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How Do We “Account” For A Volcano’s Magma Budget?

January 31, 2014

Shaded relief map of Kīlauea’s East Rift Zone in the vicinity of the Puʻu ʻŌʻō eruption site. Colors show changes in elevation that occurred due to emplacement of a lava flow at the base of Puʻu ʻŌʻō during August 3–15, 2011. Parts of this flow were over 20 m (60 ft) thick. The topographic change was measured by a satellite system that orbits Earth and acquires images every 11 days. (USGS)

The following is this week’s edition of “Volcano Watch” from the USGS Hawaiian Volcano Observatory:

We conclude our Volcano Awareness Month series on the important questions about how Hawaiian volcanoes work with an article on monitoring volcanic activity.
The effects of some of the physical manifestations of volcanic activity, such as ash, vog (volcanic air pollution) and earthquakes, are widely experienced by people in Hawaiʻi. Scientists at the USGS Hawaiian Volcano Observatory (HVO) use monitoring tools to measure and analyze these byproducts of volcanic activity and their relation to the activity itself—such as the extent of lava flows, deformation of ground surface, and the distribution and effects of earthquakes.

Perhaps the most important aspects of volcanic activity that should be monitored involve a volcano’s magma budget—that is, the balance between the amount of magma erupted from the volcano versus the amount supplied to it.

There are various methods for measuring eruption rate, but all have limitations. For example, the amount of gas emitted from a vent is proportional to the amount of lava erupted from that vent, but this relation has not been reliable since the onset of Kīlauea’s summit eruption in 2008, when the style of gas emissions from the volcano changed.

Lava cascades down a steep slope within a lava tube. The visible part of the cascade is about 2 m high; the entire cascade is about three times as high (entire cascade is hidden from view by the roof of the tube). This photograph was taken through a skylight on the coastal plain of Kilauea about 12 hours after the end of an eruptive pause. (J. Kauahikaua/USGS)

Quantifying flows through lava tubes can provide an eruption rate, but this requires the flow of lava through a single tube in an easily observed area—a condition that has not existed for several years. Thermal measurements can also indicate eruption rate but require cloud-free satellite images and surface flows (as opposed to tube-fed lava flowing directly into the ocean)—both of which are rare conditions at Kīlauea.

Satellite measurements of surface topography offer a new method of measuring lava eruption rate. A pair of satellites orbiting Earth since 2011 can generate a map of elevations approximately every 11 days. By comparing elevations acquired from successive satellite overpasses, topographic changes due to lava flow activity can be measured over time and yield the lava eruption rate.

The satellite topographic technique will work, however, only when lava-flow activity is confined to land, as is now occurring at Kīlauea. It also produces measurements only when the satellite passes overhead, as opposed to when the scientists monitoring the volcano need the data. Nevertheless, this new method of measuring eruption rates complements existing methods and confirms the value of using satellite data to monitor volcanoes.

In addition to measuring what comes out of a volcano, it is also important to assess what goes in. The supply of magma to a volcano is, in fact, the most fundamental control on eruptive activity. But how can a process occurring deep underground and out of sight be investigated? Data from Kīlauea indicate that the emission rate of carbon dioxide (CO2) may provide a solution.

Skylight in 2005 (USGS)

During 2003–2007, a surge in magma supply to Kīlauea resulted in numerous changes in eruptive activity. This surge was preceded by a near-doubling of CO2emissions from Kīlauea’s summit—a sign that an increased amount of magma was rising beneath the volcano. Unfortunately, the CO2 emission rate is difficult to measure, because detecting the gas requires special conditions at Kīlauea’s summit. Since carbon dioxide is present in Earth’s atmosphere, identifying volcanic CO2 from background atmospheric CO2 is not easy.

New technology may help to detect volcanic CO2 emissions, and volcanologists plan to use Kīlauea as a testing ground for these techniques in the coming years. Inferring the rate of magma supply to a volcano based on CO2 emissions may become commonplace in the near future. Combined with improved measurements of eruption rate, routine monitoring of the magma budget of a volcano may finally be realized.

This concludes our annual Volcano Awareness Month series of Volcano Watch articles. For more information on the big questions related to Hawaiian volcanoes, you’re invited to attend an HVO talk on the subject in Hawaiʻi Volcanoes National Park on February 4. Details are posted on the Park website (http://www.nps.gov/havo/planyourvisit/events_adip.htm).

From the staff of HVO, Hauʻoli Makahiki Hou!

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