Plate tectonic geologic time scale-Geomorhology Chapter

• Geologic Time Scale:
• Its History and Development
Scottish geologist James Hutton (1726-1797) set the stage
for the development of the geologic time scale in the late
18th century with the publication of his Theory of the Earth
(1785). In it, Hutton advanced "uniformitarianism," a
geological doctrine which basically assumes that current
geologic processes, occurring at the same rates observed
today, in the same manner, account for all of Earth's
geological features, a principle later championed by British
geologist Sir Charles Lyell (1797-1875). Next, British civil
engineer, surveyor and amateur geologist William Smith
(1769-1839) made the discovery that fossils are found
buried in a definite order. The geologic time scale was
developed shortly thereafter.

• Geologic Time Scale
• The earth's crust consists of many layers of
sedimentary rock (called "strata"). Geologists
assume that each layer represents a long period
of time, typically millions of years. This is
actually a secondary assumption based upon
the primary assumption of Uniformitarianism.
These layers of sedimentary rock contain billions
of fossil remains and some of these fossils are
unique to certain layers. The layers are
catalogued and arbitrarily arranged into a
specific order (not necessarily the order in which
they are found). This order reflects the
assumption of macro-evolution (the widely held
notion that all life is related and has descended
from a common ancestor).

• A variety of fossils from each layer of strata have been
chosen to be what are called "index fossils". Index fossils
are how we date the sedimentary rock layers.
Paleontologists assume the age of an index fossil by the
stage of evolutionary history the fossil is assumed to be
in. They guess how long it would take for one kind of life
to evolve into another kind of life and then date the
fossils and rocks accordingly. Once again, this is a
circular argument. "And this poses something of a
problem: If we date the rocks by the fossils, how can we
then turn around and talk about the patterns of
evolutionary change through time in the fossil record?"
(Niles Eldridge, Time Frames, 1985, p. 52)

• Geologic Time Scale: Circular Reasoning
• The geologic time scale employs yet another circular argument.
We determine the age of the rock by the assumed age of the index
fossils it contains, then, to determine the age of all the other fossils
in the same layer of rock, The succession of organisms has been
determined by a study of their remains embedded in the rocks, and
the relative ages of the rocks are determined by the remains of
organisms that they contain." (R. H. Rastall, "Geology",
Encyclopedia Britannica, vol. 10, 1954, p. 168)
• The remains of a once-living organism, generally taken to be one
that lived prior to the end of the last glacial period, i.e. fossils are
older than 10 000 years. The term includes skeletons, tracks,
impressions, trails, borings and casts. Fossils are usually found in
consolidated rock, but not always (e.g. woolly mammoths living 20
000 years ago were recovered from the frozen tundra of Siberia).
In its original sense, fossil meant anything dug up from the earth,
including ores, precious stones, etc. The modern use of the word
dates from the late 17th century.

Geological time scale
• The earth's crust consists of many layers of sedimentary rock
(called "strata"). Geologists assume that each layer
represents a long period of time, typically millions of years.
Scottish geologist James Hutton (1726-1797) set the stage for
the development of the geologic time scale in the late 18th
century with the publication of his Theory of the Earth (1785).
• The geologic time scale provides a system of chronologic
measurement relating stratigraphy to time that is used by
geologists, paleontologists and other earth scientists to
describe the timing and relationships between events that
have occurred during the history of the Earth.
• It is a period of time covering the physical formation and
development of Earth, especially the period prior to human
history.
• Evidence from radiometric dating indicates that the Earth is
about 4.570 billion years old.

William Smith's monograph on identifying strata based on fossils
(The remains of a once-living organism is called fossil)

Chalk Layers in Cyprus - showing sedimentary layering

• ice age
Any geologic period during which thick ice sheets cover vast areas of land.
Such periods of large-scale glaciation may last several million years and
drastically reshape surface features of entire continents. A number of
major ice ages have occurred throughout the Earth's history; the most
recent periods were during the Pleistocene Epoch (1.8 million–10,000
years ago).

• Pollination is only a
stage towards
fertilization (the
meeting of the male
and female sex
cells). The male sex
cell is inside the
pollen grain which is
on the surface of the
stigma. The female
sex cell (the ovule) is
inside the carpel in a
different part of the
flower.

• The living pollen grain of an angiosperm has a wall that
is made up of two layers. The outer layer is called the
exine and is composed of a very unusual substance,
sporopollenin.
• The inner layer, or intine, of the pollen wall is made of
cellulose and is very similar in construction to an
ordinary plant cell wall. During fossilization only the
resistant sporopollenin-containing exine remains, and it
is this that carries the characteristic form and sculpture
which permits the identification of pollen grains.
• Sporopollenin and sporopollenin-like materials are found
in the spores of such widely separated groups as algae,
fungi, pteridophytes and angiosperms. Their chemical
stability means they survive even in ancient fossils, and
a substance which appears similar to modern
sporopollenin has been recovered from some of the
oldest sedimentary rocks in the world.

