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MAY 2011
Oceanogra phy Ch 2
Plate Tectonics and the Ocean Floor
Plate Tectonics – The earth’s crust is composed of a patchwork of thin rigid plates that move horizontally
with respect to one another.
Explains thousands of Quakes and volcanic eruptions /year
Locations of the quakes and eruptions
WHY MOUNTAINS ARE NOT ERODED AWAY
Distribution of life, past and present on earth
Land forms and ocean floor features
Differences between ocean and continental crust
Continuing development of earth’s surface.
2 - 1 Evidence supporting Continental Drift.
Alfred Wegener –continents are drifting slowly across the globe (1912). i.e. Continental drift.
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Shape of matching shorelines – Fit of the Continents. Collision formed Pangaea; the huge
ocean is called Panthalassa. The Tethys Sea is a smaller body. Sir Edward Bullard discovered
the best fit using a 2000 met depth. P.37
Matching rock sequences and Mountain chains across the Atlantic. P.38
Glacial deposits and other climatic evidence. Found within the lower latitude regions of
Africa, India , Australia and S. America, they are dated 300 MY. Coal of the same age are
present in N. America and Europe. Other evidence includes grooves, plants and animals
fossils. P.39.
Organisms distribution. Several fossils (Mesosaurus), which can only be explained by drifting
continents or a land bridge.
Objections to Continental Drift.
Lack of an acceptable mechanism: Wegener proposed that the continents plowed thru the ocean
basins to their present positions; their leading edges were deformed into ridges. Another mechanism:
Gravitational attraction to the equatorial bulge and tidal forces.
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2 – 2 Evidence supporting Plate Tectonics.
Earth’s Magnetic Field.
Magnetic particles are frozen into position, recording the magnetic field at that location and
time. All rocks have some mineral magnetite.
 Paleomagnetism. Records the magnetic N-S direction and their angle relative to the earth’s
surface (Magnetic dip) p.40 . Dip is directly related to Latitude.
 Apparent Polar Wandering. Dip data show that the magnetic pole was moving through
geologic time. For all rocks, 70 MY or older the pole position relative to N. America, lies to
the west of that for Eurasian rocks.
 Magnetic Polarity Reversals. The magnetic field has reversed itself periodically throughout
geologic time. It takes several thousand years to switch (5 times/1 MY)
 Paleomagnetism and the Ocean floor. Magnetometer reveals magnetic stripes (N-S)
symmetrically located along pull apart margins.
Sea Floor Spreading and Features of Ocean Basins.
Extensive mountain ridges discovered by Harry Hess near the center of Ocean basins
(Spreading Centers) and very deep narrow trenches at the edge of the basins (Subduction
zones). Vine and Matthews combined magnetic stripes and Sea floor spreading. P.46
 Other Evidence from the Ocean floor. Age. Oldest crust 0.18 BY, is being subducted into
the mantle at the trenches and created at the mid ocean ridges. Heat Flow along the ridges
can be up to 8X greater than the average. At the trenches , it is as little as 0.1 times.
Earthquakes along plate margins. p. 48
 Acceptance of a Theory. Continental drift and Sea floor spreading are combined into the
concept of plate tectonics. Plate movement may be a function of Slab-pull generated by a
subducting plate that pulls the rest of the plate behind it, and slab-suction.
2-3 Features at Plate Boundaries.
94% of Plate boundaries are on the Sea Floor.
 Divergent – Central down dropped linear rift valley. Pull apart. P. 52. Volcanic activity
produces hi density basaltic rock and as the plates begin to move apart, a linear rift valley
forms. Examples : East Africa, Gulf of California. P.53. Faster spreading produces broader
and less rugged segments (Rises).
 Earthquakes associated with Divergent boundaries. The amount of energy released along
divergent margins is closely related to the spreading rates. Faster speed relates to lower
energy. Scale of Measurement is the Seismic Moment Magnitude, not the Richter Scale.
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Convergent Boundary Features. Ocean crust melted into the mantle. It causes an Arc
shaped row of highly active and explosive volcanoes – a volcanic arc, that parallels the
trench.
Oceanic – Continental crust Convergence. Dense oceanic plate is subducted p.55. Basalt
mixes with granite forms explosive, viscous, andesite in a continental arc. Example:
Juan de Fuca plate produce the Cascade mts.
Oceanic – Oceanic crust convergence. The older plate is subducted. Example: Marianna
Trench. Basaltic lava, not very explosive. Examples: West Indies, Aleutian Islands.
Continental – Continental crust convergence. Both plates are low density. Not subducted,
but instead form mountains (Himalayas)
Earthquakes associated with Convergence. Largest and deepest, up to 670 km deep.
