Mid-ocean ridge system. Creative Commons
Over the past few columns, we have explored submarine eruptions near the surface. Once a volcanic edifice is high enough, with lava and rock stacking higher and higher from the sea floor through repeated eruptions, it can become explosive. We can also see it. We can see the water-rich, heavy ashfall that collapses towards the ocean, the white steam plumes from seawater evaporating, and the pyroclastic surges that can race across the ocean.
Mid-ocean ridge system. National Oceanic and Atmospheric Administration, Public domain, via Wikimedia Commons
What is much more common, though, happens far deeper below the surface, where sunlight does not reach. The majority of our broad ocean floors were initially formed by volcanic activity over many millions of years. Much of this occurs at mid-ocean ridges.
Mid-ocean ridges are where tectonic plates pull apart and new seafloor is formed. The rate at which these plates rift apart varies. Faster – and slower-spreading centres form new seafloor in different ways, with varying amounts of volcanic activity and faulting.
Here, lava flows ooze out of fractures or vents, producing pillow lavas that are rounded like toes or fingers. These can stack up to build volcanic ridges or small cones. Lavas can also build steep “haystacks” that may be five to 150m high and 30 to 330m wide. These steep features can collapse to produce talus, which is a pile of sharp, broken, glassy rock.
Sheet lava flows can also cover the sea floor, forming flat surfaces. These can have lava tubes like we see on land, and they can break apart through fractures or collapse pits that produce more sharp edges.
A beautiful, peaceful, and still hazardous environment.
Janine Krippner
Very slowly, sediment can drift down through the water column to coat the glassy rock, allowing researchers to estimate relative ages. If one group of lavas has 5cm of fine sediment, and a pile next to it has 1cm, the one with less sediment is likely to be younger. It may take a thousand years, or longer, to accumulate just a few centimetres.
Because of how volcanic activity occurs, scattered across many kilometres, we do not always simply define one clear feature as “a volcano”. There may be many cones, fractures, and vents surrounded by more faults that form as the seafloor breaks to accommodate tectonic plate movement.
These areas are also where we can have hydrothermal activity, where life can thrive in darkness under enormous pressures and at high temperatures. Tall chimneys can eventually grow, with fluids shooting out the top. We do not just learn about geology in these areas, we learn more about life itself.
It is uncommon to witness eruptions at these depths. Much of the seafloor, under thousands of metres of water, has not been seen through a remotely operated vehicle camera. It is expensive and logistically challenging to collect data out in the middle of an ocean. When I am doing research in these locations, I look at every scrap of information from the area, as well as from similar areas, to piece together the puzzle of the deep.
There are still many discoveries ahead of us.
Diagram of a mid-ocean ridge with the lithosphere, asthenosphere, and primary forces contributing to ridge push labelled. Shows ridge push in young lithosphere (<90 Ma) and the lack of ridge push in old lithosphere (>90 Ma).. Photo: Creative Commons
