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Chapter 2
The Planet Oceanus
Composition of the Earth
• The Earth consists of a series of
concentric layers or spheres
which differ in chemistry and
physical properties.
• There are two different ways to
describe those layers:
Composition and Physical State
• The compositional layers of the
Earth are:
• the Crust
• the Mantle
• the Core
• The Core is subdivided into a
molten outer core and solid
inner core
Figure 02.01a: The Earth's internal
mass. An outer crust, a middle
mantle, and a center core (solid inner
core surrounded by molten outer
core).
The Effects of Pressure and Temperature
• Increasing pressure raises the melting point of a
material by forcing atoms and molecules into
tighter configurations, and holding them together
more strongly.
• Increasing temperature provides additional energy
to the atoms and molecules of matter.
• This allows them to move farther apart, eventually
causing the material to melt.
Pressure and Temperature (continued)
• Both pressure and temperature increase toward the
center of the Earth, but at variable rates.
• Divisions of the Earth based upon physical state
are:
–
–
–
–
–
the Lithosphere
the Asthenosphere
the Mesosphere
the Outer core
the Inner core
Figure 02.01b: The Earth's interior
Three spheres surround the rocky portion of
the Earth
• Hydrosphere includes all of the “free” water of
the Earth contained in:
–
–
–
–
–
–
–
Oceans
Lakes
Rivers
Snow
Ice
Water vapor
Groundwater
2-1 The Earth’s Structure
The Earth’s Spheres (continued)
• Atmosphere is the gaseous envelope that
surrounds the Earth .
– It is mainly a mixture of nitrogen and oxygen.
• Biosphere refers to all living and non-living
organic matter.
The Sea Floor
• Physiography and bathymetry (the topography of the
submarine landscape) allow the sea floor to be subdivided
into three distinct provinces:
– continental margins
– deep ocean basins
– mid-oceanic ridges
Figure 02.02: This map shows the major bathymetric
features of the ocean basins of the Pacific Ocean, the
Atlantic Ocean, and the Indian Ocean.
Courtesy of NOAA
• Continental margins are the submerged edges of the
continents.
• They 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
– the Continental rise
Figure 2-3a Continental Margin
2-2 The Physiography of the Ocean Floor
The Sea Floor (Continued)
• The Deep Ocean Province lies between the continental
margins and the mid-oceanic ridges.
• It includes a variety of features from mountainous to flat
plains:
– Abyssal plains
– Abyssal hills
– Seamounts
– Deep sea trenches
Figure 02.03b: The floor of the deep-ocean
basin.
• Midoceanic Ridge Province consists of a continuous
submarine mountain range.
• It covers about one third of the ocean floor.
• It extends for about 60,000 km around the Earth.
Figure 2-3c Midocean Ridge
2-2 The Physiography of the Ocean Floor
Continents and Ocean Basins Differ in
Composition, Elevation, and Physiographic
Features
• Elevation of Earth’s surface
displays a bimodal (two
peaks) distribution.
– About 29% above sea
level
– Much of the remainder 45 kilometers below sea
level
Figure 02.04b: Here is a frequency plot
showing the relative distribution of the Earth's
land elevations and ocean depths.
Composition, elevation, and physiographic
(Continued)
• Continental crust is mainly composed of granite.
– light colored
– lower density (2.7 gm/cm3)
– igneous rock
– rich in aluminum, silicon and oxygen
• Oceanic crust is composed of basalt
– dark colored
– higher density (2.9 gm/cm3)
– volcanic rock
– rich in silicon, oxygen, iron and magnesium
• The Moho is the boundary between rocks of the crust and the
denser (3.3 gm/cm3) rocks of the mantle, which are mostly
silicon, oxygen, iron and magnesium.
Composition, elevation, and physiographic
(Continued)
• Isostasy refers to the balance of an object “floating” upon a fluid
medium.
• Height (elevation) of the mass above the surface of the medium is
controlled by:
– the thickness of the mass
– the density of the mass (similar to ice floating in water)
– The density of the medium in (or on) which the mass is floating
Figure 02.05a: The longer the block, the
higher its top rises above the water surface
and the deeper its base extends into the
water.
Composition elevation, and physiographic
(Continued)
• The greater the density of the mass relative to the
density of the medium, the lower it will sink in the
medium.
• The greater the thickness of the mass, the higher a
portion of it will rise above the medium.
Figure 02.05b: The higher an iceberg rises above the sea
surface, the deeper the ice must penetrate into the water in
order to maintain isostatic balance.
Isostasy
• Continents are thick (30 to 40 km), have low density
and rise high above the supporting mantle rocks.
• Sea floor is thin (4 to 10 km), has greater density and
does not rise as high above the mantle.
Figure 02.05c: The oceanic crust is denser and thinner than the continental crust.
Composition, elevation, and physiographic
(Continued)
• Altimetry uses satellites to determine
bathymetry.
