Transcript Chapter 1 The Growth of Oceanography

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Introduction to
Oceanography
Geological Oceanography
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The Earth’s Structure
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Earth consists of a series of
concentric layers or spheres which
differ in chemistry and physical
properties.
The compositional layers of the
Earth are the Crust, the Mantle,
and the Core. The Core is
subdivided into a molten outer core
and solid inner core.
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The Earth’s Structure
Physical state is determined by the combined effects
of pressure and temperature.
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• Increasing pressure raises the melting point of a
material.
• Increasing temperature provides additional
energy to the atoms and molecules of matter
allowing them to move farther apart, causing the
material to melt.
• Both pressure and temperature increase toward
the center of the Earth, but at different rates.
• Divisions of the Earth based upon physical state
are the Lithosphere, The Asthenosphere, the
Mesosphere, the Outer core, and the Inner core
Layered Earth - Evidence Of Earth’s Layers
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What evidence supports the idea that Earth has layers?
The behavior of seismic waves generated by earthquakes give scientists some of
the best evidence about the structure of Earth.
(above-left) S waves cannot penetrate Earth’s liquid core.
(above-right) P waves are bent as they pass through the liquid outer core.
Layered Earth - Chemical Properties
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Layer
Continental Crust
Oceanic Crust
Mantle
Core
Chemical Properties of Earth’s Layers
Chemical Properties
Composed primarily of granite
density = 2.7 g/cm3
Composed primarily of basalt
density = 2.9 g/cm3
Composed of silicon, oxygen, iron and magnesium
density = 4.5 g/cm3
Composed mainly of iron
Density = 13 g/cm3
Note that Earth is density stratified, that is, each deeper layer is
denser than the layer above.
Layered Earth - Physical Properties
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Layer
Lithosphere
Asthenosphere
Mantle
Outer Core
Inner Core
Physical Properties of Earth’s Layers
Physical Properties
The cool, rigid outer layer
Hot, partially melted layer which flows slowly
Denser and more slowly flowing than the
asthenosphere
Dense, viscous liquid layer, extremely hot
Solid, very dense and extremely hot
So, when we examine the chemical and physical properties of
Earth’s layers, we see that a cool, rigid, less dense layer (the
lithosphere) floats on a hot, slowly-flowing, more dense layer (the
asthenosphere).
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The Earth’s Structure
Three fluid spheres surround the rocky portion of
the Earth.
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• Hydrosphere includes all of the “free” water of the Earth
contained in the ocean, lakes, rivers, snow, ice, water vapor and
groundwater.
– The ocean covers 71% of the Earth’s surface
• Atmosphere is the gaseous envelope that surrounds the Earth
and is mainly a mixture of nitrogen and oxygen.
• Biosphere refers to all living and non-living organic matter.
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The Physiography of the Ocean Floor
Physiography and bathymetry (submarine landscape) allow
the sea floor to be subdivided into three distinct provinces:
continental margins, deep ocean basins and midoceanic
ridges.
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• Continental margins are the submerged edges of the continents
and consist of massive wedges of sediment eroded from the
land and deposited along the continental edge. The continental
margin can be divided into three parts: the Continental shelf,
the Continental slope, and the Continental rise.
• Deep Ocean Province is between the continental margins and
the midoceanic ridge and includes a variety of features from
mountainous to flat plains: Abyssal plains, Abyssal hills,
Seamounts, and Deep sea trenches
• Midoceanic Ridge Province consists of a continuous submarine
mountain range that covers about one third of the ocean floor
and extends for about 60,000 km around the Earth.
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Geologic Differences between
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Continents and Ocean Basins
Continents and ocean
basins differ in
composition, elevation
and physiographic
features.
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• Continental crust is mainly
composed of granite, a light
colored, lower density (2.7
gm/cm3) igneous rock rich in
aluminum, silicon and
oxygen.
• Oceanic crust is composed of
basalt, a dark coloured,
higher density (2.9 gm/cm3)
volcanic rock rich in silicon,
oxygen and magnesium.
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Geologic Differences
Continents and Ocean Basins
Isostacy refers to the balance of an object
“floating” upon a fluid medium. Height of the
mass above and below the surface of the
medium is controlled by the thickness of the
mass and its density (similar to ice floating in
water).
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• Greater the density of the mass, the lower it will sink in
the medium.
• Greater the thickness of the mass, the higher a portion of
it will rise above the medium.
• Continents are thick (30 to 40 km), have low density and
rise high above the supporting mantle rocks.
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Continental Drift
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Based upon the fit of continental outlines and
fossil and geologic evidence, Alfred Wegner
proposed his hypothesis of continental drift.
According to Wegner, the continents are sections
of a past super continent called Pangea, which
broke apart and the fragments plowed through the
oceanic crust, to their present locations
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Sea-Floor Spreading
Sea floor spreading demonstrates that the sea floor
moves apart at the oceanic ridges and new oceanic
crust is added to the edges.
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• Rift valleys along oceanic ridge crests indicate tension, are
bounded by normal faults and are floored by recently-erupted
basaltic lava flows.
• Axis of the oceanic ridge is offset by transform (strike-slip)
faults which produce lateral displacement.
• Whereas oceanic ridges indicate tension, continental mountains
indicate compressional forces are squeezing the land together.
