Faults ?

G'day Ian, hope your studies are going well. Just pointing out the difference between descriptive words and rock types

Igneous rocks require two features to be distinguished - composition and crystal size. When describing the composition the terms can be easily interchangeable for example a granite can be described as having a rhyolitic composition and a rhyolite can be described as having a granitic composition (same with basaltic for a gabbro and and gabbroic for a basalt). To actually use the terms granite, rhyolite, gabbro and basalt to describe a rock we need to also consider the crystal size. A granite or gabbro has crystals easily visible to the naked eye and therefore can only form at depth. A basalt or rhyolite does not have individual crystals visible to the naked eye and therefore have undergone rapid cooling on the surface. Dykes are intermediate in crystal size (generally individual crystals are just visible to the naked eye) as they have formed close to the surface but have not extruded. Therefore we use the term dolerite (basaltic/gabbroic composition but not the right crystal size for either) and aplite/pegmatite (granitic/rhyolitic composition but again not the right crystal size for either).

Looking forward to catching up and discussing rocks when we are over there again

Cheers mate, good to hear from you
I am with you on the terminology now, i actually had a different post written before, but then realized i was talking more about basaltic/granitic composition rather than basalt or granite itself....
Still trying to get it all straight in my head, hopefully i am getting there though

Hope you had a good chrissy and new year, can't wait to talk rocks

Hey shivan, Ilikegold & goldierocks.
In the last 4 posts between the three of you I have learnt quite a bit about the difference between composition & deposition in ignieous rocks.
Although I dont have any formal qualifications in either earth sciences,mining or metalurgy I have completed a continuing education course Called "The geology of prospecting". I have also read a lot of old books on the subjects of mining, mineralogy and analysis of ores. Some of these are still relevant today for the hobbie propspector some are not relevant due to the risk of personal injury if some of the methods are used.

perhaps we sh0uld ask the mods to make a sticky topic for geology terms & Q & A.

Jethro.

For some reason nothing I write today gets posted - a disagreement re comments on the use of terms like basalt and granite (what I wrote originally was completely correct in its usage), a discussion of the use of aeromagnetics to reveal faults. Guess I'll have to give up at this stage

Sorry I don't have time to repeat. Basically granite and basalt can occur in dykes - it is correct terminology. Here is an example (an official United States Geological Survey website):

http:// then mrdata.usgs.gov/geology/state/ then sgmc-unit.php?unit=IDTmib%3B0

It is a bit complicated to explain why, but relates to the fact that nowadays grainsize and mineralogy alone (not position and depth) are usually used to define the rock type, so if a fine-grained "basaltic" rock occurs in a dyke, it is correct to call it basalt (one can also have rhyolite dykes). Because really coarse-grained rocks don't crystallizer at surface, in practice we don't get granite or gabbro flows, but the reason is not because of where and at what depth the rocks occur, but because of their composition and grainsize. Grainsize in an igneius rock is controlled largely by how quickly a rock cools - the faster it cools the finer-grained it is - and because things cool quicly at surface they are always fairly fine-grained (although you can commonly still see crystals in them with the naked eye, and 95% of the time with a hand lens).

This was not always the definition 50 to 100 years ago, when position/depth was also used - hence Ilikegold's claim of an error (no, I didn't fail first year undergrad, I graduated 45 years ago and have a PhD in it and have been a full-time geology lecturer, researcher (eg CSIRO gold research group) and gold explorer ever since, on four continents, and have found some mines, one gold mine still operating . However it was impractical to use that (it is often impossible to tell if something is a flow or a dyke if you only have an outcrop a few tens of centimetres in size). Also, there are two different - genetic (chemical composition) and grainsize/mineral composition - definitions that are used in slightly different ways - basically a basalt family and a granite family, which together make up most igneous rocks that we see at the Earth's surface.

goldierocks said:

For some reason nothing I write today gets posted - a disagreement re comments on the use of terms like basalt and granite (what I wrote originally was completely correct in its usage), a discussion of the use of aeromagnetics to reveal faults. Guess I'll have to give up at this stage

Click to expand...

hum I can see your post here ^_^

I was trying to convince by the strength of my argument, not by pulling rank (if you get my drift), but unfortunately I don't have the strength to re-write that carefully-worded and lengthy argument again (one of the two that disappeared), so I hope that boast is convincing enough to establish my credentials. Still doesn't prove I'm not an idiot though - there is a bit of it going around

I don't mean you Ilikegold - hope we are all mates here, learning from each other and swapping stories. However it was a bit of a shock intro as a Newbie doing a first post - someone without skin as thick as a rhino and as confident in their knowledge mught never post again (I will)!

Looks like the problem was including images and url's

"hum I can see your post here ^_^" - yes thanks, but that was my third post and short

"Hunting the Gold" - it was not just me trying to blow my own trumpet, but I had been told on my first post that "Your intro says your a geologist, but you have made statements that even a first year student would know are not right". i.e. I'm not quite a first year and thought I should say so.

No worries goldierocks glad to here your contributions to this post.
Sorry can be a bit picky on using the correct terminology having specialized in igneous petrology.

It was also my first post so don't think your being picked on by members here

No probs Ilikegold. Damn, it happened again - hit submit and it simply disappeared (and I had no urls or images in it)! Will make a copy before posting each time in future. I assume you read my post 25? Yes, I did my Honours on basaltic dykes and in the last decade published other studies on basaltic dykes (REE, Ar/Ar, petrography, Sm/Nd). Used to consult on things like the Bushveld and researched Barberton and elsewhere. We will have to agree to disagree I guess, but what I have said is correct and gerally widely used (as in the USGS paper I cited)

But the point I was actually correcting originally was that faults, dykes and quartz veins are different things (the topic of the post, not yours of course). This other sort of thing really should have a new topic, or blogs wander too much I feel.