Study of plate tectonics of the earth, or plate movement,

PLATE TECTONICS
Lec 4
a) Wegener’s Evidence (Continental Drift)
b) History of Plate Tectonics
c) Breakup and Appearence of Pangea

• WHAT IS A PLATE?
• The lithosphere is broken up into large segments what are called tectonic
plates. Tectonic plates consist of lithospheric mantle (upper part of the upper
mantle) overlain by either of two types of crustal material: oceanic crust (in
older texts called sima from silicon and magnesium) and continental crust (sial
from silicon and aluminium). Average oceanic lithosphere is typically 100 km
thick; its thickness is a function of its age: as time passes, it conductively
cools and becomes thicker. Continental lithosphere is typically ~200 km thick,
though this also varies considerably between basins, mountain ranges, and
stable cratonic interiors of continents. The two types of crust also differ in
thickness, with continental crust being considerably thicker than oceanic
(35 km vs. 6 km).
• Major continental and oceanic plates include:
the Eurasian plate, Australian-Indian plate, Philippine plate, Pacific plate, Juan
de Fuca plate, Nazca plate, Cocos plate, North American plate, Caribbean plate,
South American plate, African plate, Arabian plate, the Antarctic plate, and the
Scotia plate. These plates consist of smaller sub-plates.
PLATE TECTONICS

Types of Earth’s Crust
• Tectonic plates can include continental crust or oceanic crust, and many
plates contain both. For example, the African Plate includes the continent
and parts of the floor of the Atlantic and Indian Oceans. The distinction
between oceanic crust and continental crust is based on their modes of
formation. Oceanic crust is formed at sea-floor spreading centers, and
continental crust is formed through arc volcanism and accretion of terranes
through tectonic processes; though some of these terranes may contain
ophiolite sequences, which are pieces of oceanic crust, these are
considered part of the continent when they exit the standard cycle of
formation and spreading centers and subduction beneath continents.
Oceanic crust is also denser than continental crust owing to their different
compositions. Oceanic crust is denser because it has less silicon and more
heavier elements ("mafic") than continental crust ("felsic"). As a result of this
density stratification, oceanic crust generally lies below sea level (for
example most of the Pacific Plate), while the continental crust buoyantly
projects above sea level (see isostasy for explanation of this principle).

Plate tectonics
• Plate tectonics (from the Late Latin tectonicus) is
a scientific theory which describes the large scale
motions of Earth's lithosphere.
• The theory builds on the older concepts of
continental drift, developed during the first
decades of the 20th century by Alfred Wegener,
and seafloor spreading, developed in the 1960s.

THE DYNAMIC EARTH
• The earth is a dynamic planet, continuously changing both
externally and internally. The earth’s surface is constantly being
changed by endo-genetic processes resulting in volcanism and
tectonism, and exogenetic processes such as erosion and
deposition. These processes have been active throughout
geological history. The processes that change the surface feature
are normally very slow (erosion and deposition) except some
catastrophic changes that occur instantaneously as in the case of
volcanism or earthquakes. The interior of the earth is also in
motion. Deeper inside the earth, the liquid core probably flows at a
geologically rapid rate of a few tenths of mm/s. Several hypotheses
attempted to explain the dynamism of the earth.
• + Horizontal movement hypothesis
• + Continental drift, displacement hypothesis

Development of the theory
•Plate tectonic theory arose out of the hypothesis of continental
drift proposed by Alfred Wegener in 1912. He suggested that the
present continents once formed a single land mass that drifted
apart, thus releasing the continents from the Earth's core and
likening them to "icebergs" of low density granite floating on a
sea of denser basalt.
•Seafloor Spreading
The first evidence that the lithospheric plates did move came
with the discovery of variable magnetic field direction in rocks of
differing ages.

PANGEA
• Looking at a map of the Earth, it appears that the continents could fit
together like a jigsaw puzzle.
• Alfred Wegener (1912) proposed the idea of "continental drift."
• Wegener suggested that a single "supercontinent" called Pangaea
once existed in the past.
• Continental Drift
• Wegener developed his idea based upon 4 different types of
evidence:
• 1. Fit of the Continents
• 2. Fossil Evidence
• 3. Rock Type and Stuctural Similarities
• 4. Paleoclimatic Evidence

Global change is not a new idea
to geoscientists; they are quite
aware of phenomena like
continental drift, sea floor
spreading leading to
tremendous modifications to
our planet. Geoscience is
dealing with the recording of
the creation, accretion,
destruction and movements of
the continents. Economic
aspect of Geoscience is related
to the investigations where,
when, and how, mineral
deposits have formed and how
to locate and extract these
deposits including the non-
renewable energy resources for
the future and they are all
related to tectonics.
Drifting of Continents

Evidence for Continental Drift
• Fit of the Continents
• It was the amazingly good fit of the continents
that first suggested the idea of continental drift.
• In the 1960's, it was recognized that the fit of the
continents could be even further improved by
fitting the continents at the edge of the
continental slope — the actual extent of the
continental crust.

