Download Sea-Floor Spreading

Sea-Floor Spreading
During World
War II, geologist
Harry Hess
became
interested in
mapping the
ocean floor
using sonar.
Unexpected Finding:
Long mountain ranges existed throughout the
floors of the oceans called mid-ocean ridges.
The longest one runs through the middle of
the Atlantic Ocean and is called the Mid
Atlantic Ridge (MAR).
Hess devised a theory called sea-floor
spreading which states that at mid-ocean
ridges, molten material rises from the
mantle and erupts. It then spreads out,
pushing older rock to both sides of the
ridge.
In the 1960’s, other evidence was
found to support his theory:
• http://earthguide.ucsd.edu/eoc/teachers/t_te
ctonics/p_seafloorspreading.html
Molten Material
• Scientist aboard a submersible called Alvin dove
to the sea floor and discovered rocks called pillow
lavas along the mid ocean ridges. Such rocks can
only form from cooled molten material.
•
http://www.ocean.udel.edu/extreme2004/mission/submersiblealvin/index.html
Magnetic Stripes
• Rocks that make up the ocean floor lie in a
pattern of magnetized stripes, which show a
record of reversals of Earth’s magnetic field.
Drilling Samples
• In the year 1968, a drilling ship called the
Glomar Challenger was used to gather
samples of rocks from the ocean floor.
Scientists discovered that the further the rocks
were from the mid ocean ridges, the older they
were. The youngest rocks were always at the
center of the ridges.
Resulting Question: Are all of the
oceans getting bigger?
• Suggested Answer: Ocean floors do not keep
spreading. Parts of the oceanic crust get
plunged into deep-ocean trenches due to the
process of subduction.
Subduction-process by which the ocean floor
sinks beneath a deep-ocean trench and back
into the mantle.
http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_subduction.html
Subduction in the Pacific Ocean:
There are many trenches, so the
Pacific Ocean is getting smaller.
Subduction in the Atlantic Ocean:
There are fewer trenches, so the
Atlantic Ocean is getting wider.
• http://www.suu.edu/faculty/colberg/hazards
/platetectonics/18_Pangaea.html
Diagram of Sea-Floor Spreading: