2. Hess was in World War II
He was initially assigned to duty in New York City, where
he was responsible for estimating the positions of enemy
submarines in the North Atlantic. Hess was then assigned
to active sea duty and eventually became commander of
an attack transport ship. This vessel carried equipment
for sounding the ocean floor, and Hess took full
advantage of it. He mapped a large part of the Pacific
Ocean, discovering in the process the underwater flattopped seamounts that he named guyots, in honor of A.H.
Guyot, the first professor of geology at Princeton.
2
3. Sonar
• A device that bounces
sound waves off
underwater objects
and then records the
echoes of these sound
waves.
• Sonar mapped midocean ridges.
4. Hypothesis of sea-floor spreading
Molten rock (magma) oozes up from the
Earth's interior along the mid-oceanic
ridges, creating new seafloor that
spreads away from the active ridge
crest and, eventually, sinks into the
deep oceanic trenches.
4
6. • Continental drift reexamined in 1960’s with new
information
• New theory developed – Seafloor spreading
– Earthquakes
– Volcanism
– Age of seafloor
– Paleomagnetism
– Heat flow
• Theory combining continental drift and seafloor
spreading termed “Plate Tectonics”
7. • The submersible,
Alvin, found strange
rocks shaped like
toothpaste squeezed
from a tube. Such
rocks can form only
when molten material
hardens quickly after
erupting under water.
8. Scientists discovered that the rock
that makes up the ocean floor lies in a
pattern of magnetized “stripes”.
They hold a record of reversals in
Earth’s magnetic field
10. The magnetic field is produced by the motion
of electric charges, i.e., electric current
10
11. The Earth's magnetic field
appears to come from a giant
bar magnet, but with its
south pole located up near
the Earth's north pole (near
Canada).
The magnetic field lines come out of the Earth near Antarctica and enter
near Canada.
11
12. The liquid iron
appears to
produce electric
currents in the
opposite
direction of the
earth rotation.
The currents
create the earth
magnetism.
12
13. The earth’s magnetic field direction
The study of paleomagnetism has demonstrated that
the Earth's magnetic field has changed over time.
18. Scientists did a drilling
sample and found rocks that
the farther away from the
ridge the older the rocks
were. The younger ones
were in the center of the
ridge.
20. ocean floors - 200 million years
land - billions of years
Why is seafloor so young relative
to continents?
SUBDUCTION
21. New sea floor created at the mid-ocean
ridge and destroyed in deep ocean trenches
22. Subduction
Ocean floor plunges into deep-ocean trenches.
Subduction is the process by which the ocean floor sinks
beneath a deep-ocean trench and back into the mantle
22
24. The theory of plate tectonics
Earth’s lithosphere is broken into giant
plates that move laterally on top of
asthenosphere.
Most earthquakes and volcanic
eruptions happen at plate boundaries.
Three types of relative motions between
plates:
24
25. The plate refers to the pieces of the lithosphere.
The plates of the lithosphere moves atop the
asthenosphere
26. Current Earth’s surface layers are divided into
nine very large plates and several smaller ones
35. Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs over the
other which causes it to sink into the mantle forming a
subduction zone.
• The subducting plate is bent downward to form a very
deep depression in the ocean floor called a trench.
• The worlds deepest parts of the ocean are found along
trenches.
– E.g. The Mariana Trench is 11 km deep!
39. San Andreas Fault near Gorman, California. The grey, metamorphic quartz
monzonite on the left side of the fault are rocks of the Pacific Plate and the brown
sandstone and siltstone on the right of the fault are rocks of the North American
Plate. Photograph copyright by David Lynch.
Plate
39
40. • Plates move away from each other
• New crust is being formed
43. Continental Drift
1) Begins with view of Earth with continents in their present
positions, 2) continents move back in time to reunite as Pangaea, 3)
Pangaea label appears, 4) Locations of stratigraphic and fossil
evidence that Wegener used to argue in favor of continental drift is
added.
http://www2.nature.nps.gov/geology/usgsnps/animate/A14.gif
The 1960’s is relatively recent. Geology is a new science compared to Physics, Chemistry and even Biology. People were doing separate studies about earthquakes, volcanoes, age of sea floor, paleomagnetism. Someone put it all together. A new theory developed – Seafloor Spreading. There were a lot of evidence to support Seafloor spreading.
Note: Orange is shallow. At spreading centers, EQ only occur at the surface. At SZ, deep EQ occur, i.e. Japan, Indonesia, Philippines, Aleutians. How does seismic activity support plate tectonics? They occur along plate boundaries.
Note: Orange is shallow. At spreading centers, EQ only occur at the surface. At SZ, deep EQ occur, i.e. Japan, Indonesia, Philippines, Aleutians. How does seismic activity support plate tectonics? They occur along plate boundaries.
Convergent boundaries are where the plates move towards each other.
There are three types of convergent boundary, each defined by what type of crust (continental or oceanic) is coming together.
Therefore we can have: continent-continent collision, continent-oceanic crust collision or ocean-ocean collision….
When continental crust pushes against continental crust both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant). Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. For example, the European Alps and Himalayas formed this way.
At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust.
This is called a Subduction Zone.
The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram).
When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram).
The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes.
Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.
When two oceanic plates converge, because they are dense, one runs over the top of the other causing it to sink into the mantle and a subduction zone is formed.
The subducting plate is bent down into the mantle to form a deep depression in the seafloor called a trench.
Trenches are the deepest parts of the ocean and remain largely unexplored.
Manned or unmanned submersible vehicles (top right photo) have explored small parts of trenches discovering new species (like the fish photographed here) and amazing ecosystems.
The third type of boundary are transform boundaries, along which plates slide past each other.
The San Andreas fault, adjacent to which the US city of San Francisco is built is an example of a transform boundary between the Pacific plate and the North American plate.
Figure 5.11: Cumulus clouds form as hot, invisible air bubbles detach themselves from the surface, then rise and cool to the condensation level. Below and within the cumulus clouds, the air is rising. Around the cloud, the air is sinking.
Heat flow provided the mechanism to move the lithospheric plates.