Seafloor Spreading - A Key Idea
Seafloor spreading was an
idea proposed in 1960 to
explain the features of the
ocean floor. It explained the
development of the seafloor
at the Mid-Atlantic Ridge.
Convection currents in the
mantle were proposed as
the force that caused the
ocean to grow and the
continents to move.
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(right) The Mid-Atlantic
Ridge conforms to the
shape of the adjacent
continents. The inset shows
the central rift.
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Sea-Floor Spreading
The geomagnetic field is the magnetic field of the
Earth.
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• Magnetometers detect and measure Earth’s magnetic field.
• Moving across the ocean floor perpendicularly to the oceanic
ridges, magneometers alternately record stronger (positive)
and weaker (negative) magnetic fields (called magnetic
anomalies) in response to the influence of the sea floor rocks.
• Magnetic anomalies and the rocks causing them form parallel
bands arranged symmetrically about the axis of the oceanic
ridge.
• As basaltic rocks crystallize, some minerals align themselves
with Earth’s magnetic field, as it exists at that time, imparting
a permanent magnetic field, called paleomagnetism, to the
rock.
• Periodically Earth’s magnetic field polarity (direction)
reverses poles.
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Confirmation of the Theory of Plate Tectonics
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Paleomagnetism: strips of alternating magnetic polarity at spreading regions.
(above-left and above-right) The patterns of paleomagnetism support plate
tectonic theory. The 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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Sea-Floor Spreading
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• Sea floor increases in age away from the
ridge and is more deeply buried by sediment
because sediments have had a longer time to
collect.
• Rates of sea-floor spreading vary from 1 to
10 cm per year for each side of the ridge and
can be determined by dating the sea floor
and measuring its distance from the ridge
crest.
• Continents are moved by the expanding sea
floor.
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Global Plate Tectonics
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Because Earth’s size is constant, expansion of the
crust in one area requires destruction of the crust
elsewhere.
• Currently, the Pacific Ocean basin is shrinking as other ocean
basins expand.
• Destruction of sea floor occurs in subduction zones.
• Seismicity is the frequency, magnitude and distribution of
earthquakes. Earthquakes are concentrated along oceanic
ridges, transform faults, trenches and island arcs.
• Tectonism refers to the deformation of Earth’s crust.
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Global Plate Tectonics
• Benioff Zone is an area of increasingly
deeper seismic activity, inclined from the
trench downward in the direction of the
island arc.
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• Subduction is the process at a trench
whereby one part of the sea floor plunges
below another and down into the
asthenosphere.
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Global Plate Tectonics
Earth’s surface is composed of a series of
lithospheric plates. Plate edges extend through the
lithosphere and are defined by seismicity.
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• Plate edges are trenches, oceanic ridges and transform faults.
• Seismicity and volcanism are concentrated along plate
boundaries.
• Movement of plates is caused by thermal convection of the
“plastic” rocks of the asthenosphere which drag along the
overlying lithospheric plates.
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The Major Lithospheric Plates
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The major lithospheric plates and their direction of relative movement
are shown here.
Plate Boundaries
The lithospheric plates interact with the neighboring plates in several
ways.
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Divergent plate boundaries – Boundaries between plates moving
apart, further classified as:
Divergent oceanic crust – for example, the Mid-Atlantic Ridge
Divergent continental crust - for example, the Rift Valley of East
Africa.
(right) Extension of
divergent boundaries
causes splitting and rifting.
Plate Boundaries
Convergent Plate Boundaries - Regions where plates are pushing
together can be further classified as:
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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
(right)
Compression
at convergent
boundaries
produces
buckling and
shortening.
Plate Boundaries
Transform plate boundaries - locations where crustal plates move
past one another, for example, the San Andreas fault.
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(above) Translation at transform boundaries causes shear.
Confirmation of the Theory of Plate Tectonics
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Evidence supporting the theory of plate
tectonics:
Apparent Polar wandering: plate movement
causes the apparent position of the magnetic
poles to have shifted.
(right) The paleomagnetic fields in the rocks
would indicate a single pole until the continents
drift apart.
Confirmation of the Theory of Plate Tectonics
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Hot Spots: Surface expression of plumes of magma.
(above) A volcanic island chain can form when a plate passes over a
hot spot and a stationary mantle plume.
Confirmation of the Theory of Plate Tectonics
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Atolls and Guyots: Coral formations and submerged volcanic mountains.
(above) Guyots were once volcanic peaks above sea level. They were
eroded by wave action as they sank beneath the surface of the water.
Confirmation of the Theory of Plate Tectonics
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Age and distribution of ocean sediments: The sediment in the ocean
is thinner and younger than the age of the ocean indicates it should be.
The Oceanic ridges: Oceanic ridges are clear indicators of past events.
Terranes: Oceanic plateaus that form by uplifting and mountain building
as they strike a continent.
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Global Plate Tectonics
• Mantle plumes originate deep within the
asthenosphere as molten rock which rises
and melts through the lithospheric plate
forming a large volcanic mass at a “hot
spot”.
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Summary
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Through the great expanse of
geologic time, this slow
movement remakes the
surface of Earth, expands and
splits continents, and forms
and destroys ocean basins.
(left) The fit of the continents
at a water depth of 137
meters.