3/27/2007

RIFTCLASTIC! And Rhyolite and Basalt - Lebombo

Ha, I will punish you non-geologist by talking about rocks before talking about people and animals. Why? Because that's my job. Santa Cruz has the "Poleta Punisher", the "Polish Punisher"... What can I be? Anyway....

Beautiful quarries like this one are carved out of the Lebombo Rhyolites, in particular the Seca Beds rhyolites. They are making road base and building stone out of solid volcanic rock. The basalts in the area are generally quite rotten and spongey and make good banana soil but bad building stone. In this area, holes are drilled by hand or by little generator powered drill, then blasted, then the rocks are broken down by hand and sorted with a dump truck and a grizzly. The quarries here are richer and the people poorer than in the north. Here's an inactive quarry in pink rhyolite

One mystery not yet solved by the mappers (from geophysical data) and geochemists (from ... geochemical data) is: where's the vent? Where did this rhyolite erupt and which direction did it flow? No volcanologists have spent any time out here sorting the mess between the ignimbrites, volcanic breccias, flows, glassy blocks, etc.... so as a structural geologist I did the only thing I know how to do, measured folds... [Diversion - SILVA COMPASSES SUCK they are not fit for any geological purpose aside from being CHEAP, IMPRECISE, WORTHLESS, in short, their only value is that you can give them to UNDERGRADUATES]... hoping the flow folds will plot up nicely and give us a flow direction?! Time will tell...

While peeling back layers of pumicy rhyolite and glassy rhyolite to get at a fold axis (and by "peeling" I mean "crushing with a sledge hammer to make a satisfying glass-tinkly sound") I uncovered a tiny bat which had been trapped between layers and dessicated. I include this here because it's not a living animal, more like a fossil or a mummy, therefore geological. Inset photo: closeup on bat. Outset photo: Jodie happy to have strange dead thing to add to her display case coffee table.

As an aside, there is this bubbly-like texture on what appear to be both top and bottom surfaces of some rhyolite beds - and sometimes lining cavities - looks like bubbles but they are actually solid glass, and seem to be internally flow banded coherently with external flowbanding. A weathering spheroidal texture? Maybe but some are stretched out, like spherical prisms (if there is such a shape?) Not sure what they are, Jodie will work it out with a volcanologist friend... Cool anyway, no?

This is the only good basalt outcrop we found, it's directly under a cap of hard rhyolite and will likely weather to banana dirt soon! (Geologically soon.) BUT CHECK OUT THE GORGEOUS COLUMNS! These are among the nicest ones I've ever seen. Question: why do they change orientation? Answer: the isotherms were curved during cooling. Question: Why did the isotherms curve? Answer: Ek weet nie. HA! That's AFrikaans for I don't know. I'm learning Afrikaans phrases from an Australian. Oh dear.


And finally, but not least, the volcaniclastic sediments. These are sediment that are interbedded with the volcanics, laid down by (in this case) a river running through and within the active volcanic field, so they are very chemically immature and almost identical to the volcanic rocks themselves, only by their sedimentary structures are they different than volcanic rock. Since they are sandstone-siltstone-conglomerate and the volcanics are demonstrably rift-related I am taking the liberty of calling them RIFTCLASTIC (which will always remind me of Randi, Marci, Ian... that crew... structure 06... but I digress!) although that term is usually used (in the USA where somethings are still better than here, such as EARTHQUAKES) to describe a fanglomerate (oooh smack!) deposited off the footwall, into the graben, or similar... a molase.... but I think these qualify. It was quite gratifying to note that the sequence resembles the Connecticut Valley sediments in the neighborhood of Smith College. Riftclastic indeed. Sadly, no fossils were discovered, body or trace. True afficianados will note a tiny structural feature (or set of features) in this photograph.... At least I hope so because Jodie and I climbed a 800' cliff in thorn trees and sticky piles of bug-attracting rotten fruit to confirm this lithology and acquire said photograph...

2 comments:

McHeather said...

Why do they even make those Silva disasters anymore?!??!!

McHeather said...

Why do they even make those Silva disasters anymore?!??!!