Dedkov pruh: vulkan sredi Alp EarthCache

🇸🇮 V kamnolomu Dedkov pruh izdanja nenavadna zelena kamnina, ki se že na prvi pogled loči od bolj ali manj sivih apnencev, kakršni prevladujejo v dolini Kamniške Bistrice. Ta kamnina je vulkanskega izvora in jo strokovno uvrščamo med predornine, imenovane trahiti in rioliti, poznana je tudi pod starejšim imenom kremenov keratofir, ki pa se ne uporablja več. Na zahodnem bregu Kamniške Bistrice te vulkanske kamnine gradijo večino vznožja Kamniškega vrha, najdemo jih tudi v dolini Črne, na Kranjski Rebri, na Jezerskem in drugod na Gorenjskem, kakor tudi v Savinjski dolini. Njihova barva je poleg zelene lahko tudi rdečkasta ali vijolična.

Te kamnine so nastale v obdobju srednjega triasa, pred približno 240 milijoni let. V tem obdobju Zemljine zgodovine so bili vsi svetovni kontinenti spojeni skupaj v velikanski superkontinent, ki so mu geologi dali ime Pangea. Ravno v obdobju srednjega triasa je vzhodni del Pangee začel razpadati, raztezanje kontinentalne litosfere je ustvarjalo tektonske jarke in povzročilo obširno vulkansko delovanje. Značilnosti srednjetriasnih vulkanskih kamnin na Slovenskem jasno kažejo, da se je lava izlivala v podvodnem okolju, to torej niso bili veliki stožčasti vulkani, kakor si ljudje vulkane po navadi predstavljamo. Vulkansko delovanje je ponehalo že konec srednjega triasa, kasnejši tektonski premiki ob prelomih in narivih pa so nekdanje vulkanske komplekse razkosali in prestavili, tako da si prvotnega obsega in razprostranjenosti vulkanizma ne moremo prav dobro predstavljati.

Zelena kamnina v kamnolomu Dedkov pruh je večinoma precej homogena in drobnozrnata. Najverjetneje gre za ostanek pravega izliva lave in ne le za mešanico vulkanskih produktov in morskih sedimentov, kakršne pogosto najdemo drugod pod Kamniškim vrhom. Ponekod, zlasti pa v kosih kamnine, ki leže po tleh kamnoloma, opazimo znotraj kamnine manjše in večje bele pike – to so vtrošniki, kristali minerala plagioklaza, ki so se strdili iz tekoče lave še med njenim prodiranjem proti Zemljinemu površju. Zelena drobnozrnata do skoraj steklasta osnova kamnine se je nato zelo hitro ohladila in strdila, ko se je lava izlila na površje oziroma v morje. Če si bele vtrošnike pozorno ogledamo, bomo opazili, da imajo vsaj nekateri izmed njih štirikotno obliko, ki odraža pravilno geometrijsko obliko kristalčkov.

Vulkanske kamnine so bile skoraj takoj po nastanku kemično precej spremenjene. Reagirale so z morsko vodo in preperevale, močno pa so jih spremenile tudi vroče hidrotermalne raztopine, ki spremljajo vulkanizem. Eno sled tega hidrotermalnega delovanja predstavljajo nepravilno oblikovane kremenove žile v kamnini, ki so vijoličaste ali celo rdeče barve – v tem primeru gre za mikrokristalni različek kremena, ki ga imenujemo jaspis in velja za poldragi kamen, ki se uporablja za nakit, čeprav jaspisa iz slovenskih srednjetriasnih vulkanskih kamin niso nikoli komercialno izkoriščali v ta namen. Kamnina v kamnolomu je tudi močno razpokana in prelomljena zaradi kasnejših tektonskih deformacij.

