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12. Plate Tectonics and Continental Drift
I. Early Ideas
A. Sir Franis Bacon in 1620 mentioned the ideas that the two continents of Africa and South
America had similar shorelines but never made the connection
B. In 1858 Antonia Snider-Pelegrini used similarities of plant fossils in the coal beds of Europe and
North America to link the continents
1. He attributed the separation to the deluge of the Bible
2. Said that this split the two continents
C. An Australian geologist proposed the concept of Gondwanaland
1. Based on Paleozoic plant fossils similarities of India, Australia, Africa, Antarctica, and
South America
2. Also used evidence of glaciation in the southern continents
3. Gondwana is a province in India that has evidence of extensive glaciation and abundant
fossils
4. He thought that this was a consequence of rise and fall of land masses and the
connection of a land bridge that had sank
D. The American geologist Frank Taylor published his ideas of continental drift
1. Explained the formation of mountain ranges as the lateral movement of continent
2. He also saw the present continents as a part of a larger continent that had moved from
the north pole to the equator
3. Thought that the continents had broke apart due to gigantic tidal forces produced when
the earth captured the moon 100 million years ago
4. He did mention that the mid-Atlantic Ridge may have been the place where the
continents had broke apart
E. Alfred Wegener is generally credited with developing the hypothesis of continental drift in
1915
1. He proposed that all the continents were in a giant land mass called Pangea
2. He envisioned that continents as moving around and eventually ending in their present
position
3. He amassed a lot of information to uphold his ideas but was not generally accepted
4. Most of the problem was the mechanism was not well understood- how could granitic
continents move through denser oceanic rock
F. Alexander du Toit in 1937 further developed his ideas
1. Used the coal deposits of the same age in the Northern continents to correlate land mass
2. He named this land mass Laurasia
G. Geologist continued to refuse to accept the idea until the 1960's when oceanographic research
provided convincing evidence about seafloor spreading
II. The Evidence
A. Continental Fit
1. Many researchers have studied the continental fit and tried to imagine how they would
align
2. There are very compelling indications that the continents have coastlines that fit together
3. Critics have pointed out that the coastlines change by erosion and deposition and may
not be as close fit as it seems
4. So the coastlines would not necessarily fit exactly
5. The true continental margin is beneath the continental slope where the continental crust
changes to oceanic crust
6. Best fit occurs beneath the continental slope at about 2000m depth and it fits well there
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B. Similarities of Rocks and Mountain Ranges
1. If the continents fit together, then the mountain ranges of the same age in the adjoining
continents should closely match
2. This is true for the Gondwana continents
3. Marine, nonmarine, and glacial evidence of Pennsylvania to Jurassic age are almost
identical for all five continents
4. The mountain range of the Appalachians seems to range from North America into
Canada and terminate at the Newfoundland coast
5. Mountain ranges of the same age and deformational style occur in Greenland, Ireland,
Great Britain, and Norway
6. These mountains essentially form a single continuous mountain range when the
continents are refit together into the ancient continent of Pangea
C. Glacial Evidence
1. During Late Paleozoic era large glaciers covered area of the southern hemisphere
2. Evidence includes glacial till deposits and glacial striations in bedrock
3. Fossil evidence in the northern hemisphere indicates that it was in tropical environment
at the same time
4. Mapping evidence in Australia, India and South America indicate that the glaciers seem
to have moved from the ocean inland
a) Glaciers do not move this way and this has puzzled geologists for years
b) Large continental glaciers move outward from inland highlands to the ocean
c) If the continents are fit back together the glaciers would move from a central area
in South Africa outward
d) This also places the northern continents in an equatorial position
D. Fossil Evidence
1. This provides some of the most compelling evidence for continental movement
2. Glossopteris flora are found in Pennsylvanian and Permian coal deposits on all five
Gondwanaland continents
a) Glossopteris is a distinctive seed fern that is restricted to certain environments,
cannot be spread by wind, and may not be able to float across the Atlantic Ocean
