This piece of basalt has been recently unearthed. It literally disintegrates from erosion when exposed to the elements. I like finding specimens of this size. Strange feeling thinking about what was going on with the Earth so many years ago.

That really doesn't look like basalt, which is sedimentary. It looks like a high-iron something, like shale maybe.

CliffJ wrote:
Hi Cliff. My mistake. Roger presented this info on another thread and I had one of your senior moments. We all get them, even if we aren't quite senior yet! I find the rock interesting, never-the-less, as finding a large piece such as this isn't common for me. Most has been destroyed by the weathering. Fragments are pretty common. Neat to know what it looked like as a whole!

"Well I would say basalt is where it started out, but that piece in particular is mightily weathered. Basalts weather surprisingly quickly.

The glassy components weather first to palagonite (typically greenish-yellow or brownish-orange).

The olivine is next most susceptible, weathering to saponite (typically pale green through to brown), or nontronite (typically bright green through to olive green and rusty brown)

Then the magnesium-rich components weather out and the iron oxidises to hematite (typically black or grey through to browns, bright orange and blood red), or maghemite (typically bluish black or brown through to bright yellow), or goethite (typically dark brown through to yellows, reds and purples).

Then the pyroxene, plagioclase and sanidine components weather to various pseudomorphs of smectite. By this stage the rock is beginning to soften significantly… smectite is geologically in the “clay” family. Frequently you’ll get halloysite (typically whitish through to bright reddish pink) and kaolinite (name a colour from whitish through to pastel shades of almost anything).

I think the likelihood is that it’s a complex combination of all of the above with unweathered basalt core elements still present. Hence the wide array of attractive colours."
Category: Archeology and Fossils

P.S. The bright red is such a soft material...you can scratch it away with your thumbnail. Red chalk...so to speak. Another year and that red you're seeing would be non-existent.

CliffJ wrote:
Basalt is igneous (and also frequently high in iron). Shales are sedimentary. Geologically, shale is characterised by “fissility” (parallel lamination or bedding planes with a thickness less than 1 centimetre). When fissility is not evident, the same rock composition is called mudstone.

Heavily weathered shale could have those colours too – at least in principle – but if it weathered as much as that, the fissility would be very evident.

Although I see some very tight banding in a couple of the clasts within the rock (for example in the clast shaped a bit like South America towards top centre of the 2nd pic), I don’t see it generally in the matrix itself. I would therefore conclude that it’s igneous flow banding, not sedimentation. The clasts themselves are much more typical of rocks with a volcanic origin and I also see residual igneous melt features in the matrix. But as Pam says most of it is a long way from basalt now.

I’ve been known to be wrong (frequently! ) but I’m going by the structural composition that I can see.

Pam

That is a very pretty rock. I like it.

Jack

greywolf22 wrote:
I think they're cool. Here's a pic of both of them together. Lighting is bad in the kitchen. Makes everything look yellow. The pair make nice room ornaments.