• Fossil Evidence
• Wegener found that identical fossils were
located directly opposite on widely separated
continents.
• This had been realized previously but the idea of
"land bridges" was the most widely accepted
solution. Wegener found fossils to be convincing
evidence that a supercontinent had existed in
the past.
• Example: Mesosaurus

Rock Type and Structural Similarities
Rock Type and Structural Similarities
• We find similar rock types on continents on
opposite sides of the Atlantic Ocean.
• Similar age, structure and rock types are found
in the Appalachian Mtns. (N.A.) and
mountains in Scotland and Scandinavia.

• Paleoclimatic Evidence
• Glacial till of the same age is found in
southern Africa, South America,India
and Australia — areas that it would be
very difficult to explain the occurrence
of glaciation.
• At the same time, large coal deposits
were formed from tropical swamps in
N. America and Europe.
• Pangaea with S. Africa centered over
the South Pole could account for the
conditions necessary to generate
glacial ice in the southern continents.
• In addition, the areas with extensive
coal deposits from the same time
period occur in regions that would
have been equatorial.

Continental Drift: PALEOMAGNETISM
• Pole Wandering
• Looking at igneous rocks, the
apparent position of the
North Pole was determined
from the paleomagnetism of
the rock.
• Assuming that the magnetic
poles are approximately
coincident with the pole of
rotation, the apparent
movement of the poles must
be due to movement of the
continents.
• Curves are similar shape for
N. America & Europe except
that they were offsetby ~24° of
longitude.

Plate Boundary
• What is a plate boundary?
• The location where two plates meet is called a plate boundary, and plate
boundaries are commonly associated with geological events such as
earthquakes and the creation of topographic features such as mountains,
volcanoes, mid-ocean ridges, and oceanic trenches.
• The majority of the world's active volcanoes occur along plate boundaries,
with the Pacific Plate's Ring of Fire being most active and most widely
known.
• Tectonic plates can include continental crust or oceanic crust, and many
plates contain both. For example, the African Plate includes the continent
and parts of the floor of the Atlantic and Indian Oceans.
• The distinction between oceanic crust and continental crust is based on their
modes of formation. Oceanic crust is formed at sea-floor spreading centers,
and continental crust is formed through arc volcanism and accretion of
terranes through tectonic processes

Types of plate boundaries
• Three types of plate boundaries exist, characterized by the way the plates move
relative to each other. They are associated with different types of surface
phenomena. The different types of plate boundaries are:
• Transform boundaries occur where plates slide or, perhaps more accurately, grind
past each other along transform faults. The relative motion of the two plates is either
sinistral (left side toward the observer) or dextral (right side toward the observer).
The San Andreas Fault in California is an example of a transform boundary
exhibiting dextral motion.
• Divergent boundaries occur where two plates slide apart from each other. Mid-ocean
ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great
Rift Valley) are both examples of divergent boundaries.
• Convergent boundaries (or active margins) occur where two plates slide towards
each other commonly forming either a subduction zone (if one plate moves
underneath the other) or a continental collision (if the two plates contain continental
crust). Deep marine trenches are typically associated with subduction zones.
• The subducting slab contains many hydrous minerals, which release their water on
heating; this water then causes the mantle to melt, producing volcanism. Examples
of this are the Andes mountain range in South America and the Japanese island arc.
• Plate boundary zones occur where the effects of the interactions are unclear and the
broad belt boundaries are not well defined.

Key principles
• The outer layers of the Earth are divided into lithosphere and
aestheno-sphere. This is based on differences in mechanical
properties and in the method for the transfer of heat.
• Mechanically, the lithosphere is cooler and more rigid, while the
aestheno-sphere is hotter and flows more easily.
• In terms of heat transfer, the lithosphere loses heat by conduction
whereas the aestheno-sphere also transfers heat by convection.
• The key principle of plate tectonics is that the lithosphere exists as
separate and distinct tectonic plates, which ride on the fluid-like
(visco-elastic solid) aestheno-sphere.
• Plate motions range up to a typical 10–40 mm/ a (Mid-Atlantic
Ridge; about as fast as fingernails grow), to about 160 mm/a
(Nazca Plate; about as fast as hair grows).

