Fossil
magnets:
clues to the
past
Yet a group of scientists has come to the
conclusion that this entire coastal strip
once lay buried under more than a kilo­
metre o f younger rock. T h e sites of mod­
ern Sydney, Wollongong, Jervis Bay, and
Nadgee Nature Reserve were, the re­
searchers say, blanketed by later sedi­
ments that accumulated in the Jurassic
and Cretaceous ages. In places these de­
posits, now vanished, may have been at
least 4 km deep.
T h e evidence comes from examining
magnetism locked millions of years ago
into grains of iron compounds in the rocks.
T h e study of this 'palaeomagnetism' has
already filled some gaps in our knowledge
of the tortuous path followed by this and
other continents as they have drifted over
the earth's surface, and it is providing
mineral companies with new tools for
prospecting; but palaeomagnetism is now
being used to reconstruct an eroded
landscape.
T o appreciate how scientists can learn
so much from 'fossil magnetism', we must
understand how rocks become magnet­
ized in the first place. We are not talking
about visible magnets, but about tiny
crystal grains of iron oxides such as mag­
netite and haematite or compounds con­
taining titanium (titano-magnetites). In
certain c i r c u m s t a n c e s the m a g n e t i c
If you wander about the coast of New South Wales —
anywhere between Newcastle, say, and the Victorian
border — looking at the landscape with a geologist's
eye, you'll identify the local rocks as old underwater
sediments laid down in Palaeozoic and Triassic times.
You'll see no sign of anything younger.
structure of these grains may alter under
the influence of the earth's magnetic field.
If the grains are to tell their story to a
20th Century scientist, their magnetiza­
tion must somehow become fixed in posi­
tion, aligned with the earth's magnetic
field. Both igneous and sedimentary rocks
may come to contain such palaeomagnets.
Tiny grains in the rock
preserve a record of the earth's
magnetic field.
Igneous rocks form from hot volcanic
lava or magma welling up from below the
earth's crust. At these high temperatures,
grains of iron compounds cannot act as
magnets, but as the rocks c o o l , the grains
reach their Curie temperature — the
temperature below which they become
strongly magnetic. Each compound has
its own Curie temperature; magnetite's,
for example, is 580°C
field. As the rock matures, it fixes the
grains in their magnetized orientation.
Other sedimentary rocks, and some ig­
neous ones too, acquire palaeomagnets
when crystals such as haematite grow in
iron-rich water percolating through the
rock. As it grows, each of these crystals
responds to the earth's field until it en­
larges beyond a critical volume, above
which its magnetism remains fixed.
At a lower temperature, known as the
blocking temperature, the magnetism is
'frozen' into the grains, so by the time they
have cooled to normal ground tempera­
tures the grains have preserved a record of
the earth's field at the time and in the
place where the rock hardened.
A rock may become magnetized more
than once in its history. An igneous rock,
for instance, already holding a record of
the earth's field at the time the rock cooled,
may weather and acquire new magnetic
crystals in the presence o f percolating
groundwater.
Some sedimentary rocks become mag­
netized while the original deposit is being
packed down by the weight of overlying
material. T h e developing rock contains
tiny crevices into which water penetrates,
giving grains a somewhat fluid medium
in which they can line up with the earth's
Degree of latitude
What can a scientist learn from this mag­
netism? One important answer is: how far
away from the equator the rock was when
it became magnetized. This is because the
magnetized grains behave as tiny compass
13
tilted the r o c k f r o m t h e o r i g i n a l p l a n e i n
w h i c h it lay w h e n it b e c a m e m a g n e t i z e d ;
h e d o e s this b y m e a s u r i n g the a n g l e o f the
b e d d i n g p l a n e ,of the r o c k o u t c r o p . W i t h
this i n f o r m a t i o n h e c a n r e c o n s t r u c t
the
o r i g i n a l o r i e n t a t i o n o f the r o c k s a m p l e .
I n the l a b o r a t o r y , the s c i e n t i s t extracts
f r o m the s a m p l e a c y l i n d e r o f r o c k , 2 • 5 c m
i n d i a m e t e r a n d 2 • 2 c m l o n g , a n d p u t s it
i n t o a r o c k m a g n e t o m e t e r to d e t e r m i n e its
direction o f magnetization.
