Monterey Bay Aquarium Research Institute

Juan de Fuca Ridge Cruise
July 20 - August 1, 2000
Over 650 km (~400 miles) off the Washington-Oregon Coast
Logbook

July 30: Day #11


"Monolith", a high temperature hydrothermal vent. The "smoke" emerging from the vent is composed of many different minerals that are in the process of precipitating out of solution as the hot water cools upon contact with the cold seawater.

Log Entry: Early this morning, wax sampling took place off the A-frame in the rear of the ship until about 3 am. From 3 am until 6 am we were in transit from the southern cleft location where we were diving yesterday to a more northerly location called "Monolith". Monolith is a large, high temperature vent site. Monolith was last seen and studied in 1994, so it has been 6 years since anyone has seen it. After about an hour and a half of exploring, we finally came upon the site. Just before reaching the vent, we discovered a "breadcrumb" trail of Alvin drop-weights (pictured below) that led us to the location. Alvin is a research submersible run by Woods Hole Oceanographic Institution, which requires weights to be dropped when it first lands on the ocean bottom, and also before it ascends to the surface at the end of a dive. The drop weights actually formed a trail that led us to the vent location that can otherwise be very difficult to locate! After a video survey of the beautiful 10 m-tall structure made of smoking spires, we took some samples, which included a small spire from the base of the structure as well as some hydrothermal vent worms which had colonized parts of the vent. As the dive continued, we saw the increased expression of the western boundary fault, which began as a barely noticeable fissure or "crack" and slowly increased in elevation to become an 80 meter high fault, which looked like a giant step up on the ocean floor. We also saw evidence of off-axis fissure-fed eruptions, which appear as an underwater "highway" bordered by lava pillows in the picture below. Finally, at the end of the dive, we revisited the north cleft sheet flow that has been associated with the original megaplume which was noticed in 1986’s as a "plume" of temperature and elemental fluctuations in the water column above the ridge. This fluctuation was caused by a volcanic eruption on the seafloor at the mid ocean ridge. Knowing the age and location of this flow made it very useful to visit, since, as Dr. Perfit explained, "it gives me a ruler by which to judge the relative age of other flows." Although sediment had begun to accumulate in a fairly substantial amount, some glassy surfaces could still be seen in the reflection of the ROV’s lights. As ocean crust ages, the glass becomes thinner and thinner while eventually the manganese "crust" covers it on the outside and becomes increasingly thicker. You may notice the sediment beginning to accumulate in the picture which shows the drainback feature we appropriately named the "underwater gazebo", which was taken at the site of that original flow.

Tonight we began our 21-hour journey back home. We left for port after Tiburon was recovered and are due to arrive in Newport, Oregon at 7 pm tomorrow (Monday) evening. After 12 days of hard work, we return a weary group, eager to study the data and come to more conclusions about the research that has taken place on our journey to the mid-ocean ridge.


The lost city of Atlantis? Actually, this is a drainback feature supported by lava columns. Notice the trademark "bathtub rings" which are an indication of the levels of the lava lake.

An off axis flow, a.k.a. the underwater "superhighway."


A large fissure in the seafloor, edged by lava pillows.

A weight from the "breadcrumb" trail of drop-weights left by the submersible vehicle Alvin. The trail led us to Monolith!

 

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