The document summarizes key aspects of plate tectonics theory including continental drift, seafloor spreading, and evidence that supports these ideas. It describes how Alfred Wegener first proposed continental drift in 1912, though it was not widely accepted until the 1950s when evidence like paleomagnetism was discovered. It also explains how seafloor spreading was observed at mid-ocean ridges and helped explain continental drift, providing evidence like pillow lava, magnetic stripes in ocean crust, and dating of ocean floor rocks. Finally, it gives a brief overview of plate tectonics theory including major components like lithosphere, asthenosphere, convection cells, and different plate boundary types.

2. 1.CONTINENTAL DRIFT
THEORY
2.SEAFLOOR SPREADING
3.PLATE TECTONICS

4.  Continental drift is the movement of the
Earth's continents relative to each other, thus
appearing to "drift" across the ocean bed.The
speculation that continents might have 'drifted'
was first put forward by Abraham Ortelius in
1596. The concept was independently and more
fully developed by Alfred Wegener in 1912, but
his theory was rejected by some for lack of a
mechanism (though this was supplied later by
Holmes) and others because of prior theoretical
commitments.

6.  Alfred Lothar Wegener (November 1, 1880 –
November 1930) was a German polar
researcher, geophysicist and meteorologist.
 During his lifetime he was primarily known for his
achievements in meteorology and as a pioneer of polar
research, but today he is most remembered for
advancing the theory of continental
drift (Kontinentalverschiebung) in 1912, which
hypothesized that thecontinents were slowly drifting
around the Earth. His hypothesis was controversial
and not widely accepted until the 1950s, when
numerous discoveries such
as palaeomagnetism provided strong support for
continental drift, and thereby a substantial basis for
today's model of plate tectonics.

9.  Similar plant and animal fossils are found
around the shores of different continents,
suggesting that they were once joined.
 The fossils of Mesosaurus, a freshwater
reptile rather like a small crocodile, found both
in Brazil and South Africa, are one example;
another is the discovery of fossils of the
land reptile Lystrosaurus in rocks of the same
age at locations in Africa, India,
and Antarctica.

10.  There is also living evidence—the same
animals being found on two continents.
Some earthworm families (e.g. Ocnerodrilidae,
Acanthodrilidae, Octochaetidae) are found in
South America and Africa, for instance.
 The similarities of rock type and age along the
matching cost lines.

11. Mesosaurus

13.  Pangaea or Pangea was a supercontinent that
existed during the late Paleozoic and
early Mesozoic eras. It formed approximately
300 million years ago and then began to break
apart after about 100 million years.Unlike the
present Earth, much of the land mass was in
the Southern Hemisphere. Pangaea was the
first reconstructed supercontinent and it was
surrounded by a super ocean, known
as Panthalassa.

15.  The theory of continental drift was not
accepted for many years. One problem was
that a plausible driving force was missing. And
it did not help that Wegener was not a
geologist. Other geologists also believed that
the evidence that Wegener had provided was
not sufficient. It is now accepted that the plates
carrying the continents do move across the
Earth's surface; ironically one of the chief
outstanding questions is the one Wegener
failed to resolve: what is the nature of the
forces propelling the plates?

16.  First, it had been shown that floating masses on a
rotating geoid would collect at the equator, and
stay there. This would explain one, but only one,
mountain building episode between any pair of
continents; it failed to account for earlier orogenic
episodes.
 Second, masses floating freely in a fluid
substratum, like icebergs in the ocean, should be
in isostatic equilibrium (in which the forces of
gravity and buoyancy are in balance). But
gravitational measurements showed that many
areas are not in isostatic equilibrium.

17.  Third, there was the problem of why some
parts of the Earth's surface (crust) should have
solidified while other parts were still fluid.
Various attempts to explain this foundered on
other difficulties.

19.  Seafloor spreading is a process that occurs at mid-
ocean ridges, where new oceanic crust is formed
through volcanic activity and then gradually
moves away from the ridge. Seafloor spreading
helps explain continental drift in the theory
of plate tectonics. When oceanic plates diverge,
tensional stress causes fractures to occur in
the lithosphere. Basaltic magma rises up the
fractures and cools on the ocean floor to form new
sea floor. Older rocks will be found farther away
from the spreading zone while younger rocks will
be found nearer to the spreading zone.

21.  Harry Hammond Hess (May 24, 1906 – August 25,
1969) was a geologist and United States
Navy officer in World War II.
 Considered one of the "founding fathers" of the
unifying theory of plate tectonics, Rear
Admiral Harry Hammond Hess was born on May
24, 1906 in New York City. He is best known for
his theories on sea floor spreading, specifically
work on relationships betweenisland arcs,
seafloor gravity anomalies,
and serpentinized peridotite, suggesting that the
convection of the Earth's mantle was the driving
force behind this process

24. 
In the 1960s, scientists found evidence that new
material is indeed erupting along mid-ocean
ridges. The scientists dived to the ocean floor
in Alvin, a small submarine built to withstand the
crushing pressures four kilometers down in the
ocean. In a ridge’s central valley, Alvin’s crew
found strange rocks shaped like pillows or like
toothpaste squeezed from a tube. Such rocks form
only when molten material hardens quickly after
erupting under water. These rocks showed that
molten material has erupted again and again along
the mid-ocean ridge.

