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Chapter 2
Plate Tectonics:
A Unifying Theory
- A unifying theory is one that helps
- explain a broad range of diverse observations
- interpret many aspects of a science
- on a grand scale
- and relate many seemingly unrelated
phenomena
- Plate tectonics is a unifying theory for geology.
Unifying TheoryUnifying Theory
2.Plate Tectonics2.Plate Tectonics
Unifying theoryUnifying theory –– plate tectonics describeplate tectonics describe movement of continental and oceanicmovement of continental and oceanic plates and forces driving them. Itplates and forces driving them. It explains locations of mountain chains,explains locations of mountain chains, earthquakes, rock assemblages, andearthquakes, rock assemblages, and structures on sea floor.structures on sea floor.
For geology Plate Tectonics is similar inFor geology Plate Tectonics is similar in importance as the Discovery of DNA is toimportance as the Discovery of DNA is to biologybiology
- Plate tectonics helps explain - earthquakes - volcanic eruptions - formation of
mountains
continents
basins
Plate TectonicsPlate Tectonics
circulation,
- and climate
- geographic distribution,
- evolution and extinction
of organisms
- distribution and formation
of resources
Introduction
Why should you know about plate tectonics?
Plate tectonics affects all of us, whether in relation to the destruction caused by volcanic eruptions and earthquakes, or politically and economically due to the formation and distribution of valuable natural resources. Plate tectonics tyies together many seemingly unrelated geologic phenomena and illustrating why Earth is a dynamic planet of interacting subsystems and cycles.
2.Plate Tectonics2.Plate Tectonics
Earlier related theories:Earlier related theories:
Over the past 200 years (up to late 1960s)Over the past 200 years (up to late 1960s) geologists had developed many theoriesgeologists had developed many theories about the characteristics & location ofabout the characteristics & location of EarthEarth’’s tectonic features.s tectonic features.
Mountains, volcanoes, earthquakes, etc:Mountains, volcanoes, earthquakes, etc:
UnderstoodUnderstood
- • generally how are they formed/occur,generally how are they formed/occur,
- • (^) but notbut not -- why here, not there, relation towhy here, not there, relation to one and otherone and other
Early Ideas About Continental Drift
Alfred Wegener
and the Continental Drift Hypothesis.
The idea that continents have moved in the past is not new. The concept of continental movement was first suggested when it was noticed that Africa and South America had coastlines which appeared to be counterparts of one another This suggested they may once have been joined and drifted apart. Fig. 2.3, p. 32
- Edward Suess
- noted similarities between
- the Late Paleozoic plant fossils
- Glossopteris flora
Early IdeasEarly Ideas
about Continental Driftabout Continental Drift
glaciation
- in rock sequences of
- India
- Australia
- South Africa
- South America
- He proposed the name Gondwanaland - (or Gondwana ) for a
supercontinent
continents
- Frank Taylor (American, 1910)
- presented a hypothesis of continental drift
with these features:
- lateral movement of continents formed mountain ranges
- a continent broke apart at the Mid-Atlantic Ridge to form the Atlantic Ocean
- supposedly, tidal forces pulled formerly polar continents toward the equator,
- when Earth captured the Moon about 100 million years ago
Early IdeasEarly Ideas
about Continental Driftabout Continental Drift
meteorologist
hypothesis “
continental drift.”
were originally
united into a
supercontinent
named Pangaea.
Alfred WegenerAlfred Wegener and theand the
Continental Drift HypothesisContinental Drift Hypothesis
Early Ideas About Continental Drift
Alfred Wegener
and the Continental Drift Hypothesis.
Pangaea consisted of a northern landmass called Laurasia and a southern landmass called Gondwana.
As Pangaea broke up, the various continents moved to their present-day locations.
Fig. 2.1, p. 31
The Glossopteris fern, also known as the “Pangaea plant”
What is the Evidence for Continental Drift?
A. Continental Fit
Wegener and others amassed a large amount of evidence in support of continental drift.
There is a close fit between the continents off the coast at a depth of about 2000 m.
