This document provides an overview of plate tectonic theory and the evidence that supports it. It discusses early ideas including continental drift theory and how the development of seafloor spreading theory addressed continental drift's lack of a driving mechanism. It then summarizes key evidence for seafloor spreading including the global system of mid-ocean ridges, patterns of magnetic reversals in ocean crust, and the age progression of ocean floors. This led to the modern theory of plate tectonics unifying continental drift and seafloor spreading.

1. Plate Tectonics

2. Tectonic System
Plate tectonics is the process by which rock material is moved from
within the earth to its surface and, in some cases, back to its interior and
by which the lithosphere is broken into a series of plates that move with
respect to one another.

3. Early Ideas on Earth History
• Supernatural Intervention
• Shrinking Earth Theory
• Continental Drift Theory
• Seafloor Spreading Theory
• Theory of Plate Tectonics
Ideas concerning the earth’s physical development and the nature of
destructive processes have changed dramatically through history.

4. Supernatural Intervention
Throughout most of
human history, natural
phenomena have been
interpreted through myths
and legends to be the
results of supernatural
intervention.
• Volcanoes and earthquakes are caused by angry gods.
• Fossils were placed by the devil to confuse man.
Sawkins et al.

5. Photo by W. W. Little
Catastrophism vs.
Uniformitarianism
Early ideas considered geological features, such as
mountain ranges, to have been created through sudden,
catastrophic, events.
Uniformitarianism is the idea that the same physical,
chemical, and biological processes operating on and in
the earth today were also active in the past and that,
therefore, all geological phenomena can be explained as
natural occurrences.

6. Shrinking Earth Theory
• Both the ocean floor and the continents are ancient features and are
fixed in their positions.
• Mountain ranges formed through crustal contraction during gradual
cooling of a molten planetary surface.
• Crustal folding and sea-level changes (uplift/subsidence) are due to
cooling and contraction of the earth's interior, similar to the wrinkling
of dried fruit.

7. Theory of Continental Drift
• The Theory of Continental Drift is an early
predecessor of the Theory of Plate Tectonics
and states that all present continental masses
were once connected as single land unit
called Pangaea. Pangaea subsequently
broke apart with each segment drifting to its
present location.
• The major weakness of the theory was the
lack of a plausible driving mechanism to
explain why the continents had drifted.

8. Abraham Ortelius (1596)
• Belgian cartographer
• Published a world map based on the maps of many others that
became the basis for our modern atlases
• Suggested that the Americas had been "torn away from Europe and
Africa . . . by earthquakes and floods"

9. Francis Bacon (1620)
• British statesman and
philosopher
• Noted that the margins of
Africa and South America
are so similar that it could be
“no mere accidental
occurrence”

10. Alexander Von Humboldt (1800)
• German botanist
• Suggested that
Africa and South
America might at one
time have been joined.

11. Antonio Snider-Pelligrini (1858)
• French
• Considered by many to be the first person to seriously promote a fit
between the continents (In a book titled: Creation and Its Mysteries
Revealed)
• Based on continental fit and backed by fossil evidence from North
America and Europe
• Believed it to have been caused by Noah’s flood (Not plausible
scientifically; therefore, did not receive wide support)

12. Frank B. Taylor (1908)
• American geologist (U.S. Geological Survey)
• Presented several lines of evidence which led to the
first serious consideration of the theory
• Believed that continents were pulled to the earth’s
equator by the gravitational attraction of the Moon
following its capture.

13. Alfred Wegener (1912)
German meteorologist
First person to make an exhaustive examination and
to vigorously promote the concept.
• Published a book titled: The Origin of
Continents and Oceans (1915).
• Evidence included shapes of continents and
similarity of fossils between Brazil and Africa.
• Drew three maps showing breakup of what he
called Pangaea (“all lands”).
• Believed that lower density continental rocks
plowed through higher density oceanic rocks
due to earth’s rotational forces.
Theory rejected by most because of no known way
in which this could have occurred.

14. Evidences for Continental Drift
• Match of continental margins
• Identical fossils on land masses now separated by oceans
• Match of geological structures and rocks across ocean basins
• Matching of out of place, environmentally-sensitive features
Despite strong physical and biological evidence, the theory
of continental drift was rejected by most scientists because
there was no plausible mechanism given by which it could
have worked.

15. Match of Continental Margins
Continental margins on opposing coasts across an ocean basin have
remarkably similar shapes and fit together like the pieces of a jigsaw
puzzle.

16. Similar Patterns
The pieces of a puzzle must have both matching edges and a
matching pattern.

17. Fossil Evidence
When continents are reassembled into Pangaea, we find a remarkable
alignment of fossil remains for organisms of that age. Organisms that
developed later do not follow this pattern.