Expanding Earth theory
• A profound consequence of seafloor spreading is that new crust
was, and is now, being continually created along the oceanic
ridges. This idea found great favor with some scientists, most
notably S. Warren Carey, who claimed that the shifting of the
continents can be simply explained by a large increase in size of
the Earth since its formation. However, this so-called "Expanding
Earth theory" hypothesis was unsatisfactory because its
supporters could offer no convincing mechanism to produce a
significant expansion of the Earth. Certainly there is no evidence
that the moon has expanded in the past 3 billion years.

Explanation of magnetic striping and
Sea floor Spreading
• The discovery of magnetic striping and the stripes being
symmetrical around the crests of the mid-ocean ridges
suggested a relationship. In 1961, scientists began to theorise
that mid-ocean ridges mark structurally weak zones where the
ocean floor was being ripped in two lengthwise along the
ridge crest. New magma from deep within the Earth rises
easily through these weak zones and eventually erupts along
the crest of the ridges to create new oceanic crust. This
process, later called seafloor spreading, operating over many
millions of years continues to form new ocean floor all across
the 50,000 km-long system of mid-ocean ridges.

Evidences of Seafloor Spreading
• at or near the crest of the ridge, the rocks are very
young, and they become progressively older away
from the ridge crest;
• the youngest rocks at the ridge crest always have
present-day (normal) polarity;
• stripes of rock parallel to the ridge crest alternated in
magnetic polarity (normal-reversed-normal, etc.),
suggesting that the Earth's magnetic field has reversed
many times

Driving Forces of Plate Motion
• Tectonic plates are able to move because the Earth's
lithosphere has a higher strength and lower density
than the underlying aesthenosphere. Their
movement is driven by heat dissipation from the
mantle. Lateral density variations in the mantle result
in convection, which is transferred into tectonic plate
motion through some combination of drag,
downward suction at the subduction zones, and
variations in topography and density of the crust that
result in differences in gravitational forces.

What Drives a Plate: Convection

Subduction
• How can new crust be continuously added along the oceanic ridges
without increasing the size of the Earth?
• This question particularly intrigued Harry Hess, a Princeton University
geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a
scientist with the U.S. Coast and Geodetic Survey who first coined the term
seafloor spreading. Dietz and Hess were among the small handful who
really understood the broad implications of sea floor spreading. If the
Earth's crust was expanding along the oceanic ridges, Hess reasoned, it
must be shrinking elsewhere. He suggested that new oceanic crust
continuously spreads away from the ridges in a conveyor belt-like motion.
Many millions of years later, the oceanic crust eventually descends into the
oceanic trenches — very deep, narrow canyons along the rim of the Pacific
Ocean basin. Hess' ideas neatly explained why the Earth does not get
bigger with sea floor spreading, why there is so little sediment accumulation
on the ocean floor, and why oceanic rocks are much younger than
continental rocks.

Types of Plate Boundaries
• Three types of plate boundaries exist, characterized by the way the plates
move relative to each other. They are associated with different types of
surface phenomena. The different types of plate boundaries are:
• Transform boundaries occur where plates slide or, perhaps more
accurately, grind past each other along transform faults. The relative motion
of the two plates is either sinistral (left side toward the observer) or dextral
(right side toward the observer). The San Andreas Fault in California is an
example of a transform boundary exhibiting dextral motion.
• Divergent boundaries occur where two plates slide apart from each other.
Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such
as Africa's Great Rift Valley) are both examples of divergent boundaries.
• Convergent boundaries (or active margins) occur where two plates slide
towards each other commonly forming either a subduction zone (if one
plate moves underneath the other) or a continental collision (if the two
plates contain continental crust). Deep marine trenches are typically
associated with subduction zones. The subducting slab contains many
hydrous minerals, which release their water on heating; this water then
causes the mantle to melt, producing volcanism. Examples of this are the
Andes mountain range in South America and the Japanese island arc.

Divergent Boundaries
• At divergent boundaries new crust is created as one or more
plates pull away from each other. Oceans are born and grow
wider where plates diverge or pull apart. As seen below,
when a diverging boundary occurs on land a 'rift', or
separation will arise and over time that mass of land will
break apart into distinct land masses and the surrounding
water will fill the space between them.
• 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.

