# Volcano Watch — An Eye on Io's Volcanism

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Io, the innermost of Jupiter's five major moons, is roughly the same size as Earth's moon. When Voyager 1 and 2 sped past Jupiter in 1979, scientists were amazed to find that Io is peppered with active volcanoes.

We now know that Io is the most volcanically active body in the solar system. The Voyager spacecraft revealed giant calderas up to 200 km (124 miles) across, dozens of erupting volcanoes, and volcanic plumes reaching 500 km (310 miles) above the moon's surface. More recently, when the Galileo spacecraft flew by Io in late 1999 and early 2000, it sent back dozens of high-resolution images showing volcanic features strikingly similar to those that we are familiar with here in Hawaii.

One of Io's most unique volcanoes, named "Pele" after Hawaii's volcano goddess, was the first one discovered on another world. The vent region of Pele is defined by a bright, high-temperature hot spot that has remained remarkably persistent. Images from Galileo suggest that this hot spot is an active lava lake - much like lava lakes seen at Kīlauea - but at least 10 km (6 miles) wide. Not only are the lava lakes bigger, but the temperature of the lava at Pele and other volcanoes is hotter than that of Hawaiʻian lava - perhaps reaching 1,727° C (3,140° F), compared to about 1,140° C (2,100° F) here at Kīlauea.

Images of another of Io's volcanoes, Tvashtar Catena, show a fissure eruption inside the volcano's caldera. Though the resulting curtain of lava looks similar to fissure eruptions often seen in Hawaii, the erupting lava was reaching a height of nearly 1.5 km (1 mile).

Perhaps the most prominent feature on Io is Prometheus, a volcano seen in every image taken of that part of the moon by Voyager, Galileo, and the Hubble space telescope. The latest images of Prometheus show an enormous caldera 28 km (17 miles) long and 14 km (9 miles) wide, with a large lava flow extending toward the west.

Prometheus is characterized by two heat sources - one at the caldera, which is the lava source, and the other at the end of the lava flow. A 50- to 100-km- (30- to 60-mile-) tall volcanic plume seen over Prometheus comes, not from the caldera, but from the far end of the lava flow.

Lava is thought to erupt beneath the caldera, enter lava tubes, and flow about 100 km (60 miles) before breaking out onto the surface again. The plume at the end of the lava flow is presumably created by the interaction of the hot lava with sulfur dioxide-rich snow that blankets the area. This process is very similar to the one we see in the ongoing eruption of Kīlauea, though on a larger scale. Lava is erupted at Puu Oo, enters a lava tube, and flows to the Pacific Ocean. The plume at Prometheus is roughly equivalent to the laze plume encountered in Hawaii when lava enters the ocean.

So what causes volcanism on Io? The answer is a tidal process similar to what we see here on Earth. Living in Hawaii, we are all familiar with the daily fluctuations in water level caused by the tides. But the solid earth also has a tidal bulge that follows the same schedule as ocean tides. Fortunately, the amount of vertical displacement caused by solid earth tides is extremely small and can only be detected with the most sensitive instruments.

Io is situated between Jupiter and two of Jupiter's large moons - Europa and Ganymede. The gravitational force exerted by Jupiter and its moons creates a tidal bulge more than 100 meters (330 feet) high on Io's surface. Remember, this is a tide created in the solid rock of Io's crust! As Io rotates around Jupiter, the tidal bulge moves, Io's crust is flexed, and tremendous heat is generated - much like the heat generated in a piece of wire when it is quickly bent back and forth. This heat drives the volcanic activity so prevalent on Io.

### Volcano Activity Update

Eruptive activity at Puu Oo continues. Spatter cones in the crater of Puu Oo glow brightly on clear nights but have not produced any lava flows for several months. Spattering in the crater intensified briefly early on January 25 but diminished by the next day. The MLK vent area, at the southwest base of the cone, intermittently erupts small Pāhoehoe flows that stack up close to the vent.

The PKK flow continues to host substantial breakouts from the 2,300-ft elevation to the coastal plain. Lava is not entering the ocean. As of January 25, several lobes of the PKK flow were active on the coastal plain. The eastern arm of the PKK flow is about 1 km from the coast, about half way between Kamokuna and Wahaula. The closest activity to the end of Chain of Craters Road, in Hawaii Volcanoes National Park, is about 300 m (330 yd) inland of the shore at West Highcastle. This flow is about 2.6 km (1.6 mi) from the ranger shed at the end of Chain of Craters Road. Expect a 1- to 1.5-hour walk each way and remember to bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs.

Two earthquakes were felt in the early morning hours of January 25 on Hawai`i Island. The first occurred at 12:41 a.m. at a depth of 3.5 km (2.2 miles) east of Leilani Estates in lower Puna and was felt locally. The second occurred at 2:22 a.m. south of Kīlauea summit at a depth of 3.7 km (2.3 miles) and was reported felt at the Volcano Golf Course.

Mauna Loa is not erupting. The summit region continues to inflate. Since July 2004, the rate of inflation and number of deep earthquakes has increased. Weekly earthquake counts have varied from 5 to over 150, but have been low for the past four weeks. During the week ending January 27, six earthquakes were recorded beneath the summit area. Nearly all are 30 km (18 mi) or more deep, and most are the long-period type, with magnitudes less than 3.