• Based upon slight changes in the elevation
of the sea surface (averaged over time to
account for waves and tides).
• These changes result from the greater
gravitational attraction of large rock masses
on the sea floor, such as volcanoes or the
mid-ocean ridges.
Satellite Altimetry
Figure B02.05: Variations in the height of the sea surface reflect the presence of large
bathymetric features that affect the elevation of the sea surface.
Echo Sounding and Seismic Reflection
• Echo sounding and seismic reflection rely on
sound pulses that reflect off the ocean floor and
off sedimentary layers.
• The fundamental relationship between the speed
of sound and the density of the material in which
the sound is traveling is the key to using these
techniques.
Figure B02.03: Seismic reflection profile. The
continuous reflection of sound from
subbottom layers (see Figure B2–1) produces
a geologic cross section. In this profile, layers
of sediment have been deformed into a
broad fold.
Echo Sounding and Seismic Reflection
• Seismic refraction examines how sound waves are
bent (refracted) as they travel through material.
• They reveal:
– Densities
– Depths
– Thicknesses of rock layers
Figure B02.04: The refraction (bending)
of sound as it travels through different
rock types reveals the shape and
density structure of the underlying rock
masses.
Chapter 3
The Origin of Ocean Basins
Continental Drift
• Alfred Wegner proposed his hypothesis of
continental drift based upon
– the fit of continental outlines
– fossil evidence
– geologic evidence
• The continents are sections of a past super
continent called Pangea, surrounded by a single
ocean he called Panthalassa.
• Pangea broke apart and the continents drifted to
their present locations.
Fitting the Continents Together
Figure 03.01a: This map, published by Antonio Snider in 1858, shows the arrangement
of the continents into a large landmass.
Pangaea, 200 to 300 MYBP
Figure 03.01b: If the continents are arranged into Pangea, coal beds and glacial
deposits that are 200 to 300 million years old fall into latitudinal belts.
The Breakup of Pangaea
Figure 03.12a: This sequence of maps displays the changing world geography since the
time of Pangaea, the megacontinent of the Permian Period.
Sea Floor Spreading
• Sea floor spreading was proposed and confirmed
in the 1960s, long after Wegener’s hypothesis
became very unpopular…
• It demonstrates that:
– the sea floor moves apart at the oceanic ridges
– new oceanic crust is added (mainly through volcanic
activity) in the rift valleys at the crests of those ridges
• Rift valleys along oceanic ridge crests:
– indicate tension
– are bounded by normal faults
– are floored by recently-erupted basaltic lava flows
Ridges & Faults
Axis of the oceanic ridge (divergent, spreading
displacement) is offset by transform (strike-slip)
faults which produce lateral displacement.
Figure 03.02a: The rift valley at the crest of midocean ridges is formed by tension.
The Geomagnetic Field (Continued)
• These measurements
(called magnetic
anomalies) alternate
strong (positive) and weak
(negative) in response to
the influence of the sea
floor rocks.
• Magnetic anomalies form
parallel bands arranged
symmetrically about the
axis of the oceanic ridge.
Figure 03.03: Magnetic anomaly stripes
run parallel and are symmetrically
arranged on both sides of the midocean
ridge axis.
The Geomagnetic Field (Continued)
• Ocean floor rocks are mostly basaltic, and they
contain minerals rich in iron. These minerals
aligned themselves with Earth’s magnetic field
as it existed at the time when the lava
erupted and froze to form volcanic basalt.
Figure 03.04a: The lava
sequences in areas A, B, and C
have alternating directions of
rock magnetization.
The Geomagnetic Field (Continued)
Figure 03.04c: Based on numerous
analyses of lava flows on land, the
pattern of polarity reversals of the
Earth's magnetic field has been
accurately established.
• This imparts a permanent
magnetic field, called
paleomagnetism, to the rock.
• This is measured by looking at
magnetic rocks of known age
that are collected from one
place to construct a paleomagnetic time scale.
The Geomagnetic Field (Continued)
Figure 03.04b: During normal polarity,
the geomagnetic north pole lies near
the geographic north pole. During a
reversal, it flips with the geomagnetic
south pole.
The geomagnetic polarity
time scale, a.k.a. the
paleomagnetic time scale
shows that over geologic
time, the Earth’s magnetic
field polarity (direction)
periodically reverses poles
The Sea Floor and the Geomagnetic Field
• Because of their paleomagnetism, rocks of the sea
floor influence the magnetic field recorded by
magnetometers.
• Rocks on the sea floor with normal polarity (e.g.
the same as today) paleomagnetism locally
reinforce Earth’s magnetic field making it
stronger and producing a positive anomaly.
• Rocks on the sea floor with reverse polarity (e.g.
the opposite from today) paleomagnetism locally
weaken Earth’s magnetic field, producing a
negative anomaly.
Magnetic Reversals
Magnetic reversals
give rise to a magnetic
anomaly signature that
is the sum total of both
the modern field and
the influence of the
ocean floor rocks.
