March 29, 2000
How do underwater volcanoes erupt? Are they really dangerous?
Good question! Underwater volcanoes erupt when magma, molten rock
from the Earth’s mantle, comes up through the crust through
fissures, or cracks, in the seafloor. The magma is very hot, 1200
deg. C, but when it comes in contact with the near-freezing sea
water at the ocean floor it chills instantaneously and forms glass,
like obsidian that is found on land. The glass forms the outer
crust of the lava flows and they have many different shapes, some
like pillows, some like flattened sofa cushions, and there are
even some that look like the folds in curtains.
Because of the pressure of the weight of ocean water above the
mid ocean ridge crest, almost all of the gases in ocean floor magmas
are not allowed to escape. On land, dangerous volcanoes which explode
violently are often ones that have lots of gases in the magma.
On the seafloor, volcanic eruptions probably don’t get very
violent, but we do have evidence that they can stir up the water
and that some of the rocks are very broken up, suggesting that
they got all jumbled when they erupted. When we dove in this area
of the East Pacific Rise with Alvin in 1991, a submarine volcanic
eruption was going on and the water at the seafloor was very murky,
with lots of sooty particles in it from all the hydrothermal activity
that had started because of the eruption. So to a scientist diving
in Alvin, a submarine eruption could be very dangerous. In 1991,
there were places we could not go because the sub. pilot could
not see where he was going it was so murky - kind of like driving
in a dense fog.
I hope this answers your questions. Please be sure to check out
the Mid-Ocean Ridge Infomodule in the “Deeper Discovery” part
of the site for more information.
Questions from the 5th/6th Polish bilingual room at Reilly School,
How deep does the DSL-120 and Argo II go?
The DSL-120 sonar and Argo II mapping system can both go as deep
as 6000 meters, that is nearly 20,000 feet deep, about 3.75 miles.
Is the crust in the ocean getting deeper (thicker)?
The crust in the ocean is about 6 kilometers thick in most places.
The crust under the continents is about 30 kilometers thick. The
thickness of ocean crust does increase a bit as you go from the
ridge crest where it is formed to the ocean basins. The sediment
layers that accumulate with time on top of the volcanic rock definitely
make the whole sandwich of volcanic rock and sediment get thicker
with time, but the volcanic layer is usually about 6 kilometers
Is it fun to be there and work there?
Yes, it is a lot of fun to be out here working with my friends
and scientific colleagues on exciting science. Check out some of
the interviews with the scientists and students that we have posted
and will keep posting on the web site for what people are doing
and how they got to be oceanographers and go on this cruise.
Why can't people go inside and underwater in the Argo II?
Scientists can go down to the bottom of the ocean in Alvin. Alvin
is a mini submarine (check out the information about Alvin in the
Oceanographic Tools section under “Deeper Discovery” in
the web site). Argo is a mapping system that does not have any
place for people to sit. If they just climbed on Argo and went
to the bottom they would get squished!
How deep is the ocean where you are working?
The depth of the ocean here at the East Pacific Rise crest is between
2500 and 2600 meters. This is about the average depth of the mid-ocean
ridge crest all over the globe.
If one of the machines was broken, will the other one be broken
The DSL-120 sonar was broken, but only for a short time. That is
why we have terrific engineers out here who know a lot about how
to fix and keep the systems working. The Argo mapping system also
uses the fiber optic cable, but its electronic components are very
different from the sonar. I will be talking more about Argo in
the upcoming days so stay tuned!