Mid-ocean rifting

From GeoClasses

Mid-ocean ridges

As the pieces of continental crust continue to be pulled apart, the central rift valley will be flooded creating a new seaway. Eventually volcanoes take over the central valley and start adding new crust. Because the new crust is produced from volcanism, it is basaltic in nature. When the volcanism occurs under water, it produces pillow basalts.

Topography of mid-ocean ridge. Kios and Tilling, 1996, Dynamic Earth, USGS

The newly created crust is warmer than normal. After all it just cooled from lava. But over time the crust cools. Like most things, when rocks cool they shrink, so the older the new oceanic crust is the thinner it is, simply because of cooling. The result is that the crust at the spreading center (the divergent margin) is thicker than the rest of the ocean floor, creating a ridge. Of course the volcanoes on the ridge help to give more relief to the ridge.

Movies

Movies from From NOAA Ocean Explorer

• A Quicktime VR movie showing a spreading center
  • Media:Ridge_qtvr_CD.mov
• Quicktime movie showing a mid-ocean ridge.
  • Media:Ridge subd 320.mov

Magnetic Striping

A theoretical model of the formation of magnetic striping. New oceanic crust forming continuously at the crest of the mid-ocean ridge cools and becomes increasingly older as it moves away from the ridge crest with seafloor spreading (see text): a. the spreading ridge about 5 million years ago; b. about 2 to 3 million years ago; and c. present-day. Kios and Tilling, 1996, Dynamic Earth, USGS

Lava/magma contains magnetic minerals, like magnetite. As the lava cools the magnetic minerals often solidify before the rest of the lava. These small magnetic mineral grains, floating in the liquid rock gradually align themselves with the Earth's magnetic field. As a result, the floor of the oceans record the strength and orientation (polarity) of the Earth's magnetic field. (Remember the magnetic field).

Measurements of the magnetic stripes along the sea-floor were one of the most convincing arguements the confirmed the theory of plate tectonics.

Life at mid-ocean vents

Life at mid-ocean ridges is weird. Unlike anything at the surface which gets its energy primarily from the Sun (phototrophs), there are autotrophs at the mid-ocean ridges that get their energy from the chemicals in the hot water that erupt from hot water vents on the mid-ocean ridges.

It is possible that life originally evolved in locations like these, and that the most likely places to find life on other planets will be at hot water vents.

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