Continental Drift and the Earth's History

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Continental Drift and the Earth's History
Era Period Epoch Time (MYA) Events
Paleozoic Cambrian 600 Marine invertebrates appear
Ordovician 500 First agnathan vertebrates appear
Silurian 425 Invasion of land
Devonian 405 Amphibians and insects appear
Carboniferous 345 Primitive forests of vascular plants
Mississippian First reptiles, insects radiate
Pennsylvanian
Permian 280 Pangaea formed, marine extinctions, glaciation
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The time period we're most interested in begins
about 230 MYA, at the end of the Paleozoic Era,
and thus at the end of its last period, the
Permian. This begins our history at the time
when the continental land masses were last united
into a single great mass called Pangaea, which
was centered in the southern hemisphere. The
distributions of contemporary species have been
far more influenced by events since the break-up
of Pangaea than anything which occurred before
its formation.
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Era Period Epoch Time (MYA)
Events
Mesozoic Triassic 230 Dinosaurs appear
Jurassic 181 Angiosperms appear
Cretaceous 135 At the end mass extinctions, e.g. dinosaurs
Cenozoic Tertiary Paleocene 63 Radiation of birds and mammals, insects and angiosperms become abundant, by the end continents in or near modern positions
Eocene 58 Radiation of birds and mammals, insects and angiosperms become abundant, by the end continents in or near modern positions
Oligocene 36 Radiation of birds and mammals, insects and angiosperms become abundant, by the end continents in or near modern positions
Miocene 25 Radiation of birds and mammals, insects and angiosperms become abundant, by the end continents in or near modern positions
Pliocene 12 Radiation of birds and mammals, insects and angiosperms become abundant, by the end continents in or near modern positions
Pleistocene 2 glaciation, large mammal extinctions
Quartenary Holocene .01  
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From a biogeographic point of view, it is
apparent that long before the formation of
Pangaea there had been a wide variety of vascular
plants, amphibians, insects, lizards, and later,
while Pangaea was united, angiosperms, birds, and
mammals. All these species could wander over
most or all of the terrestrial continental areas
freely. Most of the large scale barriers to
movement of species arising since are directly or
indirectly traceable to effects of drift. Adding
to continental drift, there are 3 kinds of
barriers important in restricting large scale
distributions of groups of organisms.
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• They are
• rapid, extreme changes in climate,
• oceans or other large bodies of water and
• mountain ranges.
• The latter two are basically the result of
  dynamic geological processes which are
  collectively called plate tectonics, and which
  are the underlying cause of continental drift.
• Biogeographers separate the land masses of earth
  into 8 realms, with those latter two types of
  barriers isolating realms from each other. The
  realms are

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• the Nearctic - North America and Greenland.
• the Palearctic - Europe and Asia, but excluding
  the
• Indian subcontinent and southeast Asia.
• the Neotropical - South America Central
  America
• and southernmost Mexico.
• the Ethiopian - Africa south of the
  Mediterranean
• coastal region.
• the Oriental - India and southeast Asia. The
• Himalayas rise between the Oriental and
  Palearctic.
• the Australasian - Australia, New Guinea, New
• Zealand, and the Pacific islands southeast
  of
• Wallace's line.
• 7) the Antarctic
• 8) Oceania - Pacific Islands

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Tectonic plates and biogeographic realms
correspond closely, but not perfectly.
8
The explanation for separation of realms and many
species distributions arises from continental
drift. However, from a strictly North American
perspective other factors dominate The key
factors in the northern hemisphere are 1) the
rising of the Rocky Mountains, separating the
east and middle of North America from the west,
2) the effects of Pleistocene cycles of
glaciation on both the physical environment and
species' distributions, and 3) the repeated
presence of an extensive land bridge connecting
North America and Asia at the Bering Strait.
9
The evidence supporting the importance of
continental drift to biogeographic pattern is 1.
