Io, while being one of the most spectacular moons of Jupiter, is also one the most active, exhibiting volcanic activity on a massive scale.
The Voyager 1 spacecraft sent the first images of Io as it passed by the mysterious moon in 1979. These images shocked the scientific community because they revealed active volcano plumes rising up more than 300 kilometers above Io's surface. Since then, the Hubble Space Telescope and the Galileo spacecraft have delivered many additional images, helping illustrate that this remarkable body is dotted by hundreds of volcanic centers, about 70 of which are active.
Io's surface is extraordinarily colorful, with yellows, oranges, reds, and blacks reflecting diverse eruptions, from great outpourings of basaltic lava to massive deposits of sulfur.
The first images obtained from Voyager 1 in 1979 showed a strange umbrella-shaped feature that appeared to be protruding from Io's surface. Soon it was realized that the feature was an eruption plume, largely composed of sulfur dioxide (SO2) gas, rising 260 kilometers above the surface. Additional Voyager images revealed even more volcanic plumes. A total of 15 active plumes have now been identified from the images obtained through the Voyager and Galileo Missions, two of which are shown here:
Spacecraft instruments have provided us with important data on the nature of these explosive eruptions. Voyager's Infrared Interferometer Spectrometer (IRIS) indicates that most of the active plumes correspond with hot areas on Io's surface. Data from Galileo's Near-Infrared Mapping Spectrometer (NIMS) suggested that many of these high-temperature explosive eruptions are driven by SO2 gas emission. In contrast, explosive eruptions on earth are driven largely by H2O and CO2 gas emissions.
The Changing Face of Io
Unlike the surface of most other planetary bodies, the extensive volcanic activity on Io can generate dramatic changes to its surface over very short periods of time. Volcanic plains composed of overlapping lava flows and volcanic plume deposits largely cover the colored surface.
The red deposits on Io appear to represent young, SO2-rich pyroclastic material that will last perhaps only a few years before turning yellow or bright white. The lighter colored SO2 frost is probably coarser grained than the younger, red-colored SO2. The yellow ring in the image to the left was deposited by the volcanic plume Prometheus that has been an active site of volcanism for over 20 years. Over this time, the source of plume has shifted 70 km to the west. Most scientists believe that the source for this long-lived plume is from a vent lying at the west end of the dark lava flow in the image. However, there is some speculation that the plume might be a product of the advancing lava as it flows over and volatilizes the SO2-rich surface.
An example of a spectacular ring of red pyroclastic debris comes from the active volcanic plume Pele, shown in the two Galileo images below. These images were taken less than six months apart. Note the dramatic change that has occurred over this time period near the volcano Pillan Petera, marked by the red arrow. The April image shows the simple caldera of Pillan Petra located within the red pyroclastic ring of the Pele plume. However, the September image show circular, dark deposits about 400 kilometers in diameter surrounding the caldera. The dark deposit covers an area about as large as the state of Arizona and is most likely composed of silicate-rich pyroclastic material.
Lava Flows and Flow Fields
Lava flows are also rapidly changing the face of Io. A spectacular example of active lava production is demonstrated in the second pair of images shown below. These images, taken three months apart, lie within a chain of giant calderas known as Tvashtar Catena. The November 1999 image on the left displays a fissure eruption (arrow) that appears to be generating a "curtain of fire" which is a typical feature displayed by many Hawaiian-type eruptions on Earth. The fissure shown here is about 40 km long and the "curtain of fire" extruding from it appears to rise about 1.5 kilometers above the surface. The eruption appears to be associated with the generation of active lava flows at the base of the fissure. The second image, to the right, was taken three months later in the February 2000. At this time, the site of volcanism has shifted to the west (arrow) to generate a lava flow that is over 60 kilometers long.
Lava flows on Io have many of the same characteristics as lava flows found on earth, as demonstrated in the images below. The Amirani and Maui lava flows (left-hand image) are the longest active lava flows known to exist in the solar system. The flows travel down lava channels that extend more than 250 kilometers away from a common vent. The white deposits that encircle the vent are probably SO2-rich vapors that were ejected above the surface where they froze as solid particles that settled to the ground as snow or frost.
The right hand image (above) displays the lava flow field of Culann Patera. These flows spill out on all sides of a central caldera. The variegated colors of the flows are most likely related to the interaction of the hot lava with SO2-rich debris on the ground surface as well as SO2-rich ash fall.
The region confined to the yellow box the image above is blown up in the image shown below. The summit caldera of Culann Patera appears to be the source of a lava tube that feeds the large, dark green flow to the west. The axis of the western part of the flow is marked by what appears to skylights lying above the tube's continuation. These features are typical of basalt lavas found on Earth.
What is the Composition of the Lavas on Io?