b) The climates of these continents are too diverse today to support Glossopteris
flora and they must have been connected at that time
3. Animal fossils provide evidence by looking at Mesosaurus
a) Found in certain rocks of Permian age in Brazil and South Africa
b) These animals lived in freshwater environments and would be unable to cross a
salt water ocean to populate two continents
c) They probably lived in freshwater lakes in the two continents when they were
connected
d) Lystrosaurus is a land dwelling reptile that is found in Triassic rocks in separate
continents of Gondwanaland
e) It would be unlikely to have crossed an ocean to populate different continents
E. Paleomagnetism
1. Provided some of the most compelling evidence for plate tectonics
2. The ocean floor was found to be relatively young and the poles had moved in the past
3. These studies discovered that older rocks showed the north pole in a different place as
the younger rocks as well as the present rocks
4. When plotted on a map the lava flows from all ages in North America trace the apparent
movement of the pole over time
5. Furthermore paleomagnetic studies on other continents traced different paths
6. This could mean that there were different magnetic poles, but is highly unlikely
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7. The best explanation is that the magnetic poles have stayed at the same place and the
continents have moved around the globe
8. When the paleomagnetic data are fit to one pole, the glacial deposits match, and the
fossil evidence aligns, they all support each other
III. Seafloor Spreading
A. Interest in mapping the ocean basins increased in 1960's revealing an oceanic ridge system some
65,00 km long, making this ridge the longest mountain chain in the world
B. The mountain range in the Atlantic Ocean nearly divided the Atlantic Ocean basin into two
equal parts
C. Harry Hess in 1962 proposed the theory of seafloor spreading to account for the movement
1. He proposed that the continents do not move across oceanic crust but that they move
together
2. He used the studies of guyots in his oceanographic research to explain how the crust is
formed at the ridges and moves laterally away from the ridge
3. He also proposed a mechanism that Arthur Holms developed in 1930's of thermal
convection cells
4. This consisted of thermal upwelling at the ridges and subduction at the deep sea trenches
D. To test this hypothesis paleomagnetic readings should be recorded in the new crust forming at
the ridges, with the youngest rocks near the spreading center
1. Also magnetic reversals will be recorded in the rocks spreading apart
2. Unusual patterns were found off the coast of North America
3. These were roughly north-south parallel strips of cut and offset by east-west fractures
4. This pattern was eventually found throughout the ocean basins at spreading ridges
E. Studies did find that the youngest rocks were nearest the spreading ridges
F. The oldest oceanic rocks found were only 180 million years while continental crust was 4
billion years old
G. This confirmed that the ocean basins were young features and were responsible for the
movement of the continents
H. Deep sea drilling projects and seismic profiles have confirmed much of the ideas of seafloor
spreading
1. Also fossil sediments on the ocean floor confirm this too
2. Sediments that accumulate on the ocean floor are at most a few hundred meters thick
3. Thinnest at the mid ocean ridges as would be expected because these are the youngest
rocks with little time to accumulate sediment
4. Thickens with distance from the ridges
IV. Plate Tectonic Theory
A. Based on a simple model of the Earth
1. The rigid lithosphere consisting of the oceanic and continental crust and the underlying
upper mantle consists of numerous plates of differing sizes
2. The plates vary in thickness
a) With the upper mantle and continental crust as much as 250 km
b) The upper mantle and oceanic crust up to 100 km thick
3. The lithosphere overlies the hotter and weaker asthenosphere
4. The movement is a result of heat transfer within the asthenosphere that causes the plates
to move
5. As plates move they form and separate at the ridges, and collide at trenches where they
are subducted back into the mantle
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V. Plate Boundaries
A. Interaction of plate boundaries accounts for most of the action on the Earth
1. Earthquakes
2. Volcanoes
3. Mountain Building
B. Divergent Boundaries
1. Where plates are separating and new oceanic crust is being formed
2. This is where crust is thinned, fractured and extended and new magma is being forced to
the surface
3. This magma is mostly basaltic, derived from partial melting of the mantle, and intrudes
into the overlying crust as sheeted dikes or lava flows
4. Typically occur at ocean ridges or spreading centers and are characterized by
