© 2016 Cengage Learning. All Rights Reserved. 3 Oceanography, An Invitation to Marine Science | 9e...

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3

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

• Compelling evidence for plate movement is recorded in magnetic fields within the ocean floor

Key Concepts (cont’d.)

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• Theory proposed by Alfred Wegener

• Pangaea – all continents in one landmass

• Fossil evidence across continents

• Proposed a centrifugal force mechanism

• Dismissed as a “crank”

The Theory of Continental Drift

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Continents That Were Once Joined Formed Continuous Chains

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• Density stratified, with denser material towards the center– Density: mass per unit

volume

3.2 Earth’s Interior Is Layered

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• 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

3.3 The Study of Earthquakes Provides Evidence for Layering

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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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How Earthquakes Contributed to Our Model of the Layered Earth

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• Earth’s internal layers are based on chemical composition– Crusts, mantle, and cores

• Earth’s internal layers are also based on physical properties– Lithosphere

– Asthenosphere

– Lower mantle

– Core

3.4 Earth’s Inner Structure Was Gradually Revealed

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• Earth’s interior is heated by radioactive decay– Conduction

– Convection

• Isostatic equilibrium supports continents above sea level– Buoyancy – displacement of fluid as an object

floats

Radioactive Decay and Isostatic Equilibrium

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Stepped Art

Mountains

Mantle

Continentalcrust

Low-densitymountain root

Uplift

Subsidence

TransportDeposition

Erosion

Subsidence

Figure 3-10 p79

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Erosion and Isostatic Readjustmentin Continental Crust

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• The age of Earth has been controversial– James Hutton – uniformitarianism

– Controversy with biblical school of thought – catastrophism

– Darwin and Wallace – biological evolution

• Evidence from exploration has convinced most researchers that Earth is of great age

3.5 The New Understanding of Earth Evolved Slowly

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• Pacific Ring of Fire – Tectonic activity surrounding Pacific Ocean

• Oceanic crust dated and found to be young compared to the age of Earth

• Echo sounding used to reveal seafloor topography

3.6 Wegener’s Idea Is Transformed

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• 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

3.7 The Breakthrough: From Seafloor Spreading to Plate Tectonics

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The Tectonic System Is Powered by Heat

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• Divergent plate boundaries– Plates moving apart

– Rift valley forms as crust is pulled/pushed apart

3.8 Plates Interact 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

• Transform plate boundaries – Plates shear laterally past one another

Convergent and Transform Plate Boundaries

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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)

3.9 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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The Age of the Ocean Floors

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• Plate movement above mantle plumes and hot spots – Mantle plumes and hot spots remain

stationary under moving plates

• Sediment age and distribution, oceanic ridges, and terranes – Terranes – small pieces of different crust that

gets sutured onto a continent at a subduction zone

Plumes, Hot Spots, and Terranes Provide Evidence of Plate Tectonics

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Terrane Formation

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• Many questions remain unanswered– Why should long lines of asthenosphere be

any warmer than adjacent areas?

– Why do mantle plumes form? What causes a superplume? How long do they last?

– How far do most plates descend?

(Recent evidence suggests that much of the material spans the entire mantle, reaching the edge of the outer core)

3.11 Scientists Still Have Much to Learn about the Tectonic Process

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• Earth is composed of concentric spherical layers that are density sorted

• Layers are confirmed based on seismic evidence

• Plate tectonics suggests Earth’s surface is made up of lithospheric plates that interact in different ways at plate boundaries

Chapter In Perspective

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• What’s the potential for serious loss of life and property damage due to tectonic plate movement?

• Is plate movement a new feature of Earth?

Some Questions from Students