Figure 03.04d: A magnetometer measures
simultaneously both the Earth's magnetic field
and the fossil magnetization in the rocks.
Dating and the Magnetic Field
• Rocks forming at the ridge
crest record the magnetic field
existing at the time (and place)
when they solidify.
• Sea floor increases in age and
is more deeply buried by
sediment away from the ridge
because sediments have had a
longer time to collect.
• It also gets deeper as it cools,
because colder rocks are
denser and “float” lower
(isostasy!)
Figure 03.05a: Sea-floor speading
combined with geomagnetic polarity
reversals creates magnetic anomaly
stripes
Determining Rates of Change
• Rates of sea-floor spreading vary from 1 to 10 cm per year for each
side of the ridge.
• Rates can be determined by dating magnetic anomaly stripes of the
sea floor and measuring their distance from the ridge crest.
• Continents are moved by the expanding sea floor - they ride on
larger blocks we call “Lithospheric Plates”.
Figure 03.05b: The rate of seafloor
spreading is easily calculated using the
age and distance from the ridge crest of
any magnetic anomaly stripe.
The Changing Ocean Floor
• Because Earth’s size has not changed, expansion
of the crust in one area requires destruction
(throuh convergence) of the crust elsewhere.
• Currently, the Pacific Ocean basin is shrinking
(because the oceanic lithospheric plate is being
forced back into the mantle) as other ocean basins
expand. This process is violent, and produces
many earthquakes.
• Seismicity is the frequency, magnitude and
distribution of earthquakes.
• Tectonism refers to the deformation of Earth’s
crust.
3. Global Plate Tectonics
- Because Earth’s size is constant, expansion of
the crust in one area requires destruction
of the crust elsewhere.
- Earthquakes are concentrated along oceanic
ridges, transform faults, trenches, and
island arcs.
- Subduction is the process at a trench
whereby one part of the sea floor plunges
below another and down into the
asthenosphere
Earthquakes
Earthquakes are concentrated along oceanic ridges,
transform faults, trenches and island arcs; larger
and deeper earthquakes at trenches, smaller and
shallower earthquakes at ridges.
Figure 03.06a: Most
earthquakes (represented
by black dots) clearly
coincide with midocean
ridges, transform faults, and
deep-sea trenches.
Subduction Zones
• Destruction of sea floor occurs in subduction
zones.
• Subduction is the process at a trench whereby one
part of the sea floor plunges below another and
down into the asthenosphere.
• The final stage of subduction is the collision
between continents that ride on the lithospheric
plates… these continents are too low-density to
allow subduction to continue (isostasy again!), and
mountain belts form
Figure 03.12b: These diagrams depict
the collision and suturing of the India
and Asia plates.
Subduction Zones
• The Wadati/Benioff Zone is an area of increasingly deeper
seismic activity that runs parallel to a subduction trench and
slopes down into the mantle parallel to the down-going plate.
• It is inclined from the trench downward in the direction of the
island arc.
Figure 03.06b: Sketch map of the south Fiji Basin and cross section showing a plot of
earthquakes that clearly defines a Benioff zone.
Lithospheric Plates
Figure 03.07a: The edges of large lithospheric plates are indicated by bands of seismicity.
The arrows indicate relative plate motions.
Plate Movement
Movement of plates is caused by thermal convection
of the “plastic” rocks of the asthenosphere which
drag along the overlying lithospheric plates.
Figure 03.08: Convection in the
asthenosphere drags lithospheric plates
away from the crests of ocean ridges.
Figure 03.07b: A brittle lithospheric plate,
which includes the crust and the upper
mantle, overlies and moves relative to the
plasticlike asthenosphere.
The Wilson Cycle
• The Wilson Cycle refers to the sequence of events leading
to the formation, expansion, contracting and eventual
elimination of ocean basins. This cycle is thought to repeat
over 200-500 million year time scales.
• Stages in basin history are:
– Embryonic - rift valley forms as continent begins to split.
– Juvenile - sea floor basalts begin forming as continental
fragments diverge.
– Mature - broad ocean basin widens, trenches eventually
develop and subduction begins.
– Declining - subduction eliminates much of sea floor and
oceanic ridge.
– Terminal - last of the sea floor is eliminated and continents
collide forming a continental mountain chain.
The Wilson Cycle
Figure 03.11: The Wilson cycle depicts ocean-basin development as proceeding through a
sequence of distinct stages.
The Red Sea
• The Red Sea is a juvenile ocean basin that is forming
as the African plate diverges from the Arabian plate.
• New basaltic ocean crust is just beginning to form in
the center of the Red Sea.
Figure B03.04c: New oceanic crust is forming in
the axial trough of the Red Sea by the process of
seafloor spreading.
Figure B03.04a: This
satellite image shows the
terrain of the Red Sea
region.
Figure B03.04b: The Red Sea is a juvenile
ocean basin that has just recently opened
up as the Arabian plate separates from the
African plate.