On To Day 16Back To Day 14Day 15. The Southern Flank

By Edwin Schiele

October 10, 1998. Lets say you can walk down the spine of Puna Ridge. You pick your way across craters, fissures, and lava flows until you are 1,000 meters underwater. From there you turn right and slide for five miles down the south flank to a depth of 3,000 meters. You will arrive in a landscape of steep terraces and multiple pits.

Track Map with transponder locations
The location of the swaths along the top of the ridge and along the southern flank are shown as lines. Upcoming cross-axis lines are also shown as lines. The stars mark the position of the transponders.

We are especially interested in how these features on the south flank formed. So after we finished mapping the sixth line along the top of the ridge, we reeled in the DSL 120 and laid down three transponders along the south flank. As you remember, these transponders enable us to monitor the exact position of the fish (DSL 120) as we are towing it. We are now mapping five swaths along this interesting portion of the southern flank. Each swath takes about eight hours to map. The U-turns at the end of each swath take another three to four hours. Naturally, we were making one of these turns while I was on watch. While the navigator had to stay busy keeping track of our position, the flyer, watch leader, and I had plenty of spare time to tell our inspirational life stories and complain about the CD selection in the control van.

Although the harrowing dramas we related could fill several daily flashes, they could not compete with the myriad of craters or pits on the south flank that the sonar images revealed. Let’s review the possible ways that we think these pits could have formed.

Sidescan - Pits and a crater (L. Dolby)
Sonar image of a crater.   The deep is masked in shadow and the far wall is a bright (white) reflector. Small pits surround the crater. Horizontal lines are spaced 100 m apart. Vertical lines show 15 minutes along the track. (L. Dolby)

The first scenario is a primary eruption. A dike branches off of the main magma chamber and tunnels its way through the ground. In the process, it pries open fissures at the surface. Lava then may erupt from these fissures. If the eruption continues from a single site, the lava could build a cone. If, however, the magma supply stops and leaves nothing but a hollow space under the cone, the top of the cone could collapse and form a pit. The Pu’u O’o crater is a perfect example of this phenomenon.

The second scenario is a secondary eruption. Lava from a primary eruption flows down the side of the ridge through a lava tube. When the terrain flattens out, lava spills from the end of the tube and forms a shelf. The top of the shelf quickly solidifies in the cold water, but lava continues to flow underneath, causing the shelf to swell. Eventually, the pressure is great enough that lava bursts out the other side and drains out. Without magma to support it, the top of the shelf collapses and forms a pit. (For more information on how craters form, see day 10.)

There are examples of both of these processes on the subaerial portion of Kilauea, but it is unclear whether these same processes occurred on Puna Ridge. To find out how the craters on the southern flank may have formed, we plan on lowering ARGO II into some of the craters. Here is what we will be looking for. Primary eruptions often occur in cycles. Lava erupts, stops, erupts, stops, and so on. Each eruption adds a new layer of lava to the structure. If you go inside the crater, you can actually distinguish the layers of lava that each eruption deposits. Craters formed from secondary eruptions are not built through successive lava flows. Therefore they should not show the same type of layering as the craters from primary eruptions.

Side-scan image of seamount at MAR (D. Smith)
Side-scan image of a 220- m high seamount with a larger crater in its top  from the Mid-Atlantic ridge.   Illumination is from the right.  The crater is in a shadow, and the seamount casts a large shadow to the left.  (D.Smith)

We will also send ARGO II down to take a closer look at some of the smaller pits on the southern flank. Hopefully we can learn whether these pits formed from the wide-spread draining of magma from beneath the crust, or whether the pits might be openings in the tops of lava tubes. These are also known as skylights.

We also have the same questions about how volcanic features at the end of the ridge formed. If we can determine whether the craters at the end of the ridge were created by primary or secondary eruptions, we gain some insights on how far along the ridge the dikes have penetrated.

Ship Tracks October 8 through October 10

Ship Tracks Oct. 8-10

Go back to day 14. Go to day 16.