T h e m a g n e t i s m o f a r o c k is s o faint that
scientists
can measure
it o n l y i n
an
e n v i r o n m e n t in w h i c h they c a n c o n t r o l all
o t h e r m a g n e t i c i n f l u e n c e s . W i t h suitable
m a g n e t i c f i e l d a r o u n d an e x p e r i m e n t , b u t
T h e CSIRO r o c k m a g n e t i s m l a b o r a t o r y
has n o iron c o m p o n e n t s .
m e a s u r e m e n t s w o u l d b e i m p r a c t i c a b l e in
m u c h o f t h e detail, a l t h o u g h t h e p i c t u r e
a laboratory filled with the
r e m a i n s far f r o m c o m p l e t e .
apparatus they can c a n c e l the
A r e s e a r c h e r using the r o c k
magnetometer to measure a s p e c i m e n ' s
magnetism.
earth's
magnetic
' n o i s e ' created b y e l e c t r i c m o t o r s a n d other
sources o f magnetic disturbance.
Measurements
O f c o u r s e , p a l a e o m a g n e t i s m studies tell
u s t h e latitudes t h r o u g h w h i c h A u s t r a l i a
of palaeomagnetism
has travelled relative to t h e m a g n e t i c p o l e s ,
surroundings.
n o t the g e o g r a p h i c p o l e s . T h e g e o g r a p h i c
n e e d l e s that n o t o n l y p o i n t t o w a r d s t h e
T h e r o c k m a g n e t i s m laboratory o f the
p o l e s are t h e o n e s f r o m w h i c h l i n e s o f l o n ­
n o r t h as it a p p e a r s o n a m a p , b u t tilt as
CSIRO
D i v i s i o n o f Mineral Physics in
g i t u d e radiate o n m a p s , a n d t h e y m a r k the
w e l l , p o i n t i n g u p i n t o t h e sky i f t h e y are i n
S y d n e y has n o i r o n c o m p o n e n t s . It w a s
e n d s o f t h e a x i s a b o u t w h i c h the p l a n e t
the S o u t h e r n H e m i s p h e r e a n d d o w n i n t o
built u s i n g unreinforced c o n c r e t e , lime­
rotates. T h e m a g n e t i c p o l e s m o v e a b o u t
the g r o u n d in the N o r t h e r n H e m i s p h e r e .
s t o n e a g g r e g a t e , and e v e n c o p p e r n a i l s .
relative to the g e o g r a p h i c p o l e s ; the S o u t h
O n l y n e a r t h e e q u a t o r do t h e m a g n e t s l i e
T h e a i r - c o n d i t i o n i n g m o t o r is in t h e gar­
m a g n e t i c p o l e at p r e s e n t l i e s at a b o u t lat­
horizontal.
therefore require special
d e n , w e l l a w a y f r o m the e x p e r i m e n t a l area.
i t u d e 6 6 ° S , r i g h t o u t s i d e A n t a r c t i c a and,
T h e n e a r e r t h e y are t o o n e o f t h e p o l e s ,
W i t h o u t this l a b o r a t o r y , o n e o f o n l y t w o
indeed, relatively c l o s e to Australia.
t h e m o r e s t e e p l y t h e y d i p . A s i m p l e tri­
o f its k i n d i n A u s t r a l i a , the D i v i s i o n ' s
H o w e v e r , o n average t h e m a g n e t i c p o l e
g o n o m e t r i c a l e q u a t i o n l i n k s the a n g l e o f
p a l a e o m a g n e t i s m studies, carried o u t by
s p e n d s as m u c h t i m e to o n e s i d e o f the
i n c l i n a t i o n o f a p a l a e o m a g n e t to its g e o ­
D r B r i a n E m b l e t o n , D r P h i l S c h m i d t , and
g e o g r a p h i c p o l e as it d o e s to t h e o t h e r , and
graphical
Mr
o v e r a p e r i o d o f a b o u t 10 0 0 0 years — n o t
latitude w h e n
it
became
Dave
Clark,
w o u l d have
been
impossible.
magnetized.
Palaeomagnetism
has
therefore
be­
c o m e a valuable w e a p o n in the armoury o f
m u c h l o n g e r than a n o d o r a w i n k t o a
g e o l o g i s t — the average p o s i t i o n s o f the
Drifting Matilda
t w o p o l e s c o i n c i d e . Scientists c h o o s e their
a s t u d e n t o f c o n t i n e n t a l drift. I f y o u k n o w
T h e m o s t spectacular story to e m e r g e f r o m
s a m p l e s i n s u c h a w a y that t h e a v e r a g e
t h e a g e o f a particular
t h e s e s t u d i e s m u s t s u r e l y b e the h i s t o r y o f
m a g n e t i z a t i o n o f the s a m p l e s p r o v i d e s a
m a g n e t i s m c a n tell y o u its l a t i t u d e at that
A u s t r a l i a ' s m o v e m e n t s as t h e
continent
satisfactory e s t i m a t e o f t h e a v e r a g e m a g ­
t i m e . I f y o u c a n f i n d e n o u g h r o c k s o f dif­
drifted
surface.