The fit between South America and Africa using
contemporary coastlines. In the region of close
fit the cratons match, not only in shape,
but also stratigraphy. There is a near
absolute correspondence in the stratigraphy of
the cratons along the east coast of the
prominence of South America and the 'armpit' of
west Africa. Antonio Snider-Pelligrini (1858)
described the geometric fit, but the
stratigraphic fit was described only as late as
1968 (Hurley 1968 Rand 1969).
10
The cratons include not only emergent land, but
the continental shelves as well. These cratons
are Pre-Cambrian shield (2-3 billion years old).
The match between fragments in Brazil (and to a
lesser extent Argentina) and West Africa (Sierra
Leone, Liberia, the Gold Coast, Ivory Coast,
Nigeria, Zaire) is so close as to strongly
suggest these continents were once fused. The
match is not only structural (i.e. the thickness
and order of individual layers), but also
chemical. Similarly, the sedimentary rock of
Brazilian coast and Gabon match extremely well,
also suggesting earlier fusion.
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Close match throughout these areas
Andes rise as South America moves westward into
the Pacific plate
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2. The Evidence of Permian Flora A map of areas
in the southern continents apparently glaciated
during the Permian shows that a logical physical
alignment also matches the distributions of flora
across continents.
13
During the period from 280-230 MYA there is
strong evidence of widespread glaciation over
parts of South America, Africa, Antarctica,
Australia, India and the Falkland Islands. These
glaciers existed from the late Carboniferous, and
probably had considerable influence over the
southern flora. That flora, different from the
species composition on northern continents, was
dominated by seed-bearing tree ferns of the
genera Glossopteris and Gangamopteris. The
distribution of that flora is shown on the map.
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The flora (deciduous) is considered indicative of
cool temperate conditions, and is evidence that
India and Antarctica were both once temperate,
though today their climates represent opposite
ends of the spectrum. The flora dominated the
edges of the ice sheets and expanded during
interglacials. The northern limit of the flora
was seemingly set by warmer climates. We call
that united, supercontinental mass Gondwanaland.
The name is derived from a site in India, which
was one of the key places used to identify the
Glossopteris flora.
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3. The Evidence of the Fossil Record The
zoological fossil record distinguishes northern
from southern continents, and indicates the
northern continents were fused into a
supercontinent we call Laurasia (its name derived
from a site in the Laurentian Mountains which
characterized the fauna). The important
(negative) evidence is the fossil record of a
sheep-sized dinosaur, Lystrosaurus. It was
present during the earlier part of the Triassic
in Antarctica and at the same time in southern
Africa and India, but has not been found in North
America or Europe.
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Following (in time, and thus just above in
stratigraphy) the Lystrosaurus community was a
widespread assemblage described by the presence
of Cynognathus, present on most southern
continents, but not Antarctica or Australia.
Why? It is evidence timing the rift of
continents forming Gondwanaland. These faunas,
however, create one of the great puzzles of
biogeography. Members of both these faunas
(Lystrosaurus and Cynognathus) have been found at
the same time in the fossil record in southern
China (Sinkiang and Shansi provinces.
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While these kinds of evidence point to the
existence of Pangaea and its rift into
Gondwanaland and Laurasia, the objective is to
reconstruct continental histories. How do we
achieve that? Methods of Reconstructing
Continental History The continents are not made
up of what is, in geological terms, heavy dense
material. The rigid, outer shell of the earth (or
lithosphere) is made up of a number (between 6
and 10) of separate plates of material less dense
than either deeper layers (the asthenosphere) or
the core materials. These plates therefore float
upon the asthenosphere.
19
The movements of the plates are somewhat like
bumper cars at an amusement park they converge,
collide, rotate, slide past one another, and
occasionally one rides up over another after
collision. The plates generally consist of 2
layers. The denser and deeper layer is the rock
which forms the ocean floor (and the only layer
on non-continental plates) and the underlayer
beneath continents. The material of this layer is
called sima after its chief ingredients, silicon
and magnesium. Geologically, this rock is mostly
basalt.
20

• The emergent continents are made up of lighter
  silicaceous materials called sial after its main
  ingredients, silica and aluminum. The Canadian
  Shield is a prime example of the sial crust.
