Rock of the Week #3

Sorry, I am getting a bit behind on RoW. That's just as well, and I think I will keep the blog a week behind the student competition as they could always find this and use your guesses! And they are in direct competition for geology stickers so we can't have that.

Due to the general troubles with the first two rocks of the weeks (both online and in the office) I decided to try something a bit more accessible to students and internet viewers this week. I present:

And the solution to last week's rock:
This rock is a partially-serpentinised harzburgite from the Franciscan Complex (Ring Mountain), California, USA. Harzburgite is a rock of “depleted” mantle origin. The rock comes from an ophiolitic unit which was thrust over the complex during the Jurassic. The original mineralogy of the rock would have been about equal amounts of pyroxene and olivine. You can still see the grains of dark green pyroxene but the olivine has reacted with seawater to form serpentine (antigorite) and talc. Metallic looking bits come from weathered sulfides and oxides. These minerals weather easily, leaving the surface of the rock rough and yellow in colour.

1. (1 pt) Minerals?
: Pyroxene, serpentine, magnetite

2. (1 pt) Rock name?
: peridotite or harzburgite got a point

3. (1 pt) Tectonic environment?
: ophiolite or subduction zone got a point.


FIRST PLACE: Pia Vigletti for
performing experiments on the rock, even though results are inexplicable

RUNNER UP: Anonymous person for
“Moon rock, that’s why it smells like cheddar”


Anonymous said...

Carbonatite with some copper minerals (malachite?), maybe Phalaborwa?


Salomé said...

Hi Christie,

Am actually posting this because I would like to email you - would like some info on the "mountains" (orthogneiss?) around Nampula, in layman's terms. More out of curiosity than anything else, since I live here, and they seem to be pretty unique! :-)My email address is vanniekerk.salome@gmail.com.

Salome van Niekerk

JF said...

Hi Salomé (answering while Christie looks on the other side...),

Actualy, sorry to disapoint you, but neither the landforms nor the rocks are very unique !

The landforms (the fact that you have big, rounded hills popping out of a flat plain) is a typical morphology for about any granite or gneiss (two rock types) in tropical climate. We call that "inselbergs", and you can see a few nice examples on my website: in south India (http://jfmoyen.free.fr/spip.php?article41), in RSA (http://jfmoyen.free.fr/spip.php?article185), but also in Madagascar and south America.

The rocks are "granulitic orthogneisses". A long word meaning that they are rocks that have been deformed and foliated ("gneisses"); that the deformation occured within certain conditions of pressure and temperature (corresponding to 20-30 km deep, and > 750°, more or less) ("granulitic"); and that the rocks initially were granites or similar rocks ("ortho-"). Rocks like that are fairly common throughout Africa, forming a large part of the continent...

who can not use his gmail account to publish comments, for some reason, and has to remain anonymous

Silver Fox said...

Minerals may include chrysocolla or/and turquoise, possibly minor sulfides or Mn-oxides. Rock type could be a vein, not sure.

Fault Rocks said...

JF and Salome -
Given that this geomorphology and the rock types are not that unusual... I still feel that the landscape is not well explained! When I first saw the Nampula area on aerial photos, and then from the air when flying in, I assumed that the flat lands would be a sedimented plane. In fact, there is very thin soil cover and the flat areas are the same gneiss as the inselbergs. The rounded dome shape of the inselbergs is typical of granitic (or metagranitic) rocks but there seems to be no explanation that I have found for why the inselberg occurs in one place and not the other. JF - any insight on this? My geomorphologist friend here was no help.

JF said...

"there seems to be no explanation that I have found for why the inselberg occurs in one place and not the other. JF - any insight on this?"

Well, I'm afraid no. I have seen inselberg in different parts of the world (mostly sub-tropical belt, between say 10 and 30° lat.): India, Northern RSA, Moz from your photos, Madagascar from other people, I believe there are some in Brazil and parts of Oz too, etc. Each time there is no clear control on what causes inselbergs; the rocks between seem to be just the same as the rocks forming them.

On a larger scale, some rock types form inselbergs easier than some other. In Archaean cratons, the classical situation is a flat, featureless plain made of the TTG gneisses (no outcrops !!), and inselberg zones corresponding to potassic plutons -- but the plutons extend between and beyond individual inselbergs. Well, K-granites are more resistant to weathering because qtz and K-feldspar do not weather too easily, compared to plag, but that does not explain the local control.

In some places in India, I remember seeing zones that might have been "proto-inselbergs" : you start with a small depression, for any random reason; in it, you accumulate topsoil, water, etc., plants start growing there; the combination of water + acid from the roots causes the rock to weather more easily. So as soon as there is a small depression, it can evolve into a major one. I suppose that over long periods you can evolve significant inselbergs.

Some of the inselberg-bearing plains seem to have an ancient topography indeed, typically in Archaean terranes the base of the Proterozoic is actually at the level of the plain itself: this suggests that the topography did not change much since 2 billions years. Are the inselbergs (I mean, the landform, not the rock) as old? I'm trying to think of an example of inselbergs in younger rocks, the youngest I can think of would be panafrican, 600 Ma old...

The other thing, of course, is that inselbergs form in massive rocks -- without a prefered weakness orientation such as a foliation or a bedding.