Oceanography, An Invitation to Marine Science | 9e Tom ...€¦ · within Earth keeps asthenosphere...
Transcript of Oceanography, An Invitation to Marine Science | 9e Tom ...€¦ · within Earth keeps asthenosphere...
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Oceanography, An Invitation to Marine Science | 9eTom Garrison
Earth Structure and Plate Tectonics
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• Earth’s interior is layered, and the layers are arranged by density
• Continents rise above sea level because they float on a dense, deformable layer beneath them
• The brittle surface of Earth is fractured into about a dozen tile like “plates”
Key Concepts
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• Movement of the subterranean material on which these plates float moves them relative to one another
• Continents and oceans are formed and destroyed where the plates collide, flex, and sink
Key Concepts (cont’d.)
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• Density stratified, with denser material towards the center
• Note-Crust, Mantle, Outer Core and Inner Core
Earth’s Interior Is Layered
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Earth’s Inner StructureA cross section through Earth showing the internal layers.
Note lithosphere VS asthenosphere, and crust VS mantle.
Thin oceanic crust is primarily basalt, a heavy dark-colored rock.
Thicker continental crust is granite
Mantle is mainly of oxygen, iron, magnesium, and silicon.
Outer and inner core-mainly of iron and nickel.
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Based on chemical/physical properties of Earth’s layers:cool, rigid, less dense layer (the lithosphere) floats on a
hot, slowly-flowing, more dense layer (the asthenosphere).
Layer Physical Properties
Lithosphere Cool, rigid, outer layer
Asthenosphere Hot, partially melted layer which flows slowly
Mantle Denser and more slowly flowing than the asthenosphere
Outer Core Dense, viscous liquid layer, extremely hot
Inner Core Solid, very dense and extremely hot
Earth’s Layers May Be Classified by Composition
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Note that Earth is density stratified; that is, each deeper layer is denser than the layer above.
Layer Chemical PropertiesContinental Crust Composed primarily of granite
Density = 2.7 g/cm3
Oceanic Crust Composed primarily of basaltDensity = 2.9 g/cm3
Mantle Composed of silicon, oxygen, iron, and magnesiumDensity = 4.5 g/cm3
Core Composed primarily of ironDensity = 13 g/cm3
Earth’s Layers May Be Classified by Physical Properties
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How Earthquakes Contributed to Our Model of the Layered Earth
Earthquakes generate seismic waves
Surface waves – travel along Earth’s surface
Body waves – travel through Earth
P wave – compressional wave
S wave – shear wave
Seismograph – detects and records seismic waves
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• Shadow zones– Predicted by Richard Oldham– S waves cannot pass through liquids– P waves are refracted at density boundaries
• Data from an earthquake, a “natural experiment,” confirmed theories of Earth’s layering
Earthquake Wave Shadow Zones Confirmed the Presence of Earth’s Core
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• Earth’s interior is heated by radioactive decay– Conduction– Convection
• Isostatic equilibrium supports continents above sea level– Like- Buoyancy – displacement of fluid as an
object floats
Radioactive Decay and Isostatic Equilibrium
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Isostatic equilibrium describes the way the lithosphere is supported on the
asthenosphere.
Isostatic Equilibrium
The concept of buoyancy
is illustrated by a ship on
the ocean. The ship sinks
until it displaces a volume
of water equal to the weight
of the ship and its cargo.
Icebergs sink into water so that the same proportion
of their volume (about 90%) is submerged. The
more massive the iceberg, the greater this volume is.
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Erosion and Isostatic Readjustment in Continental Crust
Erosion and isostatic readjustment can cause continental crust to become thinner in mountainous regions.
As mountains are eroded over time, isostatic uplift causes their roots to rise.
Deposition of sediments away from the mountains often causes nearby crust to sink.
Further erosion exposes rocks that were once embedded deep within the peaks.
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• The age of Earth has been subject to debate. Scientists now use an age of 4.6 billion years.
• James Hutton-Principle of uniformitarianism was introduced in 1788. This principle sates that the forces which shaped Earth are identical to forces working today.
• Catastrophism is the thought that Earth is very young, and events described in the Bible are responsible for the appearance of Earth’s features.
Age of Earth Was Controversial and Not Easily Determined
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The Theory of Continental Drift-Continents That Were Once Joined Formed Continuous Chains
• Proposed a centrifugal force mechanism• Dismissed as a “crank”
• Theory proposed by Alfred Wegener
• Pangaea – all continents in one landmass
• Fossil evidence across continents
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The Fit Between Continental Edges Suggested That They Might Have Drifted
• Seismographs revealed a pattern of
volcanoes and earthquakes.
• Radiometric dating of rocks oceanic crust
dated and found to be young compared to
the age of Earth
• Echo sounders revealed the shape of the
Mid-Atlantic Ridge
• Sir Edward Bullard at the University of
Cambridge in the early 1960s-Fit of all the
continents around the Atlantic at a water
depth of about 137 meters (450 feet).
