Transcript Sea Floor Spreading and Convection

Review
Alfred Wegener…
 No one believed him because he did not explain…
 What force could be strong enough to move such large
masses of rock over such great distances?
 How are the continents moving?
Background
 First we’ll answer why the continents are moving.
 We began to take a good look at the ocean floor
because of submarine warfare in World War Two.
 Until the mid 1900s, most scientists thought that the
ocean floor, unlike the continents, was flat.
 WHY?
Submarine Warfare
 The demands of the war promoted periods of rapid
technological advances.
 For the first time, we began to use submarine warfare.
 Cartographers needed to make detailed maps of the
ocean floor for the military so that they may navigate
through the water.
Sonar
 Sonar (SOund Navigation And Ranging)is an echosounding technique that has been used to map oceanfloor topography.
 The time it takes for the sound waves to travel is used
to calculate the distance to the ocean floor.
Ocean Floor Topography
 Underwater mountain ridges were discovered using
sonar.
 Ocean ridges = underwater mountain ranges.
 Later we discovered that volcanism and earthquakes
are common along the ridges.
Deep Sea Trenches
 Deep sea trenches: narrow, elongated depressions in
the seafloor with very steep sides.
Magnetometers
 During WWII, the magnetometers
were used to detect the magnetic
fields generated by the steel hulls
of submarines.
 A magnetometer is a device that
can detect small changes in
magnetic fields.
 Scientists began to use the
magnetometers to measure the
magnetic strength of rocks on the
ocean floor.
 The data helped scientists discover
the theory of sea floor spreading.
Paleomagnetism
 Paleomagnetism is the study of the magnetic record.
 The magnetic fields of basalt changes with time. The
magnetic pattern matched on either side of ocean
ridges.
Dating Ocean Rocks
 Scientists also began to take samples of ocean rocks.
 The ages of rocks appeared in a predictable pattern.
 The age of rock increased the farther you moved away
from the ocean ridge.
Mid-Atlantic Ridge-New Rock
B
A
Age (millions of years)
Age of Mid-Atlantic Ridge
B
A
Distance from Mid-Atlantic Ridge
Sea Floor Spreading
 With the topographic, sedimentary and age data from
the seafloor, an American Scientist named Harry Hess
proposed the theory called sea floor spreading.
 Sea floor spreading: states that new ocean crust is
formed at ocean ridges and destroyed in deep-sea
trenches.
Magma Role
 Magma is very viscous, so it is less rigid
than surrounding material. This forces the
magma into the crack in the crust, away
from the ocean ridge.