On To Day 19Back To Day 17Day 18. Lava Flows and Magnetics

By Edwin Schiele

Tim Dulaney, Laura Kong, and Kevin Johnson (L. Dolby)
Tim, Laura, and Kevin interpret the side scan sonar images after  putting together a mosaic of the five southeast flank lines.(L. Dolby)

October 13, 1998. There was a lot of excitement around the map tables this morning. At about 4:00 a.m., we finished mapping the final swath along the southern flank of Puna Ridge and pulled the DSL 120 back on board. Laura Kong, Kevin Johnson and Tim Dulaney pieced together the sonar images from the five swaths and came up with a remarkable picture. The sonar images clearly showed a giant crater and large lava flows that had recently erupted from it.

This discovery energized the scientists on board. Laura made her interpretations of the volcanic features she was seeing and marked them on the bathymetric map. Kevin scanned the portion of the ridge above the crater for clues about whether we were seeing a primary or secondary eruption or whether there is another explanation. Mark Bulmer jotted down notes describing features on the sonar image that caught his attention and debated with Laura about the texture of the lava along part of the flow. At this stage, however, nobody is willing to draw conclusions. Each person who looks at a sonar image will come up with different interpretations. Answers won’t be forthcoming until we can analyze samples from the lava flows and send ARGO II down to photograph some of the features. Stay tuned.

Side-scan image of lava flows (L. Dolby)
Rubble flows (arrows) cascading down the slopes of a lava terrace on the southeast flank of the Puna Ridge.(L. Dolby)

Another technique we are using to study the dynamics of Puna Ridge is to measure the magnetic intensity of the uppermost crust along the axis. As lava hardens, iron minerals within the lava align along the earth’s magnetic field and produce their own magnetic signals. Over time, chemical changes in the iron minerals cause the magnetic signals to fade. We can therefore determine the relative ages of features along the ridge based on the strength of their magnetic signals. The stronger these magnetic signals, the younger the lava.

Magnetometer on the DSL 120 (L. Dolby)
Magnetometer mounted at the back of the DSL 120 instrument.

The DSL 120 has a magnetometer that measures the strength of the total magnetic field. Interpreting these measurements, however, can be tricky. The magnetic readings we get not only include the magnetic field of the ridge, but also the earth’s magnetic field, the ship’s magnetic field, and even the DSL 120’s magnetic field. To tease out the magnetic field strength along the ridge, we must determine the strength of the magnetic field from these other sources and then subtract these values from the total magnetometer reading. What remains is the magnetic signal from volcanic features along the ridge. It’s as if we wanted to find out how much somebody on the ship weighed by weighing the entire ship. To do so, we would have to know the weight of the ship and everything and everybody else on board. If we then subtracted these combined weights from the total weight, we would be left with the person’s weight.

Still we are not done. We must also take into account the alignment of the earth’s magnetic field. If you are familiar with how a compass works, you know that the needle points towards magnetic north, which is just off the North Pole. The magnetic readings we get vary depending on the direction the magnetometer is pointing. Therefore, we must make adjustments based on the heading and tilt of the DSL 120.

Right now, we are mapping and collecting magnetic data along swaths that cut across the ridge. The recent magnetic data from these lines will help us determine the width of the accretion zone (area of volcanic activity). We expect the magnetic signal to be strongest down the center of the ridge where there has been the most volcanic activity. Once we are off the ridge, the magnetic signal should decrease to the background magnetization levels of old sea floor.

We also collected magnetic readings as we mapped the swaths along the ridge. These measurements, in conjunction with the chemical analysis of wax core samples, will tell us ages of volcanic features as we move down the ridge. We are particularly interested in the age of features at the end of the ridge. If dikes really do penetrate to the end of the ridge, we would expect to see some strong magnetic signals.

Finally, the magnetic features enable us to detect volcanic features within the ridge that we cannot see. Through a set of complex calculation that I don’t even pretend to understand, it is possible to determine the size and the shape of the object producing the magnetic signal.

Ship Tracks October 11 through October 13

Ship Tracks

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