We’re at Marion Rise on the Southwest Indian Ridge, a mid-ocean ridge located along the floors of the southwest Indian Ocean and the southeast Atlantic Ocean (yellow in the diagram below). 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 ultra-slow spreading because the plates move away from each other at rates of only 1.4 to 1.5 centimeters per year- that's slower than your finger nail grows. Fast spreading corresponds to speeds of more than 7.5 cm per year and slow corresponds to speeds of less than 5.5 cm per year. At the Marion Rise the rate is only 1.4 cm per year. Now that’s slow! It’s one-tenth the speed of the speed at Pito Deep where I was two years ago. This is an ultra-slow-poke.
At mid-ocean ridges, the Earth's tectonic plates are pulled apart allowing the Earth’s mantle to rise up. Because this mantle is hot, it is less dense and more buoyant and so forms the raised area or ridge where the plates meet. As the plates pull apart the Earth's crust cracks, and 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 could be 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 descends to depths of 600m deep into the mantle. This means oceanic crust is rapidly (on a geological time scale) recycled back into 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 4 billion years old!
Remember what I told you about the striped ocean floor? Well, magnetic minerals will record the direction of the Earth’s magnetic field as magma cools to become rock. When the Earth’s magnetic field reverses and our magnetic north pole becomes a magnetic south pole, any new magma will cool and record this reversed magnetic field. That’s how the stripes are made!
Wow. That was deep!