Temporina globate(a) Corrugatisporites turpitus Tricolpites tressireticulatus Dyadoporonites sp(f) Dyadoporonites sp(f)
Pollens
Octaplata palanaensis(a) Octaplata palanaensis(a)
Dyadoporonites sp(f) Dyadoporonites sp(f) Dyadoporonites sp(f)
Dyadoporonites sp(f) Corrugatisporites turpitus(p) Striatriletes sp(p) Polypodiisporites sp.(p) Striatriletes indicus(p)

Aegicerus sp
Heritiera sp Aegicerus comiculatum
Xylicarpus sp Phoenix paludosa
Lumnitzera racemosa

• Sporopollenin is a
major component of
the tough outer (exine)
walls of spores and
pollen grains. It is
chemically very stable
and is usually well
preserved in soils and
sediments. The exine
layer is often intricately
sculptured in species-
specific patterns (see
image at right),
allowing material
recovered from (for
example) lake
sediments to provide
useful information
about plant and fungal
populations in the
past.

• Modern dating methods
• Radiometric dating has been carried out since
1905 when it was invented by Ernest Rutherford
as a method by which one might determine the
age of the Earth. In the century since then the
techniques have been greatly improved and
expanded.[14] Dating can now be performed on
samples as small as a billionth of a gram using a
mass spectrometer. The mass spectrometer
was invented in the 1940s and began to be used
in radiometric dating in the 1950s.

• Uranium-lead dating method
• The uranium-lead radiometric dating scheme has been refined to
the point that the error margin in dates of rocks can be as low as
less than two million years in two-and-a-half billion years. An error
margin of 2–5 % has been achieved on younger Mesozoic rocks.
• Uranium-lead dating is often performed on the mineral zircon
(ZrSiO4).

• Samarium-neodymium dating method
• This involves the alpha-decay of 147Sm to 143Nd with a half life of
1.06 x 1011 years. Accuracy levels of less than twenty million years
in two-and-a-half billion years are achievable.
• Potassium-argon dating method
• This involves electron capture or positron decay of potassium-40 to
argon-40. Potassium-40 has a half-life of 1.3 billion years, and so
this method is applicable to the oldest rocks. Radioactive
potassium-40 is common in micas, feldspars, and hornblendes,
though the closure temperature is fairly low in these materials, about
125°C (mica) to 450°C (hornblende).
• Rubidium-strontium dating method
This is based on the beta decay of rubidium-87 to strontium-87, with
a half-life of 50 billion years. This scheme is used to date old
igneous and metamorphic rocks, and has also been used to date
lunar samples. Closure temperatures are so high that they are not a
concern. Rubidium-strontium dating is not as precise as the
uranium-lead method, with errors of 30 to 50 million years for a 3-
billion-year-old sample.

• Uranium-thorium dating method
• A relatively short-range dating technique is
based on the decay of uranium-234 into thorium-
230, a substance with a half-life of about 80,000
years. It is accompanied by a sister process, in
which uranium-235 decays into protactinium-
231, which has a half-life of 34,300 years.
• While uranium is water-soluble, thorium and
protactinium are not, and so they are selectively
precipitated into ocean-floor sediments, from
which their ratios are measured. The scheme
has a range of several hundred thousand years.

• Radiocarbon dating method
• Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730
years, which is very short compared with those above. In other
radiometric dating methods, the heavy parent isotopes were
synthesized in the explosions of massive stars that scattered
materials through the universe, to be formed into planets and other
stars. The parent isotopes have been decaying since that time, and
so any parent isotope with a short half-life should be extinct by now.
Carbon–14 is an exception. It is continuously created through
collisions of neutrons generated by cosmic rays with nitrogen in the
upper atmosphere. The carbon-14 ends up as a trace component in
atmospheric carbon dioxide (CO2).
• An organism acquires carbon during its lifetime. Plants acquire it
through photosynthesis, and animals acquire it from consumption of
plants and other animals. When an organism dies, it ceases to take
in new carbon-14, and the existing isotope decays with a
characteristic half-life (5730 years). The proportion of carbon-14 left
when the remains of the organism are examined provides an
indication of the time elapsed since its death. The carbon–14 dating
limit lies around 58,000 to 62,000 years.

• The rate of creation of carbon-14 appears to be roughly constant, as
cross-checks of carbon–14 dating with other dating methods show it
gives consistent results. However, local eruptions of volcanoes or
other events that give off large amounts of carbon dioxide can
reduce local concentrations of carbon–14 and give inaccurate dates.
The releases of carbon dioxide into the biosphere as a
consequence of industrialization have also depressed the proportion
of carbon-14 by a few percent; conversely, the amount of carbon-14
was increased by above-ground nuclear bomb tests that were
conducted into the early 1960s. Also, an increase in the solar wind
or the Earth's magnetic field above the current value would depress
the amount of carbon-14 created in the atmosphere. These effects
are corrected for by the calibration of the radiocarbon dating scale.
Chlorine-36 dating method
• Large amounts of otherwise rare 36Cl were produced by irradiation
of seawater during atmospheric detonations of nuclear weapons
between 1952 and 1958. The residence time of 36Cl in the
atmosphere is about 1 week. Thus, as an event marker of 1950s
water in soil and ground water, 36Cl is also useful for dating waters
less than 50 years before the present. 36Cl has seen use in other
areas of the geological sciences, including dating ice and
sediments.

