Who is Harry Hess?
• Harry Hess was a
geologist and Navy
submarine
commander during
World War II.
• Part of his mission
had been to study
the deepest parts of
the ocean floor.
• Scientists began to
use magnometers to
study the ocean
floor in the 1950s.
• The Great Global
Rift (mid-ocean
ridge) was
discovered.
• What are mid-ocean
ridges?
What are mid-ocean ridges?
• Mid-ocean ridges are underwater
mountain ranges that extend into all of
Earth’s oceans.
• They curve like the
seam of a baseball
along the sea floor.
What did Harry Hess propose?
• The discoveries
inspired Hess to look
back at his data from
years before.
• In 1960, Harry Hess,
proposed that the
movement of the
continents was a result
of sea-floor spreading.
• What is sea-floor
spreading?
What Is Sea-Floor Spreading?
• In sea-floor spreading, the sea floor
spreads apart along both sides of a
mid-ocean ridge as new crust is added.
As a result, the ocean floors move like
conveyor belts, carrying the continents
along with them.
Convection Currents and
Sea-Floor Spreading
What is the evidence for Sea-Floor Spreading?
• Several types of evidence supported Hess’s theory
of sea-floor spreading: eruptions of molten material,
magnetic strips in the rock of the ocean floor, and
the ages of the rocks themselves.
• What are magnetic strips?
• The Earth
acts like a
giant
magnet.
What causes the magnetic field?
• Scientists think that
the movements in
the liquid core
create Earth’s
magnetic field.
The magnetic poles are constantly changing
and this is recorded in the Earth’s rocks.
This is a reading of the magnetic
strips along the Mid-Atlantic Ridge.
http://openlearn.open.ac.uk/file.php/2717/S279_1_006i.jpg
Magnetic minerals within igneous rocks align
themselves with the magnetic poles.
Do we track plate movement?
• Scientists use
satellites to
track the
movements of
the Earth’s
plates.
Interesting Fact!
• Because of sea-floor spreading, the
distance between Europe and North
America is increasing by a few
centimeters per year.
Rock samples
• Scientists have also collected core samples.