#petrogenesis

Every rock that crops out of the ground is a record of the past, and so, in one way or the other it tells something about the environment in which it was formed. For example, the dark grey rock with fine and smooth texture you can see on the photo, looks that way because it formed on deep water sea bottom, where waves did not disturb the sediment that make up this rock, nor coarser sandy material was brought into it. This was a clam environment which allowed the sea-bottom animals to feed on the organic matter within the sediment (hence the dark colour) and is the reason why there are so many fossils in it, which I showed in the ‘underwater lilies’ post.

So, it is all good, and all makes sense!

But, then there are those large pieces of light grey rock (not shown, but some bigger than sedan car) with rough texture and little holes. Like you can see, its pieces are larger than anything else in a vicinity and are scattered all throughout the dark grey rock. This light grey rock does not belong to the sedimentary environment it is in, it got to where it is by other means than the sediment that surrounds it. And so, the plot thickens…

Role of liquid immiscibility has immense importance in petro genesis of basaltic magmas. Roedder, Weiblen [1,2] postulated that high silica and high iron glasses of the lunar basaltic rocks have widespread petro genetic significance. In lunar basalts, generally, the globules of dark brown glass appear to have composition equivalent to Pyroxene + Iron ore. On the contrary, it is significant that Washington [3] initially recognized that the residual glass in the Deccan Trap basalts should be composition corresponding to a mixture of augite and magnetite. The role of liquid immiscibility in basaltic magmas had been graphically depicted by Roedder in terms of SiO2-(FeO+MgO+CaO)-(K2O+Na2O+Al2O3) diagram; characteristically the liquid immiscibility field in Roedder's ternary plot assumes an elongated elliptical geometry relatively away from SiO2-rich corner.The compositions of terrestrial basalts like Deccan Trap [3] Palisade diabase sill etc.