continental drift, plant tectonics, tectonic activities evidences

1. CONTINENTAL
DRIFT

2. * In 1910,Alfred Wegener
developed the hypothesis that all
the continents had been joined
together in a single land mass and
have drifted apart.
*The idea that the
continents are slowly drifting
apart is known as continental
drift.
* He published his
evidence in a book called origin of
continents in 1915.

3. Evidence From Landforms
* Wegener pieced together maps that showed mountain ranges that matched
up in South Africa and South America.
* European coal mines matched up with coal mines in North America.
Evidence From Fossils
*A fossil is a trace of an organism that has been
preserved in rock.
*Fossil for Mesosaurus and Lystrosaurus were
found in places now separated by oceans .
*Neither reptile could have swum long distances
across salt water.

4. Evidence From Climate
*Spitsbergen is an island in the Artic Ocean north of Norway.
*The island is ice-covered with harsh polar climates.
*Fossil of tropical plants were found on this island . When these plants lived
300 million years ago , the island must have had a warm and mild climate.
*Wegener concluded that the island must have been located near the equator.
*There is evidence that continental glaciers once covered South Africa.
*The climate of South Africa today is too mild to have sustained continental
glaciers.

5. Explanations
*Wegener also attempted to explain how the drift took place and
offer a new explanation for how mountains form.
*He thought that when the drifting continents collide , their
edges fold or crumble.
*The folding continents slowly push up huge chunks of rock to
form mountains.

6. Scientists Reject Wegener’s Hypothesis
*What forced or pulled the continents apart?
*Geologists would have to reject their own theories of how mountains
form.
*Geologists in the early 1900 thought that the earth was cooling and
shrinking.
*According to this idea, the crust would wrinkle all over the earth.
*Mountains are formed in narrow bands along the edges of all continents.
*Wegener’s theory was rejected until in the 1950’s when new evidence
about
the Earth’s structure caused scientists to reconsider the hypothesis.

8. * The Wilson cycle is a model that
describes the opening and closing of ocean
basins and the subduction and divergence
of tectonic plates during the assembly and
disassembly of supercontinents.
* A classic example of the Wilson
cycle is the opening and closing of the
Atlantic.

9. ●The earliest stage , called the embryonic stage , involves uplift and
crustal extension of continental areas with the formation of rift
valleys(e.g. the East African Rift System).
►This satellite view
(this photo, looking south
from the Nile Delta)show the
young ocean basin of the Red
sea. The oldest sea floor
rocks in the Red Sea are
about five million years old ,
indicating that the Red Sea
began to form at that time.

10. ●The youngest stage involves the evolution of rift valleys into spreading
centers with strips of ocean crust between the rifted continental segments.
The result is narrow , parallel–side sea , for example the Red Sea that is
opening between NE Africa and Arabia.
►This satellite
view(photo, looking south
from the Nile delta) shows the
young ocean basin of the Red
Sea . The oldest Seafloor rocks
in the Red Sea are about five
million years old, indicating
that the Red Sea began to form
at that time.

11. ● Eventually , this
expanding system becomes
unstable and , away from the
ridge , the oldest oceanic
lithosphere sinks back into the
asthenosphere , forming an
oceanic trench subduction
system with a Wadati –Benioff
zone marking the descending
plate and associated island arcs
, such as the situation in the
western Pacific Ocean , or
Andean – type Volcanism. The
onset of subduction at the
ocean boundary marks the
subduction stage of the cycle (e
.g . The Specific Ocean).

12. ●Once subduction
outpaces the formation of
new crust at the
constructive boundary , the
ocean begins to contract .
Island arc complexes ,
complete with their
inventory of sedimentary
rocks &volcanic rocks ,
collide and create young
mountain ranges around
the periphery of the ocean .
These features mark the
thermal stage of the cycle
(e.g. the Mediterranean).

13. ●The end stage occurs
once all the oceanic crust between
the continental masses has
subducted , and the continents
coverage along a collision zone
characterised by an active fold
mountain belt, such as the
Himalayas. Finally the plate
boundary becomes inactive , but
the site of the join , or suture,
between the two continental
masses is a zone of weakness in
the lithosphere that has the
potential to become the site of a
new rift and so the cycle
continues.

15. Mid-Ocean Ridge
Mid–Ocean Ridge: The undersea mountain chain where new
ocean floor is produced; a divergent plate boundary.

16. Sonar
Sonar: A device that determines the
distance of an object under water by recording
echoes of sound waves.
The sonar is used to map the ocean floor
●Sonar bounces sound waves off underwater objects and then
records the echoes of these sound waves.
●The time it takes for the echo to arrive indicates the distance
to the object.

17. Evidence for Sea-Floor Spreading
►In the 1960s, Harry Hess examined maps of
the mid ocean ridge. He proposed that the ocean
floors move like conveyor belts, carrying the
continents with them.

18. Sea-Floor Spreading
►The process by
which molten material
adds new oceanic crust
to the ocean floor.

19. What evidence did scientists find
for sea-floor spreading in the 1960s?
• Evidence from molten material
• Evidence from magnetic stripes
• Evidence from drilling samples

20. Evidence From Molten Material
• Alvin’s crew found strange rocks shaped like pillows or like
toothpaste squeezed from a tube.
• Such rocks can form only when molten material hardens quickly
after erupting under water.
• The presence of these rocks showed that molten material has
erupted again and again from cracks along the central valley of
the mid-ocean ridge.

21. Evidence From Magnetic Stripes
●Scientists discovered that the rock
that makes up the ocean floor lies in a
pattern of magnetized “stripes”
780,000 years ago, magnetic poles
reversed themselves.
● If they reversed today, the needle
in a compass would point south instead
of north.
●The rock in the ocean is made of
iron, which began as molten material.

22. Evidence From Drilling Samples
• When scientists sampled the rocks, they found that the
further away from the ridge the rocks were the older they
were.
• The younger rocks were always in the center of the ridges.

23. Subduction at Deep-Ocean Trenches

24. Deep-Ocean Trenches
● A deep valley along the ocean floor
through which oceanic crust slowly sinks
towards the mantle.
• Subduction: The process by which oceanic
crust sinks through a deep-ocean trench and
back into the mantle; a convergent plate
boundary.

25. Subduction and Earth’s Oceans

26. Subduction in the Pacific Ocean
●Subduction in the Pacific Ocean is occurring at a greater rate
than sea-floor is expanding.
●This is caused by the large amount of trenches.
Subduction in the Atlantic
●The Atlantic Ocean is expanding at a greater rate than
subducting.
●This is because of the low number of trenches in the Atlantic.
●Over time the entire ocean gets larger and pushes against the
continents.