The document discusses the theories of continental drift, seafloor spreading, and plate tectonics. It provides details on the original proponent of continental drift theory, Alfred Wegener, in 1915. It also outlines various geological, climatic, and biological evidence that supports continental drift. The document then discusses the sea floor spreading theory proposed by Harry Hess in the 1960s, which helped explain continental drift. It notes evidence that the ocean floor becomes progressively younger near mid-ocean ridges. Finally, it describes how the theories of continental drift and sea floor spreading were combined into the comprehensive theory of plate tectonics.

1. Md. Asif Hasan
Geo 47
Department Of Geology
University Of Dhaka

2. Continental Drift Theory
• Introduction
• Evidences in support
• Drawbacks
• Acceptance
Seafloor spreading
• Introduction
• Evidences in support
• Drawbacks
• Acceptance
Finally, Origin Of the unifying Plate Tectonics Theory

3.  Continents can drift apart from one another and have
done so in the past
 They were once part of a single landmass called
“Pangea” that broke apart and have moved to their
present locations.
Continental Drift Theory
Alfred Wegener (1596) first use the phrase
“Continental Drift”. The theory states that-

4. Continental Fit
Geological Evidence
Climatic Evidence
Biological Evidence
What are the evidence of Continental Drift?

5. Continental Fit:
Some continents almost fits like a puzzle if placed beside
each other, such as the Western seaboard of Africa and
the Eastern seaboard of South America seem to fit
together.

7. Geological evidence:
 Rocks of the same age and type and displaying the same
formations are found. Example: Resemblance in rock
Formations south-east Brazil and South Africa.

8.  The trends of the mountains in the eastern USA and north-
west Europe are similar when placed in their old positions.

9.  Similar glacial deposits are found in Antarctica, South
America and India, now thousands of km apart.
Climatic Evidence

10. Biological Evidence
Matching Fossils
- Fossil evidence for continental drift includes several
fossil organisms found on different landmasses.

13. Well, if the continents
are really moving
then why don’t we
notice that?
 We don’t really see what whole continents
look like in real time
 Continents move so slowly that people die
before any noticeable changes take place

14. Wegener could not provide an explanation of
exactly what made the continents move.
Drawback
Acceptance
• Initially Rejected

15. Sea floor spreading theory
In the 1960s a Scientist called Hanry Hess said that not only
were the continents moving but also newer crust is
continuously being added to the sea floor & because of that
sea floor is constantly moving.
Hess said that The movement of the sea floor was to do with
the convection currents in the mantle because the hot
magma that rises is less dense than the cooler magma; this
magma cools are forms new rocks when it reaches the seas
surface and consequently other magma that rises pulls the
sea floor apart moving the continents around. Then at the
plate boundaries the cool rocks sinks and melts from the
mantles heat.

20. Evidence in support of sea floor spreading
 Samples of the deep ocean floor show that basaltic
oceanic crust and overlying sediment become
progressively younger as the mid-ocean ridge is
approached, and the sediment cover is thinner near
the ridge.

21.  The rock making up the ocean floor is considerably
younger than the continents, with no samples found
over 200 million years old, as contrasted with
maximum ages of over 3 billion years for the
continental rocks. This confirms that older ocean
crust has been reabsorbed in ocean trench systems.

22.  Paleo-magnetism: By the mid-1960s studies of the earth's
magnetic field showed a history of periodic reversals in
polarity. Magnetic surveys conducted near the mid-ocean
ridge showed elongated patterns of normal and reversed
polarity of the ocean floor in bands paralleling the rift and
symmetrically distributed as mirror images on either side of
it.

24. If sea floor is spreading,
does it mean that the total
area of the earths crust is
increasing?
Surely not…
As the new crust is formed near the
spreading centers, on the other hand older
crusts are consumed in the subduction
zone simultaneously. That is why the total
area or volume of earth remains almost
constant over time.

26. Acceptance
The accumulation of lines of evidence such as
those mentioned in this section, along with many
other lines of evidence in support, has convinced
scientists of the validity of sea floor spreading &
as well as continental drift.
Drawbacks
Says that, the crustal part or ocean is riding over
the mentle

28. Continental
Drift
Sea Floor
Spreading
Plate Tectonics
Since the late 1960s, the concepts of continental
drift and sea floor spreading have been united into a
much more encompassing theory known as plate
tectonics

29. Plate Tectonics
• The lithosphere (Crust + Upper Mentle) is
broken into fragment which are known as
lithospheric plates.
• These lithospheric plates can contain both
continental crust, oceanic crust
• They are riding over the lower part of mentle
due to convection current

31. I would love to answer
your questions