Types of Volcanoes on Io
Io's surface shows a mosaic of volcano forms from extensive plumes which cover 5 per cent of Io's surface as black spots, to vested chains of volcano calderas (pits) with black lava flows, to fissures through which lava erupt in vertical sheets or curtains of fire. Freeman and Kaufmann state, that Io has two distinct styles of volcanic activity: unique geyser like plumes, and Earth-like lava flows.
There are thermal areas like those in Yellowstone National Park, fumeroles, and calderas. The names of some of the plumes and volcanoes come from Greek and Roman Gods and Goddesses such as Prometheus and Zamama. Other large volcanoes on Io are Acala, Pillan, Pele, Svarog and Marduk.
The plumes of Io are mostly sulfur and sulfur compounds, but the lava flows are made from different compounds. The temperatures in lava flows on Io ranges from 1700 to 2000k (2600? to 3150?F), as determined by Galileo spacecraft infrared measurements. At these temperatures, sulfur evaporates instantly. Earth volcanoes have lower temperatures (1300 to 1450) which indicate that the chemical composition of lavas on Io is not the same as those on Earth. The high temperatures of Io's lava flows indicate that they are probably ultramafic lavas rich in magnesium and iron. The presence of molten ultramafic lava on Io shows that the interior of Io is much hotter than the present day Earth. In lava beds that formed billions of years ago when the Earth was much hotter than it is today, solidified ultramafic lavas are found. (Freeman and Kaufmann III, 320.) Researchers have developed a model that explains volcanoes and mountains formations on Io. According to this model, Io has a 100-km (60mile) thick crust that floats on a global ocean of liquid magma some 800km (500 mile) deep. This magma ocean accounts for the volcanic activity found everywhere on Io's surface, whereas on Earth, volcanoes are concentrated in spots or regions. In this model, mountains are blocks of Io's crust that have tilted before sinking into the magma ocean. These mountains reach heights up to10 km (30,000 ft).
Io's substantial volcanic activity consists of over 300 active volcanoes, each ejecting some 10,000 tons of material per second into its atmosphere. This material would cover its entire surface, up to a meter thick, in about a century. Thus, Io's surface is constantly being renewed and its impact craters are eroded. Io's volcanoes exhibit longevity. In 1979, Voyager I and Voyager II viewed volcanic plumes, some four months apart. These same volcanic plumes continued to erupt at nearly the same intensity as the first viewing. In 1996, the Galileo spacecraft showed that half of the volcanoes, viewed by the earlier spacecrafts, were still very active and other new ones had formed.
Io is one of Jupiter's inner (Galilean) satellites and its orbital period is in a 1:2:4 ratio with those of Europa and Ganymede. In 1992 the Hubble Space Telescope showed features of Io as small as 150 km across when Io was 4.45 AU form the Earth. Io is roughly the same size and density as our Moon. It is thought that the Galilean satellites probably formed in a similar fashion to our Solar System but on a smaller scale, and are composed roughly of rocky materials. The Galileo observations suggest that Io has a dense core composed of iron and iron sulfide, with a radius of about 900 km, surrounded by a mantle of partially molten rock, topped by Io's visible crust. (Freeman and Kaufmann, 321)
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