n e t i c f i e l d d u r i n g the p e r i o d o v e r w h i c h
ferent a g e s , y o u c a n b u i l d u p a p i c t u r e o f
P a l a e o m a g n e t i s m d i d n o t p r o v i d e t h e first
the r o c k w a s f o r m i n g .
the c o n t i n e n t ' s n o r t h - s o u t h
e v i d e n c e f o r t h i s drift, b u t it h a s s u p p l i e d
Scientists drilling for r o c k s a m p l e s .
r o c k , its p a l a e o ­
movements
o v e r h u n d r e d s o f m i l l i o n s o f years. A n d
f r o m the h o r i z o n t a l c o m p o n e n t o f the
m a g n e t i s m y o u c a n d e t e r m i n e h o w far the
c o n t i n e n t h a s rotated, l i k e an e x t r e m e l y
slow merry-go-round.
Unfortunately
y o u c a n n o t infer
the
r o c k ' s l o n g i t u d e f r o m its m a g n e t i s m , and
y o u m u s t t u r n to o t h e r c l u e s f o r i n f o r m a ­
tion o n the e a s t - w e s t
p o s i t i o n s o f the
c o n t i n e n t at different t i m e s i n its h i s t o r y .
T h e theory may sound simple. Actual
m e a s u r e m e n t o f t h e d i r e c t i o n o f t h e pa­
l a e o m a g n e t i s m in a r o c k , h o w e v e r , turns
out to be m u c h m o r e c o m p l i c a t e d . S i n c e
a s c i e n t i s t is interested i n t h e d i r e c t i o n o f
the r o c k ' s m a g n e t i s m , h e m u s t
measure
the o r i e n t a t i o n o f h i s s p e c i m e n b e f o r e h e
r e m o v e s it f r o m its site. H e m u s t a l s o al­
l o w f o r a n y d i s t u r b a n c e s that m a y h a v e
14
a b o u t over the earth's
F r o m this v i e w p o i n t , the S o u t h P o l e
s e e m s to h a v e b e e n in A u s t r a l i a
arate e v i d e n c e that t h e o l d e r A u s t r a l i a n
2 4 0 0 m i l l i o n years a g o , b e f o r e s e t t i n g o f f
c o a l s d e r i v e f r o m plants that g r e w i n a
eastwards and then l o o p i n g north w e l l u p
cold
i n t o the A r c t i c C i r c l e . I n p r a c t i c e , this
s o m e t h i n g l i k e the tundra o f m o d e r n S i b ­
eria a n d n o r t h e r n C a n a d a . P a l a e o b o t a n -
'mirror i m a g e ' o f these m o v e m e n t s —
ists h a v e f o u n d that t h e s e p l a n t s b e l o n g e d
passing
to r e l a t i v e l y f e w s p e c i e s a n d attained n o
close
to
the
North
Pole
great s i z e , a n d t h e r o c k s
S c i e n t i s t s h a v e c o m p i l e d several alter­
native
itineraries
to a c c o u n t for
palaeomagnetism measurements
the
from
I n addition, M s M i c h e l l e S m y t h o f the
m u c h A u s t r a l i a n c o a l ( f r o m the
w o u l d have s h o w n the S o u t h P o l e m o r e or
P e r m i a n , f o r e x a m p l e — say 2 5 0 — 2 7 0
less o n the Q u e e n s l a n d - N e w
South
m i l l i o n years o l d ) w a s a l o n g t i m e i n the
W a l e s b o r d e r . F r o m there t h e p o l e s e e m e d
D i v i s i o n o f Fossil F u e l s has de­
m a k i n g , a n d m u s t h a v e l a i n o n o r n e a r the
Lower
to l o o p b e y o n d N e w Z e a l a n d and back,
surface w i t h o u t fully d e c o m p o s i n g for an
p a s s i n g c l o s e to south-eastern
extensive period. T h i s indicates perma­
Australia
frost c o n d i t i o n s .