• Fission track dating method
• Apatite crystals are widely used in fission track dating.
• This involves inspection of a polished slice of a material to
determine the density of "track" markings left in it by the
spontaneous fission of uranium-238 impurities. The uranium content
of the sample has to be known, but that can be determined by
placing a plastic film over the polished slice of the material, and
bombarding it with slow neutrons. This causes induced fission of
235U, as opposed to the spontaneous fission of 238U. The fission
tracks produced by this process are recorded in the plastic film. The
uranium content of the material can then be calculated from the
number of tracks and the neutron flux.
• This scheme has application over a wide range of geologic dates.
For dates up to a few million years micas, tektites (glass fragments
from volcanic eruptions), and meteorites are best used. Older
materials can be dated using zircon, apatite, titanite, epidote and
garnet which have a variable amount of uranium content.[26]
Because the fission tracks are healed by temperatures over about
200°C the technique has limitations as well as benefits. The
technique has potential applications for detailing the thermal history
of a deposit.

• Plate tectonic Theory
• The rigid lithospheric slabs or rigid and solid crustal layers are
tectonically called ‘plates’. The whole mechanism of the
evolution, nature and motion of plates and resultant reactions
is called plate tectonics. Moving over the weak asthenosphere,
individual lithospheric plates glide slowly over the surface of
the globe: much as a pack of ice of the Arctic Ocean drifts
under the dragging force of currents and winds.
• Plate tectonic theory, a great scientific achievement of the
decade of 1960s, is based on two major scientific concepts:
(1) the concept of continental drift and (2) the concept of sea-
floor spreading. Six major and 20 minor plates are identified so
far (Eurasian plate, Indo-Australian plate, American plate,
Pacific plate, African plate, Antarctic plate).
• These slabs rest upon a layer of heated, pliable rock called the
asthenosphere, which flows slowly like hot tar. A popular
theory is that the movement of the thick, molten material in the
asthenosphere forces the upper plates to shift, sink, or rise.

• The basic concept behind plate tectonics is simply that heat
rises. Hot air rises above cool air, and warm water currents
flow above cold water. The same is true of the heated rock
below the earth’s surface. The asthenosphere’s molten
material, or magma, pushes upwards, while cooler,
hardened matter sinks deeper into the mantle.
• Hot material in the mantle rises to the base of the
lithosphere, where it than moves laterally, cools, and
descends to become reheated, so the cycle begins again.
The regular flow circuit of rising warm fluid and sinking cold
fluid, roughly circular motion, is called a convection current.
• Sinking rock eventually reaches the extremely hot
temperatures of the lower asthenosphere, heats up, and
begins to rise again.. At diverging plate boundaries and at
hot spots in the otherwise solid lithosphere, molten material
wells up to the surface, forming a new crust.

Plate tectonics: The main features are:
1. The Earth's surface is made up of a series of large plates (like pieces of a giant
jigsaw puzzle).
2. These plates are in constant motion travelling at a few centimetres per year.
3. The ocean floors are continually moving, spreading from the centre and sinking at
the edges.
4. Convection currents beneath the plates move the plates in different directions.
The source of heat driving the convection currents is radioactive decay which is
happening deep in the Earth.

• Convection Currents
• Very slow convection currents flow in this plastic
layer, and these currents provide horizontal
forces on the plates of the lithosphere much as
convection in a pan of boiling water causes a
piece of cork on the surface of the water to be
pushed sideways

• Plate tectonic theory
• The theory of plate tectonics has done for geology what
Charles Darwin's theory of evolution did for biology. It
provides geology with a comprehensive theory that explains
"how the Earth works." The theory was formulated in the
1960s and 1970s as new information was obtained about the
nature of the ocean floor, the distribution of volcanoes and
earthquakes, the flow of heat from Earth's interior, and the
worldwide distribution of plant and animal fossils.
• The whole mechanism of the evolution, nature and motion of
plates and resultant reactions is called “plate tectonics”.
• In other words, the whole process of plate motions is referred
to as plate tectonics. “Moving over the weak asthenosphere,
individual lithospheric plates glide slowly over the surface of
the globe; much as a pack of ice of the Arctic Ocean drifts
under the dragging force of currents and winds”- A.N. Strahler
and A.H. Strahler, 1978.

It may be mentioned that the term plate was first used by Canadian
geophysicist J.T.Wilson in 1965. He postulated a ‘paving stone’
hypothesis wherein the oceanic crust was considered to be newly
formed at mid-oceanic ridges and destroyed at the trenches. It may
be highlighted that the plate margins are most important because
all tectonic activities occur along the plate margins e.g. seismic
events, vulcanicity, mountain building, faulting etc.
1. Constructive Plate Margins: These are also called as “divergent
plate margins” or accreting plate margins. These represent zones
of divergence where there is continuous upwelling of molten
material (lava) and thus new oceanic crust is continuously formed.
In fact, oceanic plates split apart along the mid-oceanic ridges and
move in opposite directions.
2. Destructive Plate Margins: These are also called “ convergent plate
margins” or “ consuming plate margins” because two plates move
towards each other or two plate converge along a line and leading
edge of one plate overrides the other plate and the overridden
plate is subducted or thrust into the mantle and thus part of the
crust (plate ) is lost in the mantle.
3. Conservative Plate Margins: These are also called as shear plate
margins. Here, two plates pass or slide past one another along
transform faults and the thrust crust is neither created nor
destroyed.
Plate Boundaries

• Plate boundaries do not necessarily match
the coastlines of continents. A plate can
consist of continental crust, oceanic crust,
or both. In most cases, continents are part
of larger plates that extend for hundreds of
miles offshore. Many plate boundaries are
far out in the middle of the ocean. There
are three types of plate boundaries:
divergent, convergent, and transform.

