October 4, 1998. Looking down from the deck into the water, it is hard to imagine the history of dramatic events that have gone on right beneath us. The sonar images give us a picture of what is down there. But there is nothing like solid evidence that you can hold in your hand.
Jennifer Reynolds had in her hands evidence of a recent eruption on Puna Ridge. The beaker she was holding contained fragments of sparkling black glass that she had collected with a wax core from Puna Ridge. She explained to me that the glass had changed very little since the eruption. There were no dull pieces of glass, and no sediment had accumulated on topboth of which would be present had the eruption happened long ago. It was an exciting find.
Each fragment of lava contains information about the history and dynamics of Kilauea and Puna Ridge. The wax core samples that Jennifer has collected are just the first of many. She, Frank Trusdell, and Kevin Johnson will be targeting fissures and craters up and down Puna Ridge where individual eruptions have taken place. They will then compare the chemical make-up of the lava from each site.
Lets look at what the chemical content of lava can tell us about how magma moves from its source into Puna Ridge. Because Kilauea is so well studied, scientists have figured out the composition of the fresh magma that rises from the mantle below the mountain. As the magma ages and cools, its composition changes. Some of the compounds join together or crystallize to form minerals.
The first and most abundant mineral that is formed is called olivine. It is made out of iron, magnesium, and silica. Over time, the materials in the magma that go into forming these crystals are used up. For example, most of the magnesium in the magma goes into forming olivine. The older magma therefore contains much less magnesium than the younger magma rising from the mantle. As this magma continues to age and cool, new types of crystals form from the materials that remain in the magma.
Jennifer, Frank, and Kevin will be comparing the relative ages of the magmas that erupted at different points along the ridge. This information will help them determine how fast the magma moves from the magma reservoir into Puna Ridge, the paths it takes, and how long it remains underground before it erupts. Lets look at an interesting possibility. Suppose they found that lava that erupted at the end of Puna Ridge had the same make-up as the magma that is rising under Kilaueas summit. That would mean that the magma would have had to travel the 130 kilometers from the summit to the end of the ridge awfully fast.
Lava samples can also provide clues about the nature of the eruption. Cores that Jennifer collected from a site close to land yielded a brittle, foamy substance called reticulite. Reticulite only forms when the eruption occurs on land. So it is reasonable to assume that the reticulite from this sample had either landed in the water or floated out.
The black glass samples that Jennifer showed me had lots of tiny air bubbles. All magma contains dissolved gas, most notably water and carbon dioxide. Under intense pressure, such as the pressure experienced under 5,000 meters of water, the gases remain dissolved. In shallower water where there is less pressure, the gases come out of solution and form bubbles. Therefore this glass likely came from an eruption in shallow water. You experience this phenomenon every time you open a bottle of soda. The pressure inside the bottle decreases. The carbon dioxide that had been dissolved in the soda form bubbles that rise to the top.
The beautiful glass samples were only one measure of the success of the first core samples. Jennifer was just as pleased that the wax corer landed within 20 to 30 meters of the target. Every time the wax corer gets dropped, there is the danger that the current will push the ship and the corer away from its target. The ship, however, held its position remarkably well and the corer traveled straight down. With this much accuracy, Jennifer is confident that they will be able to target small features along the ridge and get "the most precise rock cores ever."
The conspicuous oblong jog in the ship track south of the survey area was made in order to calibrate the magnetometer on the DSL-120 vehicle. The magnetometer measures the total magnetic field. It is necessary to calibrate the magnetometer to determine its response to the magnetic field along any ship heading in our survey area.
The total magnetic field measured by the magnetometer comprises contributions from the Earth's magnetic field, the magnetization of rocks on the seafloor, the ship, and the DSL vehicle itself. We only want to use the variations in the rock magnetization, so we must account for, and subtract, the other contributions to the total magnetic field. The calibration allows us to do this.
Rock magnetization is a function of the magnetic susceptibility of the rock itself. This is mainly controlled by the presence of ferro magnetic minerals, and also by the polarity and intensity of the Earth's magnetic field at the time the lava cooled below its Curie temperature (molten lava is non-magnetic because its temperature is too high). The magnetic data we collect from the rocks will give us clues about its eruptive age and the structure of the Earth's upper crust.