• Movements are traceable only because of the
  geological processes involved the formation and
  disappearance of parts of whole plates, i.e.
  plate tectonics. The evidence of movement and
  position comes from 2 major sources
• Paleomagnetism
• Sea Floor Spreading

21
Heres the plate map again. Now note the
directions of movement indicated by the arrows,
e.g. the continuing separation of the South
American plate from the African.
22
Paleomagnetism Paleomagnetism can determine the
latitudes (but not the longitudes) of continents
through their history. Paleomagnetism refers to
the weak magnetic orientations of magnetic
materials, elemental compasses oriented towards
the earth's magnetic poles, fixed into rocks at
the time of their formation. The magnetic
elements are iron, cobalt and nickel (and
titanium oxides). By aging rocks using isotopic
methods (carbon-14, potassium-argon, etc.) the
history of latitudes of a continental mass can be
obtained.
23
Assume the stratigraphy of our exemplar continent
includes only 3 layers beneath the present
surface, and that these layers have been aged as
respectively 50, 100, and 150 million years old.
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What orientation does a 'compass' take? It points
toward the poles. Thus, if our continent were to
be observed 150 MYA, there would be only one
layer on the continent, with a compass
orientation to the poles. If the continent were
in the far south, say 60o latitude, the
paleomagnetic orientation of iron-bearing rocks
would be at an angle of 60o to the rock (or
layer) surface. The angle of magnetic orientation
with respect to the surface of the layer would be
equal to the latitude at the time of rock
formation. 100 MYBP the continent was at the
equator, and the orientation of the compass is 0o
to the surface
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Paleomagnetism can also provide information about
twisting (rotation) of the continental masses at
any latitude (if the 'compasses' for each layer
don't point N-S, with different layers oriented
at different angles with respect to the surface,
then the mass has rotated, as well as drifted,
during its movements). The weakness of the
method is the assumption that the poles have
remained fixed in place (or it must correct for
polar wandering). Polar wandering is small
compared to latitudinal movements of the
continents. This map giving Gondwanaland
distributions shows polar wandering
26
If youre into plate tectonics, you also know
that the magnetic poles have repeatedly reversed.
More about that later.
27
The previous map shows the gradual wandering of
the poles. However, it appears there have also
been times when the poles (and the axis of
rotation) of the earth shifted rapidly, and quite
a bit. One such shift was around 84 MYBP.
This shift is believed due to shifting of mass in
the mantle, far from the axis of rotation, and
therefore having large effect.
28
Longitude and Sea Floor Spreading The basic
model
Asterisks along the portion of the plate at the
point of contact and submergence indicate the
regions of earthquake activity (the Benioff
zone).
29
To understand how longitude can be determined we
need to step back into geology for a moment. The
molten core of the earth is undergoing what is
described as sluggish thermal convection. Imagine
water heating in a pan. If you look into the pan
as the water heats, but before it boils, you see
a number of relatively separate convection cells.
Sluggish means that these currents in the earths
core move only a few cm per year. Plates of the
lithosphere ride these currents like surfboards
on the ocean.
30
These currents, like atmospheric circulation
cells, must rise and fall, as well as move
laterally. There are distinct zones where this
rise and fall is evident. At mid-ocean,
particularly in the Atlantic, ridges form a
nearly continuous path. They are zones of new sea
floor formation. There are other regions with
deep trenches the trenches are the deepest
regions of the ocean. They are zones where
convection returns sea floor into the magma. When
one plate is forced down under another, called
subduction, sea floor also descends and melts
back into the magma.
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The average thermal flow over the ocean floor is
about 1 x 10-8 calories /cm2/sec. At the ridges
the rate of heat flow is 2-8x this rate, at the
trenches heat flow is reduced from this average
by about the same amount (1/10 the
average). Faulting along the ridges indicates
that this is a 'pull-apart' zone. The mantle is
ripped by being pulled apart, riding on separate
convection currents on the two sides of the
ridge. The rift opened by this pull- apart is
filled by rising magma (the new sea floor). The
formation of new ocean floor occurs continuously
at mid-ocean ridges at a rate of a few
centimeters a year.
