Pressure in the Deep Seas

Part of the Deep Sea Vents Curriculum Collection.

Greetings from a beautiful day on the Pacific Ocean— specifically, the Pacific Ocean above the Juan de Fuca Ridge! Today the ocean is calm and the Sun is shining— a bit unusual for this part of the ocean. On the open ocean, weather conditions are a combination of local weather and also of the effects of distant weather. Sea swells build and travel over long distances, so if the ocean is choppy over the Juan de Fuca Ridge, someone's having rough weather somewhere else in the Pacific Ocean.

 So why are we here? Well, the Juan de Fuca Ridge is one of only about 120 sites around the globe where ocean floor spreading centers include the ecosystem we want to study— the deep sea vents. There are ocean floor spreading centers, including submarine volcanoes, all over the world, but not all of them include deep sea vents. Much of the ocean is still unexplored and new sites are being found, but Juan de Fuca has been studied fairly extensively since its discovery. This is not only because of what it is; it's also because of where it is— just 300 kilometers off the coast of Washington State. And because the study of deep sea vents is so new, we need to be able to study the same site over and over in order to have baseline data of what stays the same and what changes. As a result, expeditions keep coming back here!

 At 6 a.m. this morning, the ALVIN was taken from its hanger and positioned on its track at the stern of the ship, ready for launch. Today the two scientists on board the sub are making their first trip to the vents since last year; it took a while to load all the new equipment they'd deploy, as well as the tools for collecting samples. The scientists planned to use the ALVIN's mechanical arm to take the samples and place them in containers on a platform in the front of the ALVIN to be brought back up. (The team needs to collect vent samples— all kinds of living and non-living things, including specially adapted bacteria, fish, and rocks.) They also need to collect thermometers placed on the bottom last year to measure temperature variations around the site. Why can't they just stick a hand out to do it themselves? It's all about pressure!

Close-up of equipment, including lights, mounted on the top of the submersible ALVIN.
View of the cameras, lights, and other equipment mounted on the top of the submersible ALVIN. Visit the Web site for the Deep Submergence Operations Group at Woods Hole Oceanographic Institution to learn more about the ALVIN. Photo © Woods Hole Oceanographic Institution, Deep Submergence Operations Group.

 The Earth's atmosphere has weight—or force—which presses down on everything on the surface. If you are at sea level, you have one atmosphere's worth of pressure pushing down on you. In other words, the pressure inside your lungs is the same as the pressure of the air around you, 1.033 kilograms on each square centimeter, or one "atmosphere" of pressure. You can picture it as a column of air that goes from the top of your head to the edge of Earth's atmosphere. If you go higher, by climbing a mountain or flying in a hot air balloon, the column gets shorter; there is less air above you, and the air pressure decreases

 In the ocean, pressure works the same way, but instead of just having a column of air over you, you also have the weight of all the water above you, pressing down on your lungs. And water is much heavier than air. For every 10 meters you go below the surface, the pressure increases by one atmosphere. At 2,500 meters, the depth of hydrothermal vents at the Juan de Fuca Ridge, for example, you'd have 250 atmospheres of pressure on you. That’s about what your big toe would feel like if an elephant were standing on it.

 Our bodies are designed to withstand about one atmosphere of pressure. But at the deep sea vents, the weight of a mile-and-a-half's worth of ocean lies on top of the ALVIN. The ALVIN is specially designed to maintain the same pressure we have at the surface, but there can be no break between the inside of ALVIN and the environment at the sea floor—otherwise everyone inside would be crushed by all that pressure. The ALVIN has very thick walls so that it can maintain an internal crew pressure that is the same as the pressure we're used to on land— one atmosphere. As ALVIN descends to the bottom, the ocean exerts a great squeezing pressure on its hull and on the air inside it—the titanium-doped steel is the best metal for standing up to the pressure, and the spherical shape is the best structure for standing up to the pressure!


Deep sea hydrothermal vent crab being collected on sea floor.
Samples from the seafloor, like this deep sea hydrothermal vent crab, are collected with the mechanical arm of the ALVIN and then placed in the ALVIN's collection basket. Photo courtesy of J. R. Delaney, University of Washington.

And if ALVIN can't protect its crew, they don't dive. Remember that plan to collect thermometers and other samples? Well, guess what! The pilot found a tiny leak in the outer seal of the porthole when the sub had descended only about 200 feet below the surface. That's why safety measures are so important— if the pilot hadn't spotted that leak at that depth, the additional pressure on the sub at the bottom of the sea would have pushed on that crack, forcing water through the leak and possibly causing a breach in the sub to the inner porthole seals. Talk about dangerous! Thankfully, the pilot spotted the problem before it became dangerous, and the sub had to come back up to the surface without completing its dive today.

 The researchers felt pretty disappointed about today's dive, but everyone still feels really lucky to be able to be part of this new science. Before the invention in 1964 of the ALVIN, deep sea research just wasn't possible, and we didn't even know that the deep sea vents existed. Only ALVIN, with its spherical, titanium-doped steel body, was able to withstand the incredible pressure at the bottom of the sea, up to 4,000 meters down. And ALVIN has allowed humans to conduct some incredible expeditions. So far, researchers aboard ALVIN have found and recovered a lost hydrogen bomb (in 1966), found the Titanic (in 1985), and found and explored the hydrothermal vent communities— starting in 1977 and continuing today. That's why I'm here! For me, being here in any capacity truly is incredible— but I'm still crossing my eyes and fingers to help my chance of getting picked to go down to the vents aboard the ALVIN.

 From one atmosphere of pressure,