' F o s s i l m a g n e t i s m ' n o t o n l y reveals de­
tails o f A u s t r a l i a ' s p e r i p a t e t i c past, it a l s o
A u s t r a l i a d i d n o t travel i n d e p e n d e n t l y
tells u s s o m e t h i n g o f t h e t e m p e r a t u r e s to
all this t i m e . A t o n e stage t h e l a n d m a s s
w h i c h rocks have been subjected during
f o r m e d part o f a f e d e r a t i o n o f c o n t i n e n t s
their h i s t o r y . S c i e n t i s t s c a n o b t a i n t h i s i n ­
that w e c a l l G o n d w a n a l a n d ; t h e n
formation
this
by reversing the p r o c e s s by
o u p i n g g r a d u a l l y c a m e apart a n d for a
w h i c h a rock originally b e c a m e magnet­
while Australia and Antarctica remained
ized: for e x a m p l e , they can heat the r o c k
t o g e t h e r . T h i s a s p e c t o f the h i s t o r y , s o i m ­
a n d g r a d u a l l y d e s t r o y its m a g n e t i s m .
I n practice, the researchers heat a spec­
i m e n o f the r o c k to s u c c e s s i v e l y h i g h e r
T h e scientists have n o w a c c u m u l a t e d
from
of
z o i c era, s o m e 2 3 0 m i l l i o n years a g o ,
in Ecos 2 4 .
latitude
signs
glaciation.
d u c e d f r o m h e r p e t r o g r a p h i c s t u d i e s that
portant to palaeontologists, was d e s c r i b e d
historical
surrounding
show
atlas p u b l i s h e d at t h e start o f t h e M e s o ­
s o u t h w a r d s , as A u s t r a l i a i n reality drifted
tralia's
deposits often
CSIRO
north.
W h e n a researcher determines
coal
P a l a e o z o i c r o c k s , b u t they a g r e e that an
a b o u t 1 2 0 m i l l i o n years a g o , t h e n h e a d i n g
H o w the South Pole seems to have
moved during the Mesozoic and
Cainozoic eras. The dates are in
millions of years.
climate, forming communities
m e a n s that A u s t r a l i a w a s p e r f o r m i n g t h e
1 5 0 0 — 1 6 0 0 m i l l i o n years a g o .
A map of the South Pole's apparent
movements, as seen from Australia,
between 2500 and 750 million years ago.
In reality the Pole stayed still and the
Australian continent drifted.
T h i s n o t i o n fits neatly w i t h q u i t e s e p ­
about
temperatures,
typically in the
range
so m u c h information o n the palaeolati-
1 0 0 - 6 0 0 ° C , c o o l i n g it b e t w e e n h e a t i n g s
tudes t h r o u g h w h i c h Australia
a n d r e m e a s u r i n g its m a g n e t i s m e a c h t i m e .
passed,
p a r t i c u l a r l y d u r i n g t h e M e s o z o i c era, that
I n this w a y t h e y m o n i t o r c h a n g e s i n the
t h e y c a n o f t e n date a r o c k f r o m its p a l a e o ­
r o c k ' s m a g n e t i s m as t h e s p e c i m e n is p r o ­
m a g n e t i s m . B u t they h a v e to b e c a r e f u l ,
g r e s s i v e l y r a i s e d to h i g h e r t e m p e r a t u r e s .
Aus­
for t h e date at w h i c h a r o c k b e c a m e m a g ­
F r o m t h e t e m p e r a t u r e at w h i c h t h e r o c k
the
n e t i z e d is n o t n e c e s s a r i l y the date o f that
l o s e s m a g n e t i s m , t h e scientists c a n d e ­
rock's formation.
d u c e t h e t e m p e r a t u r e s at w h i c h it o r i g i ­
p a l a e o m a g n e t i s m o f a r o c k s a m p l e , there­
f o r e , h e c a n c o n f i d e n t l y m a r k that latitude
o n a m a p as i f m a g n e t i c a n d g e o g r a p h i c
nally b e c a m e magnetized.
Cold coals
For these experiments, the
scientists
O n e intriguing implication o f mapping
u s e a s p e c i a l f u r n a c e kept m a g n e t i c a l l y
regard
A u s t r a l i a ' s drift c o n c e r n s c o a l . T o m a n y
s h i e l d e d , o r , as the r e s e a r c h e r s p u t it, in a
A u s t r a l i a as m o v i n g relative t o t h e S o u t h
p e o p l e the idea o f c o a l formation c o n j u r e s
zero field.
P o l e , s c i e n t i s t s f i n d it c o n v e n i e n t t o t h i n k
u p an i m a g e o f forests w h o s e d e a d trees
poles were one.
