Transcript Oceanography Chapter 11

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►The Earth Inside and Out
►The Theory of Continental Drift
►The Theory of Seafloor Spreading
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►The Unifying Theory: Plate Tectonics
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Earth’s Internal Layers
Chapter 11 Pages 11-4 & 11-5
The Earth Inside and Out
 The current theory is that the Earth’s
interior consists of multiple layers:
 The inner core is primarily iron and nickel
plus other heavy elements. It is theorized
to be solid due to enormous pressure.
 The outer core is made up of the same
elements. However, it is theorized that
with less pressure, this layer is liquid.
 The mantle is thought to contain mostly
silicon and oxygen, with some iron and
magnesium. It consists of the upper
mantle and the lower mantle. The upper
mantle itself has two layers.
 The crust is composed mainly of oxygen,
silicon, magnesium, and iron. It varies
in thickness and is the outer layer of
the lithosphere.
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Earth’s Internal Layers (continued)
 Earth’s internal layers are further divided:
 Scientists separate the uppermost mantle from the crust because they think the
mantle’s elemental composition changes little. The crust consists of different
rock types thought to undergo change over long periods.
Chapter 11 Pages 11-5 11-6
The Earth Inside and Out
 The upper mantle also has two layers. The top part of the upper mantle is the
asthenosphere. The asthenosphere is also solid but much the same as old glass is a
solid that flows slowly over time. Hotter, less dense material (magma) rises towards
the surface where it can eventually flow from a volcano or other opening. At this point
the molten rock is lava.
 The lithosphere includes the uppermost, rigid part of the upper mantle and the crust.
This is the solid rock portion of the outer Earth that rests on the molten asthenosphere.
 Also differentiating the crust from the mantle are conditions such as temperature and
pressure.
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The Rock Cycle
Chapter 11 Pages 11-6 & 11-7
The Earth Inside and Out
 Scientists believe that rocks form
or change over long periods due to
the processes of the rock cycle.
 The rock cycle is like a recycling
machine, endlessly converting one
type of rock to another.
 Three types of rock found in the crust are:
 1. Igneous – rocks that form when magma/lava cools and hardens.
 2. Sedimentary – rock made up of particles deposited by water flow. Sediment
consists of organic/inorganic matter particles that have been compressed into rock.
This is the type of rock in which fossils are found.
 3. Metamorphic– as layers build up, this rock is formed when pressure and heat
become great enough to change the rock chemically.
 The rock cycle is completed through the tectonic process – rocks return to
the mantle, remelt, become magma, return to the crust as igneous rock.
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Isostatic Equilibrium
 With the crust resting on the mantle, there must be a balance between the
weight of the crust and the upward force of buoyancy. This is an application
of Archimedes' Principle of buoyancy.
 This balance is called isostatic equilibrium.
 As material adds to the oceanic crust from sedimentation, glaciers and volcanic
activity or from the continental crust from erosion, this balance becomes
disrupted. This is one theorized cause of earthquakes.
Chapter 11 Pages 11-7 & 11-8
The Earth Inside and Out
 To restore equilibrium, landmasses will sink or rise slightly along a weak area called a
fault.
 The accepted theory of plate tectonics
suggests that the continents move in
horizontal directions and that earthquakes
also result from that movement.
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Alfred Wegener and Pangaea
 Pangaea is the name given to the single giant continent in a proposal by
Wegener (1912).
 He suggested that in the distant past all the Earth’s continents had been a single giant
continent.
Chapter 11 Page 11-10
The Theory of Continental Drift
 Panthalassa is the name given to the single large ocean surrounding Pangaea.
 The theory of continental drift states that the continents were once a single
landmass that drifted apart and are still doing so.
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Evidence for Continental Drift
 Accurate world maps. People noticed that the continents apparently fit together like a
jigsaw-puzzle.
 Fossils of the Glossopteris fern in South
America, Africa, Australia, India, and
Antarctica. The seeds are too heavy
to travel by wind and too fragile to
survive significant sea crossings.
 Distribution of other animals and fossils were
studied, especially coal.
Chapter 11 Pages 11-11 to 11-13
The Theory of Continental Drift
 Wegener tried to support his theory with evidence:
 Wegener’s theories were not accepted because he could not explain how
continents could drift, also he was a meteorologist, not a geologist.
 The plate tectonics theory would finally provide an explanation for how
continents move, making Wegener’s theory widely accepted.
 Additionally, it was found that including the continental shelves filled the gaps in the
jigsaw puzzle.
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New Technology and Seafloor Knowledge
 Sonar made it possible to detect an otherwise invisible object. Scientists
immediately recognized that they could map the ocean bottom with sonar.
 The German Meteor expedition mapped the South Atlantic in 1925. This became the
basis for the first worldwide seafloor map of deep ocean ridges.
Chapter 11 Pages 11-14 & 11-15
The Theory of Seafloor Spreading
 The ability to map the seafloor in greater detail revealed important new features:
 Mid-ocean ridges are enormous mountain ridges on the bottom of the ocean.
 Rift valleys are deep valleys
running through the center of
mid-ocean ridges such as
the Atlantic Ridge.
 Trenches are deep ravines
in the seafloor.
 Scientists began to notice patterns
in the mid-ocean ridges, trenches,
and rift valleys that proved pivotal
in developing new theories.
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Chapter 11 Page 11-16
The Theory of Seafloor Spreading
The Creation and Destruction of Seafloor
 Hess and Dietz (1960) proposed an explanation of seafloor features. They
hypothesized that the seafloor is in a constant state of creation and destruction
through a process called seafloor spreading.
 In the theory of seafloor
spreading, new crust
emerges from the rift valley
in a mid-ocean ridge. Magma
from the asthenosphere
pushes up through the rift
and solidifies into new crust.
 The new seafloor forms at
the rift valleys and midocean ridges, spreading
away from the ridges until
it returns as part of the
rock cycle at subduction zones (trenches).
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Evidence of Seafloor Spreading

