6. Kamninski krog / Rock Cycle
FORMATION OF ROCKS
You have to visualise the structure of our planet to understand how rocks are formed (see Figure 2.1: Earth's layered structure).
Central part of the Earth is metal CORE, which is surrounded with partly fluid MANTLE, while thin, rigid outer layer called CRUST builds continents and ocean floors. Crust, as well as all Earth's interior is completely build by rocks and minerals. The movement of the material in the mantle causes movement of plates that form the crust. This produces earthquakes and active volcanoes. Our EarthCache EC14-Earthquake 1511 describes the consequences of a strong Medieval earthquake.
ROCKS are formed in several different ways - from explosive during volcanic eruption, to thousands of years slow deposition and binding. And rocks don't remain unchanged after they are formed. On the contrary, they are changed all the time! You could say they cycle form formation to destruction and another formation in new rocks. Forming, dissolving, eroding transforming, melting, and changing through logn periods of time is called ROCK CYCLE. Simplified "cycle" of rocks is shown at Figure 6.1.
You could say, that Rock Cycle is never-ending battle: Inner Earth forces build new, high mountain ranges all the time, and those are eroded and wasked away under the relentles attacks of water, weather and other influences of nature. To see this with your naked eye, you would have to live a very long life - millions of years!
Rocks can be divided into three broad categories - depending on the way of their formation: igneous, metamorphic and sedimentary.
ROCK TYPES
IGNEOUS ROCKS are formed from hot magma.
Magma is hot, molten chemical mixture of elements that organise themselves into several minerals as the magma cools down and crystallises. Magma usually lies beneath Earths crust, but sometimes breaks trough in volcanoes. Batches of magmas that didn't make it to the Earth's surface before cooling and solidifying form Intrusive igneous rocks. GRANODIORITE is the most abundant intrusive igneous rock in Slovenia (rock cubes at Figure 6.2).
Intrusive igneous rocks are very resilient and solid, often used and valued at roadworks, and buildings (curbstones, paving stones, window ledges, stairs).
Extrusive igneous rocks are formed when magma erupts at the surface. There are not many in Slovenia. Otherwise BASALT and OBSIDIAN (also known as volcanic glass) are well known.
Obsidian forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture. It has been used as cutting tool and arrow tips at first. Today it is still used in cardial surgery.
SEDIMENTARY ROCKS are formed by laying down, or deposition of grains.
Sedimentary grains are formed when rocks at the Earth’s surface are weathered or broken up by water, wind, frost and ice. Pieces of rock, fragments and individual mineral grains are transported from one place to another by water or wind before being deposited as roughly horizontal layers of sediment. With time these layers of sediment may be buried by yet more sediment and compacted. When this happens and water is squeezed out of the sediment, new minerals may grow in the spaces between the grains and the loose sediment grains become cemented together forming a solid sedimentary rock such as SANDSTONE.
CONGLOMERATE formes from river sediments (rounded pebbles cemented together), BRECCIA from material accumulated under mountain slopes (sharp-edged rock pieces cemented together), or TUFF from volcanic ash. You can find more about conglomerate visiting our EarthCache EC26-Millstones quarry, and about tuff at EC13-Peračica tuff.
But sedimentary rocks don't always form from old rock pieces. LIMESTONE, which is the most common rock in Slovenia, is often of "organic" origin. That means, it is formed from large quantities of small marine animals remains - more exaokamnelctly - their shells and skeleton remains. Fossils are common in our limestone, which proves the rock origin and also help to determine the age of rocks.
Fossils are petrified remains of animals or plants; There are foraminiferae at Figure 6.4, found in limestone from highway building site - they are about 50 million years old. You can see a nice exhibition of 300 million years old plant fossils at our EC09-Castle Hill Secrets EarthCache, and petrified footprints of an early dinosaur at EC15-Dinosaur fossil footprints.
DOLOMITE is another common rock in Slovenia. It looks similar to limestone, but is much less susceptible to accids. It is usually white or pink shaded. Dolomite is used in glass production, as well as soil acidity neutralizer, and source of magnesium.