18. Climatic Belt Evidence
Earth today has climatic belts that vary with latitude. These show a trend of cooling
from the equator to the poles and pattern of a humid equatorial region, followed by
arid low latitudes, moderately-wet mid latitudes, and arid polar regions. When
continents are reassembled into Pangaea, a similar pattern is seen.

19. Glacial Evidence
Present Configuration Pangaea Reconstructed
Ancient glacial deposits are found at the southern end of all southern
hemisphere continents. When Pangaea is reassembled, the boundaries of
these deposits match and a common center of ice flow is manifest.

20. Rock Evidence
Rock units of similar type and
age line up when continents
are reassembled into Pangaea.
This alignment has aided in
the discovery of important
mineral resources across
continental boundaries.

21. Structural Evidence
Structural features, such as
ancient mountain ranges, are
continuous when continents
are reassembled into Pangaea.
Present Configuration
Pangaea Reconstructed

22. Hypothesis Rejected
Despite all of the physical and biological evidence, the theory
was rejected by most of the scientific community.
“Wegener’s hypothesis in general is of the foot-loose type, in that it takes
considerable liberty with our globe, and is less bound by restrictions or tied
down by awkward, ugly facts than most of its rival theories. Its appeal seems
to lie in the fact that it plays a game in which there are few restrictive rules and
no sharply drawn code of conduct (R. T. Chamberlain).”
“Utter damned rot (W. B. Scott, Pres. of the American Phil. Soc.)!”
The reason for the rejection was that there was no conceivable
way to make it work.

23. Theory of Seafloor Spreading
The seafloor is spreading at oceanic ridges and being subducted along
deep sea trenches.

24. Robert Dietz (1962)
Robert Dietz, of the US Coast Guard, proposed the idea of sea-floor
spreading in 1962. He also coined the terms lithosphere and
asthenosphere.

25. Harry Hess (1962)
Harry Hess, of Princeton
University, proposed mantle
convection as the driving
mechanism for sea-floor
spreading.

26. Evidences for Seafloor Spreading
• Oceanic ridge system parallel to continents discovered by sonar
• Volcanism and earthquakes concentrated along ridge and continent margins
• Oceanic crust younger than continental crust
• Oceanic crust increases in age away from ridges
• Sediment thickness increases away from ridges
• Sea mounts become submerged away from ridges
• Oceanic crust mostly basaltic and continental crust mostly granitic
• Paleomagnetic reversals in bands parallel to ridges
• Apparent polar wandering
Seafloor spreading provides the mechanism that makes
continental drift work. Plate tectonics is the combination
of the theories of continental drift and seafloor spreading.

27. Oceanic Ridge System
Through sonar studies, a large
submarine mountain range was
found to run along the center of the
Atlantic Ocean parallel to the
continental margins on either side.
Similar features have subsequently
been located in other ocean basins.

28. Oceanic Ridges & Spreading Rate

29. Oceanic Ridges and Volcanism
As oceanic crust separates and moves apart, a fissure is produced
which becomes filled with basaltic magma, similar to this Hawaiian
lava lake.

31. Ocean Ridges and Earthquakes
Separation of the crust along oceanic ridge systems generates
earthquakes. Because of the thinness of oceanic crust, most of
these quakes are shallow in nature.

32. Drummond Matthews & Fred Vine (1963)
Matthew Drummond
Fred Vine
Drummond Matthews and his graduate student Fred Vine of
Cambridge University hypothesized and later confirmed the presence
of “stripes” on the ocean floor related to Earth’s magnetic reversals.

33. Ocean Ridges and Magnetic Reversals
Surveys of the seafloor have revealed a symmetrical pattern of
alternating normal and reversed magnetic polarity parallel to
oceanic ridges.

34. Earth’s Magnetic Field
Because of convection within the core, the earth has developed a
polarized magnetic field.

35. Paleomagnetism
Earth’s magnetic properties are recorded by some rocks, permanently
recording magnetic conditions at the time of their formation.

36. Magnetic Reversals
For reasons not yet understood, the earth’s magnetic polarity
periodically changes direction.

37. Magnetic Reversal Records
Volcanic rocks record the polarity of the
earth’s magnetic field at the time of their
formation, producing the alternating stripes
now present on the ocean floor.

39. Magnetic Reversal Record and Time
As magnetic reversals are easily preserved in igneous rocks, they can
be dated and show the same age relationships along all oceanic ridge
systems.

40. Age of the Ocean Floor
Rocks of the
ocean floor are
much younger
than those of
the continents.
The seafloor is youngest along oceanic ridges and becomes
progressively older as it approaches continental margins, where it has
a maximum age of about 180 my.

41. Apparent Polar Wandering
Polarity records from continental masses make it appear as though polar positions have
shifted over time and that, until recently, the poles were located at different places for
each of the continents. However, as continents are migrated backward toward Pangaea,
it becomes clear that it is the continents that have moved and the poles, in reality, have
remained stationary.

