Transcript Continental Drift

Continental Drift and the Earth's History
Era Period
Time (MYA)
Events
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
Paleozoic Cambrian
Epoch
First reptiles,
insects radiate
Mississippian
Pennsylvanian
Permian
280
Pangaea formed,
marine extinctions,
glaciation
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.
Era
Period
Epoch
Time (MYA)
Events
Mesozoic Triassic
230
Dinosaurs appear
Jurassic
181
Angiosperms appear
135
At the end: mass
extinctions, e.g.
dinosaurs
Cretaceous
Cenozoic Tertiary
Paleocene
63
Eocene
58
Oligocene
36
Miocene
25
Pliocene
12
Pleistocene
2
Quartenary Holocene
.01
Radiation of birds and
mammals, insects and
angiosperms become
abundant, by the end continents in
or near modern positions
glaciation, large
mammal extinctions
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.
They are:
1) rapid, extreme changes in climate,
2) oceans or other large bodies of water and
3) 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:
1) the Nearctic - North America and Greenland.
2) the Palearctic - Europe and Asia, but excluding the
Indian subcontinent and southeast Asia.
3) the Neotropical - South America + Central America
and southernmost Mexico.
4) the Ethiopian - Africa south of the Mediterranean
coastal region.
5) the Oriental - India and southeast Asia. The
Himalayas rise between the Oriental and Palearctic.
6) the Australasian - Australia, New Guinea, New
Zealand, and the Pacific islands southeast of
Wallace's line.
7) the Antarctic
8) Oceania - Pacific Islands
Tectonic plates and biogeographic realms correspond
closely, but not perfectly.
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.
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).
The cratons include not only emergent ‘land’, but the
continental shelves as well. These cratons are PreCambrian 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.
Close match
throughout
these areas
Andes rise as
South America
moves westward
into the Pacific
plate
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.
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.
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.
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.
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.
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.
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.
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:
1.Paleomagnetism
2.Sea Floor Spreading
Here’s 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.
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.
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.
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…
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…
If you’re into plate tectonics, you also know that the
magnetic poles have repeatedly reversed. More about
that later.
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.
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).
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 earth’s core move only a
few cm per year.
Plates of the lithosphere ride these currents like
surfboards on the ocean.
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.
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 'pullapart' 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 pullapart 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.
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…
Mid-Atlantic ridge
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)…
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.
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.
A break – this is Tristan de Cunha, a photogenic
view of its main active volcano.
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…
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.
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).
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 169:1197-1201.
Hurley, P.M. 1968. The confirmation of continental drift. Scientific American
218:52-62
Rand, J.R. 1969. Pre-drift continental nuclei. Science 164:1229-1242
Schopf, J.M. 1970. Relation of the floras of the Southern Hemisphere to continental
drift. Taxon 19:657-74.