Scientists have found three forms of evidence that
support the theory of seafloor spreading.


Chapter 11 Pages 11-17 to 11-20
The Theory of Seafloor Spreading

1. In 1968, scientists began drilling holes in the
seafloor to gather sediment samples – the layers
were thin or absent at the ridges, and thicker away
from the ridges.
2. Radiometric dating was used to determine the age
of rocks. Scientists have found seafloor rock to be
significantly younger than rock in the center of the
continents. According to the theory, continental rock
doesn’t subside, where as seafloor rock does.
3. Evidence from magnetometer data. This instrument
measures the polar orientation and intensity of
magnetism of minerals. After towing magnetometers
around the seafloor beginning in 1950, scientists
discovered a symmetrical pattern in the polar
orientation of magnetism of seafloor rock, the seafloor
on either side of the mid-ocean ridges roughly mirrors
each other’s polar orientation.
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Seafloor Spreading and Continental Drift Combine
 The theory of plate tectonics united the theories of continental drift and
seafloor spreading.
 In plate tectonic theory the Earth’s lithosphere consists of more than a dozen
separate plates. The plates are rigid and float on the asthenosphere.
 At a spreading or divergent boundary, two plates are moving apart. As this
happens, the crust pulls apart and forms valleys. Magma flows up through the
rift valleys creating new crust and widening the seafloor.
 Mid-ocean ridges and rift valleys mark divergent boundaries.
Chapter 11 Pages 11-22 to 11-25
The Unifying Theory: Plate Tectonics
 It combines ideas from both theories, along with some of the original isostatic
equilibrium concepts.
 At a colliding or convergent boundary, two plates push together.
 They are also called destructive boundaries because movements along these destroy
crust.
 At a transform boundary or fault, two plates slide past each other.
 Earthquakes result as rocks move when the plates slide next to each other.
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Seafloor Spreading and Continental Drift (continued)
 Island arcs (divergent boundary).
 Subduction zones (convergent boundary).
 A trench forms as a more dense oceanic plate moves
under a less dense continental plate. As subduction
occurs, some of the material from the melting oceanic
plate rises upward to form volcanoes on the continent.
Chapter 11 Pages 11-22 to 11-25
The Unifying Theory: Plate Tectonics
 When two oceanic plates collide, the denser plate will
be subducted. Some of the material from the melting
oceanic plate rises upward to form a volcanic island
arc.
 Convergent boundary (transform boundary).
 Mountain formation at continental plate collision.
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Chapter 11 Pages 11-25 & 11-26
The Unifying Theory: Plate Tectonics
Hot Spots
 The hot spot theory states that
hot spots are small melting areas
within the mantle where thermal
plumes cause magma columns to
push up intensely, breaking through
the crust.
 What makes this theory significant
is that hot spots do not move with
tectonic plates because they
originate in the mantle.
 Volcanic island chains, are the
result of the plate moving over a
hot spot.
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Plate Movement
 Convection currents form as hot material rises and cold material sinks. This occurs in
the mantle.
 This creates a current that moves the plates away from each other at the divergent
boundaries, toward each other at the convergent boundaries, and past each other at
the transform boundaries.
 A second driving force comes from seafloor spreading.
Chapter 11 Pages 11-26 & 11-27
The Unifying Theory: Plate Tectonics
 Convection is the primary force driving seafloor spreading and
continental drift.
 As new seafloor forms, the plates
tend to slide away from
the elevated mid-ocean ridges.
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Plate Movement (continued)
Chapter 11 Pages 11-28 & 11-29
The Unifying Theory: Plate Tectonics
 Predicted changes over the next 50 million years:
 The Baja Peninsula will have moved past and apart
from the North American Plate.
 Southern California will pass San Francisco as it
moves to the northwest.
 A new sea will form in eastern Africa.
 Australia will move toward Eurasia/the Equator.
 The Mediterranean Sea will close as Africa pushes
towards Europe.
 The Atlantic and Indian Oceans will continue to grow
while the Pacific will become smaller.
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