• Oceanic crust (sima-Si+Mg), (depth of 0-10 kilometres)
the floor of the deep oceans, is thin, about 7km thick, and made of
relatively dense rocks like basalt. average density is between 2.9
and 4.7.
• Continental crust (sial-Si+Al) (depth of 0-75 kilometres)
is much thicker, averaging 33km, and is composed of relatively light
material such as granite. Average density is 2.9.
Basalt
Granite

• Convergent Plate Boundary –
• Oceanic and Continental Plates: When continental and oceanic
plates collide the thinner and more dense oceanic plate is
overridden by the thicker and less dense continental plate. The
oceanic plate is forced down into the mantle in a process known as
"subduction". As the oceanic plate descends it is forced into higher
temperature environments. At a depth of about 100 miles (160 km)
materials in the subducting plate begin to approach their melting
temperatures and a process of partial melting begins.

• Convergent Plate Boundary - Oceanic:
• When a convergent boundary occurs between two oceanic plates
one of those plates will subduct beneath the other. Normally the
older plate will subduct because of its higher density. The
subducting plate is heated as it is forced deeper into the mantle and
at a depth of about 100 miles (150 km) the plate begins to melt.
Magma chambers are produced as a result of this melting and the
magma is lower in density than the surrounding rock material.

• When an oceanic plate pushes into
and subducts under a continental
plate, the overriding continental plate
is lifted up and a mountain range is
created. Even though the oceanic
plate as a whole sinks smoothly and
continuously into the subduction
trench, the deepest part of the
subducting plate breaks into smaller
pieces. These smaller pieces
become locked in place for long
periods of time before moving
suddenly and generating large
earthquakes. Such earthquakes are
often accompanied by uplift of the
land by as much as a few meters.
When two oceanic plates converge
one is usually subducted under the
other and in the process a deep
oceanic trench is formed. The
Marianas Trench, for example, is a
deep trench created as the result of
the Phillipine Plate subducting under
the Pacific Plate.

• Convergent Plate Boundary – Continental:
• This is a difficult boundary to draw. First it is complex and second, it
is poorly understood when compared to the other types of plate
boundaries. In this type of convergent boundary a powerful collision
occurs. The two thick continental plates collide and both of them
have a density that is much lower than the mantle, which prevents
subduction (there may be a small amout of subduction or the
heavier lithosphere below the continental crust might break free
from the crust and subduct).

• When two continents meet head-on, neither is subducted
because the continental rocks are relatively light and, like
two colliding icebergs, resist downward motion. Instead,
the crust tends to buckle and be pushed upward or
sideways. The collision of India into Asia 50 million years
ago caused the Eurasian Plate to crumple up and override
the Indian Plate. After the collision,- the slow continuous
convergence of the two plates over millions of years
pushed up the Himalayas and the Tibetan Plateau to their
present heights. Most of this growth occurred during the
past 10 million years.

The concept of sea-floor spreading was first
propounded by Prof. Harry Hess of the Princeton
University in the year 1960. He discovered that
the mid-oceanic ridges were situated on the
rising thermal convection currents coming up
from the mantle. The oceanic crust moves in
opposite directions from mid-oceanic ridges.
These molten lavas cool down and solidify to
form new crust along the trailing ends of
divergent plates (oceanic crust). Thus, there is
continuous creation of new crust along the mid-
oceanic ridges and the expanding crust (plates)
are destroyed along the oceanic trenches.
These facts proves that the continents and
ocean basins are in constant motion.

• Sea-floor spreading is the
process in which the ocean floor is
extended when two plates move
apart. As the plates move apart,
the rocks break and form a crack
between the plates. Earthquakes
occur along the plate boundary.
Magma rises through the cracks
and seeps out onto the ocean
floor like a long, thin, undersea
volcano.
•
• As magma meets the water, it
cools and solidifies, adding to the
edges of the sideways-moving
plates. As magma piles up along
the crack, a long chain of
mountains forms gradually on the
ocean floor. This chain is called
an oceanic ridge.

• As the rift valley expands two
continental plates have been
constructed from the original one. The
molten rock continues to push the crust
apart creating new crust as it does.
This is an example of a divergent plate
boundary (where the plates move away from
each other). The Atlantic Ocean was created by
this process. The mid-Atlantic Ridge is an area
where new sea floor is being created.
As the rift valley expands, water collects
forming a sea. The Mid-Atlantic Ridge is
now 2,000 metres above the adjacent
sea floor, which is at a depth of about
6,000 metres below sea level.
The sea floor continues to spread
and the plates get bigger and
bigger. This process can be seen
all over the world and produces
about 17 square kilometres of new
plate every year.

