Slo-Mo Stripy Seafloor

By Theresa Williams

Slackjaw Sally here with an update on the “rapidly” spreading seafloor – if you’re a slug.  My friend Sandy, a bottom dwelling squid told me all about it.

We’re at the East Pacific rise which is a mid-ocean ridge that runs through the eastern Pacific Ocean.  The ridge is a plate boundary where plates are moving away from each other so it’s called a divergent boundary.  This ridge is considered rapidly spreading because the plates move away from each other at rates of 6 to 16 centimeters per year.  Fast spreading is speeds of more than 7.5 cm per year and slow is speeds of less than 5.5 cm per year.  At Pito Deep the rate is about 14 cm per year.  Now that’s fast!  It would take four years for your fingernails to grow that much or about two hours for a banana slug to travel that distance.  For comparison, the Mid-Atlantic Ridge is spreading at about the same rate as your fingernails are growing and a banana slug could cross that distance in ½ hour.  It’s a slow-poke.

At mid-ocean ridges heat from Earth’s mantle makes the crust softer and less dense so it rises forming a raised area where the plates meet.  As the crust cracks, hot magma bubbles up to fill the cracks and spills onto the surface where it is cooled by ocean water forming new ocean floor.  At the same time across the ocean the other side of the plate is subducting under continental crust.  That means that where oceanic plates and continental plates push against each other, the oceanic plate moves underneath because it is denser.  As gravity pulls it down, the whole plate is pulled away from the mid-ocean ridge.  The older crust melts and becomes part of the mantle.  Rocks of the ocean floor are young whipper-snappers compared to rocks on land.  The oldest ocean floor crust is only about 200 million years old, but there are rocks on land as much as 3 billion years old!

Another cool thing that Sandy told me is that the ocean floor is striped.  What!? Striped?  No, not like black and white or colored stripes you see with your eyes.  There are stripes of normal and reversed polarity of the magnetic minerals in the rock of the ocean floor.  Where do these come from?  As new rock cools the magnetic minerals in it line up with Earth’s magnetic field.  When Earth’s magnetic field changes, the arrangement of magnetic minerals in new rock line up with it, but the old rock is already solid and keeps its magnetic orientation.  Scientists map the magnetic stripes using data from magnetometers pulled behind boats or run close to the seafloor determining the direction of magnetic alignment in the rocks.  The nearly symmetrical pattern of stripes moving in both directions from and paralleling the rift is a time stamp of magnetic reversals on our planet.  There have been about 170 magnetic flip flops in the last 76 million years.  These stripes help geologists figure out how the ocean floor has changed since it was formed.

Wow.  That was deep!

Sally