A l t h o u g h it s e e m s l o g i c a l t o
o f the c o n t i n e n t as s t a y i n g i n o n e p l a c e
were s o m e h o w prevented from
w h i l e the p o l e h a s m o v e d a b o u t . T h e y
c o m p l e t e l y d e c o m p o s e d . C e r t a i n l y all c o a l
being
h a v e t h e r e f o r e p r o d u c e d d i a g r a m s o f the
d o e s derive from the i n c o m p l e t e decay o f
'apparent polar wander', s h o w i n g h o w the
o r g a n i c p l a n t matter, b u t t h e p l a n t s n e e d
S o u t h P o l e w o u l d h a v e a p p e a r e d , to an
n o t h a v e b e e n trees.
A u s t r a l i a n o b s e r v e r gifted w i t h b o t h l o n g
M u c h o f Australia's abundant c o a l was
life a n d l o n g s i g h t , t o h a v e drifted to a n d
laid d o w n d u r i n g the P e r m i a n p e r i o d ,
fro.
w h e n , a c c o r d i n g to the e v i d e n c e from
p a l a e o m a g n e t i s m , the S o u t h P o l e o c c u ­
p i e d a p o s i t i o n near T a s m a n i a ,
before
The older Australian coals
c r o s s i n g the southern coast o f the m a i n ­
derive from plants that grew
W a l e s . I n o t h e r w o r d s , the great c o a l - m i n ­
in a cold climate.
land and m o v i n g north into N e w South
i n g districts o f eastern A u s t r a l i a e n d u r e d
a p o l a r c l i m a t e in t h o s e d a y s , a n d s e e m
u n l i k e l y to h a v e s u p p o r t e d f o r e s t s .
During the earth's history, the magnetic
poles have seldom coincided with the
geographic poles ( N , S). Magnets align
themselves with the lines of force.
North of the magnetic equator they dip;
south of it they tilt up.
15
T h e s e c o n c l u s i o n s find strong support
T h e CSIRO g r o u p c a n relate t h i s a n o m ­
in s o m e q u i t e different o b s e r v a t i o n s that
aly to their p a l a e o m a g n e t i c d e t e c t i v e w o r k .
had previously been unexplained a n o m ­
T h e n e a r e r apatites are to the c o a s t , t h e
alies. O n e s u c h line o f e v i d e n c e involves
h i g h e r the temperature to w h i c h t h e y w e r e
f i s s i o n tracks in apatites.
raised ( a n d t h e m o r e t h o r o u g h l y , there­
A p a t i t e s are naturally o c c u r r i n g c r y s ­
fore, their fission tracks w e r e
erased)
tals o f c a l c i u m p h o s p h a t e ( h y d r o x y a p a t i t e
d u r i n g that subterranean 'heat w a v e ' a b o u t
m a k e s u p m u c h o f o u r teeth). T r a p p e d i n ­
9 0 m i l l i o n years a g o .
s i d e s o m e crystals lie traces o f r a d i o a c t i v e
m i n e r a l s , w h i c h e m i t alpha p a r t i c l e s
as
they d e c a y . T h e s e f i s s i o n p r o d u c t s l e a v e
T h e same geological p h e n o m e n o n would
tracks as they pass t h r o u g h t h e crystal,
a c c o u n t for a n o t h e r c u r i o u s o b s e r v a t i o n ,
a n d a s c i e n t i s t c a n , m i l l i o n s o f years later,
this t i m e i n v o l v i n g c o a l . A c u t , p o l i s h e d
m e a s u r e t h e d e n s i t y o f t h e tracks u n d e r a
surface o f c o a l s h i n e s , a n d s c i e n t i s t s h a v e
m i c r o s c o p e and s o arrive at an e s t i m a t e o f
accumulated m u c h information on
t h e c r y s t a l ' s a g e . T h e o l d e r the crystal, the
shininess o f c o a l s from c o l l i e r i e s in the
m o r e tracks there w i l l b e to c o u n t .
B u t t h e r e is a c a t c h . I f a n apatite crystal
A map of the vitrinite r e f l e c t a n c e
('shine') of surface coals in the Sydney
Basin supports the idea that much
erosion has occurred. Coals with the
highest values must have been most
deeply buried in the past.
Sydney buried
Experiments
t h e d e p t h o f burial o f t h e c o a l a n d h o w
m u c h it has b e e n heated.
all t h e tracks are erased. T h e d e n s i t y o f
W h e n the r e f l e c t a n c e v a l u e s f o r c o a l s at
t h e s e tracks t h e r e f o r e tells a s c i e n t i s t n o t
the s u r f a c e are p l o t t e d o n a m a p , t h e y f o r m
the age n e c e s s a r i l y o f the crystal, but rather
concentric contour lines centred o n a
h o w m u c h t i m e has e l a p s e d s i n c e t h e
coastal point between Sydney and W o l ­
crystal w a s last w a r m e d u p t o 65° C or
l o n g o n g , w h e r e the highest values o c c u r .
higher.
