Dr. Dziak
Bob Dziak

This post comes to us from Dr. Bob Dziak, Acoustics Program Director:

So, why should people care what’s at the bottom of the ocean? Because humans are, by nature, explorers; we want to know what’s behind the next turn in the road – what new adventure awaits. Given that we know more about the surface of the moon than what lies beneath the vast ocean, it’s not only in our best interests to explore the ocean floor, it’s in our DNA!

The images posted here today give us a glimpse of what is at the bottom of the ice cold ocean: its volcanic fire! What you see is the latest compilation of bathymetric (i.e. topography of the seafloor) data of a chain of seamounts that is located 300 miles west of the Oregon coast and 1 mile under water.

Under Sea Volcanoes off Coast of Oregon
Under Sea Volcanoes off Coast of Oregon

This bathymetric data was collected over a 20 year period by several research ships. A transducer on the hull of the ship sends out an acoustic ping into the water below. The sound wave travels through the water, reflects off the seafloor, and travels back to the ship. Since we know the speed of sound in water, by precisely measuring the time it takes for the sound wave to travel to the bottom and back to the ship, we can calculate how deep the seafloor is. Then, by having the ship go back and forth, spanning a large area of the ocean (a process we call “mowing the lawn”), we can build comprehensive maps of the seafloor.

The images show the unique shapes of two extinct volcanoes (Cobb and Brown Bear) and one still actively erupting volcano (Axial). The seamounts are the youngest part of a chain of seamounts formed when the overriding Pacific plate passed over a large mantle hotspot plume called the Cobb hotspot. This is exactly how the Hawaiian Islands, the youngest seamounts in the Explorer Seamount chain, were formed.


Each seamount in the Cobb chain shown here have volcanic cones, craters and big lava flows that give the mounts their unique shapes. Axial Seamount is the youngest (500K years), and got its name because it straddles the axis of the Juan de Fuca Ridge, a place where the Pacific and Juan de Fuca plates move away from each other and the magmatic upper mantle is really close to the seafloor. Axial has erupted 3 times since 1998, the most recent occurring just last April 2015:  PMEL Axial Seamount Expedition.

Cobb is the oldest Seamount (3.3 million years) and has a very distinct flat top, which was created by wave action and erosion when Cobb was above sea level. Recent studies show human migration from Siberia to the Americas occurred in one big migration 23,00 years ago. Cobb was an island during this migration, and I’ve always wanted to go explore the summit of Cobb to see if we could find evidence humans landed there long ago!

The seamounts also show an odd feature. According to classic plate tectonic theory, the depth of the seafloor should increase as you move farther away from the source ridge. But Cobb is much shallower than Axial. How can that be? We surmise that Cobb is much shallower than the other seamounts because it formed at a time when eruptions were much more voluminous, with a much higher supply of magma from the mantle magma hotspot plume. Axial is also deeper because it’s younger and hasn’t had a chance to build up yet; when Axial erupts, the magma/lava tends to get spread out along the faults of the Juan de Fuca Ridge.

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