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Item #187

This rock is an intrusive igneous form called gabbro which makes up much of Earth’s oceanic crust. This rock, sometimes called “black granite” in the ornamental stone industry, helped prove the theory of plate tectonics in the 1960s.

When researchers mapped the mid-ocean ridges in the 1950s, they noticed the magnetism of the rock formed zebra stripes of alternating polarity.

It was later confirmed that these stripes reflected the slowly shifting polarity of Earth’s magnetic field of the past.

As magma cools in the crust to form magnetite-rich gabbro, the rock is magnetized in the direction of the planet’s magnetic field at the time. As it is pushed up to become new seafloor, the continents spread apart and these magnetic stripes continue to form.

Through this process, a record of Earth’s shifting magnetic field is saved similarly to how a magnetic hard drive saves digital information.


Please note, the images of gabbro shown are representative of what you will receive. Please allow for slight variation in size and color. 

Size: 1" and up


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