• Sea-floor spreading:
• The rocks nearest the
ridge were relatively
young, but the rocks
aged as the distance
from the ridge increased.
In addition, marine
sediment was thicker
and older further from
the ridge, whereas the
ridge itself had virtually
no deposits of sediment.
These observations,
added to those of the
heat flow at the ridge,
confirmed the creation of
new crust at mid-ocean
ridges and the
mechanism of seafloor
spreading.

• Iceland is splitting along the
Mid-Atlantic Ridge - a
divergent boundary between
the North American and
Eurasian Plates. As North
America moves westward
and Eurasia eastward, new
crust is created on both sides
of the diverging boundary.
While the creation of new
crust adds mass to Iceland
on both sides of the
boundary, it also creates a rift
along the boundary. Iceland
will inevitably break apart into
two separate land masses at
some point in the future, as
the Atlantic waters eventually
rush in to fill the widening and
deepening space between.

Sea floor Spreading.
The boundaries where the plates move apart are 'constructive' because
new crust is being formed and added to the ocean floor. The ocean floor
gradually extends and thus the size of these plates increases. As these
plates get bigger, others become smaller as they melt back into the
Earth in the process called subduction.
The Marianas Trench, just east of the Mariana Islands in the western
Pacific, is the deepest seafloor depression in the world at 11,033 metres
(36,198 feet). The Marianas Trench is one of many deepwater trenches
formed by the geologic process of subduction. During subduction, the
edges of plates are subducted, or forced under, other plates. Ocean
crust is drawn down into the mantle and partially melted.
On the whole, sea-floor spreading is basically volcanic, but it is
a slow and regular process, without the explosive outbursts of the
volcanoes on land.

Subduction
• The Marianas Trench, just east of the Mariana Islands in the
western Pacific, is the deepest seafloor depression in the world at
11,033 metres (36,198 feet). The Marianas Trench is one of many
deepwater trenches formed by the geologic process of subduction.
During subduction, the edges of plates are subducted, or forced
under, other plates. Ocean crust is drawn down into the mantle and
partially melted.
• An important effect of the melting of subducted ocean crust is the
production of new magma. When subducted ocean crust melts, the
magma that forms may rise from the plane of subduction deep
within the mantle, erupting on the earth’s surface. Eruption of
magma melted by subduction has created long, arc-shaped chains
of volcanic islands, such as Japan, the Philippines, and the
Aleutians. Where an oceanic plate is subducted beneath continental
crust, the magma produced by subductive melting erupts from
volcanoes situated among long, linear mountain chains, such as the
Andes in South America.

• Earthquakes can occur at these plate margins, shifting
plates by up to 5 metres (about 15 feet) at once. Such
faults exist in Chile, Japan, Taiwan, the Philippines,
New Zealand, and Sumatra. When two continental plates
collide, the crust from both plates thrusts upwards,
creating mountain chains. The collision of India with the
Asian continent formed the Himalayas. In fact, the
mountain range is still growing in height today because
India and Asia are still converging.
• At a transform boundary, plates move past each other in
opposite directions. Little volcanic activity accompanies
transform boundaries, but large, shallow earthquakes
can occur. The San Andreas Fault in California (USA), is
the most famous example of this type of boundary. Mid-
ocean ridges are offset by hundreds of small transforms.

• After molten rock reaches the seafloor as
lava, deep ocean water quickly cools and
consolidates the material. To make room
for this continual addition of new crust, the
plates on either side of the ridge must
constantly move apart. In the North
Atlantic, the rate of movement of each
plate is only about 1 to 2 centimetres (0.4
to 0.8 of an inch) per year. In the Pacific,
the rate can be more than 10 centimetres
(about 4 inches) annually.

• Additional evidence for plate tectonics came in
the 1950s and 1960s. During this period,
scientists discovered that all rock fragments
maintain a set magnetic pattern based on when
the rocks formed. Geophysicists also learned
that the earth’s magnetic field had reversed
between north and south dozens of times over
millions of years. With this knowledge, they
examined both sides of ocean ridges and found
that the rocks on one side of the ridge produced
a mirror-image geomagnetic pattern of the rocks
on the other side.

• Period North Pole South Pole
• Silurian 14 d N Lat to the north-west of madagascar
• 124 d W Long
• Carboniferous
Shifting of the position of the poles

• It may be mentioned that the term plate was first used by
Canadian geophysicist J.T. Wilson in 1965. Meckenzie and
Parker discussed in detail the mechanism of plate motions
on the basis of Euler’s geometrical theorem in 1967. They
postulated “a paving stone” hypothesis wherein the
oceanic crust was considered to be newly formed at mid-
oceanic ridges and destroyed at the trenches. Isacks and
Sykes confirmed the “paving stone hypothesis” in 1967.
The hypothesis suggested that “the earth was covered with
rigid plates that moved relative to one another, bearing the
continents along with them.”
• Plate tectonic theory, a great scientific achievement of the
decade of 1960s, is based on two major scientific
concepts (1) the concept of continental drift and (2) the
concept of sea-floor spreading.

