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.
• A device that bounces
sound waves off
and then records the
echoes of these sound
• Sonar mapped midocean ridges.
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.
• Continental drift reexamined in 1960’s with new
• New theory developed – Seafloor spreading
– Age of seafloor
– Heat flow
• Theory combining continental drift and seafloor
spreading termed “Plate Tectonics”
• The submersible,
Alvin, found strange
rocks shaped like
from a tube. Such
rocks can form only
when molten material
hardens quickly after
erupting under water.
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
The earth’s magnetic field does not point exactly north
The magnetic field is produced by the motion
of electric charges, i.e., electric current
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
The magnetic field lines come out of the Earth near Antarctica and enter
The liquid iron
currents in the
direction of the
create the earth
The earth’s magnetic field direction
The study of paleomagnetism has demonstrated that
the Earth's magnetic field has changed over time.
The portion of the ocean crust nearest the
mid-ocean ridge must have what type
of magnetic polarity?
We don’t know!!!
Can you find a solution to this
tough scientific problem?
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
ocean floors - 200 million years
land - billions of years
Why is seafloor so young relative
New sea floor created at the mid-ocean
ridge and destroyed in deep ocean trenches
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
The theory of plate tectonics
Earth’s lithosphere is broken into giant
plates that move laterally on top of
Most earthquakes and volcanic
eruptions happen at plate boundaries.
Three types of relative motions between
The plate refers to the pieces of the lithosphere.
The plates of the lithosphere moves atop the
Current Earth’s surface layers are divided into
nine very large plates and several smaller ones
Earthquake distribution matches plate boundaries
Volcanoes match some plate boundaries;
some are hot spots
• There are three styles of convergent plate
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
• Forms mountains, e.g. European Alps, Himalayas
Continent-Oceanic Crust Collision
• Called SUBDUCTION
Earthquake depth indicates subduction zones
Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs over the
other which causes it to sink into the mantle forming a
• 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
– E.g. The Mariana Trench is 11 km deep!
Ocean-Ocean plate collision
Modern example: Japan
• Where plates slide past each other
Above: View of the San Andreas
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.
• Plates move away from each other
• New crust is being formed
An example Atlantic Ocean
also can rip
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
What are the
driving forces of
Fig. 5-11, p. 118
Street Clouds over City College of New York taken
by Professor Gedzelman
How does Earth work?
Theory of Plate Tectonics
How does convection work? No one knows
but they aren’t afraid to propose models!
Two mantle convection cells