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The Sea Floor
Marine Biology
Chapter 2
A portion of
the midAtlantic ridge
about the sea
surface in
Iceland
Water Planet
• Oceans cover 71% of the globe
– 61% of the Northern hemisphere
– 80% of the Southern hemisphere
• Regulates the climate and atmosphere
Fig. 2.1
Ocean Basins
• Classified traditionally into four
large basins:
–Atlantic Ocean
–Pacific Ocean
–Indian Ocean
–Arctic Ocean
Tab. 2.1
Which ocean is the largest and deepest?
Which two are of average depth?
Which ocean is smallest and shallowest?
World Ocean
• All four basins are connected
• The interconnected system is
referred to as the world ocean
Southern Ocean
• Oceanographers refer to the
continuous body of water that
surrounds Antarctica as the
Southern Ocean.
Fig. 2.2
Where did the Earth come from?
• The Big Bang caused a
cloud of dust to form.
(13.7BYA)
• The dust cloud forms into
the Earth and the solar
system (4.5BYA)
• The universe began with
a massive expansion
• It is continuing to expand
http://www.charliewagner.net/big.htm
http://www.charliewagner.net/big.htm
• Any particle with mass has
an attraction for any other
particle with mass.
• Small pieces collide and
clump into larger pieces.
• Larger pieces clump
together, forming rocks,
meteors, and eventually
planets or stars.
DENSITY
• So much heat was generated as the
Earth formed that it was probably
molten.
• Matter settled within the planet based
on density.
• Density is the mass of a substance
divided by its volume.
Density
density =
mass
volume
www.johnniemoore.com/blog/im
ages/icecubes.jpg
www.ramdh
When two substances are
mixed, the denser material
sinks and the less dense
material floats.
www.ramdhanyk.com
Fig. 2.3
Earth’s Internal Structure
• Core is mostly iron
– Pressure a million times that at the surface
– 4000oC (7,200oF)
– Solid inner core, liquid outer core
– Swirling motion in the outer core produces
the earth’s magnetic field
Mantle and Crust
• Mantle solid but nearly at the melting point of
rock
–
–
–
–
Mantle slowly flows like a liquid
Upper and lower layers
Uppermost part is sold
Outermost portion of mantle and crust form the
lithosphere
– Mantle layer just under lithosphere is the
asthenosphere (relatively fluid)
• Crust is the outermost layer
– Relatively thin
Tab. 2.2
The nature of the rock at any area in the crust determines
the elevation and whether it is covered with water.
Ocean and Continental Crusts
• Both crusts are less dense than
underlying mantle
• Continents float on the mantle like
icebergs
• Ocean crusts float on the mantle, but
not as high (why continents are dry)
• Oceanic crust is much thinner than
continental crust.
Lithosphere and Aesthenosphere
• Lithosphere includes the crust and the
upper rigid mantle
• Aesthenosphere
– 100-250 km deep
– Highly viscous, flexible
– flows
Continental drift
• Alfred Wegener - geophysicist 1912
• Wegner’s hypothesis
– All continents once joined together in a
supercontinent
– Pangaea
– Began breaking up 180 million years ago
– Did not explain how continents moved
Evidence of Plate Tectonics
• Continents on opposite sides of Atlantic
fit together like puzzle pieces
• Coal deposits and geologic formations
on opposite sides of Atlantic match up
• Fossils match
• Mid-Ocean Ridges
Fig. 2.4
Fossil evidence of
supercontinent
orange indicates the fossil
remains of Cynognathus, a
Triassic land reptile. Dark
blue indicates fossil
remains of the freshwater
reptile Mesosaurus. Green
indicates fossils of the fern
Glossopteris, found in all
of the southern
continents. Brown
indicates fossil evidence of
the Triassic land reptile
Lystrosaurus. (Map
courtesy United States
Geological Survey)
Mid-Ocean Ridges
• Discovered by sonar after World War II
• A continuous chain of volcanic
mountains
• The world’s largest geological feature
Fig. 2.6
Fig. 2.5
Deep depressions in the seafloor are called trenches.
Fig. 2.7
Trenches and ridges are geologically active.
Mid-Ocean Ridges
• Associated with earthquakes and
volcanoes
• Sediments are thicker farther from ridge
• rock older farther from the ridge
• Bands of rock alternating between
normal and reversed magnetism
– Parallel to the ridge
Fig. 2.8
Magnetic
Anomalies
Particles in
molten
rock act
like tiny
compasses.
Field reversals
occur naturally
over hundreds
of thousands
of years.
Fig. 2.9
Sea floor
spreading and
magnetic fields
Fig. 2.10
The lithosphere is broken up into plates.
Plates can contain oceanic crust, continental crust, or both.
Lithospheric plates float on the aesthenosphere.
