The Changing Earth
Early on, it was
recognized that the
layers in geological
strata represent
periods in the earth’s
history and contain
fossils of organisms
characteristic of that
time.
Some of the fossils
were widespread, and
could be used to relate
widely separated
strata. These are
called index or guide
fossils.
Some index fossils
Initially, fossil
correlations could
provide only relative
estimates of ages.
Initially, the earth was
thought to be only a few
thousand years old
(based on Biblical
doctrine). Alfred Russel
Wallace estimate the
age at some 400 million
years.
It took radioactive dating
techniques to provide
more accurate ages for
rock layers.
For example, the radioactive breakdown of 40K19 to
the more stable 40Ca20 and the intert gas 40Ar18
occurs with a half-life of 1.31 billion years. This
technique is very useful for extremely old rocks.
For slightly younger rocks, the
Rubidium/Strontium method is
preferred. The decay of 87Rb to
86Sr is useful for rocks older than
100 million years.
For very recent material,
radiocarbon dating is widely used.
Carbon 14 decays to stable
Carbon 12 rapidly, with a half life
of approximately 5730 years. This
can be used for carbon-containing
materials less than 50,000 years
old.
Many trees form an annual ring
during the growing season. This
allows the use of tree growth rings
(in temperate latitudes) as a means
of dating fossils formed with the last
10,000 years.
Tree rings can be used to calibrate
radiocarbon dating techniques.
The values match very closely.
A combination of these
methods has been used
to produce estimates of
the major geological
events in the history of
the earth. We call this
the geological time
scale.
The time scale is
hierarchical, with eons
divided into eras, eras
divided into periods, and
periods divided into
epochs.
Each transition
corresponds to a
transition in the
geological strata and
associated fossils.
There is always some debate about the exact timing
of transitions. Don’t be surprised if you see other
time scales with different ages.
The theory of continental drift has
probably had more impact on the
science of biogeography than
any other set of ideas.
In its simplest form, the theory
states that the continents have
been carried across the surface
of the planet by the movement of
the mantle beneath the crustal
plates.
In recent years, the science of
plate tectonics has developed
which explains the mechanisms
of plate movement.
The theory probably originated with early scholars who noted the close match
between the Americas and Africa. These included Abraham Ortelius and Sir
Francis Bacon. Even Benjamin Franklin speculated that the Earth’s crust was
shell-like and floated on a fluid interior.
In 1912, Alfred Wegener presented the
idea of continental drift. He spent the
next two decades developing the
ideas.
Wegener integrated ideas from
geology, geophysics, paleoclimatology,
paleontology, and biogeography.
Initially, Wegener’s ideas were ignored
or ridiculed.
With subsequent publications, the
ideas became more widely known.
Alfred Wegener
Wegener’s Basic Conclusions:
1. Continental rocks (sial – consisting of largely silicon and aluminum) are
fundamentally different from oceanic rocks (sima – composed largely of
silicon and magnesium). They are less dense, thicker, and less magnetized.
The lighter sialic blocks (continents) float on a layer of fluid mantle.
2. The major landmasses were once united as a single supercontinent (which
he called Pangaea). Pangaea broke into smaller continental blocks, which
moved apart as they floated on the mantle. The breakup began in the
Mesozoic Era.
3. The breakup began as a rift valley, which gradually widened into an ocean.
The mid-oceanic ridges mark the places where the continents were once
joined.
4. The continental blocks have retained their basic shape.
5. Rates of movement of the blocks range from 0.3 to 36 meters per year.
6. Radioactive heating in the mantle may be a cause of block movement, but
other factors may be involved.
The Evidence
Examination of the submarine contours of the continental shelves showed an even
better fit for the existing continents.
Topgraphic features (mountains, oceanic ridges, and island chains), rock strata,
and fossil deposits were found to be aligned along Wegener’s hypothesized
connections.
Paleoclimatic evidence is also in line. For example, evidence of glacial movement
corresponds to what would be predicted by the Wegener model.
Paleontological evidence – Evidence indicates that Glossopteris (a fossil
gymnosperm) was likely associated with glacial margins. Fossils of Glossopteris
are consistent with Gondwanaland reconstructions.
Disjunct distributions of some living taxa suggest an ancestral radiation across
Gondwanaland.
Glacial movement as
suggested by current
distribution of
Glossopteris fossils.
Glacial movement as
suggested by
Glossopteris fossils using
a Gondwanaland
reconstruction.
The disjunct distribution of
the Southern temperate
beetles of the tribe
Megadopini of the family
Carabidae….
…and fishes of the
superfamily Galaxioidea,
which are restricted to warm
nontropical waters in the
Southern Hemisphere.
Disjunct distribution of
the plant family
Proteaceae, found on
all of the southern
continents but barely
reaching the Northern
Hemisphere.
Disjunct distribution of the
clawed aquatic frogs of
the family Pipidae. The
family is composed of two
subfamilies, the Pipinae
in tropical South America
and the Xenopipinae in
tropical Africa. This
suggests an ancestor
once distributed in
western Gondwanaland.
The global system of
mid-ocean ridges,
which are the sites of
seafloor spreading.
The earth acts like a giant magnet.
Latitudinal position can be read as
declination in a compass needle.
The same phenomenon influences
the orientation of crystals during the
formation of magnetically active
rock.
Such information can be used to
reconstruct the positions of the
continents at the time of rock
formation.
Studies of paleomagnetism revealed shifts in the orientation and
latitudinal position of Africa and Australia over the last 200 million
years.
The paleomagnetism seen in the rocks can only be explained as the
result of these movements. movements
At the beginning of the 20th Century, Bernard Bruhnes
discovered magnetic reversals when he found lavas in
which the magnetic orientation was opposite that in newly
formed lavas.
