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Lecture #07- Mid-Ocean Ridges

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Mid-Ocean Ridges (MOR)

Mid-Ocean ridges are divergent plate boundaries; these are regions where plates move away from one another

As you might imagine, they occur only between oceans and oceans (continents are not involved)

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Mid-Ocean Ridges

MORs are the “birthplace” of oceanic lithosphere

The newly formed oceanic crust and lithosphere are pushed sideways to make room for hotter upwelling material

This sideways motion continues as the plates age and become cooler and heavier

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Mid-Ocean Ridge (MOR)

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Rocks & Magnetism

When rocks form in the presence of a magnetic field (like that on Earth), they are influenced by the field.

In an igneous rock, which forms from cooling lava or magma, the iron rich atoms are aligned with Earth’s field, and then frozen into the rock.

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Magnetic Time Scales

By studying many rocks of different ages, they were able to map out the “polarity” of the geomagnetic field back into Earth’s history.

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Irregular Patterns

Note that the pattern is irregular - that makes it like a fingerprint. We can use the pattern to estimate the age of rocks that show similar magnetic signatures.

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Magnetic Stripes

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Sea Floor Spreading

One way to explain these observations was to have a system operating that created new ocean floor at the mid-ocean ridges.

As the material cooled, it inherited the polarity of Earth’smagnetic field, & then the sea floor spread apart like a conveyor belt.

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Rift Zones

The continental analog to a MOR is called a rift zone– A major rift zone exists in east Africa; some have already

named the new plate the “Somalian Plate”

At a rift zone a continent is essentially being broken apart– If the rifting is successful a spreading ridge (MOR) will

develop and ocean basins will form– If the rifting does not succeed the continent will remain

whole and no new plate boundary will be formed (failed rift)

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The East-African Rift Zone: A New Plate Boundary Being Developed Right Now!

Stage 1. Plume upwelling reaches surface

Stage 2. Continental breakup begins

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MORs – Like Seams on a Baseball

East AfricaRift Zone

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The Age of the Sea FloorIceland

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Iceland - A “Dry” Ridge

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Icelandic Fissure Eruptions

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The Age of the Sea Floor

Juan de Fuca Ridge

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Closer to Home – The Juan de Fuca Ridge

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MORs Up Close

When we look at mid-ocean ridges in detail we find some interesting things:

Hydrothermal systems Pillow basalts Plumes of chemical precipitation (black

smokers)

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Hydrothermal System

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Pillow Basalts

When magma directly encounters water it is supercooled into pillow basalt

Basalt refers to the chemical composition of the rocks, and pillow refers to their smooth, rounded shapes

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Direct Water-Magma Interaction (Hawaii)

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Pillow Basalts

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More Pillow Basalts …

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Deep Sea Vents (Smokers)

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Deep Sea Vents (Smokers)

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MOR Summary

Mid-ocean ridges are divergent plate boundaries where new oceanic lithosphere is being created

Rifting refers to the breaking up of a continent; rifts may or may not turn into spreading ridges