O n c e a g a i n , the p a l a e o m a g n e t i s m w o r k
W e l l i n l a n d f r o m the e a s t e r n c o a s t o f
o f this kind e n a b l e d
N e w S o u t h W a l e s , d a t i n g r o c k s b y t h e fis­
s i o n tracks in apatites g i v e s t h e s a m e re­
studies h a d already s u g g e s t e d , n a m e l y that
sults as d a t i n g b y o t h e r m e a n s , b u t for
A u s t r a l i a ' s eastern c o a s t o n c e lay u n d e r
s o m e t i m e s c i e n t i s t s at t h e U n i v e r s i t y o f
d e e p Jurassic and Cretaceous deposits.
M e l b o u r n e h a v e b e e n p e r p l e x e d by the
T h e y examined rock samples from nine
o b s e r v a t i o n that t o w a r d s the c o a s t apa­
sites i n t h e r e g i o n k n o w n to g e o l o g i s t s as
tites s e e m t o o y o u n g for their r o c k s . T h e
Sydney Basin, roughly from
New­
nearer y o u a p p r o a c h the c o a s t , the greater
c a s t l e t o W o l l o n g o n g , a n d f o u n d that all
the d i s c r e p a n c y y o u find. In P a l a e o z o i c
n i n e c o n t a i n e d , in a d d i t i o n to t h e i r o r i g i ­
granites, for e x a m p l e , apatite f i s s i o n tracks
nal high-temperature
g i v e a g e s r a n g i n g f r o m 3 6 0 m i l l i o n years
magnetization,
'overprint' o f low-temperature
an
magneti­
z a t i o n a c q u i r e d m u c h later.
progressively
scientists
c o n c l u d e d that the r o c k s that n o w m a k e
u p the s u r f a c e l a n d s c a p e a l o n g t h e c o a s t
o f the S y d n e y B a s i n m u s t h a v e b e e n heated
to a b o u t 2 2 0 ° C s o m e 9 0 m i l l i o n years a g o .
F r o m m a g n e t i c and o t h e r e v i d e n c e , the
s c i e n t i s t s b e l i e v e the S y d n e y B a s i n r o c k s
m u s t h a v e b e e n kept h o t for s o m e t h i n g
l i k e 5 - 1 0 m i l l i o n years. C l e a r l y t h e r o c k s
c o u l d n o t h a v e b e e n e x p o s e d t o the
at­
mosphere. T h e y must have been well
b u r i e d : j u s t h o w d e e p is a little h a r d t o say,
as the c a l c u l a t i o n i n v o l v e s , a m o n g o t h e r
things, making assumptions
about
the
g e o t h e r m a l g r a d i e n t — that i s , the rate at
w h i c h the temperature
changed
with
d e p t h . B u t the r e s e a r c h e r s c o n f i d e n t l y as­
sert that m o r e than 1 k m o f s e d i m e n t m u s t
h a v e lain o v e r the m o d e r n s u r f a c e r o c k s ,
a n d that t h e c o v e r i n g i n p l a c e s w a s several
t i m e s that t h i c k — p e r h a p s m o r e than
16
100 k m i n l a n d to 8 0 m i l l i o n years a l o n g
the c o a s t .
demagnetizing
s a m p l e s f r o m t h e s e sites, t h e
4 km deep.
S y d n e y Basin. T h i s property, t e c h n i c a l l y
k n o w n as vitrinite r e f l e c t a n c e , i n d i c a t e s
s i o n tracks b e g i n to d i s a p p e a r ; at 1 2 5 ° C
the
By
the
for any r e a s o n heats u p a b o v e 6 5 ° C , its fis­
CSIRO t e a m to c o n f i r m w h a t s o m e o t h e r
the
Shiny coals
What caused these dramatic
changes some 90 million years
ago?
Fossil radioactive fission tracks in a
crystal of apatite from a granite on King
Island in Bass Strait. The tracks,
photographed by D r Andrew Gleadow of
the University of Melbourne, measure
up to 16 μm. T h e long, faint lines are
polishing scratches.
