The mountains of Io are rugged and isolated. They are separated by the plains and the mountains and plains cover about 2% of the surface. Individual mountains can be up to 100 km long and have relief of 9 km. The mountains do not appear to be volcanic in origin (although they are mantled in sulfur) and are thought be older than both the plains and volcanoes. The mountains have been modified by both tectonic and erosional processes. The presence of the mountains suggest Io has a rigid lithosphere, possibly up to 30 km thick.
About 40% of Io is covered by plains with low relief and light and dark areas. The plains are probably layers of pyroclastic material erupted from volcanoes and possibly lava flows of different compositions or ages. Layering can be seen on the edges of some plains. Other plains contain plateaus with smooth tops and escarpments from 150 to 1700 m high. The escarpments are evidence for erosion.
Only about 5 percent of Io is covered by volcanic vents. About 500-700 volcanic centers have been identified but, over the last decade, most of the energy has been released at only four centers. The energy is released from these centers at enormous rates. Carr (1997) reported that 356 calderas had been identified in the Voyager and Galileo coverage. The largest volcanoes have diameters of more than 250 km and are closer to the equatorial region. Volcanoes at higher latitudes tend to be smaller, less than 100 km in diameter. The random distribution of Io's volcanoes suggests a lack of mantle convection, which is partially responsible for linear hot spot tracks and island arcs on Earth.
Paterae, low-profile volcanic shields are the most common type of vent. Their flows can cover large areas and reach lengths of 700 km. Such long flows suggest high eruption rates and/or low viscosity material. Some patera have summit calderas with relief of 1-2 km from floor to rim. The images above compare the 1979 Voyager 1 image of Loki Patera with Galileo images taken in 1996. The patera is at the center of the images. A dark fissure is just above and right of the patera. Voyager observed an eruption from this fissure in 1979.
Ra Patera covers an area of 760 x 480 km and has numerous long, narrow flows that radiate from the summit. These views of Ra Patera show changes seen on by Voyager 1 (upper left and upper right), Galileo (bottom right), and Voyager 2 (bottom left). The Galileo images reveal the detailed morphology of new deposits. Dark materials are interpreted as the overflow of lava flows from the caldera. New bright deposits, also though to be lava flows, cover an area of about 40,000 square kilometers and surround the dark materials. Images courtesy of The Jet Propulsion Laboratory and NASA. Maasaw Patera, another shield volcano with summit caldera, has been compared to Volcan Alcedo in the Galapagos. The patera are thought to be made mostly of silicate lava flows with interbedded sulfur lava flows and pyroclastics.
Io may have calderas with active lava lakes and fissures erupting silicate lava flows. These calderas may be up to 200 km in diameter and are located on the surface of the plains. Volcanic plumes originate in some calderas.