1. Plate Tectonics
Part 1
Dr. Mark McGinley
Honors College and Department of
Biological Sciences
Texas Tech University

2. Scientific Revolutions
• Sometimes a new viewpoint revolutionizes
how we look at the world
– Copernican Revolution
• Altered how we look at Astronomy
– Atomic Revolution
• Altered how we look at Chemistry
– Darwinian Revolution
• Altered how we look at Biology
– Plate Tectonics
• Altered how we look at Geology

3. Plate Tectonics
• So far we have learned
– The Earth is very old
– Earth has change over time
• The position of the continents has changed
over time!!!

4. Plate Tectonics
• History of the idea
– Alfred Wegner
• Continental drift
• What has happened?
• Why it happens?
– Mechanisms
• What plate tectonics allows us to understand?

5. Alfred Wegener
• German
Meteorologist
– 1880 – 1930

6. Wegener- Continental Drift
• The jigsaw fit of the
continents,
especially South
America and Africa,
suggest that they
were together at one
time
http://www.eoearth.org/article/Wegener,_Alfred

7. Wegener- Continental Drift
Mountain ranges and rock types match when South
America and Africa are fit together, providing
additional support that they were once connected.
• In Wegener’s words “It is just as if we were to refit the
torn pieces of a newspaper by matching their edges
and then check whether the lines of print run smoothly
across”

8. Wegener- Continental Drift
Plants and animals are similar in Africa and South
America, suggesting a connection in the past; this
connection is better explained by drifting continents
than by the prevailing theory of the time that a land
bridge across the Atlantic has sunk below sea level.
– http://www.classzone.com/books/earth_science/t
erc/content/investigations/es0802/es0802page02.
cfm?chapter_no=investigation

9. Wegener- Continental Drift
Evidence for climates of the past, like glacial
deposits in tropical regions, suggests that the
continents must have been in different locations in
previous times.

10. Wegner- Continental Drift
• To explain the drift, he argued that the
continents are large blocks floating on the ocean
crust, somewhat like icebergs floating in the
ocean.
– A possible driving mechanism for the movement is the
force of the spinning earth driving the continents
away from the poles and toward the equator.
– He knew that this was the weakest part of his theory
and he later replaced it with the suggestion of others
that convection currents in the mantle are the cause,
an idea now generally accepted as the most likely
explanation.

11. Wegener- Continental Drift
• Wegener’s ideas were largely rejected by
geologists of the day
• Continental Drift was not take seriously until
the 1960s

12. Plate Tectonics
• http://www.youtube.com/watch?v=QDqskltCi
xA

14. Evidence

15. Good Info About Plate Tectonics
• http://www.bbc.co.uk/schools/gcsebitesize/science/21c_pr
e_2011/earth_and_space/continentaldriftrev1.shtml
• http://www.ucmp.berkeley.edu/geology/tectonics.html
• http://www.classzone.com/books/earth_science/terc/cont
ent/investigations/es0802/es0802page06.cfm?chapter_no=
investigation
• http://pubs.usgs.gov/gip/dynamic/dynamic.html#anchor19
978839

16. What is a Tectonic Plate?
• A tectonic plate (also called lithospheric plate) is a
massive, irregularly shaped slab of solid rock, generally
composed of both continental and oceanic lithosphere.
• Plate size can vary greatly, from a few hundred to
thousands of kilometers across
– the Pacific and Antarctic Plates are among the largest.
• Plate thickness also varies greatly
– ranging from less than 15 km for young oceanic
lithosphere to about 200 km or more for ancient
continental lithosphere (for example, the interior parts of
North and South America).
http://pubs.usgs.gov/gip/dynamic/dynamic.html#anchor19978839

