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Chapter 5 Lecture Outline
Plate Tectonics:
A Scientific
Revolution Unfolds
From Continental Drift to Plate Tectonics
• Before 1960 most geologists saw the positions of
ocean basins and continents as fixed
• A new model of tectonic processes developed
– A scientific revolution
– Tectonics – study of large-scale deformation and
structures in the outer portion of the Earth.
– Tectonic processes deform crust and create major
structural features
– Plate Tectonics Theory relates such deformation to the
existence and movement of rigid “plates” over a weak or
partly molten layer in the earth’s upper mantle
– It became the unifying theory of geology
Continental Drift: An Idea Before Its Time
• In 1915, Alfred Wegener outlined the hypothesis
of continental drift
– Published :The Origin of Continents and Oceans
– Single supercontinent of all of Earth’s land: Pangaea
– Fragmented ~200 mya and smaller landmasses drifted to
their present positions
However, Wegener was not the first to suspect
the continents were on the move.
• World maps in the 1600s suggested that South
America and Africa fit together
• 1596 – Dutch map maker Abraham Ortelius
– Thesaurus Geographicus – Americas were,
“torn away from Europe and Africa…by
earthquakes and floods…the vestiges of the
rupture reveal themselves, if someone
brings forward a map of the world and
considers carefully the coasts of the three
(continents).”
Antonio Snider-Pelligrini – 1858
-geographer
Continental Drift: An Idea Before Its Time
• Geographic evidence
• Similarity between coastlines on opposite sides of
the Atlantic
• Opponents argued that coastlines are modified
through time by erosion and deposition
• Continental shelf is a better approximation of the
boundary of a continent
Continental Drift: An Idea Before Its Time
• Fossil Evidence
• Identical fossils found in South America and
Africa
• Paleontologists agreed: land connection
necessary to explain fossil record
Continental Drift: An Idea Before Its Time
• Mesosaurus
– Small Permian aquatic freshwater reptile
– Found in eastern S. America and western Africa
• Glossopteris
– Seed fern
– Africa, Australia, India, S. America, and Antarctica
• Opponents explain fossil patterns by rafting, oceanic
land bridges, and island stepping stones
Much Fossil Evidence
Continental Drift: An Idea Before Its Time
• Geologic Feature Evidence
• Rock types and mountain features match up
– 2.2 billion-year-old igneous rocks in Brazil and Africa
– Mountain belts end at coastlines and reappear across
oceans
Continental Drift: An Idea Before Its Time
• Glacial Evidence
• Evidence for glaciation on continents now at
tropical latitudes
– Can be explained by
supercontinent located
near the South Pole
Paleoclimate evidence
Fossil/Rock Sequences
The Great Debate
• Wegener’s hypothesis of continental drift was met
with criticism
• Objections were based on lack of mechanism for
continental drift
– Wegener proposed that tidal forces of the Moon moved
continents and that sturdy continents broke through thin
oceanic crust
– Wegener incorrectly suggested that continents broke
through the ocean crust, much like ice breakers plowed
through ice
– Strong opposition to the hypothesis from the scientific
community
• The Fatal Flaw!
– Geophysicists found it in defiance of the laws of physics
– Sir Harold Jefferies – “what force could cause the continents
to plow though the oceanic crust?”
The Theory of Plate Tectonics
• Idea lay dormant for almost 50 yrs
• Technology out of WWII led to rebirth of the study
of the ocean basins
• Oceanographic exploration increased
dramatically following World War II
– Discovery of global oceanic ridge system
– Earthquakes at great depths in western Pacific ocean
trenches
– No oceanic crust older than 180 million years
– Thin sediment accumulations in deep-ocean basins
A New View!
• Harry Hess -
Professor of Geology, Princeton
– During WWII, did echo soundings of Pacific between battles!
» Paper – “History of Ocean Basins” – 1962
» Resulted in the hypothesis of
Seafloor Spreading
What Problems with Wegener’s
Model Did Seafloor Spreading
explain?
