MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 27 update posted by Lionel Wilson
Tiburon dive 888 - Sediment sampling in Escanaba Trough - West transect
Today ROV Tiburon continued the exploration of the Northern Escabana site where we started work yesterday. As someone who has not worked on marine geology before, I have been struck by the fact that the ocean floor is a place of great diversity, not just in terms of the creatures that live there but also of the scenery. All we have seen from Tiburon's cameras for most of yesterday and today is a smooth or gently rolling plain of mud, with only a very few exciting rock outcrops. Yet only two days ago we were looking at great piles of recently-erupted volcanic rocks with very little sediment in sight.
But, boring or not as regards scenery, the thick sediments we are working on at the moment are really important, because they have acted as a convenient smooth surface to collect volcanic fragments from explosive activity at a nearby vent. The vent fed a lava flow that erupted at least 300 years ago, but we have good evidence from a few samples taken during earlier work in this area that significant explosive activity was involved in the eruption. Yesterday we took about 30 samples of the surface mud during the day and analyzed some of them during the evening. These showed for sure that there are volcanic particles of the kinds we expected. So today we have been extending the area from which samples are taken to get a better idea of the pattern of particle distribution.
This sketch shows a volcano producing a high eruption plume - the plume spreads sideways in all directions as it rises until it is pushed over by the current (underwater) or wind (on land). The long dashed lines show the paths of the largest particles released part-way up the plume, which fall quickly to the surface and are not blown sideways much by the current. The short-dashed lines show the paths of somewhat smaller particles released further up the plume, which fall more slowly to the surface and are blown sideways a greater distance by the current. Finally the dotted lines show the paths of much smaller particles released from the top of the plume, which fall very slowly and are carried a long way by the current. The thick solid lines represent the contours on the horizontal surface of the landing points of successively smaller particles. The arrows show how the width of the deposit mimics the increasing width of the plume as it rises.
This microphotograph shows some of the coarse material from one of our cores. The largest fragment in this photo is about 8 mm across. The glassy, brown-black particles marked with the letter "L" are limu o Pele. Other particles in this core include other volcanic rocks, sediments, and the shells of tiny marine organisms called foraminiferans.
Any explosive eruption, on the ocean floor or on land, throws hot particles up above the vent. These heat up the surrounding fluid--water or air as the case may be--and create a convecting cloud, usually called an eruption cloud or eruption plume when it is in the atmosphere. The rise height of the plume is a direct measure of the rate at which heat is being released from the magma coming out of the vent. The plume spreads mainly in the direction of the wind (or the current, under water) but also at right angles to this direction because it gets wider as it gets higher.
In the past, eruption plumes above volcanoes on land have received all the attention, mainly because they are easy to observe. By looking at how far particles of a given size have traveled from these land-based vents, both with the wind and at right angles to it, we have worked out how high the plume went and how fast the wind was blowing for dozens of eruptions, both recent and ancient. Now we plan to do exactly the same with this submarine eruption, and work out the height of the eruption plume in the water and the speed that the current was traveling.
Oceanographers will be interested in how strong the current was when the eruption occurred, and we volcanologists will use the height of the plume to work out how fast heat was being released from the vent. The quantity of heat released in an eruption (under the ocean or on land) is proportional to the amount of magma erupted, so the heat release rate gives the eruption rate - the volume being erupted per second. So if we can survey the lava flow from this eruption well enough to get its volume, we can divide the volume by the eruption rate to get the duration of the eruption. Durations and volumes of eruptions are key pieces of information we need to help understand patterns of volcanic activity on the ocean floor.
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Jenny writes: Miles and miles of mud! The mud seemed endless, and we sought excitement from the variety of critters we saw, and from the sedimented swales and hummocks that punctuated the flat expanse. It could have been a dive anywhere else on the vast, abyssal plain of the deep sea. But we are still on a mid-ocean ridge, and we were jolted back to that realization by the surprise at the end of the dive: a hydrothermal-vent site at NESCA’s Central Hill.
We took 30 pushcores today, 30 yesterday, and we anticipate taking 30 tomorrow and 30 more the next day! We want to see how far the limu o Pele lava bubble-wall fragments got dispersed during the eruption, and in which direction. We can do this here because there has been just one lava flow since turbidites flooded this site in the Pleistocene. We will compare the amount of glass in each core by distance from the flow, taking into account vertical displacement due to bioturbation.
This photo shows one day's worth of push-core tubes loaded on ROV Tiburon. Each evening, after the dive, we must process sediment samples from all of these tubes, so that they can be used again on the next day's dive.
After traversing acres of mud, we were rewarded by a close-up view (and samples) of these tubeworms near the an old eruption site in the Escanaba Trough.
Once the ROV is recovered, the cores must be extruded right away so
the push-corers can be returned to the ROV sample drawer for the next day’s
dive. We have developed a routine for this now, and the process takes us
several hours. We have 2 extrusion stations of 2 people each, plus runners,
baggie-labelers, and core-tube washers and reassemblers. We are careful
to get all of the mud from each core-depth interval into its baggie.
When the ROV Tiburon returns to the surface, the sediment cores are removed as quickly as possible so that we can start processing them.
Stephanie and Larry separate (extrude) the middle section of a core, which will be placed in a carefully labeled plastic bag.
After the cores are extruded, we begin sieving the particles out from the sticky mud. This process is very time-consuming, and will undoubtedly continue once we get back to shore. During the dive today we finished sieving the top 8cm of all the cores from yesterday and, as we anticipated, the amount and size of particles increased as we approached the flow!
Image
to left: Dave and Jenny focus on the seemingly endless process of sieving the
cores to remove the mud and separate out larger particles of different
sizes.
This lava flow and eruption is relatively isolated from other volcanic sites. In addition, it sits in the midst of a plain of thick turbidite silt and mud that fills the axis of the Escanaba Trough (to a depth of 450 m). For this reason, it is an ideal site to investigate the dispersal of limu o Pele and other pyroclastic glass fragments that may have been released during the volcanic eruption. During the next five dives (over the next five days), we will attempt to measure the amount, size distribution, and morphology of the glass particles emitted during the eruption, and to find out how these characteristics vary depending on the distance and direction from the eruptive vents.