Fig. 2.11
Some trenches are formed by the collision of oceanic and
continental crusts.
One plate slips below the other.
Subduction
• Plate descends into the mantle
• Heat and pressure breaks up plate
– causes earthquakes
• More heat melts the plate
• Some molten material rises to form
volcanoes
• Denser oceanic crust sinks
– Rock recycled
– Very old rocks are always continental
Fig. 2.12
When two oceanic plates collide, one dips below the other
Trenches curve due to earth’s spherical shape.
Creates island arcs
Aleutian islands
Mariana islands
Fig. 2.13
Mt. Veniaminof,
part of Aleutian
Island Chain
Three types of plate
boundaries
• Mid-Ocean Ridges
• Trenches
• Shear boundaries
– Two plates move past each other
– Intense friction
– Sometimes “locks”, builds pressure, then
slips = EARTHQUAKE!!
Fig. 2.14
San Andreas Fault is
the largest example
of a shear boundary.
Fig. 2.15
Driving forces of tectonic plate movement:
primarily “slab pull” and partially convection currents.
Slab pull: cooling lithosphere gets denser, slips into
mantle, pulling the rest of the plate behind it.
Causes separation at mid-ocean ridge.
Fig. 2.16
Geologic
history of the
Earth.
Sediments
The seafloor is carpeted
with muddy sediment.
Two most abundant types of sediment:
Lithogenous
made from rock
physical and chemical breakdown
Biogenous
made from living things
skeletons of diatoms, foraminiferans
radiolarians and coccolithophorids
Fig. 2.17
Fossil shell of a
foraminiferan, used
as an indicator of
ocean temperature
in the past.
Types of Biogenous Sediment
• Siliceous ooze
– Made of silica (SiO2)
– Glasslike
• Calcareous ooze
– Made of calcium carbonate (CaCO3)
Fig. 2.18
Using the magnesium/calcium ratio of foraminiferans
in core samples, scientists determine average sea
surface temperatures.
White and blue lines indicate glacial and
interglacial periods.
Fig. 2.19
The Continental Margin
Continental Margins Part 1
• Continental shelf
– Shallowest part of the margin
– 8% of the ocean surface area
– Composed of continental crust
– 1 km – 750km wide
• Shelf break
– Slope abruptly gets steeper
– Starts at 120-400m deep
Continental margins Part 2
• Continental slope
– Exact edge of the continent
– Submarine canyons channel sediment to
deep-sea floor.
• Continental rise
– Sediment moving through submarine
canyon accumulates
• Deep-sea fan
Abyssal plain
•
•
•
•
•
Deep sea floor
Oceanic crust
Relatively flat
Abyssal hills
Seamounts
– Submarine volcanoes
– Guyots (gee-oh’s) are flat-topped
seamounts
Fig. 2.20
The continental shelf off
of Atlantic City
Scours are scars made by
icebergs during the last iceage.
Fig. 2.21
Multibeam sonar image of California continental
margin. Arrow indicates Monterey Canyon
Active and Passive Margins
• Depends on plate tectonics
• Active margins
– Continental/Oceanic crust margins form
trenches
– Highly active geologically
• Earthquakes, volcanoes
• Passive margins
– Continental edge is not a plate boundary
Fig. 2.22
Fig. 2.23
Steep,
rocky
shorelin
es are
typical
of active
margins.
California
Coast at
Monterey
Fig. 2.24
A broad coastal plain with wide
shelf, gentle slope indicates a
passive margin (Atlantic Coast)
Fig. 2.25
Plates
pulling
apart at
mid-ocean
ridge.
In the center of the mid-ocean
ridge is the central rift valley
Central rift valley and
Hydrothermal Vents
• Cracks and crevices in floor and sides of
valley
• Sea water seeps down cracks
• Sea water heated by hot mantle
material
• Heated water forces back up through
crust as hydrothermal vents
– Deep-sea hot springs
Hydrothermal Vents
• Discovered in 1977
• Warm water
– 10-20oC (50-68oF)
– Some 350oC (660oF)
• Dissolved minerals
– Sulfides
– Rapidly cool and solidify
– “black smokers”
• Biologically rich
Fig. 2.26
Fig. 2.27
Formation of the Hawaiian
Islands
• The Hawaiian Ridge Chain
• Connected to the Emperor Seamount
chain
• Volcanoes progressively older northwest
along the ridges
• Newest island: Loihi
– Not yet above the surface
Formation of the Hawaiian
Islands
• Hot spot
– Plume of hot magma rises from deep in the
mantle
– Pacific plate moves to north and east
above the mantle.
– New volcano forms above current location
• The “debate”
– Skeptics say volcanoes occur because of
stress or weakness in tectonic plate
causing cracking.
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Kilauea