42. Depth of the Ocean
As the seafloor moves away from oceanic ridges, it cools, increases in
density, and subsides to greater depths. Therefore, the seafloor
becomes progressively deeper away from the ridge.

43. Seamounts
Seamounts are volcanoes located on the seafloor. These often form
along an oceanic ridge and become submerged as they are moved
away from the ridge.

44. Thickness and Age of Sediment
Deep sea sediment is
derived primarily from the
accumulation of animal
skeletons and from wind
blown dust that originated
over continents.
As soon as the lava has erupted onto the surface, it begins to be covered
by sediment. Therefore, the older the lava flow, the thicker the
accumulation of sediment above it and the older the sediment that is in
contact with the lava.

45. Plate Tectonics & the
Scientific Method
Observation: The continents fit together like puzzle pieces
Hypothesis: A single large continent split and drifted apart
Tests: Fossils, mountain ranges, rock types, etc. match; however,
lack of a plausible mechanism led to rejection by most
New observation: Mid-oceanic ridge
Revised hypothesis: The seafloor is spreading.
New tests: Paleomagnetism, age of seafloor rocks, sea mounts, etc.

46. Plate Tectonic Model
The plate tectonic model proposes that new crust is formed at divergent
plate boundaries, where plates move away from each other, and is
consumed along convergent boundaries, where plates move toward one
another. Transform boundaries exist where plates slide laterally past
each other, neither producing nor destroying crustal material.

47. Plate Movement
Plates move at a variety of rates and directions with respect to one
another.

48. Convection
As a material is heated, it expands lowering its density. This causes the
material to rise, where it cools, contracts, increases in density, and
subsequently sinks.

49. Mantle Convection
convection
cell
convection
cell
Rocks are heated in the mantle, causing them to expand. Expansion
lowers density and weakens the rock’s structure. This allows the rock
to flow and rise toward the earth’s surface.

50. Seiya Uyeda (1994)
Seiya Uyeda, a professor at Tokai
University in Japan developed the
concepts of slab pull and ridge
push as mechanisms for plate
movement.

51. Slab Pull/Ridge Push
Coupled with connection, and probably of greater significance in their
continued movement is the idea of slab pull and ridge push, in which
gravity becomes the primary driver of plate motion.

52. Divergent Plate Boundaries
Spreading
Convection
Divergent plate boundaries are locations where the crust is being pulled apart by
spreading convection cells. Most divergent boundaries occur on the sea floor and are
represented by oceanic ridges; however, they can also form on land as continental rift
systems. New crustal material is produced along these boundaries, and they are the
sites of frequent shallow focus earthquakes.

53. Divergent Boundaries & Earthquakes
Earthquakes occur along the entire length of divergent boundaries. Because of the
thinness of oceanic crust and thinning of continental crust, these earthquakes are mostly
shallow in nature.

54. Continental Rift Systems
As continental crust is stretched, it becomes thinner and eventually produces an ocean
basin. Rift systems are characterized by linear mountain ranges, shallow focus
earthquakes, and basaltic volcanism. Over time, the rift system will evolve into an
oceanic ridge.

55. Early Rift Development
Spreading beneath continents pulls the
crust apart, forming linear mountain ranges
and intervening valleys.

56. Basin and Range Structure
Mountains and valleys of continental rift systems are bounded by faults. As the crust is
stretched, it is thinned by dropping valleys down between the mountain ranges.

57. Driggs
Graben Jackson
Hole
Graben
Teton
Horst

58. Basin and Range Province
Spreading
The Basin and Range Province of the western United States is an example of a
continental rift system. This system extends northward from the Gulf of California to
northern Idaho and southern Montana.

60. Continental Rifts and Volcanism
Like oceanic ridges,
continental rift zones
are characterized by
basaltic volcanism.
Volcanoes typically
occur in a line and can
produce flows that
spread in layers over
wide areas.

62. New Ocean Basin
Continental rifting eventually leads to the creation of a new ocean
basin, as the crust thins and becomes composed primarily of basalt and
gabbro.

64. Red Sea Rift
The Red Sea Rift is an example of
a continental rift system that has
evolved to a young ocean basin.

65. Triple Junction

66. From Tarbuck and Lutgens
Oceanic Basins
Mid-Atlantic Ridge
East Pacific Rise
Passive Margin
Rifting eventually leads to the development of a full ocean basin,
including a ridge system running down the center and passive continental
shelves along the margin.

67. Convergent Plate Boundaries
Collision
Convection
Convergent plate boundaries are locations where the crust is being pushed together by
converging convection cells. Most convergent boundaries occur on the sea floor and
are represented by oceanic trenches; however, they can also form on land as continental
suture zones. Crustal material is consumed along most of these boundaries, and they
are sites of frequent shallow, intermediate, and deep focus earthquakes.

