Geologists out there?

[avodude]

mallacootasmoothie, whoops thought you were in NZ not Australia. Duh. We could actually narrow it down as to what type of plutonic rock your round rock is by using a Q, A, P triangle. It is the upper half of the QAPF triangle attached. The lower triangle of rock types are pretty rare. The QAP triangle (upper half) uses a ratio of Quartz, Alkali Feldspar and Plagioclase Feldspar to determine what kind of plutonic rock it is. Saying it was a granodiorite was a safe bet because most of the continental crust is made up of granodiorite. Australia is pretty unique in that it doesn't have a lot of tectonic activity. You must have discovered an area where most of the sedimentary rock had been eroded away leaving a granite type bedrock. Some of the oldest rocks on earth have been found in Australia. Last I saw was a zircon that they dated to 4.4 billion years ago in the Jack Hills region.

We geologists have a cool name for the process that made your round rock and the bowl that it formed but I cannot remember it. Pothole was a lame substitute. That can be a pretty common find around rivers with a certain kind of geology.

 

[avodude]

I was trying to remember, exactly, what rock I saw in Alaska that seemed very much like Colonel00's rock. We would find them heavily rusted apart depending on weathering. I remember the rock now - amphibolite. It is a metamorphic rock. Here is a photo of bore cross-section attached.

Here is another photo of an unrounded amphibolite:

Here is a rounded gabbro cobble just to show that it is tough to tell what kind of rock it is by a picture:

 

[Rockhound]

Thanks for the interest in my rock, great to hear some professional opinions about its origins.
From my limited understanding it is not a sedimentary rock, it is much harder than sandstone (or the sandstone I've seen.)
I would not be able to scratch this rock easily like sandstone.
I found it in a bowl shaped hole, on the south east coast of Australia while fishing for wild bass.
I actually looked at the hole and how it was spherical inside and thought something must have spent a long time rumbling around in there to carve out that bowl. The opening at the top was about 12" and sphere at its largest diameter was about 20", I wish I had a photo of it but I don't.
When I looked closely into the bowl I saw the round rock in the bottom. The river bed where I found it usually only floods every second year.
I left the rock in the hole and hiked back in a year later with a framed pack to haul it out.
I placed a remotely round rock back in the hole just because it felt like the right thing to do.

Hmm....that is a sweet rock man. More speculation, but a lot of times in streams, scouring occurs, creating these holes that are super round. What might have happened is a scour hole develops, a rock of the right size gets caught in it, over time the circulating water (moreover, the suspended sediment) rounds the rock, while still working on the hole. So I would be surprised if the rock actually created the hole....again, just a theory.

Here are a couple photos of scour holes....this kind of look like where you found it?

 

[Rockhound]

I was trying to remember, exactly, what rock I saw in Alaska that seemed very much like Colonel00's rock. We would find them heavily rusted apart depending on weathering. I remember the rock now - amphibolite. It is a metamorphic rock. Here is a photo of bore cross-section attached.

Yep....I'd buy that. Good call you westerner

Actually we have a lot of metamorphic stuff in the Appalachians (including amphibolite here in NC) but volcanics? Few a far between for us.

 

[avodude]

Actually we have a lot of metamorphic stuff in the Appalachians (including amphibolite here in NC) but volcanics? Few a far between for us.

Do you guys have any plutonic rocks? A batholith of some sort? Any granite type rocks in the Appalachians? It is probably such an old mountain chain that most of the magma chambers have been eroded away.

Mt Stuart Batholith 360:

Mt Stuart:

 

[avodude]

We are kind of spoiled, here in Idaho, with the Idaho batholith (taken from Idaho State U Geology site).

 

[rmh2282]

I am ashamed of myself. Of course that's an igneous rock.

We do have some plutons out east. All these sedimentary rocks had to come from somewhere. Shenandoah National Park in VA has some pretty sweet granite, most notably Old Rag Mountain.

 

[jtw]

I studied a lot of geology in college. I majored in env science. Worked as a wetland scientist for awile. Got laid off and now I'm an office worker. I miss field work.

 

[Rockhound]

I studied a lot of geology in college. I majored in env science. Worked as a wetland scientist for awile. Got laid off and now I'm an office worker. I miss field work.

Field work can be cool. I did a tiny amount of wetland stuff (delineations) and here in NC, during the summer, I think I'd sooner jump into a woodchipper before doing that full time. Props to you sir if you did that in a warm climate! I really don't have it bad, like some oil guys, but I've probably spent 20-25 weeks on the road this year. Nice to enjoy my own bed sometimes. Also I make it a priority to be home as much as possible during the fall

Also here is a sweet sweet granitic pluton we have in NC, one of the best rock climbing destinations in the southeast, Looking Glass Rock:

 

[MCKROB]

Geophysicist here 20+ years, working in mineral exploration mostly and specialising in gravity surveys.

 

[avodude]

No need to be ashamed rmh2282, I can't tell you how many times I've been wrong before I got it right and I'm still wrong a lot. Haha

That is a sweet photo Rockhound. I think I may have heard of Looking Glass Rock before. I remember hearing that the Appalachians, in their prime, would have put the Rockies to shame.

Good to see all the fellow geologists/env. scientists/civil e's/geophysicists, etc! There is a special glint to the eye that a person gets after they look at so many rocks!

 

[Blockcaver]

Mining Engineer here....you the geologist find and I mine it!

 

[Rockhound]

Geophysicist here 20+ years, working in mineral exploration mostly and specialising in gravity surveys.

That is way cool man, can you elaborate? What are you looking for where grav surveys are the best way?

 

[MCKROB]

That is way cool man, can you elaborate? What are you looking for where grav surveys are the best way?

At a given depth, the acceleration due to gravity increases over denser rock units, gravity mapping is quite useful for mapping geological trends and structure; generally speaking ore is always denser than the surrounding country rock it lies in. While in itself it's usually not used to spot drill holes, gravity anomalies where they occur with coincident magnetic, electromagnetic, or chargeability/resistivity anomalies are statistically better places to drill.

This is a bouguer gravity anomaly from a highly prospective gold area near Elko, Nevada with 13g/t gold and 105g/t silver from trench sampling (from the internet).

 

[Rockhound]

^^^^^^ Wow man way cool. I did not realize relatively little ore (i.e. a few grams per ton) would alter gravity results so much. We're working on a 2-3g/t gold deposit in the SE USA and I am learning so much. Blows me away we have the tech to mine 2,000 pounds of material and cost effectively extract a few grams of the money mineral.

Our mining lead always says "If it can't be grown, it has to be mined."

 

[rmh2282]

Avo - Good to hear.

Rock - Check out this book. I recently had to write 10 pages on geophysics in mineral exploration and I'd have made this my one and only source if it were permitted. I have limited knowledge on it, but the book seems pretty comprehensive. See if you can loan it from a near by university?

http://www.amazon.com/Geophysics-Mineral-Exploration-Geoscientist-Professor/dp/0521809517