CompuServe Thread

Hello, Mr. Titus: Actually, ultramafic lava flows are present in the geologic record. Have you heard of "komatiites"? These were first described from a type locality along the Komati River in the Barberton Mtns. of southern Africa. Similar rocks have since been recognized in most areas of the world where v. old terranes (i.e., Precambrian shields) are exposed, for example Australia and Canada. Textural evidence from komatiites indicates that these features were rapidly chilled, as would be expected from subaerial or subaqueous eruption (i.e., precisely as would be expected in a lava flow). As alluded above, ultramafic lavas are almost exclusive to v. old rocks (the youngest example known, if memory serves, occurs in an Ordovician formation). This situation suggests that the formation of ultramafic lavas depended on conditions peculiar to the young earth, such as much higher heat flow (higher load of radiogenic elements that have since decayed), a thinner crust, possible differences in the source region (mantle). There are technical references that address komatiites in great detail. I could dig out one or two if you were interested in pursuing the topic. Hope that this helps! Cheers, Scott M.

Hi, Wayne– Welcome to the forum! My "Simon & Schuster's Guide to Rocks & Minerals" lists melilite and carbonatite as extrusive ultramafic igneous rocks. Both are associated with alkaline environments, carbonates are mentioned in the formation of both, and the pictures of both look like intrusive rocks to my untrained eye, having marked crystal grains (I am a mineral collector with no formal geology education beyond a quick introductory course). To Scott's more knowledgeable remarks (thanks Scott!), let me add one wild flying guess: maybe if they have not reacted with continental rocks to loss their ultramafic nature, might they tend have too much gas (I am thinking of CO2 from carbonates) to often erupt "peacefully" as lava and tend to produce explosions and ashes instead? –Doug