a) Rugged topography and high relief
b) Shallow focus earthquakes
c) High heat flow
d) Basaltic flows and pillow lavas
5. Divergent boundaries also occur in the continental areas
a) Magma intrudes beneath the crust creating a general rise in topography
b) It is fractured, stretched, thinned, producing elongated rift valleys
c) These valleys are places where basaltic lava flows, and dikes and sills form
beneath the surface, associated with shallow earthquakes
d) If allowed to proceed, the rift valley will continue to separate and form a shallow
sea like the Red Sea in the Arabian Peninsula
e) The Gulf of California is another example of a rift valley although it is unique(1) Large transform faults are present
(2) Rifting is small segments of spreading ridge
(3) Movement is north-south not east-west
f) Divergent boundaries are recognized in Eastern US where the continent was split
apart and formed rift valleys
(1) Palisades sill in New Jersey is an example of a sill that intruded the
continental crust
(2) Other lava flows, dikes, fault blocked basins where thousands of meters
of sediment accumulated
C. Convergent boundaries are where plates collide
1. Complicated geologic processes including
a) Igneous activity
b) Metamorphism
c) Deep focus earthquakes
d) Crustal deformation
e) Mountain building
2. The specific process depends on the type of crust involved in the collision
3. Oceanic-continental collision
a) If one plate is oceanic and the other continental, the denser oceanic plate will be
thrust beneath the other and subduction will occur
b) The continental plate will experience compression resulting in folding and
mountain building
c) Metamorphism will occur in the deep mountain roots
4. Oceanic-oceanic boundary
a) When two oceanic plate collide one is subducted beneath the other
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b) A subduction complex forms
(1) Slices of folded and faulted oceanic rock and oceanic sediments form
wedge shaped slices at the inner wall of the overriding plate
(2) As the oceanic plate is subducted it heats and melts forming andesitic
magma that rises through the crust
(3) This lighter magma rises above the overriding plate and forms volcanoes
called a volcanic island arc
(4) This island arc forms because a flat plane intersects a sphere forming an
arc
(5) A trench forms several hundred kilometers offshore, depending on the
angle and speed of the subducting slab
(6) Aleutian Islands, Japanese Islands
5. Continental-continental collision
a) Starts out as a continent riding on a subducting oceanic slab
b) As the continents approach and collide they cannot be subducted like the oceanic
plate because the continental crust is lighter than oceanic lithosphere
c) They will collide and be welded or sutured together
d) This forms an interior mountain belt formed of
(1) metamorphic rock,
(2) deformed sediments,
(3) igneous intrusions, and
(4) fragments of oceanic crust
e) Or one may slide under the other, doubling the crustal thickness and forming
large mountain ranges like the Himalayas
f) The Himalayas are the youngest, highest mountain range on Earth and resulted
from a collision between India on the Australian plate and Asia
D. Transform boundaries
1. These form along fractures in the oceanic plate and separate or offset spreading ridges
2. They transform or change the type of motion along a ridge or subduction zone
3. They create a plate boundary when the transform fault is parallel to plate motion
4. Does not create or destroy plates but does form an intensely fractured and sheared zone
5. Most are in oceanic plates but they can extend into continental crust
a) California is a perfect example along the San Andreas Fault
b) Connects the spreading ridge in the Gulf of Cal.- the Cocos Plate off Mexico and
the spreading ridge off the coast of Oregon and Washington- the Juan de Fuca
c) Most of the earthquakes in California are a result of this boundary
d) Creates large displacements in rock types but does not make diagnostic features
like the other plate boundaries
E. Ancient convergent boundaries create a mixture of complicated rocks called a melange
1. These are common on the West Coast of North America
2. Ophiolites are slices of oceanic crust that are caught up in this melange
3. They are formed of oceanic crust overlain by deep sea sediments
a) Oceanic crust is composed of pillow lavas, dike complexes, massive gabbro,
layered gabbro, and peridotite
b) Deep sea sediments are made of graywackes, black shales, and chert
4. Where ever these sequences are found indicate that there was convergence at some time.
5. These are found in California, Oregon, the Andes, the Himalayas, and even the
Appalachians
VI. Plate Motion
A. The geometry of a curved plate on a sphere was worked out hundreds of years ago
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VII.
