mantle
.
Cooling
plates
and
sink-
ing
slabs
can
also
be
regarded
as
driving
them-
selves,
and
driving
convection
in
the
underlying
mantle;
they
create
chemical,
thermal
and
den-
sity
anomalies
in
the
n"lantle.
Plate
tectonics
qualifies
as
a
branch
of
complex-
ity
theory.
Plate
tectonics
may
be
a
far-from-
equilibrium
self-organized
system
powered
by
heat
and
gravity
from
the
mantle
and
organized
by
dissipation
in
and
between
the
plates
.
Mantle
convection,
below
the
plates,
may
not
drive
or
organize
the
plates;
it
may
be
the
other
way
around.
Plate
buoyancy
and
dissipa-
tion
control
plate
motions,
stresses,
and
locations
of
plate
boundaries
,
intraplate
extensional
zones
and
volcanic
chains.
The
cold
stiff
outer
shell
of
Earth
is
the
active
element
and
the
template;
the
underlying
convectiv
e
mantl
e
is
passive
.
The
outer
shell
of
the
Earth
is
not
just
a
ther-
mal
boundary
layer
or
a
cold
strong
layer
. It
is,
in
part,
the
accumulated
buoyant
residue
of
man-
tle
differentiation
including
the
on-going
pro-
cess
of
seafloor
spreading
and
building
of
island
arcs.
It
is
composed
of
fertile
melts,
dikes,
sills
and
cumulates,
and
infertile
refractory
residues
.
It
is,
in
part,
isolated
from
the
low-viscosity
fertile
interior.
Earthquakes
and
volcanoes
not
34
EARTH:
THE
DYNAMIC
PLANET
only
mark
plate
boundaries
but
they
antiCI-
pate
new
plate
boundaries,
changes
in
boundary
conditions
and
dying
plate
boundaries
.
If
the
shallow
mantle
is
close
to
the
melting
point
or
partially
molten,
volcanoes
have
a
simple
cause,
stress.
The
mode
of
convection
in
the
Earth
depends
on
the
distribution
of
radioactive
elements
and
physical
properties
and
how
these
properties
depend
on
temperature
and
pressure
and
melt-
ing
point.
An
Earth
with
most
of
the
radioac-
tive
elements
in
the
crust
and
upper
mantle,
and
with
strongly
pressure-dependent
thermal
properties
will
not
behave
as
a
uniform
fluid
being
heated
on
a
stove.
These
effects,
plus
continents
and
sphericity,
break
the
symmetry
between
the
top
and
bottom
thermal
boundary
layers.
The
main
plate
tectonic
cycle
is
the
ridge-
trench-slab
system,
primarily
playing
out
in
the
ocean
basins
.
There
is
a
secondary
cycle
involving
underplating
,
freezing
at
depth,
delamination
and
asthenospheric
upwelling
. A
mafic
lower
crust
,
if
it
thickens
and
cools
sufficiently,
will
convert
to
a
high
density
mineral
assemblage
,
leading
to
a
gravitationally
unstable
configura-
tion
in
which
the
lower
crust
can
sink
into
the
underlying
lower-density
mantle,
cooling
it
and
fertilizing
it.
The
solid
Earth
can
rotate
rapidly
underneath
its
spin
axis
through
a
process
known
as
true
polar
wander
(TPW
).
The
spinning
Earth
continuously
aligns
its
maximum
moment
of
inertia
with
the
spin
axis.
Melting
ice
caps,
plate
motions,
con-
tinental
uplift
and
drift
and
ridge-trench
anni-
hilations
can
all
cause
TPW.
The
magnetic
and
rotational
poles
are
good
terrestrial
reference
systems;
the
hotspot
frame
is
not.
But
fertile
patches
in
the
asthenosphere
move
more
slowly
than
the
plates
and
plate
boundaries
and
there-
fore
melting
anomalies
appear
to
be
relatively
fixed.
Part
Ill
Radial
and
lateral
structure
Descend
into
the
crater
of
Yocu
l
of
Sneffels,
Which
the
shade
of
Scartaris
caresses,
before
the
kalends
of
July
,
Audacious
traveler;
and
you
will
reach
the
center
of
the
Earth.
I
did
it.
Arne
Sok.nussemm
Overview
The
Australian
seismologist
Keith
Bullen
intro-
duced
the
nomenclature
for
the
subdivisions
of
the
Earth's
interior
. Table
8 .1 gives
these
subdi-
visions.