The theory states that Earth's outermost layer, the lithosphere, is broken into 7 large,
rigid pieces called plates: the African, North American, South American, Eurasian,
Australian, Antarctic, and Pacific plates. Several minor plates also exist, including
the Arabian, Nazca, and Philippines plates.

• The plates are all moving in different directions and at
different speeds (from 2 cm to 10 cm per year--about the
speed at which your fingernails grow) in relationship to
each other. The place where the two plates meet is called
a plate boundary. Boundaries have different names
depending on how the two plates are moving in
relationship to each other
• crashing: Convergent Boundaries,
• pulling apart: Divergent Boundaries,
• or sideswiping: Transform Boundaries

• Perhaps initiated by heat building up underneath the vast
continent, Pangaea began to rift, or split apart, around 200
million years ago. Oceans filled the areas between these
new sub-continents. The land masses continued to move
apart, riding on separate plates, until they reached the
positions they currently occupy. These continents are still on
the move today.

• Exactly what drives plate tectonics is not known. One theory
is that convection within the Earth's mantle pushes the
plates, in much the same way that air heated by your body
rises upward and is deflected sideways when it reaches the
ceiling.
• Another theory is that gravity is pulling the older, colder, and
thus heavier ocean floor with more force than the newer,
lighter seafloor.
• Whatever drives the movement, plate tectonic activity takes
place at four types of boundaries: divergent boundaries,
where new crust is formed; convergent boundaries, where
crust is consumed; collisional boundaries, where two land
masses collide; and transform boundaries, where two plates
slide against each other.

• The Earth's Magnetic Field — The Earth's magnetic field is
thought to arise from the movement of liquid iron in the outer core
as the planet rotates. The field behaves as if a permanent magnet
were located near the center of the Earth, inclined about 11 degrees
from the geographic axis of rotation (Fig. 4). Note that magnetic
north (as measured by a compass) differs from geographic north,
which corresponds to the planet's axis of rotation.
• Placing a bar magnet beneath a piece of paper with iron filings on it
will create a pattern as the filings align themselves with the magnetic
field generated by the magnet. The Earth's magnetic field is similar
to that generated by a simple bar magnet. At present, the lines of
force of the Earth's magnetic field are arranged as shown in Figure
4; the present orientation of the Earth's magnetic field is referred to
as normal polarity. In the early 1960s, geophysicists discovered
that the Earth's magnetic field periodically reverses; i.e. the north
magnetic pole becomes the south pole and vice versa. Hence, the
Earth has experienced periods of reversed polarity alternating with
times (like now) of normal polarity. Although the magnetic field
reverses at these times, the physical Earth does not move or
change its direction of rotation.

• Basaltic lavas contain iron-bearing minerals such as magnetite
which act like compasses. That is, as these iron-rich minerals
cool below their Curie point, they become magnetized in the
direction of the surrounding magnetic field. Studies of ancient
magnetism (paleomagnetism) recorded in rocks of different
ages provide a record of when the Earth's magnetic field
reversed its polarity.
Return to top
•
• During World War II, sensitive instruments called
magnetometers were developed to help detect steel-hulled
submarines. When research scientists used magnetometers to
study the ocean floor, they discovered a surprising pattern.
Measurements of magnetic variations showed that, in many
areas, alternating bands of rocks recording normal and reversed
polarity were arranged symmetrically about mid-ocean ridges
(Fig. 5).
• In 1963, F. Vine and D.H. Matthews reasoned that, as basaltic
magma rises to form new ocean floor at a mid-ocean spreading
center, it records the polarity of the magnetic field existing at the
time magma crystallized. As spreading pulls the new oceanic
crust apart, stripes of approximately the same size should be
carried away from the ridge on each side (Fig. 5). Basaltic
magma forming at mid-ocean ridges serves as a kind of "tape
recorder", recording the Earth's magnetic field as it reverses
through time. If this idea is correct, alternating stripes of normal
and reversed polarity should be arranged symmetrically about
mid-ocean spreading centers. The discovery of such magnetic
stripes provided powerful evidence that sea-floor spreading
occurs.

• The age of the sea-floor also supports sea-floor spreading. If sea-
floor spreading operates, the youngest oceanic crust should be
found at the ridges and progressively older crust should be found in
moving away from the ridges towards the continents. This is the
case. The oldest known ocean floor is dated at about 200 million
years, indicating that older ocean floor has been destroyed through
subduction at deep-sea trenches.
• It took exploration of the ocean floor to discover sea-floor spreading,
the mechanism for the movement of continents that Alfred Wegener
lacked. The hypothesis of continental drift gained renewed interest
and, when combined with sea-floor spreading, led to the theory of
plate tectonics. The history of thought about the movement of
continents provides a wonderful example of how hypotheses such
as continental drift and sea-floor spreading are thoroughly tested
before a new theory emerges. For an overview of the history of plate
tectonics, see Tarbuck and Lutgens (1994).

