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I was walking on a reef in northern Italy near Genoa. (44.3230136N, 9.1462828E) The rock was composed of rocks of varying sizes, colors and shapes held together by some kind of a seemignly softer substrate. The substrate was still rock-hard on touch, but I was able to break some of the smaller rock out of it. (The photos below will say it best.)

The reef had several "splits" that ran across its whole length. These were white in colour and, I think, were composed of quartz. Interestingly, these splits ran straight through both the substrate and the rocks as if someone sliced the reef with a laser.

I would like to know how these formed. I imagine high pressures and shear forces were at play but I still don't understand how it's possible that the splits don't go around the rocks (at least the smaller ones) but straight through. Especially if I assume the rocks are harder than the substrate. But that might not be the case?

See the photo captions for more details.

Sorry, for not calling things by their proper names. I'm no geologist so this questions doubles as an identification one. I guess.

View of the rock with one of the splits going across vertically roughly in the middle: View of the rock with one of the splits going across vertically roughly in the middle.

Wider view of the same split with a person's legs for scale. The split continues behind: Wider view of the same split with a person's legs for scale. The split continues behind.

One of the small rocks being split in half by a narrow split. My finger on the left for scale: One of the small rocks being split in half by a narrow split. My finger on the left for scale.

Overall view of the reef showing a structure of plates laid on each other: Overall view of the reef showing a structure of plates laid on each other.

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That's a conglomerate.

The infilled fractures will have formed when it was buried at depth and thus under considerable pressure. The substrate may be physically softer or it may be more chemically reactive and thus erodes more easily at the surface. Regardless at depth there will have been no practical difference in their physical properties so faults will have cut substrate and clasts equally. Faulting is the result of differential movement, usually but not always because of flexion.

The filler material will have been deposited by geothermal fluid flowing through the fault. This fluid is hot, high pressure, water saturated with a number of dissolved minerals from the rocks it has been flowing through. When it reaches a fault the pressure, and it's ability to keep those minerals dissolved drops and they precipitate out along the fissure. The nature of the minerals so deposited depends on the depth of burial at time of deposition, at great depth silicates like quartz are dominant, shallower deposits are usually carbonates like calcite, sulfates like gypsum that have a higher solubility and thus can stay in solution at lower pressures. Given the rocks are probably Mesozoic in age they could well have been buried deeply enough for the deposits to be silica but I'm not really familiar with the deep history of that part of Europe so possibly not. I expect that the larger cracks are older and contain one mineral, probably quartz, while the smaller cracks are younger and have a different infill, probably calcite.

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  • $\begingroup$ Thanks! Regardless at depth there will have been no practical difference in their physical properties this is what I was thinking. But I still don't understand why this is the case. Is it the pressure alone? Do high pressures "equalize" physical properties? How? $\endgroup$
    – Jen
    Feb 22 at 10:06
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    $\begingroup$ @Jen It's more a case that at high pressure the rocks are all compressed to the point where they act as a single whole with a uniform density; when the pressure comes off the different types of material decompress to, very slightly, different degrees and their different densities and strengths become apparent once more. $\endgroup$
    – Ash
    Feb 23 at 23:15
  • $\begingroup$ Don't forget temperature. High temperature has a critical role in weakening (any) rock to the point that minor stresses can fracture rock and infill voids with dissolved materials. $\endgroup$ Feb 26 at 0:45
  • $\begingroup$ @KnobScratcher Temperature tends not to be a major factor where brittle fractures like that are a possibility but it does have a role in the processes that go into forming any rock. $\endgroup$
    – Ash
    Feb 26 at 7:55

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