The
lower
mantle,
starting
at
1000-km
depth,
includes
Regions
D'
and
D".
The
latter
is
the
only
designation
in
common
use
today
.
Using
his
nomenclature,
the
lithosphere
and
the
low-
velocity
zone
are
in
Region
B.
The
650
km
discon-
tinuity
is
in
Region
C
-
the
Transition
Region
-
rather
than
being
the
boundary
between
the
upper
and
lower
mantles.
The
transition
Region
extends
from
410
to
1000
km
depth.
The
upper
boundary
is
primarily
a
phase
change
and
the
lower
boundary
may
be
a
chemical
change
and
a
geodynamic
barrier.
Standard
geochemical
and
geodynamic
mod-
els
of
the
mantle
involve
one
or
two
large
vig-
orously
convecting
regions.
Petrological
models
of
the
mant
le
tend
to
be
more
comp
lex.
High-
reso
l
ut
i
on
seismic
techniques
involving
reflected
and
converted
phases
show
about
10
discontinu-
ities
in
the
mant
le,
not
all
of
which
are
easily
explained
by
solid-solid
phase
changes.
They
also
sh
ow
some
deep
low-velocity
zones
that
may
be
eclogite
layers.
It
is
increasingly
clear
that
the
upper
mantle
is
heterogenous
in
all
parameters
at
all
scales
.
The
parameters
include
seismic
scatter-
ing
potential,
anisotropy,
mineralogy,
major
and
trace
element
chemistry,
isotopes,
melting
point
and
temperature.
An
isothermal
homogenous
upper
mantle,
however,
has
been
the
underly-
ing
assumption
in
much
of
mant
le
geochemistry
for
the
past
35
years.
Derived
parameters
such
as
degree
and
depth
of
melting
and
the
age
and
history
of
mantle
'reservoirs'
are
based
on
these
assumptions.
There
is
now
evidence
for
major
element,
mineralogical,
trace
element
and
iso-
topic
heterogeneity,
on
various
scales
(grain
size
to
hemispheric)
and
for
lateral
variations
in
tem-
perature
and
melting
point.
The
large-scale
features
of
the
upper
mantle
are
well
known
from
global
tomographic
studies.
The
mantle
above
200-300
km
depth
correlates
very
well
with
lu1own
tectonic
features
.
There
are
large
differences
between
continents
and
oceans,
and
between
cratons,
tectonic
regions,
back-arc
basins
and
different
age
ocean
basins.
90
RADIAL
AND
LATERAL
STRUCTURE
High-velocities
appear
beneath
cratons
-
archons.
Continental
low-velocities
appear
in
tectonica
lly
extending
regions
such
as
the
Red
Sea
r i
ft
and
in
backarcs-
tectons.
Lithospheric
thickening
and
asthenospheric
thinning
with
age
are
evident
beneath
the
oceans.
Low-velocity
zones
occur
beneath
ridges,
tec-
tonic
regions,
Yellowstone,
and
other
places
at
depth
s
less
than
200
km.
Yellowstone
is
not
a
particularly
prominent
anomaly
when
placed
in
the
context
of
the
western
North
American
upper
mantle,
and
does
not
extend
below
200
km
depth.
At
depths
greater
than
200
km
there
are
low-velocity
zones
beneath
India
,
Ic
e
land
and
so
me
ridges
and
back-ar
c
basins.
Significant
fea
-
tures
include
a
widespread
and
pronounced
low-
velocity
zone
beneath
the
western
United
States,
and
high-velocity
anomalies
associated
with
sub-
ducting
slabs.
The
upper
mantle
scatters
seism
ic
energy,
indicating
that
it
is
hetero
ge
nous
on
the
scale
of
seismic
waves,
~
10
km.
At
depths
between
800
and
1000
km
there
is
good
correlation
of
seismic
ve
l
ocities
with
inferred
regions
of
past
su
bdu
ction
. Below
the
Repetti
discontinuity,
at
about
1000
km
depth
- Bullen's
lower
mantle
-
the
mantle
is
relatively
homo
ge
nous
and
uncor
-
related
with
surface
processes
.
D"
is
heterogenous
and
may
be
chemically
distinct
from
D'
and
C.
Altho
u
gh
geodynamicists
and
geochemists
are
concentrating
on
one-
and
two-layer
mod-
els
of
the
mantle,
high-resolution
seismic
tech-
niques
suggest
that
it
is
actually
multilayered
or
laminated
.
Part
IV
\ Sampling
the
Earth
PartV
[
Mineral
physics