Fig. 2.3, p. 32
Evidence for Continental Drift
E. Glacial Evidence
Glacial tills and striations on the bedrock beneath the till provide evidence of glaciation at the same time on all the Gondwana continents, with South Africa located at the South Pole.
Fig. 2.5, p. 33^ E. Correlation of Glaciation Fig. 3-3, p. 35
Evidence for Continental DriftEvidence for Continental Drift
Evidence for Continental Drift Evidence for Continental Drift
F. Fossil evidenceF. Fossil evidence
(distribution of(distribution of
certain fossils)certain fossils)
- One of the strongest examples is the Mesosaurus , a fresh water reptile.
Evidence for Continental Drift
F. Fossil Evidence
evidence comes
from fossils like
the Glossopteris
fern.
Evidence for Continental Drift Evidence for Continental Drift
** support of theory by otherssupport of theory by others
- Alexander du Toit (South African
geologist, 1937)
- Proposed that a northern landmass called
Laurasia consisted of present-day
- North America
- Greenland
- Europe
- and Asia (except India).
- Provided additional fossil evidence for
Continental drift
**Evidence for Continental Drift
- Problems with the theory**
Wegener could not provide a convincing mechanism to demonstrate ‘how’ the continents could have moved. His ideas were largely ignored.
Fig. 2.2, p. 31
Evidence for Continental Drift Evidence for Continental Drift
- Problems with the theory* Problems with the theory
- Most geologists did not accept the idea of moving continents - because no one could provide - a suitable mechanism to explain - how continents could move over Earth’s
surface
- Interest in continental drift revived when
- new evidence of Earth’s magnetic field
- and oceanographic research
- showed that the ocean basins were
geologically young
PaleomagnetismPaleomagnetism
* Earth* Earth’’s Magnetic Fields Magnetic Field
dipole magnet
poles
essentially
coincide
geographic
poles
from different
rotation
and mantle
(Paleo (Paleo) Magnetism) Magnetism
Earth’Earth’s Magnetic Fields Magnetic Field • Strength and orientation of the magnetic field varies
- weak and horizontal at the equator
- strong and vertical at the poles
Paleomagnetism Paleomagnetism
and
strength
from the
equator
poles
- Paleomagnetism is
- a remanent magnetism
- in ancient rocks
- recording the direction of Earth’s magnetic
poles
- at the time of the rock’s formation
- Paleomagnetism in rocks
- records the direction
- and strength of Earth’s magnetic field
- When magma cools
- below the Curie point temperature
- magnetic iron-bearing minerals align
- with Earth’s magnetic field
PaleomagnetismPaleomagnetism^ Paleomagnetism & Polar WanderingPaleomagnetism & Polar Wandering
- Magnetic poles apparently moved. - Their trails were called
polar wandering paths.
had different paths.
- In 1950s, research revealed - that paleomagnetism - of ancient rocks
showed
- orientations different
- from the present
magnetic field
- The best explanation
- is stationary poles and
- moving continents
Paleomagnetism and Seafloor Spreading
Earth’s present magnetic
field is considered normal
The existence of such magnetic reversals was discovered in continental lava flows by A) age dating B) determining the orientation of the remnant magnetism.
Fig. 2.10, p. 37 Fig. 2-10, p. 37
Stepped Art
North magnetic pole (reversed)
North magnetic pole (reversed)
South magnetic pole (normal position)
South magnetic pole (normal)
South magnetic pole (reversed)
South magnetic pole (reversed)
North magnetic pole (normal)
North magnetic pole (normal position)
Magnetic Reversals and Seafloor Spreading
Harry Hess proposed the theory of
seafloor spreading in 1962.
He suggested that the seafloor separates at oceanic ridges, where new crust is formed by upwelling magma. As the magma cools, the newly formed oceanic crust moves laterally away from the ridge. Fig. 2.11, p. 38
Continental Drift, Seafloor Spreading, Continental Drift, Seafloor Spreading,
and Plate Tectonicsand Plate Tectonics
Early explanation of motion/driftEarly explanation of motion/drift
Wegener (1915) suggested continentsWegener (1915) suggested continents
drifted (floated) on oceanic crust due todrifted (floated) on oceanic crust due to
sun/moon tidal forces (too weak) and hesun/moon tidal forces (too weak) and he
was meteorologist so mostly ignoredwas meteorologist so mostly ignored
Sea floor spreadingSea floor spreading (1928, Arthur Homes,(1928, Arthur Homes,
British) first to speculate that convectionBritish) first to speculate that convection
currents within mantle moved continentscurrents within mantle moved continents
apart (little evidence or details)apart (little evidence or details)
Seafloor Spreading, ContinentalSeafloor Spreading, Continental
Drift and Plate TectonicsDrift and Plate Tectonics
PostPost--WWIIWWII
(1950s)(1950s)
- new techniques help map seafloor
- • geological activitygeological activity in mid-in mid-oceanocean ridgesridges
- • ring of firering of fire
- • seafloor mapsseafloor maps
- Ocean mapping revealed
- a ridge system
- 65,000 km long,
- the most extensive mountain range in the
world
- The Mid-Atlantic Ridge
- is the best known
- and divides Atlantic Ocean basin
- in two nearly equal parts
Seafloor Spreading &Seafloor Spreading &
Mapping Ocean BasinsMapping Ocean Basins
Atlantic Ocean Basin Atlantic Ocean Basin
Mid-Atlantic Ridge
Seafloor SpreadingSeafloor Spreading
Seafloor SpreadingSeafloor Spreading
Theories on Seafloor spreadingTheories on Seafloor spreading
No accepted theory until 60’No accepted theory until 60’s.s.
Harry Hess, submariner & professor, Princeton,Harry Hess, submariner & professor, Princeton,
showed (post-showed (post-WWII) sonar & other geophysicalWWII) sonar & other geophysical
maps of sea floor (early ‘maps of sea floor (early‘60s).60s).
Plate Tectonics TheoriesPlate Tectonics Theories (1960(1960--68)68)
Mid-Atlantic sea floor ridge formed by upwelling
crust due plate divergent motion (1962, H.
Hess, Princeton Univ. & Bob Dietz, Scripps)
Seafloor Spreading orSeafloor Spreading or Plate Tectonics?Plate Tectonics?
Seafloor SpreadingSeafloor Spreading
Seafloor Seafloor
spreadingspreading
new crust new crust formed dueformed due expandingexpanding
earth?earth?
Seafloor Spreading orSeafloor Spreading or Plate TectonicsPlate Tectonics
Plate Tectonics Theories (1960Plate Tectonics Theories (1960--68)68)
1. Mid-Atlantic sea floor ridge formed by
upwelling crust due plate divergent motion
(1962, H. Hess, Princeton Univ. & Bob
Dietz, Scripps) – possible expanding earth
2. Ring of Fire - mapping & correlation of
volcanoes & (more important) the deep sea
trenches along Pacific margins (1962-64)
- In addition to mid-ocean ridges,
- ocean research revealed
- magnetic anomalies on the sea floor
- A magnetic anomaly is a deviation
- from the average strength
- of Earth’s Magnetic field
Confirmation of Hess’Confirmation of Hess’ss
HypothesisHypothesis
- The magnetic anomalies were discovered
to be
Confirmation of Hess’Confirmation of Hess’ss
HypothesisHypothesis
striped, parallel to the oceanic ridges
and symmetrical with the ridges
How Do Magnetic ReversalsHow Do Magnetic Reversals
Relate to Seafloor Spreading?Relate to Seafloor Spreading?
Magnetic Reversals and Seafloor Spreading
Deep-Sea Drilling and the Confirmation of
Seafloor Spreading
Sea floor spreading is confirmed by the ages of fossils in sediments overlying oceanic crust radiometric dating of rocks on oceanic islands. These indicate that oceanic crust is youngest at the spreading ridges and oldest at the farthest points from the ridges.