Lava composition is partly a function of its melting temperature. Data from Galileo's NIMS instrument indicated that the average temperature of active lavas on Io is about 1600 degrees Celsius. Since sulfur boils vigorously on Io's surface at about 500 degrees Celsius, the lavas on Io also are thought to be made up of silicates, which are the most common group of rock-forming minerals, made of silicon (Si), oxygen (O), iron (Fe), magnesium (Mg), potassium (K), sodium (Na), and calcium (Ca). Therefore, it is accepted that the lavas on Io are certainly of a silicate composition, but more data is needed in the future, for us to know more about this intriguing, volcanic moon.
Source: Vic Camp. "How Volcanoes Work." http://www.geology.sdsu.edu/how_volcanoes_work/io.html
Glossary of Terms
Pyroclastic Materials: The rapid eruption of expanding gases results in the obliteration and fragmentation of magma and rock. The greater the explosivity, the greater the amount of fragmentation. Individual eruptive fragments are called pyroclasts ("fire fragments"). Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation. Whereas tephra is unconsolidated, a pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type.
CLASSIFICATION OF PYROCLASTS - Tephra is classified on the basis of pyroclast size:
ASH -- Very fine-grained fragments (< 2 mm), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments. Courtesy of USGS.
LAPILLI -- Pea- to walnut-size pyroclasts (2 to 64 mm). They often look like cinders. In water-rich eruptions, the accretion of wet ash may form rounded spheres known as accretionary lapilli (left). Courtesy of USGS.
BLOCKS AND BOMBS -- Fragments >64 mm. Bombs are ejected as incandescent lava fragments which were semi-molten when airborne, thus inheriting streamlined, aerodynamic shapes. Blocks (not shown) are ejected as solid fragments with angular shapes. Courtesy of J.P. Lockwood, USGS.
Within this size classification, specific types of tephra can be further defined by physical attributes. For example, lapilli-size fragments of basaltic lava may cool quickly while airborne, to form glassy teardrop-shaped lapilli called Pele's tears. During strong winds, these molten fragments can be drawn out into fine filaments called Pele's hair. Nonexplosive Hawaiian-type eruptions often produce lapilli- to bomb-size fragments, called spatter which remain airborne for only a short amount of time so that are still liquid when they hit the ground surface. Some lapilli- to bomb-size pyroclasts maintain a frothy appearance due to escaping volcanic gases and the rapid accumulation of bubbles (vesicles). These highly vesicular rock types include mafic and felsic varieties. Vigorous gas escape in felsic lavas produces pumice, whereas similar gas escape in mafic lavas produces reticulite. The high vesicularity of pumice lowers the density of this rock type so that it can literally float on water. Reticulite has a still lower density, with vesicles occupying up to 98% of the total volume. Unlike pumice, however, most of the bubble walls in reticulite are broken so that it sinks in the presence of water. Reticulite, however, is not as common as scoria, a denser mafic rock containing a smaller abundance of relatively large vesicles. Click the links below to view images of these pyroclast types.
Silicates: All rocks are composed of minerals, and minerals are composed of one or more chemical elements. The primary elements that make up rock-forming minerals are silicon (Si), oxygen (O), iron (Fe), magnesium (Mg), potassium (K), sodium (Na), and calcium (Ca). Various combinations of these elements form the most common mineral group, the silicates. The most important silicate minerals are quartz, feldspar, mica, amphibole, pyroxene, and olivine.
All silicate minerals contain oxygen and silicon atoms, and these atoms organize into a structure called the silicon-oxygen tetrahedron. This unit is made up of one silicon atom surrounded by four oxygen atoms. The silicon-oxygen tetrahedron is the fundamental building-block of silicate minerals, and all silicates contain this structure.
Ninety-two percent of the Earth's crust is composed of rocks that are made up of silicate minerals. Most of these rocks formed when molten material from the Earth's interior solidified. Rocks that form in this manner are catagorized as igneous rocks. There are many types of igneous rocks, although basalt and granite are the most familiar.
Molten rock, or magma, contains the chemical elements that form silicate minerals. Once these elements begin to form chemical bonds within the magma, crystals start to develop. Some minerals, such as pyroxene and plagioclase feldspar (which form basalt), crystallize at relatively high temperatures. Seventy percent of the Earth's surface is basalt, with most of it found on the ocean floors. Other silicate minerals, such as potassium feldspar and quartz, crystallize at lower temperatures. These minerals are the primary constituents of granitic rocks. Granite is the rock that makes up the majority of the continental crust. Granite is pictured (top) with its primary constituent silicate minerals--(left to right) plagioclase feldspar, mica, potassium feldspar, hornblend (amphibole), and quartz.
Last Updated: 22 February 2011