🇬🇧 In the Dedkov pruh quarry, an unusual green rock is exposed, which at first glance stands out from the more or less gray limestones that dominate the Kamniška Bistrica Valley. This rock is of volcanic origin and is classified among volcanic rocks called trachytes and rhyolites. It is also known by the older name “quartz keratophyre,” which is no longer in use. On the western bank of the Kamniška Bistrica, these volcanic rocks form most of the foothills of Kamniški vrh. They can also be found in the Črna Valley, on Kranjska Reber, at Jezersko, and elsewhere in Gorenjska, as well as in the Savinja Valley. In addition to green, their color may also be reddish or violet.

These rocks formed during the Middle Triassic period, about 240 million years ago. At that time in Earth’s history, all the world’s continents were joined together into a vast supercontinent that geologists have named Pangaea. It was precisely in the Middle Triassic that the eastern part of Pangaea began to break apart. The stretching of the continental lithosphere created tectonic rifts and caused extensive volcanic activity. The characteristics of Middle Triassic volcanic rocks in Slovenia clearly indicate that the lava erupted in an underwater environment; they were therefore not large cone-shaped volcanoes, as people usually imagine them. Volcanic activity had already ceased by the end of the Middle Triassic, while later tectonic movements along faults and thrusts fragmented and displaced the former volcanic complexes, so we cannot easily reconstruct the original extent and distribution of volcanism.

The green rock in the Dedkov pruh quarry is mostly quite homogeneous and fine-grained. It most likely represents the remains of an actual lava outflow and not just a mixture of volcanic products and marine sediments, such as are often found elsewhere beneath Kamniški vrh. In some places, especially in pieces of rock lying on the quarry floor, smaller and larger white spots can be observed inside the rock – these are phenocrysts, crystals of the mineral plagioclase, which solidified from molten lava while it was still ascending toward the Earth’s surface. The green fine-grained to almost glassy groundmass of the rock then cooled and solidified very rapidly when the lava flowed out onto the surface or into the sea. If you look closely at the white phenocrysts, you will notice that at least some of them have a rectangular shape, reflecting the regular geometric form of the tiny crystals.

The volcanic rocks were chemically altered almost immediately after their formation. They reacted with seawater and weathered, and were also strongly modified by hot hydrothermal solutions associated with volcanism. One trace of this hydrothermal activity is represented by irregularly shaped quartz veins in the rock, which are violet or even red in color – in this case, a microcrystalline variety of quartz known as jasper, considered a semi-precious stone used for jewelry, although jasper from Slovenia’s Middle Triassic volcanic rocks has never been commercially exploited for this purpose. The rock in the quarry is also heavily fractured and faulted due to later tectonic deformations.

Naloge

Za uspešen vpis morate odgovoriti na naslednja vprašanja. Odgovore pošljite prek profila, preden ali takoj ko vpišete najdbo:

1. V katerem geološkem obdobju so nastale zelene vulkanske kamnine v kamnolomu Dedkov pruh in kakšen je bil takrat položaj kontinentov?
    
2. Kakšne barve kamnine opaziš?
    
3. Ali je kamnina v enakomernih plasteh ali ne? Po svojih besedah opiši, zakaj je do tega prišlo.
    
4. Kaj predstavljajo bele pike (vtrošniki), ki jih opazimo znotraj zelene kamnine, in kakšno obliko imajo nekateri izmed njih?
    
5. Objavite fotografijo sebe ali osebnega predmeta na lokaciji in jo objavite skupaj s svojim dnevniškim vpisom.

Logging tasks

For a successful log, you must answer the following questions. Send your answers through the profile, before or immediately after logging your find:

1. In which geological period did the green volcanic rocks in the Dedkov pruh quarry form, and what was the position of the continents at that time?

2. Is the rock wall in uniform layers or not? Describe in your own words why this happened.

3. What do the white spots (phenocrysts) observed within the green rock represent, and what shape do some of them have?

4. What colors of rocks do you notice?

5. Take a photo of yourself or a personal item at the location and include it with your log.

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Vir podatkov: prof. dr. Marko Vrabec Oddelek za geologijo Univerza v Ljubljani, Naravoslovnotehniška fakulteta