• Continental Drift theory:
• The theory of plate tectonics was not widely accepted until the
1960s and 1970s. Before that time, most scientists believed the
earth’s continents and oceans to be stationary. At the beginning of
the 20th century, German meteorologist Alfred Wegener suggested
that all continents had been part of one huge supercontinent,
Pangaea. According to Wegener, about 200 million years ago
Pangaea broke into separate plates that slowly drifted away from
each other, leading to today’s continental arrangement.
• One of Wegener’s most convincing pieces of evidence was the
almost perfect fit between the eastern coast of South America and
the western coast of Africa. To support his theory, he pointed out
that rock formations on opposite sides of the Atlantic—in Brazil and
West Africa—match in age, type, and structure. Also, the formations
often contain fossils of the same terrestrial creatures, indicating that
South America and Africa must have previously been connected.
• In subsequent years, scientific discoveries steadily began to support
the fundamental aspects of Wegener’s theory. Geologists
demonstrated the existence of the slowly moving asthenosphere,
underlying the crust at depths of 50 to 150 kilometres (30 to 80
miles). In addition, scientists in the 1920s used sonar, an echo-
sounding device, to determine ocean depths and map the seafloor.
They concluded that the Mid-Atlantic Ridge, detected in the 19th
century, was part of a worldwide ocean ridge system.

• Alfred Wegener first thought of this idea by noticing that the different
large landmasses of the Earth almost fit together like a jigsaw.
America fit closely to Africa and Europe, and Antarctica, Australia,
India and Madagascar fitted next to the tip of Southern Africa.
Wegener proposed this in 1912, but it wasn't considered to be
sufficient evidence in itself. He analysed either side of the
Atlantic Ocean for rock type, geological structures and fossils. He
noticed that there was a significant similarity.
• From 1912, Wegener publicly advocated the theory of "
continental drift", arguing that all the continents were once joined
together in a single landmass and have drifted apart.
• In 1915, in The Origin of Continents and Oceans (Die Entstehung
der Kontinente und Ozeane), Wegener published the theory that
there had once been a giant continent, he named "Pangaea"
(meaning "All-Lands" or "All-Earth") and drew together evidence
from various fields. Expanded editions during the 1920s presented
the accumulating evidence. The last edition, just before his untimely
death, revealed the significant observation that shallower oceans
were geologically younger.

• In 1915, the German geologist and meteorologist Alfred Wegener
first proposed the theory of continental drift, which states that parts
of the Earth's crust slowly drift atop a liquid core. The fossil record
supports and gives credence to the theories of continental drift and
plate tectonics.
•
Wegener hypothesized that there was a gigantic supercontinent 200
million years ago, which he named Pangaea, meaning "All-earth".

• In 1915, the German geologist and meteorologist Alfred Wegener
first proposed the theory of continental drift, which states that parts
of the Earth's crust slowly drift atop a liquid core. The fossil record
supports and gives credence to the theories of continental drift and
plate tectonics.
Wegener hypothesized that there was a gigantic supercontinent 200
million years ago, which he named Pangaea, meaning "All-earth".
Pangaea started to break up into two smaller supercontinents,
called Laurasia and Gondwanaland, during the Jurassic period. By
the end of the Cretaceous period, the continents were separating
into land masses that look like our modern-day continents.
Wegener published this theory in his 1915 book, On the Origin of
Continents and Oceans. In it he also proposed the existence of the
supercontinent, and named it (Pangaea means "all the land" in
Greek).

Fossil Evidence in Support of the Theory
Glossopteris, a tree-like plant from the
Permian Period through the
Triassic Period. It had tongue-shaped
leaves and was about 12 ft (3.7 m) tall. It
was the dominant plant of Gondwana.

• Eduard Suess was an Austrian geologist who
first realized that there had once been a land
bridge connecting South America, Africa, India,
Australia, and Antarctica. He named this large
land mass Gondwanaland (named after a district
in India where the fossil plant Glossopteris was
found). This was the southern supercontinent
formed after Pangaea broke up during the
Jurassic period. Suess based his deductions on
the fossil plant Glossopteris, which is found
throughout India, South America, southern
Africa, Australia, and Antarctica.

• Fossils of Mesosaurus (one of the first marine reptiles, even older
than the dinosaurs) were found in both South America and South
Africa. These finds, plus the study of sedimentation and the fossil
plant Glossopteris in these southern continents led Alexander
duToit, a South African scientist, to bolster the idea of the past
existence of a supercontinent in the southern hemisphere,
Eduard Suess's Gondwanaland. This lent further support to A.
Wegener's Continental Drift Theory
Plate Boundaries Fit
Plate tectonics was first
proposed in the 1800's by
Antonio Snyder-Pellegrini
based on the fact that the
continents of the Earth fit
together like pieces of a
jigsaw puzzle. Based on
the fit, he proposed that the
continents were once
together, forming a single
large landmass.

• Evidence for continental drift is now extensive. Simila
fossils
are found around different continent shores, suggest
Mesosaurus
, a freshwater reptile rather like a small crocodile, fou
Brazil and South Africa
, are one example; another is the discovery of fossils
reptile Lystrosaurus from rocks
of the same age from locations in
South America, Africa, and Antarctica
. There is also living evidence — the same animals b
earthworm
found in South America and South Africa.
Continental drift

If you look at the continental boundaries there is much overlap of fossil
assemblages, structures, and rock types up to the time in geologic history when
the plate split apart.

• If you look at glacial deposits from the
past, before the split, there is also
overlap.

The Triassic Period
248 - 206 million years ago
The Jurassic Period
206-144 million years ago
The Cretaceous Period
144-65 million years ago

• Bangladesh is divided into two major tectonic units: i)
Stable Pre-Cambrian Platform in the northwest, and ii)
Geosynclinal basin in the southeast. A third unit, a
narrow northeast-southwest trending zone called the
hinge zone separates the above two units almost
through the middle of the country. This hinge zone is
currently known as palaeo continental slope.
Stable Pre-Cambrian Platform in Bangladesh, Stable Pre-
Cambrian Platform refers to the stable shelf of the bengal basin. In
Bangladesh part of the Bengal Basin, the stable shelf can be divided
into three major zones. They are Dinajpur slope, Rangpur Saddle
and Bogra slope. It is composed of continental crust overlain by
Cretaceous (144 to 66 million years ago) to Recent sediment.
However, in isolated basins on the stable shelf, there is Permo-
Carboniferous (360 million years to 245 million years ago)
sediments with considerable amount of coal. The thickness of
sedimentary column on the stable shelf of Bengal Basin varies from
less then 200m to 8,000m.

• foreland basins, which occur on the plate that is subducted or
underthrust during plate collision (i.e. the outer arc of the orogen)
• Examples include the North Alpine Foreland Basin of Europe, or the
Ganges Basin of Asia
• Foreland basins form because as the mountain belt grows, it exerts
a significant mass on the Earth’s crust, which causes it to bend, or
flex, downwards. This occurs so that the weight of the mountain belt
can be compensated by isostasy at the upflex of the forebulge.
• Foreland basin systems:
• The wedge-top sits on top of the moving thrust sheets and contains
all the sediments charging from the active tectonic thrust wedge.
• The foredeep is the thickest sedimentary zone and thickens toward
the orogen. Sediments are deposited via distal fluvial, lacustrine,
deltaic, and marine depositional systems.
• The forebulge and backbulge are the thinnest and most distal zones
and are not always present. When present, they are defined by
regional unconformities as well as aeolian and shallow-marine
deposits.

• Geosynclinal Basin the geosynclinal basin in the
southeast is characterised by the huge thickness
(maximum of about 20 km near the basin centre) of clastic
sedimentary rocks, mostly sandstone and shale of Tertiary
age. It occupies the greater Dhaka-Faridpur-Noakhali-
Sylhet [Silet]-Comilla [Kumilla]-Chittagong areas.
Geosyncline a part of the earth crust that sank deeply through time.
Its length may extend for several thousand kilometres and may
contain sediments thousands of metres thick, representing millions of
years of deposition. A geosyncline generally forms along continental
edges, and is destroyed during periods of crustal deformation, which
often produces folded mountain ranges. Bengal Geosyncline is one of
the world's largest geosyncline that includes the bengal basin and the
bay of bengal. The evolution of the Bengal Basin started in the
Permo-Carboniferous with the sedimentation in the faulted Gondwana
Basins. The break up of the Gondwanaland in the Cretaceous Period
and the marine transgression led to the sedimentation in the Bengal
Geosyncline.

• In geology, a trough generally refers to a linear structural
depression that extends laterally over a distance, while being less
steep than a trench. A trough can be a narrow basin or a geologic
rift. There are various oceanic troughs, troughs found under
oceans.
• A fold can be divided into hinge and limb portions. The
limbs are the flanks of the fold and the hinge is where
the flanks join together. The hinge point is the point of
minimum radius of curvature for a fold. The crest of the
fold is the highest point of the fold surface, and the
trough is the lowest point.

For Bangladesh, the most important one is the Cenozoic era,
because most of the sedimentary layers as well as the geological
structures of the country were formed during this era.
The Cenozoic tectonic evolution of Bangladesh mainly took place at
the end of the Eocene, Middle Miocene, close of the Pliocene and the
early Pleistocene.
During the early Tertiary period (Palaeocene and Eocene epochs)
most of Bangladesh was under open marine condition, which
resulted in the formation of fossiliferous limestone and shale with
some sandstone.
During the late Tertiary period (Oligocene, Miocene and Pliocene
epochs) the open sea retreated to the south in response to the uplift
of the himalayas in the north and river systems built out a large
deltaic land that formed the backbone of the present bengal delta.
Huge successions of alternating sandstone and shale layers now
found on the surface and in the subsurface of eastern and southern
Bangladesh were formed in this delta environment during the Tertiary
period.

• Bengal Basin a term used to describe both a surface
physiographic feature and a geological feature deep
underground. The first is familiar to geographers and the
other to geologists. The geological feature is a
downwarping of the basement rocks beneath central and
southern Bangladesh under pressure of sediments laid
down since the Cretaceous age (63 to 135 million years
before the present). This Basin represents the most
important of the three tectonic divisions of the country.
The deep Basin began forming when Late Cretaceous
orogeny caused the rise of the Arakan Yoma mountains
along the eastern border of Bangladesh, and formed the
Assam Gulf. This Gulf apparently had an ecosystem
conductive to the profuse growth of phyto- and zoo-
planktons and benthic organisms.

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