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Rate of formation is verified by ageing lava rock
on volcanic islands (or samples of sea floor),
which should be of differing ages because they
are at different distances from the ridge.
Ascension and other south Atlantic islands lie
along an arc perpendicular to the south Atlantic
ridge. The arc reflects the existence of a 'hot
spot', a local weakness that leads to
particularly strong upwelling alongside the
ridge, and the formation of volcanic islands
above the weakness. The islands are about 50
million years old/1000 km from the ridge. A
similar situation has produced the Hawaiian arc
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Mid-Atlantic ridge
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The convective flow is not terribly even. The
ages of sea floor elsewhere along the south
Atlantic ridge line do not follow the same age
regression, i.e. 50 x 106 years old/100 km. Rates
also differ when we look across different ridges
(and at different times)
37
Continents riding on the plates are 'twisted'
(rotated) with respect to each other when the
rate of sea floor formation differs near them, or
even along their extension parallel to the
ridge. The most recent islands along the south
Atlantic and Hawaiian arcs (and others around the
world) are frequently actively volcanic (another
indication of active upward flow from beneath the
island), while older islands, carried along with
the sea floor away from the zone of upward flow
at the ridge or weakness, are older, inactive
volcanically, and, among the very oldest, may
have 'subsided' to become sub-surface seamounts
or guyots.
38
Island arcs are path maps for the continents that
lie at their ends. Such continents were once
together (e.g. South America and Africa), but are
now separated by sea floor spreading which has
occurred between them. Along the south Atlantic
arc, Tristan de Cunha lies just to the east of
the mid-Atlantic ridge is less than 1 million
years old and still has active volcanoes. From it
to Angola extends the lateral Walvis ridge, and
to southern Brazil the Rio Grande ridge. The
places where these lateral ridges reach their
respective continents are a perfect geographic
fit when the continents are united.
39
A break this is Tristan de Cunha, a
photogenic view of its main active volcano.
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In the absence of island arcs the positions of
continents historically can still be
reconstructed. Sea floor spreading is the basic
geological process used, but additional data is
obtained from 'anomalous' reversals in the
magnetic polarity of the earth. Reversal has
happened many times - 171 times in the last 76
million years. Reversals in polarity are recorded
on the sea floor as stripes of alternating
magnetic polarity running parallel to mid-oceanic
ridges the magnetic orientation frozen into the
rock of the sea floor. Look at the banding in the
north Pacific and the south Atlantic in the
earlier diagram
42
Bands in the two areas are in the same order and
basically proportional. The rates of spread are
much different, however. Sea floor spreading is
more rapid in the Atlantic. Also, rates are not
constant over time. Along the margins of plates
being moved at different rates by their
underlying convection cells they must somehow
slide past one another. There is enormous
friction between the plates, and the 'slippage'
is a major source of what are termed
strike-slip earthquakes.
43
Finally, one plate can ride up on another and
force it downwards. The plate on which North
America rides and the Pacific plate collide at
the western boundary of the continent. The
Pacific plate is being forced down, and as the
sea floor slides down into the trenches which rim
the Pacific, the sliding friction generates both
heat (which results in volcanic activity like the
relatively recent eruption of Mt. St. Helens in
Washington) and earthquakes (since the sliding
process is not even. Collision and friction can
also cause orogenesis, the rising of mountain
ranges (Himalayas and Urals are examples of
purely plate-driven orogenesis).
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References Brown, J.H. and A.C. Gibson. 1983.
Biogeography. Mosby, N.Y., N.Y. Chapter 5.
Eliot, D.H. et al. 1970. Triassic tetrapods from
Antarctica evidence for continental
drift. Science 1691197-1201. Hurley, P.M. 1968.
The confirmation of continental drift. Scientific
American 21852-62 Rand, J.R. 1969. Pre-drift
continental nuclei. Science 1641229-1242
Schopf, J.M. 1970. Relation of the floras of the
Southern Hemisphere to continental drift. Taxon
19657-74.