Ask an Explorer

Questions were sent to the science party during this expedition. Selected questions and answers are offered below.

Question from: Cindy, Wilmington, North Carolina

The August 1 log by Bryan Davy mentions plans to find expected magnetic anomalies at Brothers Volcano. What is a "magnetic anomaly," and why it is significant and worth investigating?

Answer from: Bryan Davy, Institute of Geological and Nuclear Sciences
Lower Hutt, New Zealand

An “anomaly” is basically a deviation from the “background” value of something. The Earth has an overall magnetic field generated by the movement of its molten iron core. A magnetic anomaly is the amount of deviation in the measured magnetic field strength at any point from the strength of the field predicted by a global model of the Earth’s magnetic field. Deviations from the predicted magnetic field are most likely caused by the nearby presence of magnetic minerals, which can either enhance the Earth’s field or have recorded within them the historical Earth’s field. This historical field strength of the Earth may differ in strength and direction from today’s field, but it can be frozen in place within rocks when the aligned magnetic grain orientations become "locked" as lava solidifies.

The field measured on the Earth’s surface also includes local effects, such as variations due to local rock types (iron-rich rocks, for example). Magnetic anomalies come in various "flavors," from very large ones caused by reversals in the Earth’s magnetic field down to small ones due, for example, to a sudden change in the magnetic properties of the underlying rocks. Solidified lavas usually provide a strong magnetic anomaly signature and a positive magnetic anomaly. Geothermal alteration of such rocks (known as “cooking”), however, generally destroys the ability to enhance or record the Earth’s magnetic field as many of the crystalline mineral phases are altered to clays. Such altered rocks generally have a low magnetic anomaly.

Often the magnetic anomaly observed will provide important clues about the nature (including volcanic and geothermal history) of buried rocks where there are few surface features to observe visually. This type of variation can occur over scales of hundreds of meters to kilometers and will likely be seen at Brothers Volcano, where there has been long-term high-temperature cooking of the rocks.

Question from: BJ, Charleston, South Carolina

The July 31 log contains figures published in "Economic Geology." Is one of the goals of this expedition to obtain information for the purpose of extracting minerals from hydrothermal vent sites? How are environmental concerns in this regard being addressed?

Answer from: Bob Embley, Chief Scientist – Submarine Ring of Fire 2007
Geophysicist, NOAA Vents Program, Pacific Marine Environmental Laboratory

We are conducting basic research and do not have any direct ties to commercial interests. Our interest in the exploration of submarine volcanoes, and in particular those found behind the great oceanic trenches, is to answer basic questions such as: 1) What is their basic geologic structure and form, 2) what is the chemistry of the fluids emitted by their hot springs, and 3) the nature and extent of hydrothermal activity and how these outputs interact with the chemistry and biology of the upper ocean. The data we collect will be open to the general community and will be published in peer-reviewed scientific journals. Authors try to submit manuscripts to the most prestigious journal that best fits the subject they’re writing about, because they will likely get the most constructive peer reviews. The field of “economic geology” includes specialists in the subject of the manuscript mentioned, such as geothermal systems and volcanic-hosted mineral concentrations.

Question from:  Tom Resing

The log of August 3 says ABE [the autonomous benthic explorer] generated a map of the "wall of Brothers where there is active hydrothermal venting." I'm curious if your research scientists are able to use this new data to modify the deployment of their current experiments while still at sea.

Answer from:  Joe Resing, Pacific Marine Environmental Laboratory

That is a very good question. What makes our studies so enjoyable is the fact that we do adjust our plans based on our findings. The ABE does many things while it is in the water doing a survey. It makes the map that you mentioned, but it also has sensors that detect geothermal springs (hydrothermal vents) and another sensor that measures the magnetic field.

When our sensors tell us where new hydrothermal vents are we then can visit that area with other tools, including our water sampler (CTD rosette, which measures conductivity, temperature, and depth) and the new German remotely operated vehicle. This vehicle is a robot that operates at the end of a long power and communications cable. It can take high definition video of the springs and collect samples of the sulfide deposits.

The sensor that measures the magnetic field identifies areas where hot water has passed through the volcanic rocks.  These areas may be the ones that are currently active or those that were active in the past. We will try to visit these areas as well to confirm our findings.

Question from:  Ed, Hawaii

Your July 31 log mentioned that several of your instruments attached to the camera array confirmed that hot metal-rich fluids were being expelled inside the caldera at Brothers. Can you be more specific as to which metals would probably be found in these fluids?

Answer from:  Joe Resing, Pacific Marine Environmental Laboratory

Thanks for your question, Ed. The dissolved materials that we find in the fluids are directly related to both the chemistry of the volcanic rocks found at Brothers Volcano and to the reaction conditions present within the volcano. For instance, in 1999, when the first New Zealand-American cruise took place, the fluids were rich in aluminum, iron, and manganese. When it was visited again in 2002 and 2004, the fluids still had a similar amount of manganese but only 10% of their 1999 iron content and very little aluminum. The difference is related to the amount of sulfuric acid present in the fluids. The sulfuric acid does a good job of dissolving most of the rock, producing the aluminum-rich fluids. However, when the sulfuric acid isn't present, the fluids are no longer capable of removing the aluminum from the rock. The sulfuric acid comes from volcanic gases rich in sulfur dioxide. We surmise that in 1999, the source of volcanic gases — the magma chamber — might have been closer to the top of the volcano.

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