East Pacific Rise Part 1

Today we passed the ½ way point: completed half our casts, half our stations, and half of our days at sea!

Spirits are high, not just because of how far we have come, but also because we’re occupying a station on top of the East Pacific Rise. The following comes from my interview of our co-chief scientist, Dr. Chris German (Woods Hole Oceanographic Institute).

The image above shows the height of the crust (or “ground”) above the earth’s mantle with continents in reds and greens and seafloor features in blues. Lighter blues indicate more shallow parts of the ocean. The 3 black arrows point to the East Pacific Rise which you can see snaking down from North America nearly all the way to Antarctica. It is the world’s FASTEST spreading mid-ocean ridge where fresh ocean crust forms.

Here’s a similar image our ship generated of seafloor height as we arrived at the station. The red band is our ridge, rising up over 1,000m above the rest of the seafloor (in blue). Previous stations were in ~4,000m of water while this station on top of the ridge is only 2,600m deep.

The East Pacific Rise spreads at an average of 20 cm per year, but in actuality we believe lava spurts out to the surface in large, 1 m blows once every 5 years. Chris says, “That’s kind of cool, because everything we’re doing today is based on research work from 20 years ago – which is 4 or 5 eruptions back. So the seafloor probably isn’t the same shape it was before.”

How did we know to come here?

Helium-3 (³He) is a particularly special isotope because it is the heaviest atom that can escape Earth’s gravity and fly off into outer space, never to return. There is no ³He “laying around” on the Earth or in its atmosphere unless it was recently released from inside our Earth. This means it can only come from volcanic eruptions (above or below the ocean surface). It makes a perfect tracer because it only has 1 source, and it is a noble gas. Digging deep in our minds, we remember our Chemistry teachers telling us the noble gases are inert (as in non-reactive) so the only way to change the ³He is through dilution.

³He is released from an underwater eruption, and this gas quickly dissolves in the surrounding seawater. Ocean currents carry it away with them, and we call this a “hydrothermal plume”. 40 years ago, oceanographers set out to measure the levels of ³He in the ocean; this told us that there was “hydrothermal activity” originating at the East Pacific Rise, and its plume carries all the way past Tahiti.

I asked Chris what it would take to not find a hydrothermal plume here. He flashed a big smile and said either plate tectonics would have to stop existing, or the world would have to spin in the other direction, “and I’m pretty sure we would have noticed that!”