25.  When scientists studied patterns in the rocks of the ocean floor,
they found more support for sea-floor spreading. You read earlier
that Earth behaves like a giant magnet, with a north pole and a
south pole. Surprisingly, Earth’s magnetic poles have reversed
themselves many times during Earth’s history. The last reversal
happened 780,000 years ago. If the magnetic poles suddenly
reversed themselves today, you would find that your compass
needle points south.
 Scientists discovered that the rock that makes up the ocean floor
lies in a pattern of magnetized “stripes.” These stripes hold a
record of reversals in Earth’s magnetic field. The rock of the ocean
floor contains iron. The rock began as molten material that cooled
and hardened. As the rock cooled, the iron bits inside lined up in
the direction of Earth’s magnetic poles. This locked the iron bits in
place, giving the rocks a permanent “magnetic memory.”

27.  The final proof of sea-floor spreading came from rock
samples obtained by drilling into the ocean floor.
The Glomar Challenger, a drilling ship built in 1968,
gathered the samples. The Glomar Challenger sent
drilling pipes through water six kilometers deep to
drill holes in the ocean floor. This feat has been
compared to using a sharp-ended wire to dig a hole
into a sidewalk from the top of the Empire State
Building.
 Samples from the sea floor were brought up through
the pipes. Then the scientists determined the age of the
rocks in the samples. They found that the farther away
from a ridge the samples were taken, the older the
rocks were. The youngest rocks were always in the
center of the ridges. This showed that sea-floor
spreading really has taken place.

30.  Plate tectonics is the theory that the outer rigid layer of the earth
(the lithosphere) is divided into a couple of dozen "plates" that
move around across the earth's surface relative to each other, like
slabs of ice on a lake.
 The drawing above is a cross section of the earth showing the
components that lie within plate tectonic theory. The cross section
should really be curved to correspond to the earth's curvature, but
it has been straightened out here.
 Note the continental craton (stable continent) in the middle of
the drawing. Note the line under the craton; that is the lower
boundary of the plate. Everything above that line is the plate. All
similar lines in the cross section mark the bottom of the plates.
Technically, everything above that line is lithosphere, the rigid,
brittle shell of the earth. Everything below is asthenosphere, the
hot, plastic interior of the earth.

31.  Within the asthenosphere are convection cells,
slowly turning over hot, plastic rock. The
convection cells bring heat from the earth's interior
out to the surface, but slowly. Movement is about
10 centimeters a year. When the convection cells
reach the base of the lithosphere they release heat
to the surface at the divergent plate boundary to
escape to space. The cooled plastic rock then turns
sideways and moves parallel to the earth's surface
before descending back into the earth at
subduction zones to become reheated. It is this
turning over of the convection cells the drives the
plate movements.

32.  Simplistically, the earth consists of the plates, and plate
boundaries, those zones where the plates contact and
interact. Observe that 7 different plates are labeled in
the cross section. Plates are combinations of two
units, continents and ocean basins. A plate may be an
ocean basin alone, or a continent alone, or a
combination of ocean basin+continent (common).
It is possible a plate could be a continent alone, but
for this to occur all edges of the continent would have
to be a plate boundary (very rare, perhaps not
practically possible). Note that in the cross section
several different ocean basin/continent combinations
are present, but that it is difficult to get a continent
with all plate boundaries.

34.  A divergent boundary occurs when two tectonic
plates move away from each other. Along these
boundaries, lava spews from long fissures and
geysers spurt superheated water. Frequent
earthquakes strike along the rift. Beneath the rift,
magma—molten rock—rises from the mantle. It
oozes up into the gap and hardens into solid rock,
forming new crust on the torn edges of the plates.
Magma from the mantle solidifies into basalt, a
dark, dense rock that underlies the ocean floor.
Thus at divergent boundaries, oceanic crust, made
of basalt, is created.

36.  When two plates come together, it is known as
a convergent boundary. The impact of the two
colliding plates buckles the edge of one or both
plates up into a rugged mountain range, and
sometimes bends the other down into a deep
seafloor trench. A chain of volcanoes often
forms parallel to the boundary, to the
mountain range, and to the trench. Powerful
earthquakes shake a wide area on both sides of
the boundary.

38.  Two plates sliding past each other forms
a transform plate boundary. Natural or human-
made structures that cross a transform boundary
are offset—split into pieces and carried in opposite
directions. Rocks that line the boundary are
pulverized as the plates grind along, creating a
linear fault valley or undersea canyon. As the
plates alternately jam and jump against each other,
earthquakes rattle through a wide boundary zone.
In contrast to convergent and divergent
boundaries, no magma is formed. Thus, crust is
cracked and broken at transform margins, but is
not created or destroyed.

42.  The essence of plate tectonic theory is that the plates (ocean
basins plus or minus continents) slide around over the earth
surface, interacting as they do at the plate boundaries. Thus,
any time there is a divergent plate boundary where two
plates are separating, there must be a convergent plate
boundary (subduction zone) where the earth comes together
again. And convergent boundaries always, eventually, lead
to collisions between continents, or continents and terranes
(island arcs plus or minus microcontinents). Observe the
subduction zones in the cross section. Next to each one is a
remnant ocean basin (ROB). An ROB is one that is
disappearing down a subduction zone; it is a remnant of its
former self. But all subduction zones must eventually
disappear completely and when they do the floating blocks
on either side will collide, and create a mountain range. The
continent-continent collision in the cross section is a case
where the collision has already occurred.

43.  But in the larger picture, it is common for a
divergent plate boundary to come into
existence and create a new ocean basin, and
then for that ocean basin to close again along a
convergent plate boundary until two
continents collide. This opening and closing of
ocean basins is the Wilson Cycle, and is the
simplest model we have of how the earth
operates historically.