Convergent Boundaries
• Crust is destroyed and recycled back into the interior of the Earth as one
plate dives under another. These are known as Subduction Zones -
mountains and volcanoes are often found where plates converge. There
are 3 types of convergent boundaries: Oceanic-Continental
Convergence; Oceanic-Oceanic Convergence; and Continental-
Continental Convergence.
• 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 generating large earthquakes. Such
earthquakes are often accompanied by uplift of the land by as much as a
few meters.

Oceanic-Continental Convergence
• When a thin, dense oceanic plate collides with
a relatively light, thick continental plate, the
oceanic plate is forced under the continental
plate; this phenomenon is called subduction.

Oceanic-Continental Convergence

Oceanic-Oceanic Convergence
• When two oceanic plates collide, one may be pushed under the
other and magma from the mantle rises, forming volcanoes in the
vicinity. 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.
• Oceanic-oceanic plate convergence also results in the formation of
undersea volcanoes. Over millions of years, however, the erupted
lava and volcanic debris pile up on the ocean floor until a
submarine volcano rises above sea level to form an island volcano.
Such volcanoes are typically strung out in chains called island
arcs.

Continental-Continental Convergence
• 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. As
a result of two continental plates collision, mountain ranges are
created as the colliding crust is compressed and pushed upwards.
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.

Continental-Continental Convergence

Transform-Fault Boundaries
• Transform-Fault Boundaries are where two plates are sliding horizontally past
one another. These are also known as transform boundaries or more
commonly as faults.
• When two plates move side ways against each other (at a transform plate
boundary), there is a tremendous amount of friction which makes the
movement jerky. The plates slip, then stick as the friction and pressure build up
to incredible levels. When the pressure is released suddenly, and the plates
suddenly jerk apart, this cause an earthquake.
• Most transform faults are found on the ocean floor. They commonly offset
active spreading ridges, producing zig-zag plate margins, and are generally
defined by shallow earthquakes. A few, however, occur on land.
• The San Andreas fault zone in California is a transform fault that connects the
East Pacific Rise, a divergent boundary to the south, with the South Gorda --
Juan de Fuca -- Explorer Ridge, another divergent boundary to the north. The
San Andreas is one of the few transform faults exposed on land.
The San Andreas fault zone, which is about 1,300 km long and in places tens
of kilometers wide, slices through two thirds of the length of California. Along it,
the Pacific Plate has been grinding horizontally past the North American Plate
for 10 million years, at an average rate of about 5 cm/yr. Land on the west side
of the fault zone (on the Pacific Plate) is moving in a northwesterly direction
relative to the land on the east side of the fault zone (on the North American
Plate).

Significance of Plate Tectonics
• The theory of plate tectonics (meaning "plate
structure") was developed in the 1960's. This
theory explains the movement of the Earth's
plates (which has since been documented
scientifically) and also explains the cause of
earthquakes, volcanoes, oceanic trenches,
mountain range formation, and many other
geologic phenomenon.

The plate tectonic super-cycle is a theory to explain a sequence of events that have
repeated at least three times. Formation of super-continents Pangea and Rodinia
occurred 300 million years ago and 900 million years ago, suggesting a super-cycle time
span for formation and breaking up of super-continents of about 600 million years.
The following is a very general description of possible super-cycles.
During plate tectonic development, a super-continent breaks up and the two new
continents become separated by the new oceanic lithosphere that is produced at a mid
ocean ridge between them. As the oceanic lithosphere grows, the continents drift further
apart. If a subduction zone forms near the edge of one of the continents, the oceanic
lithosphere will be consumed in the subduction zone. The continents will be drawn back
together, eventually to collide producing a super-continent again.
If a subduction zone develops on the far side of one of the continents, oceanic
lithosphere will be consumed. This may eventually cause the continent to collide with
another continent producing a new super-continent.
The following is another super-cycle scenario, using Pangea as an example:
•Begin with a small super-continent, like Pangea, completely surrounded with ocean.
(Pangea occupied 30% of the Earth's surface with the other 70% being ocean.)
•Spreading at a mid ocean ridge some distance from the super-continent will cause the
oceanic lithosphere near the super-continent to begin to subduct beneath it.
•This subduction produces the characteristic andesitic volcanoes. The volcanism at the
edges of the super-continent causes some weakness in the crust there.
The plate tectonic super-cycle

