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


• 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

Presenter

Presentation Notes

Table 2-1 Principles of the Cell Theory


• 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.)


• Density stratified, with denser material towards the center

• Note-Crust, Mantle, Outer Core and Inner Core

Earth’s Interior Is Layered

Presenter

Presentation Notes

Figure 3.5 A cross section through Earth showing the internal layers


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.


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


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


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

Presenter

Presentation Notes

Figure 3.7 How earthquakes contributed to our model of the layered Earth.

https://www.youtube.com/watch?v=uA_OLKfQpYA


• 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


• 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


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.


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.

Presenter

Presentation Notes

Figure 3.11 Erosion and isostatic readjustment can cause continental crust to become thinner in mountainous regions. As mountains are eroded over time (a–c), isostatic uplift causes their roots to rise. (The same thing happens when a ship is unloaded or an iceberg melts.) Further erosion exposes rocks that were once embedded deep within the peaks, sometimes exposing once-buried structures like Half-Dome in Yosemite Valley (d).


• 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


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


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).


• Pacific Ring of Fire Tectonic activity surrounding Pacific Ocean

The New Understanding of Earth Evolved Slowly


• 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


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.

Presenter

Presentation Notes

Figure 3.14 The tectonic system is powered by heat. Some parts of the mantle are warmer than others, and convection currents form when warm mantle material rises and cool material falls. Above the mantle floats the cool, rigid lithosphere, which is fragmented into plates. Plate movement is powered by gravity: The plates slide down the ridges at the places of their formation; their dense, cool leading edges are pulled back into the mantle. Plates may move away from one another (along the ocean ridges), toward one another (at subduction zones or areas of mountain building), or past one another (as at California’s San Andreas Fault). Smaller localized convection currents form cylindrical plumes that rise to the surface to form hot spots (like the Hawai’ian Islands). Note that the whole mantle appears to be involved in thermal convection currents.


– Rift valley forms as crust is pulled/pushed apart

Divergent plate boundaries-Plates moving apart

Presenter

Presentation Notes

Figure 3.18 A model for the formation of a new plate boundary: the breakup of Pangaea and the formation of the Atlantic.


The breakup of Pangaea shown in five stages beginning about

225 million years ago.

Ocean Basins Are Formed at Divergent Plate Boundaries


~10,000 Seismic events worldwide, January 1977 through December 1986.

Most Tectonic Activity Occurs at Plate Boundaries


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.


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.



The lithospheric plates interact with the neighboring plates in

several ways. (1) Divergent, (2) Convergent, (3) Transform.



• Convergent plate boundaries– Plates coming

together• Ocean to ocean

convergence• Ocean to continent

convergence• Continent to

continent convergence

Convergent and Transform Plate Boundaries


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.


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.


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


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.



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.



• Transform plate boundaries – Plates shear laterally past one another

Transform Plate Boundaries


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.


• 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


• 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


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.


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


• 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


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

Presenter

Presentation Notes

Figure 3.32 Terrane formation. Oceanic plateaus usually composed of relatively low-density rock are not subducted into the trench with the oceanic plate. Instead, they are “scraped off,” causing uplifting and mountain building as they strike a continent (a–d). Though rare, assemblages of subducting oceanic lithosphere can also be scraped off (obducted) onto the edges of continents. Rich ore deposits are sometimes found in them.

oceanography, an invitation to marine science | 9e tom ...€¦ · within earth keeps asthenosphere...

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