Download Seafloor Spreading Hypothesis

Seafloor Spreading Hypothesis
Harry Hess
How do the continents move?
Harry Hess was a geology professor and a naval officer. He
commanded an attack transport ship during WWII. Hess was
intrigued by the seafloor maps produced with the ship's echo
sounder. He thought about all of the evidence for continental drift.
He thought about all of the unusual features of the seafloor. And he
found the mechanism to explain them all.
The Evidence Comes Together
World War II allowed scientists to make some puzzling
observations. The observations came from seafloor bathymetry and
magnetism.
These observations are:
• The seafloor has a large mountain range running through it.
Deep trenches are found far from the ridges. Guyots have
eroded tops that are deep below sea level.
• The magnetic polarity of the seafloor changes. The center of
the ridge is of normal polarity. Stripes of normal and reverse
polarity are found symmetrical on both sides of the ridge.
• The youngest seafloor is at the ridge. The oldest is farthest
from the ridge. The oldest seafloor is much younger than the
oldest continent.
Scientists needed to explain these observations.
Mantle Convection
Not long after Wegener's death, scientists recognized that there is
convection in the mantle. Deeper material is hotter and so it rises.
Near the surface, it becomes cooler and denser so it sinks. This
creates a convection cell in the mantle.
Seafloor Spreading
After the war, Harry Hess put together the ideas and evidence he
needed. Hess resurrected Wegener's continental drift hypothesis.
He reviewed the mantle convection idea. He thought about the
bathymetric features and the patterns of magnetic polarity on the
seafloor. In 1962, Hess published a new idea that he called seafloor
spreading.
Hess wrote that hot magma rises up into the rift valley at the midocean ridges. The lava cools to form new seafloor. Later more lava
erupts at the ridge. The new lava pushes the seafloor horizontally
away from the ridge axis (Figure below). The seafloor moves!
Magnetite crystals in the lava point in the direction of the magnetic
north pole. The different stripes of magnetic polarity reveal the
different ages of the seafloor.
In some places, the oceanic crust comes up to a continent. The
moving crust pushes that continent away from the ridge axis as
well. If the moving oceanic crust reaches a deep sea trench, the
crust sinks into the mantle. The creation and destruction of oceanic
crust is the reason that continents move.
Magma at the mid-ocean ridge creates new seafloor.
• As oceanic crust moves away from the ridge crest, it pushes a
continent away from the ridge axis.
• If the oceanic crust reaches a deep sea trench, it sinks into the
trench.
• The oldest crust is coldest and lies deepest in the ocean.
The flat topped guyots were once active volcanoes that were above
sea level. They were eroded at their tops. As the seafloor moved
away from the ridge, the crust sank deeper. The tops of the guyots
went below sea level.
The Mechanism for Continental Drift
Seafloor spreading is the mechanism that Wegener was looking
for! Convection currents within the mantle drive the continents.
The continents are pushed by oceanic crust, like they are on a
conveyor belt. Over millions of years the continents move around
the planet’s surface. The spreading plate takes along any continent
that rides on it.
Vocabulary
convection cell: Hot material rises and cool material sinks in a
circular pattern.
seafloor spreading: Mechanism for moving continents. The
formation of new seafloor at spreading ridges pushes lithospheric
plates on the Earth's surface.
Summary
• Seafloor spreading is a mixture different ideas and data.
• Continental drift and mantle convection are supported by
bathymetric and magnetic data from the seafloor.
• Harry Hess called his idea “an essay in geopoetry." This
could be because so many ideas fit together so well. It could
also be because, at the time, he didn’t have all the seafloor
data he needed for evidence.
• Seafloor spreading is the mechanism for the drifting
continents.
Click on the following links to view the animations that show the
creation of magnetic stripes of normal and reversed polarity at a
mid-ocean ridge:
http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A49.gif
http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A55.gif.
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