• Subduction continues until the subduction zone becomes choked and
ceases, causing a new subduction zone to develop a few hundred
kilometres offshore. This new subduction zone will result in a chain of new
andesitic volcanoes, and thus new continental material developing offshore.
The weakness in the continental margin between the new island chain and
the original super-continent allows spreading to occur creating a trough
called a back-arc basin. The area west of the islands of Japan is an
example of this.
• Now, marginal seas and island arcs surround the super-continent. Back-
arc basins eventually fill with sediment, thus extending the size of the
super-continent.
• Eventually, due to the presence of weaknesses in the zones that were
once marginal seas, the super-continent is able to split up, allowing the
formation of separate continents, like we see today.
• The cycles continue for each continent. If subduction of the oceanic plates
continues, it may bring continents together once again creating a
supercontinent and thus the cycle can continue.
Continued

Rifting and break-up of Pangaea
There were three major phases hypothesized in the break-up of Pangaea. The first
phase began in the Early-Middle Jurassic, when Pangaea created a rift from the
Tethys Ocean from the east and the Pacific from the west. The rifting took place
between North America and Africa. The rift produced multiple failed rifts, the
Mississippi River being the largest. The rift resulted in a new ocean, the Atlantic
Ocean.
The Atlantic Ocean did not open uniformly; rifting began in the North-Central
Atlantic. The South Atlantic did not open until the Cretaceous. Laurasia started to
rotate clockwise and moved northward, with North America to the north, and
Eurasia to the south. The clockwise motion of Laurasia also lead to the closing of
the Tethys Ocean. Meanwhile, on the other side of Africa, new rifts were also
forming along the adjacent margins of east Africa, Antarctica, and Madagascar that
would lead to the formation of the Southwest Indian Ocean that would also open
up in the Cretaceous.
The second, major phase in the break-up of Pangaea began in the Early Cretaceous
(150-140 million years ago), when the minor supercontinent of Gondwana
separated into four multiple continents (Africa, South America, India, and
Antarctica/Australia).

About 200 million years ago, the continent of Cimmeria, as mentioned above, collided with
Eurasia. However, a subduction zone was forming, as soon as Cimmeria collided. This subduction
zone was called the Tethyan Trench. This trench might have subducted what is called the
Tethyan mid-ocean ridge, a ridge responsible for the Tethys Ocean's expansion. It probably
caused Africa, India, and Australia to move northward. In the Early Cretaceous, Atlantica, today's
South America and Africa, finally separated from Eastern Gondwana (Antarctica, India, and
Australia), causing the opening of a "South Indian Ocean." In the middle Cretaceous, Gondwana
fragmented to open up the South Atlantic Ocean as South America started to move westward
away from Africa. The South Atlantic did not develop uniformly, rather it rifted from south to
north like a zipper.
Also, at the same time, Madagascar and India began to separate from Antarctica and move
northward, opening up the Indian Ocean. Madagascar and India separated from each other 100-
90 million years ago in the Late Cretaceous. India continued to move northward toward Eurasia
at 15 centimeters per year (a plate tectonic record), closing the Tethys Ocean, while Madagascar
stopped and became locked to the African Plate. New Zealand and New Caledonia began to
move from Australia in an eastward direction towards the Pacific, opening the Coral Sea and
Tasman Sea. They have been independent islands since.
The third major and final phase of the break-up of Pangaea occurred in the early Cenozoic
(Paleocene to Oligocene). North America/Greenland broke free from Eurasia, opening the
Norwegian Sea about 60-55 million years ago. The Atlantic and Indian Oceans continued to
expand, closing the Tethys Ocean.
Continued

Meanwhile, Australia split from Antarctica and moved rapidly northward—just as India did more
than 40 million years earlier—and is on a collision course with Eastern Asia. Both Australia and
India are currently moving in a northeast direction at 5-6 cm/year. Antarctica has been near or
at the South Pole since the formation of Pangaea (since about 280 mya). India started to collide
with Asia beginning about 35 million years ago, forming the Himalayan orogeny, and also finally
closing the Tethys Seaway; this collision continues today. The African Plate started to change
directions, from west to northwest toward Europe, and South America began to move in a
northward direction separating itself from Antarctica, allowing complete oceanic circulation
around Antarctica for the first time, causing a rapid cooling of the continent and allowing
glaciers to form.
Other major events took place during the Cenozoic, including the opening of the Gulf of
California, the uplift of the Alps, and the opening of the Sea of Japan. The break-up of Pangaea
continues today in the East Africa Rift; ongoing collisions may indicate the creation of a new
supercontinent.
Continued

Study of plate tectonics of the earth, or plate movement,

Study of plate tectonics of the earth, or plate movement,

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