17. Tectonic Plates
• How do these massive
slabs of solid rock float
despite their tremendous
weight?
• Continental crust is
composed of granitic rocks
which are relatively
lightweight
• Oceanic crust is composed of
basaltic rocks, which are
much denser and heavier.
• Because continental rocks
are much lighter, the crust
under the continents is
much thicker (as much as
100 km) whereas the crust
under the oceans is generally
only about 5 km thick

18. mechanism
• http://www.youtube.com/watch?v=ryrXAGY1
dmE

19. Tectonic Plates

20. What’s Going on With the
Ocean Floor?
• In 1947, seismologists on the U.S. research
ship Atlantis found that the sediment layer on the
floor of the Atlantic was much thinner than
originally thought.
• Scientists had previously believed that the oceans
have existed for at least 4 billion years, so
therefore the sediment layer should have been
very thick.
• Why then was there so little accumulation of
sedimentary rock and debris on the ocean floor?

21. Mid-Ocean Ridge

22. Mid-Ocean Ridge
• In the 1950s, oceanic exploration greatly expanded.
• Data gathered by oceanographic surveys conducted by many
nations led to the discovery that a great mountain range on the
ocean floor virtually encircled the Earth.
• Called the global mid-ocean ridge, this immense submarine
mountain chain -- more than 50,000 kilometers (km) long and, in
places, more than 800 km across -- zig-zags between the
continents, winding its way around the globe like the seam on a
baseball.
• Rising an average of about 4,500 meters(m) above the sea floor, the
mid-ocean ridge overshadows all the mountains in the United
States except for Mount McKinley (Denali) in Alaska (6,194 m).
Though hidden beneath the ocean surface, the global mid-ocean
ridge system is the most prominent topographic feature on the
surface of our planet.

23. Mid-Ocean Ridges

24. Magnetic Striping
• Beginning in the 1950s, scientists, using magnetic
instruments (magnetometers) adapted from airborne devices
developed during World War II to detect submarines, began
recognizing odd magnetic variations across the ocean floor.
• This finding, though unexpected, was not entirely surprising
because it was known that basalt -- the iron-rich, volcanic rock
making up the ocean floor-- contains a strongly magnetic
mineral (magnetite) and can locally distort compass readings.
• This distortion was recognized by Icelandic mariners as early as the
late 18th century. More important, because the presence of
magnetite gives the basalt measurable magnetic properties, these
newly discovered magnetic variations provided another means to
study the deep ocean floor.

25. Magnetic Striping

26. Magnetic Reversals
• The Earth has a magnetic field, as can be seen by using a
magnetic compass.
• It is mainly generated in the very hot molten core of the
planet and has probably existed throughout most of the
Earth's lifetime.
• The magnetic field is largely that of a dipole, by which we
mean that it has one North pole and one South pole. At
these places, a compass needle will point straight down, or
up, respectively. It is often described as being similar in
nature to the field of a bar (e.g. fridge) magnet.
• By magnetic reversal, or 'flip', we mean the process by
which the North pole is transformed into a South pole and
the South pole becomes a North pole.

27. Seafloor Spreading
• How does the magnetic striping pattern form?
• Why are the stripes symmetrical around the crests of
the mid-ocean ridges?
• In 1961, scientists began to theorize 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 has built the 50,000 km-long
system of mid-ocean ridges.

28. Seafloor Spreading

29. Seafloor Spreading
• This hypothesis was supported by several lines of
evidence.
– (1) at or near the crest of the ridge, the rocks are very
young, and they become progressively older away
from the ridge crest
– (2) the youngest rocks at the ridge crest always have
present-day (normal) polarity
– (3) stripes of rock parallel to the ridge crest alternated
in magnetic polarity (normal-reversed-normal, etc.),
suggesting that the Earth's magnetic field has flip-
flopped many times.

30. Seafloor Spreading
• By explaining both the zebralike magnetic
striping and the construction of the mid-ocean
ridge system, the seafloor spreading
hypothesis quickly gained converts and
represented another major advance in the
development of the plate-tectonics theory.

