Volcano Watch — Fields with potential for volcano monitoring

The month of January 2011 marks the second annual Volcano Awareness Month. Throughout the month, HVO, in cooperation with Hawai‘i Volcanoes National Park, Hawai‘i County Civil Defense, and the University of Hawai‘i at Hilo, will hold special volcano awareness activities around Hawai‘i Island, including public lectures, guided hikes, and a teacher workshop. A full calendar of events and more information about Volcano Awareness Month can be found at HVO's Web site.

Volcano Awareness Month provides opportunities to acknowledge the important role that volcanoes play in our daily lives, in addition to learning more about how volcanoes work and how scientists study volcanic activity. Throughout the month of January, this column will explore the different techniques used by the staff of HVO to monitor Kīlauea, Mauna Loa, Hualālai, Mauna Kea, and Haleakalā.

This week, we will focus on potential-field and electromagnetic geophysical methods, which are used to give geoscientists an indirect way to "see" beneath the Earth's surface by sensing different physical properties, like density, electrical resistivity, or magnetization.

The magnetic properties of rocks vary depending on iron content. Aerial surveys of a volcano's magnetic field can, therefore, be used to map subsurface properties and identify areas of subsurface magma, water, and other structures or substances. In Hawai‘i, aerial magnetic surveys have identified rift zones with solidified magma, which is highly magnetic, and subsurface molten magma, which is non-magnetic.

The electrical resistivity of subsurface rocks can be measured by applying an electro-magnetic (EM) field to the surface in one location and measuring the strength of the signal that is transmitted through the ground. EM techniques have been used to identify zones of magma storage beneath Kīlauea's summit.

A particular EM technique commonly used to monitor volcanic activity in Hawai‘i is VLF, or Very Low Frequency electromagnetic mapping. VLF signals, used by the U.S. Navy to communicate with submarines, can be measured anywhere on Earth. In the vicinity of a lava tube, the VLF field induces electrical currents in flowing lava within the tube. Measuring the VLF field variations across the top of the tube allows HVO scientists to estimate the amount of lava within a tube.

The gravitational pull of the Earth across its surface varies according to the density of the rocks underground. In places where rocks are very dense, the gravitational pull is a little stronger than where rocks are less dense. As a result, mapping gravity variations can provide density information about the subsurface structure of a volcano. At Kīlauea, gravity measurements reveal a dense core within the volcano, which is evidence of dense magmatic intrusions and mineral accumulations.

Changes in gravity over time can be used to map magma movement beneath the surface. For instance, between 1975 and 2008, scientists measured a persistent increase in gravity near Halema‘uma‘u Crater within Kīlauea caldera, suggesting accumulation of magma at shallow levels. In addition, continuous gravity monitoring at Kīlauea caldera has detected changes associated with the rise and fall of the lava column within the summit eruptive vent.

Geophysical techniques that can detect variations in magnetic and electrical properties and density have great value in volcanology, and continuous measurements of these parameters are on the cutting edge of volcano monitoring and research. We will doubtless see increased use of these techniques at Kīlauea in the years to come.

Next week we will discuss how geologic measurements are used to monitor volcanic activity in Hawai‘i and around the world.

In the meantime, be sure to check out this week's Volcano Awareness Month activities, including a talk about Kīlauea's 1790 eruption at the University of Hawai‘i at Hilo on Friday (January 7), a guided hike to Mauna Ulu on Saturday (January 8), and several National Park programs throughout the week.

Details about these and other Volcano Awareness Month activities are available at http://hvo.wr.usgs.gov or by calling 808-967-8844.

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Volcano Activity Update

Lava flows have been active on the pali and coastal plain west of Kalapana over the past week. The easternmost of the active flow lobes has advanced to within about 300 m (330 yards) of the end of the Kalapana access road, and this flow continues to slowly move eastward towards residences. The ocean entry remains inactive. Lava flows from an active vent in the northwest corner of Pu‘u ‘Ō‘ō crater continue to cover more of the western portion of the crater floor.

At Kīlauea's summit, the circulating lava lake in the collapse pit deep within the floor of Halema‘uma‘u Crater has been visible via Webcam throughout the past week. The circulation pattern was interrupted sporadically by abrupt increases in the height of the lava surface. These periods of high lava level have been short-lived, lasting from ten minutes up to several hours, and each ended with a sudden drop of the lava surface back to its previous level. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Three earthquakes beneath the Hawai‘i Islands were reported felt during the past week. A magnitude-2.4 earthquake occurred at 10:01 p.m. HST on Tuesday, December 28, 2010, and was located 5 km (3 mi) southeast of Kīlauea summit. A magnitude-2.0 earthquake occurred at 2:08 p.m. on Wednesday, December 29, 2010, and was located 4 km (3 mi) southwest of Kīlauea summit. A magnitude-2.9 earthquake occurred at 3:49 a.m. on Thursday, December 30, 2010, and was located 4 km (3 mi) northwest of Ka‘ena Point (Kīlauea south flank).