METAMORPHIC ROCKS are transformed existing rocks.
Rocks are transformed under high temperatures and high pressure: along the faults in the Earth's crust plates, in the volcano areas and hotspots in the crust, everywhere deep enough under the surface and at the edges of forming mountains.
Maybe the most reknown metamorphic rock is MARBLE, which is transformed limestone or dolomite. SLATE is also common in Slovenia.
Marble is suitable for carving and very valued among sculptors. It is used primarilly for flooring and coating of facade walls.
Slate is easily split into flat plates used for roof tiles.
/ Figure 6.2: Intrusive Igneous rocks:Rock cube used for road pavement - granodiorite (photo: Vane).
/ Figure 6.3: Intrusive Ignenous rocks: Window ledge from granite;Minerals are clearly visible in this rock (photo: Vane).
/ Figure 6.4: Sedimentary rock - Limestone:Fossil foraminiferae fill almost whole volume ot this rock from highway bulding site near Divača.They are about 50 million years old (photo: Vane).
/ Figure 6.5: Sedimentary rock - Conglomerate and Breccia (photo: Vane).
/ Figure 6.6: Sedimentary rock - Tuff:Tuff rock layers at Peračica (photo: Vane) and relieff carving of wall coating.
NASTAJANJE KAMNIN
Če želimo razumeti, kako nastanejo kamnine, si moramo najprej pogledati, kako je zgrajen naš planet (zgradbo si lahko ogledate tudi na sliki 2.1: Sestava Zemlje.
V središču Zemlje je kovinsko JEDRO, ovija ga delno tekoči PLAŠČ, SKORJA pa je trdna, tanka zunanja plast, ki gradi celine in dno oceanov. Kamnine skupaj z minerali sestavljajo celotno zemeljsko skorjo in vso notranjost Zemlje. Gibanje snovi v plašču povzroča premike plošč, iz katerih je sestavljena skorja. Zato nastajajo potresi in bruhajo ognjeniki. Posledice močnega potresa opisuje naš Geološki zaklad EC14-Potres 1511.
KAMNINE nastajajo na različne načine, vse od eksplozivnega pri izbruhu ognjenika, do tisočletij trajajočega počasnega usedanja in sprijemanja. Tudi po nastanku kamnine ne ostanejo za vselej enake. Nasprotno! Ves čas se spreminjajo, lahko bi rekli, da krožijo od nastajanja do uničenja in ponovnega nastajanja v novi kamnini. Nastajanje, razpadanje in spreminjanje kamnin skozi dolga časovna obdobja, imenujemo KAMNINSKI KROG.
Ker kamnine na Zemlji dejansko gredo skozi cikle nastajanja, razpadanja, preoblikovanja, raztaljevanja in ponovnega nastajanja, to imenujemo KAMNINSKI KROG. Preprost "pot" kamnin je prikazana na sliki 6.1.
Lahko bi rekli, da gre za pri kamninskemu krogu za neodločen boj: zemeljske sile ves čas ustvarjajo nova, visoka gorovja, ta pa pod napadom vode, vremena in ostalih vplivov okolja, vedno znova razpadajo in se nižajo. Da bi to videli s svojimi očmi, bi morali živeti precej dolgo milijone – let!
Kamnine glede na način nastajanja delimo na magmatske, sedimentne (usedline) in metamorfne (preobražene) kamnine.
VRSTE KAMNIN
MAGMATSKE KAMNINE nastanejo iz vroče magme.
Magma vroča, raztaljena kemična mešanica sestavin, ki se oblikujejo kot minerali ob ohlajanju magme in kristalizaciji. Leži v nekaterih delih pod Zemljino skorjo. Kadar se magma ohladi in strdi že pod zemeljsko površino, nastanejo magmatske kamnine, ki jim pravimo globočnine. Najbolj znana globočnina v Sloveniji je GRANODIORIT (kamnite kocke na sliki 6.2), sicer je v svetu najpogostejši GRANIT (okenska polica na sliki 6.3).