This indicates reversals in the Earth’s magnetic field which
occur a periods ranging from 10,000 to 1,000,000 years.
These changes appear to result from changes in magma
flows through the mantle.
On the ocean floor,
alternating patterns of
normal and reversed
magnetized basalt
form “magnetic
stripes”.
Marine geologists discovered
that:
1. Basaltic rocks at the edge of
the midocean ridge have
normal field magnetic
properties.
2. Widths of the magnetic
stripes on opposite sides of
the ridge are usually
symmetrical, and the stripes
are generally parallel to the
axis of the midocean ridge.
3. The banding pattern of one
ocean matches that of
others.
The makeup of the
earth’s interior.
The current model
proposes that
crustal blocks are
moved about on the
mantle.
Three forces have been suggested to drive the motion:
1. Ridge push, the force generated by molten rock rising through the mantle
2. Mantle drag, the tendency of the crust to ride the mantle because of friction.
3. Slab pull, the force generated by the subduction (downward flow) pulling the trailing crust
behind it.
Currently, 16 major plates
are recognized. They range
in size from the relatively
small Gorda Plate (about
750 km2) to the Pacific Plate
(> 100 million km2). Rates
of movement are also highly
variable.
Portions of the Pacific Plate
move as rapidly as 5 cm per
year.
Plate boundaries can take three basic forms:
1. Spreading zones.
2. Collision zones.
3. Transform zones.
Midoceanic ridges are
the sites were two
plates are drifting apart.
Because of this, the age
of islands tends to
increase with increasing
distance from the ridge.
Midocean ridges
Midocean ridges, however, are
not the only locations where
spreading is taking place.
On continents, plates can also
diverge at rift zones.
Such areas have resulted in the
formation of the Red Sea, Lake
Baikal, and the East African Rift
Valley.
On the other sides of the plates,
away from the spreading zone,
the leading edge will typically
collide with another plate.
If they are of equal density, this
will lead to uplift and mountain
building.
The collision of the Indian Plate with
the Eurasian Plate led to the uplift of
the Himalayas.
More commonly,
dense oceanic
plates collide and
sink beneath lighter
continental plates to
form a subduction
zone.
The result is a deep
oceanic trench.
Subduction zones are marked with parallel bands of earthquakes and volcanism
near their active edges, and mountain ranges and accumulations of marine
sediments, or terranes, which increase in age as we move upstream from the
active zone.
The deepest spot in the world
ocean, the Marianas Trench, is the
result of such a subduction zone.
The Challenger Deep in the
Marianas Trench is 10,923 m
below the ocean’s surface.
Terranes mark the
locations of former
subduction zones along the
west coast of North
America.
These are accumulations
of marine sediments,
progressively older as we
move inland.
The Earth’s Tectonic History
These processes have been occurring throughout the
earth’s history, and will continue to occur.
The movement of the crustal plates has played a
major role in the distribution of the earth’s organisms.
By moving quickly through the next few slides, you can follow the
movement of the continents from the Pre-Cambrian (some 660 ma) to
the present day, and even a projection into the future.
Things to watch:
Pangaea – a “supercontinent” which occupied 1/3 of the earth’s
surface during the late Paleozoic and early Mesozoic (the best slide
to look at is D (Earliest Triassic)
Laurasia – the northern “half” of Pangaea made up of what would
ultimate become North America, Europe, and northern Asia.
Gondwanaland – the southern portion of Pangaea, which would
become South America, Africa, Australia, and southern parts of Asia.
Also note the separation of South America from Africa and that of
North America from Europe. Watch the transition in Slides F - I.
Read the discussion on the tectonic history of the continents (p. 256262), and relate that to the diagrams on the following slides.
The stages in the breakup of Gondwanaland
occurring from about 200 ma to about 100 ma.
Another interesting
story..
The history of the
Caribbean region is
important in
understanding the
opportunities for
exchange of organisms
between the Nearctic
and Neotropical regions
after the breakup of
Pangaea.
After the breakup of Pangaea, the Neotropics and the Neartic remained
connnected until about 160 ma. During the early Cretaceous, a chain of volcanic
islands began to emerge. By the late Cretaceous, these islands had created a
“stepping stone” route for some exchange between the Nearctic and Neotropics.
The formation of a “land bridge” was significant in allowing for more migration.
More on this later…
After Africa was separated from
North and South America, it began
to swing counterclockwise toward
Eurasia (go back to slides 59 and
60). It eventually closed the
Tethyan Seaway and a bridge was
formed between Asia and Africa.
Through Arabia.
As Africa moved toward southern
Europe, the Mediterranean was
confined (and became more salty).
The Red Sea opened as a result of
the rift system that is apparently
continuous with the Great Rift
Valley. This may eventually open a
inland seaway through eastern
Africa.
Hotspots are weak
spots in the mantle at
which magma is
released to the
surface.
As plates move
across hotspots, this
can result in island
chains.
The Hawaiian
Islands are a good
example The oldest
of islands are in the
Emperor Seamount
chain, while the
active volcanoes of
the “big island” are
currently over the
hotspot.
The “track” of the island chain provides a history of
the movement of the plates over the hotspot.
Climate has been strongly
influenced by continental drift
as continents change in
latitudes and seaways are
opened up.
Many major extinction
events in the marine
realm are linked to sea
level changes.
The diversity of
ammonites (a common
molluscan group in
prehistoric oceans) is
directly correlated with
the area of the oceans.
As North America and South
America were separated by
the formation of the Atlantic
Ocean, the similarity between
invertebrate fossils on the two
sides of the ocean decreased.
Here you’re seeing the
Simpson coefficient (a
similarity index) comparing the
fossils on the American side to
those on the African side.
Note that the similarity goes
down with increasing ocean
width, which is related to the
time of separation.