Transform Boundary Features. The offset on Mid-Ocean ridges result from Transform
faults.
Oceanic – Continental crust transform faults. P.59.
Earthquakes associated with transform boundaries. These are shallow but often strong in
the lithosphere. Example: San Andreas fault, stronger than those in the Oceanic crust.
2-4 Applications of Plate Tectonics.
Explains the origin of features:
 Hot Spots and Mantle Plumes. Volcanoes near the middle of plates (Hawaii). Some plumes
are sourced from the core- mantle boundary, while others are from much shallower depths.
Many >100 hot spots do not coincide with plate boundaries. Many occur near divergent
boundaries where the lithosphere is thin. Galapagos Islands, Iceland (hot spot 150 km wide).
The Emperor Seamount Chain, p.62. >100 volcanoes stretch 5800 km. Based on age, the
oldest is to the NW.
 Sea Mounts and Table Mounts. The former has a conical top, while the latter (Guyots) are
flat topped, due to erosion.
 Coral Reef Development – Darwin notes 3 stages of development. Fringing – associated with
active volcanoes, not well developed. The process stops at this stage. Barrier – linear or
circular, separated from the land mass by a lagoon. Fast growing 10-15’ /1000 years. The
Great Barrier Reef is 150 km wide 2000 km long. Atoll, an enclosed lagoon with open
circulation with the Ocean.
Satellites are used to detect plate motion today. P.65.
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2 – 5. How has the earth changed in the past and how will it look in the future?
Past paleogeography p.67.
540 MY barely recognizable
540- 300 MY accretion, continents coming together.
180 to present - Continents start to break up and drift.
Future p.68
2 – 6. Plate Tectonics p. 69
The Wilson Cycle
Embryonic – uplift – Linear rifts on the Continent
Juvenile – Spreading - Narrow sea with matching Coasts
Mature – Ocean basins- Ocean basin with Continental margins
Declining – Sub duction / convergence – Island Arcs and trenches around basin edge
Terminal – Up lift / collision – Narrow irregular seas with Young Mountains
Suturing – Uplift Convergent – Young to mature mountain belts
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Oceanography Ch # 2
PLATE TECTONICS
Introduction
2-1 Evidence supporting Continental Drift
Fit of the Continents – best at 2000 met depth.
Pangaea/ Panthalasa / Tethys
Matching Sequence of Rocks and Mtn Chains
Glacial Ages and other Climate Evidence (300 MY old Coal)
Distribution of Organisms – Mesosaurus
Objections to the Continental Drift Model (no mechanism)
2-2 What Evidence Supports Plate Tectonics
Earth’s Magnetic Field and Paleomagnetism (magnetic materials are aligned)
Rocks Affected
Paleomagnetism (determine latitudes)
Apparent Polar Wandering
Polarity Reversals. (every 2000 years)
Paleomagnetism and Ocean Floor (Magnetic stripes)
Sea Floor Spreading and Features of Ocean Basins
Sea Floor Spreading (convection cells)
Mid-Ocean Ridge (exposed on Iceland)
Spreading Centers
Ocean Trenches (Mariana Trench 36,161 ft deep)
Sub-duction Zones
Vine & Mathews ( (sea floor spreading)
Other Evidence from the Ocean Basins
Age of Ocean Floor (max 200 MY ; Mid –Ocean ridge is youngest)
Heat Flow (Loss is lowest in trenches)
Worldwide Earthquakes (along trenches and at Mid Ocean Ridges)
The Acceptance of a Theory
Lithosphere (crustal plates floating on the Asthenosphere)
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2-3 What Features Occur at Plate Boundaries
Divergent Boundary Features
Ocean Ridges/ Rises
Rift Valley (Red sea , Baja California, East Africa)
Earthquakes (intensity related to spreading rate)
Convergent Boundary Features
Continental-Continental - Arc (Uplift)
Oceanic-Oceanic Convergence – Island Arc or Deep linear trench.
Earthquakes.
Transform Boundary Features
Oceanic vs Continental Transform Faults
Earthquakes
San Andreas Fault.
2-4. Testing the Model. What are some applications of Plate Tectonics
Hot Spots and Mantle Plumes – Hawaiian Islands; Nematath
Sea mounts and Table Mounts/ Guyots
Coral Reef Development (Darwin)
Fringing; Atolls; Barriers
Detecting Plate Motion
2-5. How has the Earth Changed
Past – Paleography Continental accretion
Future - Some bold Predictions
2-6. Plate Tectonics
The Wilson Cycle.