Download 3 - Sea Floor Spreading

Harry Hess
Naval Officer in World War II
Sea Floor Spreading Theory
-mid-ocean ridges
-magnetic striping
- recycling of the seafloor
http://pubs.usgs.gov/gip/dynamic/HHH.html
Biography of Harry Hess
Seafloor Spreading
Harry Hess
1960s – the theory of seafloor spreading
proposed to explain presence of midocean ridge.
– SONAR (sound, navigation, and range)
used to map the ocean floor
– A system of underwater mountain ranges
discovered around the world
Seafloor Spreading
• Questions Hess wanted answers to:
– Why is there so little sediment deposited on
the ocean floor? If the oceans have existed for
at least 4 billion years, as most geologists
believed, shouldn’t there be more?
– Why are fossils found on the seafloor no
more than 180 million years old? Marine
fossils in sedimentary rocks on land -- some of
which are found high in the Himalayas, over
8,500 m above sea level – are much older.
– How do the continents move?
Seafloor Spreading
• Hess’ reasoning:
– Sediment has been accumulating for about 300
million years at most.
• Matches time needed for the ocean floor to move
from the ridge crest to the ocean trenches,
where oceanic crust descends into the trench
and is destroyed.
– Magma is continually rising along the midoceanic ridges, "recycling" crustal material by
creating new oceanic crust.
• Recycling of the seafloor explains why the oldest
fossils found on the seafloor are no more than
about 180 million years old.
Seafloor Spreading
• Explains how continents moved
– Wegener thought the continents must simply
"plow" through the ocean floor, critics argued
this was physically impossible.
– With seafloor spreading, the continents did
not have to push through the ocean floor
but were carried along as the ocean floor
spread from the ridges.
Sonar - Echo Sounding
Evidence for Seafloor Spreading
• molten material - erupting along midocean ridges
• magnetic striping - reversals of Earth’s
magnetic field is recorded in stripes
parallel to the ridges (last was 780,000
years ago)
• drilling samples
– Glomar Challenger – drilling ship built in 1968,
youngest rocks were always found at the midocean ridges
What’s goin’ on?
• Hot, less dense material below Earth’s
crust rises upward to the surface at the
mid-ocean ridges.
• It then flows sideways, carrying the
seafloor away from the ridge.
• As the seafloor spreads apart, magma
moves up and flows from the cracks,
cools, and forms new seafloor.
Seafloor Spreading
Magnetic Striping
Seafloor Spreading
& Magnetic Striping
1) Mid-ocean ridge topography with magma chamber
below,
2) Magma rises and new ocean plate spreads away
from ridge.
3) Magnetic stripes form in igneous rocks containing
iron as they move away from ridge.
Magnetic Reversals
• What is the process?
– reversals happen infrequently—on average
every 250,000 years
– it's been over 700,000 years since the last
reversal, and the next one may be currently
underway
– NOVA Animation of Magnetic Pole Reversals
click on “Launch Interactive”
Age of Crust
Glomar Challenger