Fig. 2.13, p. 40
- Seafloor spreading theory indicates that
- oceanic crust is geologically young because
- it forms during spreading
- and is destroyed during subduction
- Radiometric dating confirms the youth
- of the oceanic crust
- and reveals that the youngest oceanic crust
- occurs at mid-ocean ridges
- and the oldest oceanic crust
- is less than 180 million years old
- whereas oldest continental crust
Oceanic Crust Is YoungOceanic Crust Is Young^
Magnetic Reversals and Seafloor Spreading
Deep-Sea Drilling and the Confirmation of
Seafloor Spreading
Further evidence confirming seafloor spreading came from the Deep Sea Drilling Project are the age and thickness of the sediments overlying the oceanic crust.
Fig. 2.14, p. 41
Plate Tectonics: A Unifying Theory
Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early 1970's.
The theory is widely accepted because it explains so many geologic phenomena, including volcanism, seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the distributions of natural resources.
For these reasons, it is known as a unifying
theory.
• Plate tectonic theory is based on the
simple model that
- the lithosphere is rigid & consists of both
- oceanic crust with upper mantle
- continental crust with upper mantle
- variable-sized pieces called plates
- that move as a unit
- that can be continental, oceanic or both
- some regions containing continental crust
- whereas regions containing oceanic crust
- are up to 100 km thick max
Plate TectonicsPlate Tectonics
Plate Tectonics Map Plate Tectonics Map
Numbers represent average rates of relative movement, cm/yr
Plate Tectonics MapPlate Tectonics Map
Plate TectonicsPlate Tectonics
Mosaic of PlatesMosaic of Plates
1.1. 12 major plates with numerous smaller plates12 major plates with numerous smaller plates
with three main boundarieswith three main boundaries
2.2. Divergent boundaries: where plates movedDivergent boundaries: where plates moved
apart and new lithosphere is createdapart and new lithosphere is created
3.3.^ Convergent boundaries: where plates comeConvergent boundaries: where plates come
together and plates are recycledtogether and plates are recycled
4.4.^ Transform boundaries: plates slide past eachTransform boundaries: plates slide past each
otherother
Plate Tectonics: A Unifying Theory
The lithosphere overlies the asthenosphere, and through some type of heat-transfer system within the asthenosphere, moves the plates.
As the plates move over the asthenosphere, they separate mostly at oceanic ridges and collide and are subducted into Earth’s interior at oceanic trenches.
Fig. 14 p. 41
- Divergent boundaries are also present
- under continents during the early stages
- of continental breakup
Divergent BoundariesDivergent Boundaries
- Beneath a continent - when
magma wells
up
initially
- elevated,
- stretched
- and thinned
- The stretching produces fractures and rift valleys.
Rift ValleyRift Valley
- During this stage, - magma typically - intrudes into the
fractures
the valley floor
- Example: East African rift valleys
Narrow SeaNarrow Sea
- As spreading proceeds, some rift valleys
- will continue to lengthen and deepen until
crust eventually
breaks
sea is formed,
continental
blocks
- Examples:
- Red Sea
- Gulf of California
Modern DivergenceModern Divergence
View looking down the Great
Rift Valley of Africa.
Little Magadi soda lake
The Three Types of Plate Boundaries
Divergent Boundaries
An Example of Ancient Rifting
Characteristic features of ancient continental rifting include faulting, dikes, sills, lava flows, and thick sedimentary sequences within rift valleys. Pillow lavas and associated deep-sea sediments are evidence of ancient spreading ridges.
Fig. 2.17, p.
OceanOcean
- As a newly created narrow sea
- continues to spread,
- it may eventually become
- an expansive ocean basin
Atlantic Ocean
basin is today,
- separating North and South America
- from Europe and Africa
- by thousands of kilometers
Atlantic Ocean Basin Atlantic Ocean Basin
Europe
Africa
North America
South America
Atlantic Ocean basin
Thousands
kilometersof
An Example of Ancient RiftingAn Example of Ancient Rifting
- What features in the rock record
- can geologists use to recognize ancient rifting?
- faults
- dikes
- sills
- lava flows
- thick sedimentary sequences
- within rift valleys
- Example:
- Triassic age fault basins in eastern US
Ancient RiftingAncient Rifting
Palisades of Hudson River
sill
- These Triassic fault basins - mark the zone of rifting - between North America
and Africa
thousands of
meters of
continental
sediment
with dikes and
sills
The Three Types of Plate Boundaries
Convergent Boundaries are places where
two plates collide
There are three types of convergent boundaries.