31. Plate Boundaries
• Plates are moving so what happens at the
boundaries between two plates?
– Divergent boundaries -- where new crust is
generated as the plates pull away from each other.
– Convergent boundaries -- where crust is destroyed
as one plate dives under another.
– Transform boundaries -- where crust is neither
produced nor destroyed as the plates slide
horizontally past each other.

32. Divergent Boundaries
• Divergent
boundaries occur
along spreading
centers where plates
are moving apart
and new crust is
created by magma
pushing up from the
mantle.

33. Review of the Mechanisms Again
• http://www.youtube.com/watch?v=ryrXAGY1
dmE

34. Divergent Boundaries
• Perhaps the best known of the divergent boundaries is
the Mid-Atlantic Ridge.
– This submerged mountain range, which extends from the
Arctic Ocean to beyond the southern tip of Africa, is but
one segment of the global mid-ocean ridge system that
encircles the Earth.
– The rate of spreading along the Mid-Atlantic Ridge
averages about 2.5 centimeters per year (cm/yr), or 25 km
in a million years.
– Seafloor spreading over the past 100 to 200 million years
has caused the Atlantic Ocean to grow from a tiny inlet of
water between the continents of Europe, Africa, and the
Americas into the vast ocean that exists today.

35. Divergent Boundaries

36. Convergent Boundaries
• The size of the Earth has not changed
significantly during the past 600 million years,
and very likely not since shortly after its
formation 4.6 billion years ago.
• The Earth's unchanging size implies that the
crust must be destroyed at about the same
rate as it is being created, as Harry Hess
surmised.

37. Convergent Boundaries
• Such destruction
(recycling) of crust takes
place along convergent
boundaries where plates
are moving toward each
other, and sometimes
one plate sinks
(is subducted) under
another. The location
where sinking of a plate
occurs is called
a subduction zone.

38. Convergent Boundaries
• The type of that takes place between plates
depends on the kind of lithosphere involved.
– oceanic and a continental plate
– two oceanic plates
– between two continental plates

39. Oceanic-Continental Convergence

40. Oceanic-Oceanic Convergence
• two oceanic plates
converge, one is
usually subducted
under the other
– in the process a
trench is formed

41. Mariana Trench
• The Marianas Trench
(paralleling the Mariana
Islands), for example, marks
where the fast-moving Pacific
Plate converges against the
slower moving Philippine
Plate.
• The Challenger Deep, at the
southern end of the Marianas
Trench, plunges deeper into
the Earth's interior (nearly
11,000 m) than Mount
Everest, the world's tallest
mountain, rises above sea
level (about 8,854 m).

42. 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

43. Continental-Continental Convergence
• The collision of India into Asia 50 million years
ago caused the Indian and Eurasian Plates to
crumple up along the collision zone.
• After the collision, the slow continuous
convergence of these 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.

44. Mountain Building
• http://www.youtube.com/watch?v=loFxYSHxT
f0

45. The Himalayas
• The Himalayas, towering as
high as 8,854 m above sea
level, form the highest
continental mountains in the
world.
• Moreover, the neighboring
Tibetan Plateau, at an average
elevation of about 4,600 m, is
higher than all the peaks in the
Alps except for Mont Blanc
and Monte Rosa, and is well
above the summits of most
mountains in the United
States.

46. Transform Boundaries
• The zone between
two plates sliding
horizontally past one
another is called
a transform-fault
boundary, or simply
a transform
boundary.

47. Transform Boundaries
• Most transform faults are found on the ocean
floor. They commonly offset the active
spreading ridges, producing zig-zag plate
margins, and are generally defined by shallow
earthquakes.

48. Transform Boundaries
• However, a few occur on land, for example the
San Andreas fault zone in California. This
transform fault 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.

49. San Andreas Fault
http://www.youtube.com/watch?v=ZxPTLmg0ZCw&feature=related

50. Summary of Convergence Zones