Dacites are rich in silica, aluminium, calcium and sodium, but relatively low in iron. They often have high water contents, which is why they are often very destructive in their eruptions (Mt St Helens is a dacite volcano). It is also one reason why they are associated with hot spring systems (although often the water in thermal strings are not magmatic, but normal groundwater).<br /><br />Their lavas are usually very sticky at their normal eruption temperatures (600-800 degrees C) because of their high silica contents which leads which polymerisation. This stickiness also aids the explosive eruption process. Lower silica lavas (e.g. basalts) are richer in metal oxides and behave more like liquid metals and are thus more fluid. If superheated (to 900-1000 degrees) dacite lavas can be quite fluid. This is rare though.<br /><br />They are close cousins to rhyolites, which have higher potassium (and sometimes sodium) and lower calcium. Rhyolites and dacites are the volcanic equivalents of granite and granodiorite, respectively. Together they are called felsic (light coloured) igneous rocks.<br /><br />Typically felsic rocks are thought to be rare on Mars because they are associated with plate tectonics. However recently large areas of felsic rocks have been identified in THEMIS data from Nili Paterea. These are probably formed by a different process to flate tectonics, naminly mafic underplating and partial melting of the crust. In english this means that very large bodies of mafic (dark coloured, ie basaltic) magma pond in the lower crust and heat enough to undergo partial melting. The resulting partial melts are felsic in composition and rise though the crust forming incrusions and volcanoes. this process is thought to have been very common in the Archaean earth and is probably responsible for some mid-continent volcanism today. Some rhyo-dacites associated with underplating are the superfluid superheated felsic lavas mentioned above.<br /><br />Hope this ma <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em> Arthur Clarke</p> </div>
Does a cosmic 'glitch' in gravity challenge Albert Einstein's greatest theory?
Goodbye infinity and all that infinite singularity and infinite density descriptions
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