B. It rotates around an axis of rotation with one point being the pole of rotation
1. No relation to the earth's rotation pole
2. No relation to the Earth's magnetic pole
3. Different parts of the plate move at different velocities
a) Maximum velocity occurs at the equator
b) Minimum velocity at the pole
c) Transform faults are parallel to the latitude relative to motion
d) Therefore the orientation of the transform faults help locate the pole of rotation
4. Spreading ridges are oriented perpendicular to the plate motion
5. The pole does not have to be on the plate itself
C. The direction of movement relative to the neighboring plate can be determined by
1. Seismic data
2. Relative ages of different regions of the seafloor
3. Ages of chains of volcanoes and seamounts
4. Hot Spots can help determine the direction as they seem to be stationary
5. Hot spots like Hawaiian Island and Yellowstone help determine plate motion
D. Rates of movement can be determined by looking at the magnetic reversals on the seafloor
1. These reversals are irregular varying between 20,000 years to 10 million years
2. By finding the ages of these rocks and the distance from the spreading ridge the velocity
of the plate at that point can be determined
3. Lasers bounced off satellites onto neighboring plates
4. The plates are moving at different rates
a) Fastest moving plates are those that have a large part of them being subducted
b) Slowest moving plates are those that don't have subducting boundaries or have a
large tract of continental block
c) This has been used as evidence that the plates are part of a large convection
system and that plate motion is largely a result of cold, dense slab being subducted
into the mantle
The Driving Mechanism of Plate Tectonics
A. Ultimately, the energy that drives the plate motion is heat transported out of the hot core and
mantle to the earth's surface
1. Plate motion is a type of convection and is a result of the earth cooling and trying to
reach equilibrium with cold space
2. This convection energy release from the interior combined with gravity translates the
motion into horizontal movement
B. The first model developed was that convection cells in the mantle cause the plate motion
1. Rising limbs of the cells caused the crust to split at the spreading ridges
2. The convecting mantle would cause the plate to move to the subduction zone
3. The descending portion of the cell would mark the trench and drag the slab downward
C. Another model considers the plates to be the active participants of the convection system
1. The lithosphere is the cold upper layer of the model
2. Because the oceanic plate is denser it sinks into the mantle
3. The plates essentially move independent of the convection system
D. The forces that effect plate movement
1. Slab-pull- Pull on the plate as the slab descends under its own weight
2. Ridge-push- Gravity make the lithosphere slip off the elevated ridge
3. Basal drag- Resistance to flow at the bottom of plate with the asthenosphere
4. Mantle resistance- resistance to movement of the slab into the mantle
5. Friction- resistance along transform faults and in subduction zones
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E. Most researchers hypothesize that the forces that drive the plates are approximately equal to the
forces that resist the movement
F. Since the plates with the largest subducting plate boundaries have the highest rates of movement
the conclusion points to the slab-pull as being the major driving force
G. Slab-pull is also aided by the metamorphic phase transition of minerals from low density to
high density
1. This density change occurs inside the mantle where pressures increase with depth
2. Creates a sinker that pulls the slab deeper into the mantle
H. Ridge-push is believed to be less important but significant
1. The hot asthenosphere acts as a slippery layer under the crust
2. The lithosphere simply slides downhill from the raised ridges
I. These observations lead to the question, what slows the plates down?
1. Major retarding force is probably the resistance in the mantle to slab movement
2. Friction at plate boundaries must slow movement
3. Basal drag, once thought to be the primary motivator, may actually slow the movement
of the plates
J. Basically you can think of this as a single, though complex, system with each plate effecting the
others
1. Convective flow may have a different aspect that the pattern of plate movement
2. Plate movement may have an influential effect on convective flow
3. Basically plate motion is determined by the movement of the fast plates with the others
moving in response to them