Geological time
era
period
Cainozoic
Quaternary
Tertiary
2
65
Mesozoic
Cretaceous
Jurassic
Triassic
130
195
240
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
290
350
400
435
500
575
Palaeozoic
Scientists can often date a
rock from its
palaeomagnetism
time since
start of
period
(million years)
T h e c o a s t o f W e s t e r n A u s t r a l i a repre­
sents a n o t h e r similar rift m a r g i n , a n d the
C S I R O r e s e a r c h e r s h o p e to e x a m i n e r o c k s
i n that r e g i o n b e f o r e l o n g . A b o u t 140 m i l ­
l i o n years a g o the plate c a r r y i n g
drifted
away from Australia's
India
western
m a r g i n , and the scientists w i l l therefore
b e e x a m i n i n g r o c k s for s i g n s o f an ' e v e n t '
a b o u t 140 m i l l i o n years a g o . T h e y already
k n o w w h a t o t h e r A u s t r a l i a n r o c k s o f that
age ' l o o k ' l i k e p a l a e o m a g n e t i c a l l y , and
ties in w e l l . T h e m o r e the c o a l s h i n e s , the
n a t i o n s s m a c k to e v e r y b o d y e l s e o f u n d e r ­
h i g h e r the t e m p e r a t u r e to w h i c h it has
statement, say there m u s t h a v e b e e n an
s h o u l d t h e r e f o r e h a v e little t r o u b l e spot­
b e e n raised, and the m o r e d e e p l y it m u s t
'event', presumably associated with
ting any o v e r p r i n t there m a y b e .
therefore h a v e b e e n b u r i e d at s o m e t i m e
p r o c e s s e s that separated A u s t r a l i a
in the past.
N e w Z e a l a n d and L o r d H o w e Island.
W h e r e c o a l w i t h the h i g h e s t r e f l e c t a n c e
the
from
B e s i d e s u n r a v e l l i n g the r o u t e o f A u s ­
tralia's c o m p l e x 'driftabout' a n d t h r o w i n g
T h i s s e p a r a t i o n resulted f r o m the c r e ­
n e w l i g h t o n the g e o l o g i c a l history o f the
v a l u e o c c u r s near the surface, t h e r e f o r e ,
a t i o n o f n e w sea f l o o r as m a g m a o o z e d o u t
eastern c o a s t , p a l a e o m a g n e t i s m
the greatest e r o s i o n m u s t h a v e o c c u r r e d .
o f a l o n g c r a c k in the earth's c r u s t and
have a s i g n i f i c a n t c o n t r i b u t i o n to m a k e to
I n o t h e r w o r d s , the e v i d e n c e f r o m c o a l
spread s i d e w a y s . A l l this m u s t h a v e fol­
mineral exploration. By enabling people
s u p p o r t s the tale t o l d by p a l a e o m a g n e ­
l o w e d m i l l i o n s o f years o f g e o l o g i c a l
to u n d e r s t a n d the m a g n e t i c p r o p e r t i e s o f
t i s m : that a c o n s i d e r a b l e d e p t h o f r o c k s
d r a m a relatively near the s u r f a c e . T h e o ­
l o c a l r o c k s , t h e w o r k is h e l p i n g to a c h i e v e
has b e e n e r o d e d f r o m the S y d n e y B a s i n
retical m o d e l l i n g o f the w a y plates o f the
better interpretation o f b o t h aerial and
— particularly near the c o a s t .
earth's c r u s t split and the t w o p i e c e s m o v e
g r o u n d m a g n e t i c surveys. S i n c e many
apart ( m o r e c o n c i s e l y , the w a y A t l a n t i c -
e c o n o m i c a l l y important minerals
surface d e s e r v e s a
type rift m a r g i n s f o r m ) s u g g e s t s that h i g h
in a s s o c i a t i o n w i t h o r e s c o n t a i n i n g i r o n ,
m e n t i o n . A l t h o u g h Jurassic s e d i m e n t s d o
t e m p e r a t u r e s , u p l i f t i n g , and e r o s i o n m u s t
a c c u r a t e interpretation o f the
n o t m a k e u p any o f the l a n d s c a p e that w e
all h a v e taken p l a c e o n the eastern e d g e o f
' p r o f i l e s ' o f s u r v e y e d r o c k s s h o u l d lead
see in the S y d n e y district, they d o o c c u r in
the S y d n e y B a s i n b e f o r e the plates carry­
p r o s p e c t o r s faster
s o m e o f the l o c a l b r e c c i a — a c o m p o s i t e
i n g A u s t r a l i a and N e w Z e a l a n d b e g a n to
— to v a l u a b l e d i s c o v e r i e s .
rock, made u p o f m a g m a and v o l c a n i c
b e p u s h e d apart, a b o u t — as i n d e p e n d e n t
f r a g m e n t s , f o r m e d by the rapid ' b o i l i n g ' o f
e v i d e n c e s h o w s — 7 6 m i l l i o n years a g o .
