Yes you are at the correct site and no, I haven’t lost the plot – yet!
The following post is intended as a ‘mouth-waterer’ just to show the great variety of volcanism available.
Volcanism is not restricted to the Earth, as evidenced by various probes sent to study other members of the Solar System. Obviously, there is an absence of rock samples – other than a few meteorites and collected lunar samples – and no direct on-the-ground observations, thus features and rock types have to be by analogy to terrestrial types. The planets of our System are divided into the Rocky planets – Mercury, Venus, Earth and Mars with a relatively thin or no atmosphere surrounding a dense ‘rock-ball’; the remainder classed as Gas Giants with very thick atmospheres around a proportionally small rock core (Pluto excluded as it is now classified as a ‘captured’ Kuiper Belt object). Throughout the System there is usually evidence to some extent of massive impact cratering associated with the Late Heavy Bombardment (LHB) 4.1-3.8 billion years ago. On our Moon the cratering seen dates from this event and it can be argued, that the Mare are ‘volcanic’ as massive lava floods released by impactors rupturing through the crust. The magmatic origin of these is not in doubt, as nunatak-like mountain peaks and relic ‘ghost’ crater rims protrude the surface; similarly some lava-flooded craters, eg Plato, have lava filled floors. These structures have not been dealt with below, not having true volcanic origins – ie extrusion of liquid material not being related to mantle plumes or to tectonism.
The information given herein has been gleaned from various web sites and acknowledge by posting as necessary.
The smaller form below, with a 35km crater, is clearly a collapse caldera with both radial and ring faults well developed seen cutting paler coloured lava flows. Terracing within the crater suggests the presence of a lava lake.
Shield volcanoes, mostly associated with tectonic rifting, are well represented, with Sif Mons and Gula Mons, at 300 and 400km across respectively, but with corresponding heights of only 2 and 3km amongst the largest .
The smaller Sapas Mons, with a diameter of a mere 120km, vertical image shows well developed flows which from their geometry, appear to be of a fairly fluid lava, possibly basaltic. The false colour radar image shows light colours over rough reflective surfaces, darker over smooth and indicates a change of lava type with decreasing age. The arrows locate impact craters.
Anemone 1, 40km across, one of the small variety of shields, again shows the radar reflective blocky lavas radiating from the cone, with an extensive reflective lava field beyond, possibly a-a type basalt.
Volcanic domes each approximately 25km across, below, show the characteristic light radar reflectance of blocky material and this allied to the absence of obvious lava flows and the radially stretched surfaces to the domes suggests a very viscous magma as rhyolite building the domes from within.
Moving to the Earth – not dealt with here – and the Moon, volcanic activity, other than the Mare flood events, is not seen to any great extent on the Moon being limited to a few relatively small domes and cones. As the Moon is composed almost entirely of basaltic material, it is thought domes are the result of extrusion of viscous cool basalt lava and not rhyolite. However photographs of these features are very poor.
Unlike the almost totally volcano covered surface of Venus, Mars has volcanism concentrated in 4 provinces together with extensive volcanic plains. One of the greatest shield volcanoes so far located in the Solar System, Olympus Mons, with a basal diameter around 600km (Iceland has a width – from Snaefellsness to Egilsstaðir – of around 500km) with a height of some 25000m is located in the Tharsis Province. The vast size has been attributed to the presence of a very long-lived mantle hotspot and a lack of plate tectonics, the latter as having prevented Hawaii reaching similar proportions. The periphery of Mons Olympus is a cliff-like edge some 8km in height the origin of this feature as yet unknown. High resolution images show fresh surfaces to flows, as recent as 20 million years has beed suggested.
Olympus Mons off to the apparent north-west of 3 volcanoes – Arsia Mons, Pavonia Mons and (top) Ascraeus Mons. The green coloured area to the ‘north and west’ of Olympus Mons is a vast lava field from this feature.
Alba Mons, also on Tharsis – the very large red region at the top of the diagram above, is perhaps less obvious than Olympus Mons, having a very shallow slope around 0.5 degree and 7km high, but the volcanism covers a much greater area, with flow fields of highly fluid lava extending up to 1300km from the eruptive centres. The age of this structure has been suggested at c3.2×10^9 years.
Images for some Martian structures can be seen here:
The Gas giants
Moving to the Gas Giants, visible ‘volcanism’ is restricted to their satellites owing to their dense, thick atmospheres. In the case of Jupiter, of the 4 major satellites (Io, Europa, Callisto and Ganymede), the innermost, Io (with a diameter of 3642km) is perhaps the most active of all bodies in the Solar System, the volcanism/magma generation being a result of the massive tidal forces induced by Jupiter’s gravity field 422,000km distant. By comparison, the Earth-Moon mean distance is 384,400km.
Density measurements of Io (mean density 3.5g/cc) indicate a probable ultra-basic mantle surrounding an iron core whilst the surface is covered with volcanic silicate rock together with much sulphur derived from sulphide minerals. There is a marked absence of impact events and indicates the surface has been much reworked. Volcanoes, here called Paterae, resemble terrestrial calderas
Tvashtar Paterae showing lava extrusion
Galileo image, the orange and red colours are derived from sulphur allotropes of the vent Pele, the dark grey from silicate eruptions from Pilan Patera.
Further out still, satellites of Saturn and Neptune may show cryo-volcanic activity, mainly evidenced by the Voyager probes. This activity seen on Triton, one of Neptune’s moons takes the form of ‘geysirs’ of nitrogen gas and dust particles.
If this post has sparked the imagination to look at the heavens and you want to purchace a telescope, do not buy a cheap one or one sold on magnification only; you will be disappointed. Look into astronomy magazines first; Meade and Celestron are excellent instruments.