Volcano Watch — Past magnetism helps understand Kialuea's explosive history

This year, we were joined for 10 days by Duane Champion, an expert in paleomagnetism headquartered at the USGS office in Menlo Park, CA. Paleomagnetism? What in the world is that and why is it pertinent to a study of Kilauea's explosive history?

The earth's magnetic field is constantly changing. Most readers know that magnetic north and true north are different. North on a map is often shown as two arrows; the difference between the two, the magnetic declination, is the angle by which you correct your compass in order to measure map directions accurately. In Hawai`i today, the declination is about 11 degrees east, that is, the north-seeking compass needle points about 11 degrees east of true north.

This is only temporary. Over time, the declination and inclination (the difference from horizontal) change. In the past, the magnetic direction (both declination and inclination) has moved tens of degrees, sometimes moving slowly in one direction for several hundred years and other times darting back and forth over a similar time period.

To determine the direction of "paleomagnetism" (old magnetism), carefully oriented samples are drilled from lava flows. In a specialized laboratory, the samples are stripped of any recent magnetism acquired by sitting in the modern magnetic field. What is left is the magnetic direction acquired at the time the rock cooled from lava.

But when did the rock cool? That is determined by radiocarbon dating of burned vegetation (charcoal) under the flow in question. With this age, you can then tell when the earth had the particular magnetic direction measured.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. The two shields built over the main tube system above Pulama pali are still topped with active lava ponds. Glow from lava in the crater of Pu`u `O`o and the active ponds frequently light the night sky. Overflows from the ponds feed multidirectional flows radial to the shields. Two parallel lava flows were observed wending their ways down Pulama pali on February 7. The distal end of the western flow was at the top of the lava fan at the base of the pali, and the eastern flow was approaching the bottom of the pali. No active flows were seen in the coastal flats. The ocean entry at Kamoamoa stopped last week, and no lava is entering the ocean anywhere along the coast.

There were no earthquakes reported felt during the week ending on February 7.

Putting many such age-magnetic direction pairs together, a curve can be constructed showing changes in the direction of the magnetic field for the past several thousand years.

Such a curve then becomes another way to date a lava flow. If no charcoal can be found below a flow, its paleomagnetic direction can provide a good estimate of the age. This works best if there are no crossovers or duplications in the curve. In practice, however, the curves loop back upon themselves, so there is generally no unique age assignment. But, knowing the overall geologic context, reasonable age estimates can be made.

The curve is only as good as the number of different ages that define it. If a particular time period is not represented in the curve, the magnetic directions during that time period can be only guessed at.

This is where the explosive deposits come in. They form an excellent potting mix for vegetation. On young flows, most soil develops in explosive deposits. Looking for lava flows that overlie explosive deposits increases the chances of finding charcoal, dating it, and filling out the paleomagnetic curve.

Of course, heavy rain can wash away volcanic ash, leaving only a barren surface to be covered by the next flow. So, explosive deposits are commonly missing from areas where they once were. Now the paleomagnetic curve comes into play. By assigning ages based on the curve, one can figure out which lava flows to look under to find the incompletely preserved explosive deposits. Or, if there is dispute about whether a particular deposit is Deposit A or Deposit B, the paleomagnetic directions above and below will help resolve the uncertainty.

Using both radiocarbon ages and paleomagnetic direction, we get excellent ideas of where to look for explosive deposits. By cataloging the occurrence and characteristics of deposits from each of Kilauea's explosions, we learn much about how often they occur, how powerful they were, what might have caused them, and whether future conditions might reasonably lead to another explosion.

This is one way in which volcanology and paleomagnetism are linked. At Kilauea, they combine to make the explosive past more readable and understandable.