O n e o t h e r c l u e that y o u n g e r r o c k s o n c e
c o v e r e d the p r e s e n t
steam and h o t gases in a n a r r o w v e n t . T h e
S c i e n t i s t s s u s p e c t that the e r o s i o n o v e r
studies
occur
magnetic
— and m o r e c h e a p l y
John Seymour
M o r e a b o u t the topic
v i o l e n t activity p l u c k s p i e c e s o f r o c k f r o m
the S y d n e y area m a y have b e e n swift, m o s t
A r e v i e w o f the p a l e o m a g n e t i s m o f A u s ­
the s i d e s o f the v e n t and d e p o s i t s s o m e o f
o f it taking p l a c e i n s i d e 10 m i l l i o n years.
tralia a n d A n t a r c t i c a . B.J.J. E m b l e t o n .
t h e m at a d e e p e r l e v e l .
W h e r e h a s all this d i s p l a c e d
M u c h o f the b r e c c i a a r o u n d S y d n e y has
sediment
In ' P a l e o r e c o n s t r u c t i o n o f the C o n t i ­
g o n e ? P e r h a p s it w a s h e d i n t o the T a s m a n
n e n t s ' , Geodynamics Series V o l . 2 , e d .
M . W . M c E l h i n n y and D . A . V a l e n c i o .
l o n g s i n c e d i s a p p e a r e d , but s o m e Jurassic
Sea, where sediments o f u n k n o w n
fragments fell d e e p e n o u g h to a v o i d e r o ­
r e a c h t h i c k n e s s e s o f u p to 1 k m , or per­
(American Geophysical Union: Wash­
sion. Scientists have dated these
haps d o w n a n e t w o r k o f rivers i n t o b a s i n s
ington 1981.)
frag­
age
m e n t s f r o m the traces o f p o l l e n and s p o r e s
( s u c h as the O t w a y ) o n the s o u t h e r n mar­
they c o n t a i n .
g i n o f the c o n t i n e n t , w h e r e Jurassic, C r e ­
A u s t r a l i a and the t h e r m a l h i s t o r y o f its
t a c e o u s , and
rifted m a r g i n . P . W . S c h m i d t a n d B.J.J.
A l l o f w h i c h leaves the q u e s t i o n : w h a t
Tertiary
deposits
have
M a g n e t i c o v e r p r i n t i n g in
south-eastern
caused these dramatic c h a n g e s s o m e 90
a c c u m u l a t e d to depths o f 8 k m . T h e s e de­
E m b l e t o n . Journal of Geophysical
m i l l i o n years a g o ? G e o l o g i s t s , w h o r o u ­
posits contain Australia's m a i n source o f
search, 1 9 8 1 , 86, 3 9 9 8 - 4 0 0 8 .
tinely study s u c h h u g e p h e n o m e n a
h y d r o c a r b o n s , particularly o i l .
and
s u c h vast p e r i o d s o f t i m e that their e x p l a -
P a l e o t h e r m o m e t r y o f the S y d n e y B a s i n .
further
M . F . M i d d l e t o n and P . W . S c h m i d t .
studies to find o u t h o w m u c h o f A u s t r a l i a
Journal of Geophysical Research, 1 9 8 2 , 8 7 ,
The
scientists
are p l a n n i n g
w a s affected by the 'overprint e v e n t ' . V a r ­
A considerable depth of rock
has been eroded from the
Sydney Basin — particularly
near the coast.
Re­
i o u s types o f e v i d e n c e already s h o w that
5351-9.
E v o l u t i o n o f the T a s m a n R i f t : apatite fis­
r o c k s w e r e m o d i f i e d a l o n g a s t r e t c h o f the
sion track dating e v i d e n c e f r o m
eastern coast from about
south-eastern
Newcastle
Australian
the
continental
s o u t h w a r d s i n t o V i c t o r i a , but traces o f the
margin. M . E . Morley, A.J.W. G l e a d o w ,
9 0 - m i l l i o n - y e a r - o l d ' b a k i n g ' m a y lie wait­
a n d J.F. L o v e r i n g . I n ' G o n d w a n a F i v e ' ,
i n g to b e d i s c o v e r e d as far apart as T a s ­
ed. M . M . Cresswell and P. V e l l a . ( A . A .
m a n i a and Q u e e n s l a n d .
Balkema: Rotterdam 1981.)
17