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• Pacific Ring of Fire Tectonic activity surrounding Pacific Ocean
The New Understanding of Earth Evolved Slowly
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• Echo sounding used to reveal seafloor topography• Seafloor spreading – new hypothesis
– Mid-ocean ridges are spreading centers
• Subduction zones – areas where oceanic crust plunges down into the mantle
• Plate tectonic theory – John Tuzo Wilson– Lithospheric plates “float” on asthenosphere– Plate movement
• Form at mid-ocean ridges• Pulled downward into the mantle by leading edge
The Breakthrough: From Seafloor Spreading to Plate Tectonics
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The Tectonic System Is Powered by Heat
Heat from radioactive decay
within Earth keeps asthenosphere
flowing and lithosphere moving.
• Some parts of the mantle are warmer than others.
• Convection currents form when warm mantle material rises and cool material falls.
• Fragmented lithosphere floats above the mantle.
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– Rift valley forms as crust is pulled/pushed apart
Divergent plate boundaries-Plates moving apart
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The breakup of Pangaea shown in five stages beginning about
225 million years ago.
Ocean Basins Are Formed at Divergent Plate Boundaries
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~10,000 Seismic events worldwide, January 1977 through December 1986.
Most Tectonic Activity Occurs at Plate Boundaries
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Most Tectonic Activity Occurs at Plate BoundariesThe major lithospheric plates, showing their directions of relative movement
and the location of the principal hot spots. Most of the million or so
earthquakes and volcanic events each year occur along plate boundaries.
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Plate boundaries in action.
As Plate A moves to the left (west)
1. A gap forms behind it = DIVERGENT,
2. An overlap with Plate B forms in front of =CONVERGENT.
3. Sliding occurs along the top and bottom sides =TRANSFORM.
Most Tectonic Activity Occurs at Plate Boundaries
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The lithospheric plates interact with the neighboring plates in
several ways. (1) Divergent, (2) Convergent, (3) Transform.
Most Tectonic Activity Occurs at Plate Boundaries
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• Convergent plate boundaries– Plates coming
together• Ocean to ocean
convergence• Ocean to continent
convergence• Continent to
continent convergence
Convergent and Transform Plate Boundaries
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Convergent Plate Boundaries -• Oceanic crust toward continental crust - for example, the west coast of South America.
• Oceanic crust toward oceanic crust - occurring in the northern Pacific.
• Continental crust toward continental crust – one example is the Himalayas.
Most Tectonic Activity Occurs at Plate Boundaries
The subducting oceanic plate becomes more dense as it descends, its downward slide propelled by gravity and heat drives water and other volatile components from the subductedsediments into the overlying mantle.
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Formation of an island arc along a trench as two oceanic plates converge. The volcanic islands form as masses of magma reach the seafloor.
Convergent Plate Boundaries
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Shallow, intermediate, and
deep earthquakes for part of
the Pacific Ring of Fire in the
vicinity of the Japan trench.
Note that earthquakes occur
only on one side of the
trench, the side on which the
plate subducts.
Convergent Plate Boundaries
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Convergence of two continental plates. Neither plate is dense enough to
subduct; instead, their compression and folding uplift the plate edges. The
massive supporting “root” beneath the emergent mountain is needed for
isostatic equilibrium.
Convergent Plate Boundaries
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• Transform plate boundaries – Plates shear laterally past one another
Transform Plate Boundaries
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Seafloor spreading explains the development
of the seafloor at the Mid-Atlantic Ridge.
Convection currents in the mantle are the
force that caused the ocean to grow and the
continents to move.
Ocean Basins Are Formed at Divergent Plate Boundaries
The Mid-Atlantic Ridge
shows conformance to the
coastlines of the adjacent
continents.
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• Divergences– Divergent oceanic crust (Mid-Atlantic)– Divergent continental crust (rift valleys)
• Convergences– Oceanic-continental (South America)– Oceanic-oceanic (northern Pacific)– Continental-continental (Himalayas)
A Summary of Plate Interactions
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• The history of plate movement is captured in residual magnetic fields– Paleomagnetism – recording of Earth’s past
magnetic field• Iron-bearing minerals in ocean-floor basalts align
with the magnetic field and remain when rock solidifies
– Magnetometer – records residual magnetism
3.10 The Confirmation of Plate Tectonics
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A History of Plate Movement Has Been Captured in Residual Magnetic Fields
Paleomagnetism:strips of alternating magnetic polarity at spreading regions.
Molten rocks at the spreading center
take on the polarity of the planet
while they are cooling. When Earth’s
polarity reverses, the polarity of
newly formed rock changes.
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The age of the ocean floors. Seafloor spreading over the last 200 million years as revealed by paleomagneticpatterns. The Atlantic basin is symmetrical in contrast with the asymmetrical Pacific,
A History of Plate Movement Has Been Captured in Residual Magnetic Fields
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• Plate movement above mantle plumes and hot spots
Plumes, Hot Spots, and Terranes Provide Evidence of Plate Tectonics
Mantle plumes and hot spots remain stationary under moving plates
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Terrane Formation
Sediment age and distribution, oceanic ridges, and terranes
Terranes – small pieces of different crust that gets sutured onto a continent at a subduction zone