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October 14, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

A Glimpse into the Structure of Haleakala Crater

At the end of their shield-building stage, the summits of Hawaiian volcanoes commonly have large collapsed areas called calderas. Both Kilauea and Mauna Loa have these summit calderas. After this stage, Hawaiian lava rapidly changes chemistry and evolves into a more pasty form. These later eruptions produce short, thick lava flows that cap and subsequently bury the summit calderas resulting in volcanoes with steeper slopes covered with cinder cones. Mauna Kea, Kohala, and Hualalai are good examples of Hawaiian volcanoes at this postshield stage of development. If they ever had summit calderas, those calderas are now buried beneath postshield lava.

Haleakala, another postshield volcano, may have evolved differently. The famous crater on its top is actually within the capping lava flows, not the shield-building lava flows. Because of this, many geologists have concluded that Haleakala crater was not formed by collapse, but instead by lots of erosion. It has even been suggested that glaciers may have carved out the crater. But definitive evidence of the crater's origin has always been lacking.

To test whether the crater was indeed formed by collapse or not, we devised a test using measurement of the Earth's magnetic field across the crater floor. This test relies on the discovery of reversely magnetized lavas in the base of the crater walls, beneath normally magnetized lavas. Lava flows that cool when the north and south magnetic poles are where they are today are normally magnetized. That is, the magnetic grains within the lava align north-south, like little compass needles, before they are completely frozen into the cooled flow. The reversely magnetized flows cooled during a time in the Earth's history when the north and south magnetic poles were switched - but that's another story.

Since Haleakala has these two differently magnetized layers, a summit collapse should drop the normally magnetized flows like a receding piston into the reversely magnetized core of the volcano. This would be similar to pushing your palm into the top of a chocolate cake covered with thick white icing. The icing would end up surrounded by chocolate cake (and stuck to your hand). In the Haleakala cake, however, younger lava flows cover the crater floor so that we can't actually see how it formed. But magnetic field measurements across those surface flows can allow us to "see" through them revealing the presence and location of the edges of any collapse that brought normally magnetized lavas right next to reversely magnetized lavas.

The survey required 2.5 days and covered 5 km (3.1 miles), using four people and two magnetometers to make a total of 2500 individual measurements. The data along our route do not reveal the presence of any vertical buried contacts within the crater. We must conclude that there is no magnetic evidence that Haleakala crater formed by collapse and that it probably did form by erosion. The survey cannot offer any help in deciding whether water or ice was the more important agent of erosion.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a tube to the southeast in the direction of the sea coast. As of late Thursday morning, breakouts from the tube occur 3.5 km (2.1 mi) from Pu`u `O`o near the 640-m (2100-ft) elevation. The breakouts feed a 1-km-long (.6-mi-long) flow that has traveled down to the 560-m (1850-ft) elevation. No lava was flowing over the pali. Occasionally, the lava pond within Pu`u `O`o becomes very active, and a bright red glow, visible throughout Puna, is cast upon the fume clouds overhead.

There were no earthquakes reported felt during the week ending on October 14.

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Updated: 18 Oct 1999