29 April 2010
Posted by Callan Bentley
Ice… serpentine… halite… What do they all have in common?
I’ve discussed mineral “ghosts” here before — really, those are only pseudomorphs, where one mineral’s chemistry becomes unstable due to a change in conditions, and then a new mineral forms in the same space. I’ve also brought up the issue of clasts of minerals which are unstable over the long term (ice).
Last night, at the final meeting of the Geological Society of Washington for the spring season, Bob Hazen of the Carnegie Institution of Washington gave the Bradley Lecture. Bob discussed his ideas about mineral evolution, and gave a compelling talk.
One of the key implications about thinking about minerals evolving over time is that new mineral species can evolve when conditions change and permit their growth, but so too can old mineral species go ‘extinct’ when conditions change and no longer promote their growth.
This got me thinking about that ice-clast breccia again (link above), and how that would be interpreted by future geologists, assuming the ice itself has melted away. Consider the geologic record of a superwarm planet, where temperatures never dip low enough to form ice. Would we be imaginative enough to invoke ice as the cause of glacial landforms, of striations and deposits of till? How would we explain dropstones and ice wedges if ice were an “extinct” mineral on Earth?
And so after the talk was over, I went up to Bob and introduced myself and asked him if he could think of (or imagine) other minerals which could profoundly affect the geologic record, yet disappear after they have done their work. As we were talking, it occurred to me that halite in the form of salt domes could perturb the local stratigraphy, then the salt diapirs could rise up to the surface and be eroded (or re-dissolve into the ocean), leaving a piercing trail of destruction in their wake.
Bob came up with another one: serpentine at a subduction zone: hydrothermal alteration of oceanic crust produces serpentine, but then the serpentine is unstable when it gets subducted. It dehydrates (gives off water), and (poof!) there’s no more serpentine minerals. However, this dehydration is super duper important geologically: the addition of that water to the hot rocks of the subduction zone lowers the melting temperature of the rocks, and helps generate magma: the magma that rises to feed volcanic arcs. If we didn’t have oceanic crust to look at, would we have imagined serpentine beneath our convergent boundaries, a humble transformer of the world above?
Readers, I put the same question to you: Which minerals cause big effects, but then disappear? Who are the prime movers who flee the scene of the crime? These are minerals that aren’t just ghostly; they’re downright phantasmic! I’ll be eager to read your suggestions, or hear your thoughts on the three I’ve noted here.
Minerals that undergo chemical weathering cause big effects as they disappear. Processes like the conversion of feldspars into clays, the oxidation of pyrite, and the dissolution of carbonates, profoundly influence landscape development and soil formation, among other things.
Be careful! You are treading close to the line between science and science fiction. Ice-nine comes to mind. Nothing wrong with that, so long as you keep in mind what you are doing.
Uniformitarianism requires that we start with things we know in the present and work our way back into the unknown of the past. Or is that Actualism? Either way you are stuck with needing a scheme of falsification relative to disappearing evidence.
Of course, Earth history brought us forth without the benefits of philosophy and so follows no one’s dictates. The problem is that the mind can generate many fantastical things- like Ice-nine- and falsification is our best tool for sorting out the good stuff.
But in the spirit of your query, consider a non-mineral substance (why limit yourself to solids?). Let the water disappear!! After all, its existence on Earth is fairly tenuous. That would be as striking as letting it all freeze up-I can’t seem to get away from Ice-nine.
Thanks for the admonition… I don’t want to be pseudoscientific; only to consider possibilities that have no present-day analogue. J. Harlan Bretz taught us that strict uniformitarianism can close off interpretations of the past which appear to be true. (For readers unfamiliar with Bretz, watch the NOVA special “Mystery of the Megaflood,” which details Bretz’s work in eastern Washington state, work which came to the non-uniformitarian conclusion that an unparalleled flood had taken place there.)
I think it’s fair to ‘think outside the box’ in the interest of generating ideas which we might not otherwise think to test. We don’t know if we can test the idea, of course, until we’ve thought it up in the first place. In other words, it’s one thing to imagine a possible explanation of the past, another thing entirely to conclude that’s actually what happened.
Box?? What box? There’s a box?
Tell me about this box!
[…] couple of weeks ago the very prolific Callan Bentley at Mountain Beltway raised some speculative questions inspired by the concept of mineral evolution, the idea that the […]