Globočnine so zelo odporne in trdne kamnine, zelo uporabne pri gradnji cest in objektov (za izdelavo robnikov in tlakovcev, polic, stopnic).
Kadar magma iz ognjenika prodre na površje, nastanejo magmatske kamnine imenovane predornine. Pri nas je predornin zelo malo. Znani predornini sta BAZALT in OBSIDIAN (imenovan tudi vulkansko steklo).
Obsidian nastane, ko se lava zelo na hitro strdi. Je zelo trd. Iz njega je moč izdelati izjemno ostra rezila, še bolj ostra kot jeklena. Včasih so ga uporabljali za izdelavo konic puščic, danes pa rezila iz obsidiana uporabljajo v srčni kirurgiji.
USEDLINE (ali sedimentne kamnine) nastanejo z usedanjem delcev.
Sprva trdne kamnine na površju postopoma preperevajo in razpadajo, s pobočij se krušijo in valijo v doline, vode in ledeniki jih odnašajo v nižine in morja. Z usedanjem delcev razpadlih kamnin in ostankov organizmov nastajajo usedline ali sedimentne kamnine. Običajno traja na milijone let, da se sipke usedline pod težo vedno novih plasti postopoma spremenijo v trdo kamnino.
Iz delcev starejših kamnin v vznožjih gora nastaja BREČA (povezani - med seboj z mineralnim lepilom zlepljeni ostrorobi koščki), iz rečnih usedlin KONGLOMERAT (povezani obrušeni koščki kamnin), iz peskov PEŠČENJAK. Iz vulkanskega pepela, ki se usede, nastane TUF. Pri nas je poznan zeleni tuf iz bližine kraja Peračica, ki so ga uporabljali za izdelovanje kamnitih portalov vrat. Nekaj o konglomeratu izveste v našem Geološkem zakladu EC26-Kamnolom mlinskih kamnov, o tufu pa v EC13-Peračiški tuf.
Usedline pa ne nastajajo vedno le z usedanjem starejših kamnin. APNENEC, ki je v Sloveniji najpogostejša usedlina je večinoma organskega izvora. To pomeni, da je nastal z usedanjem ostankov velike množice školjk in skeletov malih morskih živali. Pogoste najdbe fosilov v njem to dokazujejo. Fosili so tudi v pomoč pri določitvi starosti kamnin.
Fosili so okamneli ostanki živali ali rastlin; na sliki 6.4 so luknjičarke v apnencu iz cestnega useka pri Divači, stare so preko 50 milijonov let. Rastlinske fosile stare 300 milijonov let predstavi naš Geološki zaklad EC-09-Skrivnosti Grajskega hriba, okamnele sledove prednika dinozavrov pa EC15-Dinozavrove stopinje.
Ena od pogostejših sedimentnih kamnin v Sloveniji je tudi DOLOMIT. Po tej kamnini so včasih Polhograjsko hribovje imenovali kar Polhograjski Dolomiti. Dolomit je na videz podoben apnencu, a je manj občutljiv na kisline. Običajno je bel ali rožnato obarvan. Veliko se uporablja v proizvodnji stekla, a tudi za zmanjšanje kislosti prsti in kot vir magnezija.
METAMORFNE KAMNINE nastanejo s preobrazbo iz drugih kamnin.
To se zgodi pod vplivom spremenjene temperature in tlaka: na stikih plošč zemeljske skorje, v bližini ognjenikov in vročih točk v zemeljski skorji, povsod dovolj globoko pod površjem ter na obrobju nastajajočih gora.
Nam najbolj znana metamorfna kamnina je MARMOR, ki nastane iz apnenca ali dolomita. Pri nas je pogost tudi SKRILAVEC.
Marmor se da dobro klesati in obdelovati. Uporabljajo ga pri gradnji, predvsem za oblogo zgradb, iz marmorja izdelujejo tudi kipe in spomenike. Najbolj cenjen je čisto beli marmor.
Skrilavec je moč razklati v lepe, ravne plošče, ki jih uporabimo kot strešno kritino.