69. Folded Mountain Belts
Collision causes plates to shorten and thicken along their margins. The result is a deep
trench produced where the oceanic plate is subducted and an adjacent high mountain
belt that forms along the continental margin.

71. Convergent Boundaries & Earthquakes
Because a subducting plate moves downward beneath the overriding plate,
earthquakes become progressively deeper landward of the trench.

72. Benioff Zones
Progressively deeper earthquakes landward of the deep sea trench support the idea of
subduction and is another evidence in favor of the Theory of Plate Tectonics.

73. Three Types of Convergent Plate
Boundaries
Ocean/Ocean
Ocean/Continent
Continent/Continent

74. Ocean/Continent Convergent Boundaries
As the subducting plate moves back into the mantle, it begins to melt, producing
magma that feeds large composite volcanoes. These volcanoes form in a line parallel
to the plate boundary and have highly explosive eruptions of mostly ash.

75. Mt. St. Helens

78. Ocean/Ocean Convergent Boundaries
As with ocean/continent boundaries, one plate subducts beneath the other and begins
to melt, producing magma that feeds volcanoes. In this case, the volcanoes form an
island chain.

81. The Pacific Plate
subducts beneath the
Asian Plate to form
the Kurile Trench and
Kurile Islands.

82. Continent/Continent Convergent Boundaries
Because both plates have a density that is lower than that of the mantle, neither is
subducted. Instead, continental plates crush one-another, becoming shorter producing
large, highly folded mountain ranges that lack volcanism.

83. Formation of Continent/Continent Boundaries
Continent/continent convergent plate
boundaries are formed as continents
converge from opposite sides of an
ocean basin. Initially, the boundary
will be of ocean/continent character;
however, as the basin closes, the
continents will collide, eliminating the
subduction zone. The collision will
produce a high mountain range that
parallels the boundary. Due to the
lack of a magma source (thick crust
and no subduction), there is no active
volcanism.

84. Collision of India and Asia
The collision of India with Asia is
responsible for the formation of the
Himalayan Mountains, the highest
mountain range on earth. Unlike most
other major mountain ranges, the
Himalayas lack volcanoes.

87. Transform Plate Boundaries
Transform plate boundaries are locations
where plates slip laterally past one another.
They receive their name from the fact that
they connect other types of plates together
and transfer motion between them. While
they are characterized by large numbers of
earthquakes, they experience little or no
volcanic activity.
San Andreas Fault System

89. Offset Streams
Movement on transform faults can offset the path of a stream.

90. San Andreas Fault

91. Transform Fault Basins & Mountains
Bends in transform faults lead to extensional and compressional zones
along the the fault trace, creating basins and mountain ranges,
respectively

92. Divergent/Divergent Transform Boundaries
These boundaries are common on the sea floor where they separate
segments of an oceanic ridge.

94. Divergent/Convergent Transform Boundaries
These boundaries connect
divergent plate boundaries to
convergent boundaries, such
as the San Andreas Fault that
transfers divergent motion
from the Gulf of California
to convergent motion along
the coast of the Pacific
Northwest.

95. Convergent/Convergent Transform Boundaries
This type of boundary connects two convergent plate boundaries,
such as the Alpine Fault that crosses New Zealand.

96. Hotspots
Hotspots occur where plates move over a stationary heat source.
This forms a volcanic chain as one volcano moves off of the heat
source and another develops above it.

98. Mantle Plumes
Hotspots are believed to be caused by mantle plumes that originate
deep within the mantle and rise to the surface.

99. A Problem
How do you reconcile these two models?

100. Hotspot Volcanic Island Chains
As an oceanic plate
moves over a hotspot, it
leaves a trail of volcanic
islands. Only the
volcano directly above
the plume is active, with
other volcanoes of the
chain becoming
progressively older the
further they are from the
hotspot.

101. Hawaiian
Volcanic
Chain

103. Yellowstone Hotspot

104. Ash Falls

105. Isostatic Adjustment
Isostatic adjustment (isostacy) refers to the vertical uplift or subsidence
of the crust in response to changes in thickness. As material is added, the
crust thickens and sinks further into the mantle. As material is removed
by erosion, the crust thins, and material formed at depth rises toward the
surface.

106. Glaciers and Isostacy
Ice bergs are a good
example of isostatic
adjustment, in that the bulk
of the ice is actually
present beneath the sea
surface. As the exposed
portion of the ice melts, the
submerged portion rises
toward the surface.

107. Isostacy and Erosion
One of the significant
aspects of isostacy is that
it brings rocks and
structures formed at depth
to the surface where they
can be observed.