Sea-Floor Spreading

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TOPIC OBJECTIVES FOR GROUP REPORTERS:C. Sea-floor Spreading Enumerate and explain the initial and

unusual observations that led to the formulation of the sea-floor spreading hypothesis.

Enumerate and explain the evidences that support the Sea-Floor Spreading Process.

Differentiate continental crust from oceanic crust and explain why the continental crust is older than the oceanic crust.

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Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart.    

As magma seeps out between the plates, it solidifies and extends the ocean floor and creates a long chain

mountain called the Mid-Oceanic Ridge.

• Seafloor spreading is the movement of two oceanic plates away from each other (at a divergent plate boundary), which results in the formation of new oceanic crust (from magma that comes from within the Earth's mantle) along a mid-ocean ridge.

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Where the oceanic plates are moving away from each other is called a zone of divergence.

Sea Floor Spreading Hypothesis

• Proposed by Harry H. Hess in 1960, he hypothesized that the formation of underwater mountains and new ocean floors were caused by the Earth’s mantle rising up between two plates.

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HYPOTHESIS

The world's ocean floors are spreading outward from the mid-ocean ridges--at a geographically varying rate of 1 to 2

inches per year-

-and then subducting under

the less dense continental

plates, destroying old crust

at roughly the same rate as

the rifts are creating new

crust.

The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean floor. 

The ocean floor gradually extends and thus the size of these plates increases.

As these plates get bigger, others become smaller as they melt

back into the Earth in the process called

Subduction.

Sonar - a device that bounces sound waves off under-water objects and then records the echoes of these sound waves. The time it takes for the echo to arrive indicates the distance to the object.

Supporting Evidence for Seafloor Spreading

1. Molten material

2. Drilling samples

3. Magnetic stripes

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Evidence from Molten Material

At the mid-ocean ridge, molten material rises up from the mantle and spreads out, pushing the older rocks to both sides of the ridge.

Rocks shaped like pillows (rock pillows) show that molten material has erupted again and again from cracks along the mid-ocean ridge and cooled quickly

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Evidence from Drilling SamplesSamples of the deep ocean floor show that basaltic oceanic crust and overlying sediment become progressively younger as the mid-ocean ridge is approached, and the sediment cover is thinner near the ridge.

The rock making up the ocean floor is considerably younger than the continents, with no samples found over 200 million years old, as contrasted with maximum ages of over 3 billion years for the continental rocks. This confirms that older ocean crust has been reabsorbed in ocean trench systems.

By the mid-1960s studies of the earth's magnetic field showed a history of periodic reversals in polarity.

The magnetic history of the earth is thus recorded in the spreading ocean floors as in a very slow magnetic tape recording, forming a continuous record of the movement of the ocean floors.

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Evidence from Magnetic Stripes

• Magnetic surveys conducted near the mid-ocean ridge showed elongated patterns of normal and reversed polarity of the ocean floor in bands paralleling the rift and symmetrically distributed as mirror images on either side of it.

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•   "A dramatic proof of sea-floor spreading was discovered in the mid 1960s when data revealed alternating stripes of magnetic orientation on the sea floor, parallel to the mid-ocean ridges and symmetric across them -- that is, a thick or thin stripe on one side of the ridge is always matched by a similar stripe at a similar distance on the other side. This mirror-image magnetic orientation pattern is created by steady sea-floor spreading combined with recurrent reversals of Earth's magnetic field. Iron atoms in liquid rock welling up along a mid-ocean ridge align with Earth's magnetic field. "

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Oceanic Crust• The oceanic crust is

that part of the Earth’s crust that covers the ocean basins. It consists of dark-colored rocks made up of basalt.

• The oceanic crust scarcely floats on the mantle.

Continental Crust• The continental crust

accounts for 40% of the surface of the Earth. It is made up of granite rock. This rock is rich in constituents like silicon, aluminum, and oxygen.

• The continental crust is much thicker when compared to the oceanic crust.

• The continental crust floats much more freely on the magma.

• The density of the continental crust is much less as compared to the oceanic crust. It has an approximate value of 2.6 g/cm3. Due to this difference in densities in magma between the oceanic crust and the continental crust, the continents stay in their places, and both crusts are able to float on the magma.

• With age, the oceanic crust gathers a layer of cooled mantle on the underside. This causes the two-layered structure to sink into the hot, molten mantle. Once in the mantle, the oceanic crust melts and is thus recycled. Due to this process there is an absence of aged oceanic crust.

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• The best theory fitting all this evidence together is a mechanism known as "sea-floor spreading," an idea formalized in the 1960s by North Americans Harry Hess, Robert Dietz, and Tuzo Wilson.

• This developing theory received further confirmation by the mapping of 20th-century seismic activity and recognition that it is concentrated at shallow depths of the lithosphere along the "axial rift" of mid-ocean ridges and at deeper depths around subduction zones, just as the sea-floor spreading theory would predict.

• Once again, the military played a central role in these scientific discoveries, for it was the 1963 treaty banning above-ground nuclear-weapons testing that created the need for a Worldwide Standardized Seismograph Network (WWSSN), the very instruments used to detect this faint earthquake activity all around the planet.

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References• http://library.thinkquest.org/17457/platetectonics/4.php• http://www.britannica.com/EBchecked/topic/530828/seafloor-spread

ing-hypothesis• http://mail.colonial.net/~hkaiter/MCASreview.html• http://library.thinkquest.org/17175/current.html• http://www.ucmp.berkeley.edu/fosrec/Metzger3.html• http://www.sciencephoto.com/image/167059/530wm/E3500088-Sea

_floor_spreading-SPL.jpg• http://www.aoi.com.au/bcw/Seafloor/index.htm• http://www.ucmp.berkeley.edu/fosrec/Metzger3.html• http://www.aoi.com.au/bcw/Seafloor/index.htm• http://www.geosociety.org/educate/LessonPlans/SeaFloorSpreading

.pdf• http://home.comcast.net/~steveriddle2/6th/documents/!workbooks/c

h4sec4KEY.pdf• http://www.encyclopedia.com/topic/seafloor_spreading.aspx• http://library.thinkquest.org/17457/platetectonics/4.php

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