An oceanic-oceanic boundary is where two oceanic plates collide, one ocean plate will subduct beneath the margin of the other plate.
An oceanic-continental boundary is where an oceanic plate and a continental plate collide, the oceanic plate will subduct.
A continental-continental boundary occurs when two continents collide
The Three Types of Plate Boundaries
Convergent Boundaries
Oceanic-Oceanic Boundaries
One oceanic plate is subducted beneath the other and a volcanic island arc forms on the non-subducted plate An oceanic trench forms parallel to the volcanic island arc where the subduction is taking place. The volcanoes result from rising magma produced by the partial melting of the subducting plate. (^) Fig. 2.18a, p. 47
The Three Types of Plate Boundaries
Convergent Boundaries
Oceanic-Continental Boundaries
An oceanic plate and a continental plate converge, with the denser oceanic plate being subducted under the continental plate. Just as with an oceanic-oceanic boundary, a chain of volcanoes forms on the nonsubducted plate. (^) Fig. 2.18b, p. 47
Plate Movement and Motion
Determining rate and direction of plate
movement
Hot spots enable geologists to determine absolute motion because they provide an apparently fixed reference point from which the rate and direction of plate movement can be measured.
Fig. 2.23 a-b, p. 51
Plate Movement and Motion
Determining rate & direction of plate movement
Fig. 2.23a, p. 51
The average rate of plate movement is most commonly determined by dividing the distance from an oceanic ridge axis to any magnetic anomaly in the crust of the seafloor by the age of that anomaly. Satellite-laser ranging techniques are also used to determine the rate of movement and relative motion of one plate with respect to another.
The Driving Mechanism of Plate Tectonics
What drives the plates?
Most geologists agree that some type of convective heat system is the basic process responsible for plate motion.
Fig. 2.25, p. 52
The Driving Mechanism of Plate Tectonics
How do thermal convection cells move plates?
Two models involving thermal convection cells have been proposed to explain plate movement. 1. thermal cells are restricted to the asthenosphere 2. the entire mantle is involved.
Problems with both models involve the source of heat for the convection cells and how heat is transferred from the outer core to the mantle. (^) Fig. 2.25, p. 52
The Driving Mechanism of
Plate Tectonics
Gravity driven plate motion
Some geologists think a gravity-driven mechanism also plays a major role.
“Slab-pull” involves pulling the plate behind a subducting cold slab of lithosphere “Ridge-push” involves gravity pushing the oceanic lithosphere away from the higher spreading ridges and toward the subduction trenches (^) Fig. 2.26, p. 53
The Driving Mechanism of Plate Tectonics
The Supercontinent Cycle (Wilson cycle)
In the early 1970s J. Tuzo Wilson put forth the hypothesis of a large-scale global cycle of supercontinents. Supercontinents like Pangea form, break up, and re-form in a cycle spanning approximately 500 million years. The breakup forms rift valleys within the supercontinent that eventually becomes a long, linear ocean basins as the crust is depressed below sea level. As the width of the narrow sea continues to expand an open ocean develops. As the ocean basins close, another supercontinent forms.
Plate Tectonics and the Distribution of
Natural Resources
Mineral Deposits
Many metallic mineral deposits are related to igneous and associated hydrothermal activity, so it is not surprising that a close relationship exists between plate boundaries and the occurrence of these valuable deposits. Many of the world’s major metallic ore deposits are associated with convergent and divergent plate boundaries. Copper, iron, lead , zinc, gold and silver ore deposits are associated with plate boundaries. Fig. 2.27, p. 54
Plate Tectonics and the Distribution of Life
Fossil evidence provided one of the first proofs for plate tectonics. Together, plate tectonics and evolution have changed the way we view our planet.
The world’s plants and animals occupy biotic provinces controlled mostly by: Climate Geographic barriers
The location of these provinces is mostly controlled by plate movement.
Fig. 2.28, p. 56
End of
Chapter 2