Continental crust was riding
along on a spreading oceanic
crust
The Theory of Plate Tectonics
• Lithosphere is the crust and uppermost (coolest)
mantle
– Oceanic lithosphere varies in thickness
• Thin at ridges, up to 100 km thick
• Mafic composition
• More dense than continental lithosphere
– Continental lithosphere 150–200 km thick
• Felsic composition
– Responds to forces by bending or breaking
The Theory of Plate Tectonics
• Asthenosphere is the hotter, weaker mantle
below the lithosphere
– Rocks are nearly melted at this temperature and pressure
– Responds to forces by flowing
– Moves independently from lithosphere
The Theory of Plate Tectonics
• Lithosphere is broken into irregular plates
– Plates move as rigid units relative to other plates
– 7 major plates make up 94% of Earth
The Theory of Plate Tectonics
• Interaction between plates at plate boundaries
– Divergent boundaries (constructive margins)
• Two plates move apart
• Upwelling of hot material from mantle creates new seafloor
– Convergent boundaries (destructive margins)
• Two plates move together
• Oceanic lithosphere descends and is reabsorbed into mantle
• Two continental blocks create a mountain belt
– Transform plate boundaries (conservative margins)
• Two plates slide past each other
• No lithosphere is created or destroyed
The Theory of Plate Tectonics
Divergent Plate Boundaries and Seafloor
Spreading
• Most divergent plate boundaries are along
the crests of oceanic ridges
• New ocean floor is generated when mantle
fills narrow fractures in oceanic crust
Divergent Plate Boundaries and Seafloor
Spreading
Divergent Plate Boundaries and Seafloor
Spreading
• Most divergent plate boundaries are associated
with oceanic ridges
– Elevated seafloor with high heat flow and
volcanism
– Longest topographic feature on Earth’s surface
(covers 20% of surface)
– Crest is 2 to 3 km higher than adjacent basin and
can be 1000 to 4000 km wide
• Rift valley is a deep canyon along the crest of a
ridge resulting from tensional forces
Divergent Plate Boundaries and Seafloor
Spreading
Divergent Plate Boundaries and Seafloor
Spreading
• Seafloor spreading is the process by which
new seafloor is created along the ocean ridge
system
– Average spreading rate is ~5 cm/year
• Up to 15 cm/year or as slow as 2 cm/year
– New lithosphere is hot (less dense) but cools
and subsides with age and distance from the
ridge system
Divergent Plate Boundaries and Seafloor
Spreading
• Continental rifting occurs when divergent
boundaries develop within a continent
– Tensional forces stretch and thin the lithosphere
– Brittle crust breaks into large blocks
– Eventually become ocean basins
Divergent Plate Boundaries and Seafloor
Spreading
Divergent Plate Boundaries and Seafloor
Spreading
Convergent Plate Boundaries and
Subduction
• Convergent plate boundaries occur when two
plates move toward each other
– Convergence rate is equal to seafloor spreading
– Characteristics vary depending on subducting crust
– Subduction zones
• Lithosphere descends into the mantle
– Old oceanic crust is ~2% denser than asthenosphere
– Continental crust less dense than asthenosphere
Convergent Plate Boundaries and
Subduction
• Deep ocean trenches
– Long, linear depressions
– Result of subduction
• Angle of subduction varies
– Nearly flat to nearly vertical
– Depends on density of crust
• Older crust is cooler and denser
Convergent Plate Boundaries and
Subduction
• Characteristics of convergent plate boundaries
vary depending on type of crust being subducted
– Oceanic + continental
– Oceanic + oceanic
– Continental + continental
Convergent Plate Boundaries and
Subduction
• Oceanic – Continental Convergence
– subduction of oceanic lithosphere
– Continental lithosphere is less dense
– Water from descending oceanic crust triggers
partial melting of asthenosphere at ~100 km
– Molten material is less dense and rises
• Continental volcanic arcs
Convergent Plate Boundaries and
Subduction
Convergent Plate Boundaries and
Subduction
• Oceanic-Oceanic Convergence
– One slab subducts under another
– Volcanism because of partial melting
– Generates volcanic island arcs
• Volcanic cones underlain by oceanic crust
Convergent Plate Boundaries and
Subduction
Convergent Plate Boundaries and
Subduction
• Continent-Continent Convergence
– Neither plate subducts
– Continental crust is buoyant
• Folding and deformation of rocks
• Mountain building
Convergent Margins: India-Asia Collision I
Transform Plate Boundaries
• Transform plate boundaries form when two
plates slide horizontally past one another
– Transform faults
– No lithosphere is produced or destroyed
– Connect spreading centers and offsets oceanic ridges
• Linear breaks in the seafloor are fracture zones
– Fracture zones are inactive
– Active faults occur between offset ridge segments
Transform Plate Boundaries
A few Transform Faults ,the San Andreas fault and the Alpine fault
of New Zealand, cut through continental crust
Transform Plate Boundaries
Testing the Plate Tectonics Model
• Evidence from Deep Sea Drilling Project
– Collect sediment and oceanic crust
– Date fossils in sediment
• Sediment age increases with distance from
ridge
• Sediment is thicker with increased distance
from the ridge
• Oldest seafloor is 180 million years old
Sedimentary Evidence
DSDP -Glomar Challenger
1968-1983
JOIDES Resolution
IODP –Chikyu
Testing the Plate Tectonics Model
Testing the Plate Tectonics Model
• Volcanoes in the Hawaiian Island-Emperor
Seamount Chain increase in age with
distance from Hawaii
– A cylinder of upwelling hot rock (mantle plume) is
beneath Hawaii
– A hot spot is an area of volcanism, high heat flow,
and crustal uplift above a mantle plume
– A hot-spot track formed as the Pacific Plate moved
over the hot spot
Testing the Plate Tectonics Model
• Paleomagnetic Evidence
Testing the Plate Tectonics Model
• Today North and South magnetic poles align
approximately with geographic North and South poles
• Iron-rich minerals influenced by magnetic pole
– Basalt erupts above the curie temperature, so
magnetite grains are nonmagnetic
– Grains align to magnetic field during cooling
– Rocks preserve a record of the direction of magnetic
poles at the time of formation
• Paleomagnetism or fossil magnetism
Position of paleomagnetic poles appears to change through time
because of continental drift
Testing the Plate Tectonics Model
• Magnetic field reverses polarity during a magnetic
reversal
– Rocks with same magnetic field as today have normal
polarity
– Rocks with opposite magnetism have reverse polarity
• Polarity of lava flows with radiometric ages was used
to generate a magnetic time scale
– Divided into chrons ~1 million years long
– Finer-scale reversals within each chron
• Vine and Matthews (1963) suggested stripes of
normal and reverse polarity are evidence of seafloor
spreading
• Vine and Matthews
– Observed reversals in the earth’s magnetic
field
Testing the Plate Tectonics Model
Testing the Plate Tectonics Model
Testing the Plate Tectonics Model
Magnetic Patterns of the Sea Floor
What Drives Plate Motions?
• Mantle is solid, but hot and weak enough to flow
• Convection occurs as hot, less dense material
rises and surface material cools and sinks
• During slab pull, cold, dense oceanic crust
sinks because it is denser than the
asthenosphere
• During ridge push, gravity causes lithospheric
slabs to slide down the ridge
• Drag in the mantle also affects plate motion
• Convection
What Drives Plate Motions?
What Drives Plate Motions?
• Mantle convective flow drives plate motion
• Subducting plates drive downward
component of convection
• Upwelling of hot rock at oceanic ridges drives
upward component of convection
• Convective flow is the heat transfer
mechanism from Earth’s interior
What Drives Plate Motions?
• Multiple models for convective flow:
– Whole-mantle convection
• Cold oceanic lithosphere sinks and stirs entire mantle
• Subducting slabs sink to core-mantle boundary
• Balanced by buoyant mantle plumes
– Layer cake model
• Subducting slabs do not sink past 1000 km
What Drives Plate Motions?
How Do Plates and Plate Boundaries
Change?
• Total surface area of Earth is constant
• Size and shape of individual plates changes
– African and Antarctic Plates are growing
• Surrounded by divergent boundaries
– Pacific Plate is being consumed
• Surrounded by convergent boundaries)
• Plate boundaries move and change through time
How Do Plates and Plate Boundaries
Change?
• New ocean basins were created during the
breakup of Pangaea
How Do Plates and Plate Boundaries
Change?
• Present plate motions can be used to predict
future continental positions
• 50 million yrs
• 250 million yrs