Igneous, Plutonic and Intrusive EarthCache

This EarthCache brings you to the bust of dr. Franjo Tudjman. He was the first president of the Republic of Croatia, from 1990 to 1999. For this EarthCache we are interested for the plinth of this bust. The plinth of this bust represents a specimen of rock that is categorized as igneous, plutonic and intrusive.

Plutonic or Volcanic

Igneous rocks are one of three large groups of rocks (the other two are sedimentary and metamorphic). Igneous rocks are formed by cooling of hot magma or lava and by its solidification. Although both magma and lava are essentially molten rock, the magma is molten rock while it is still below the surface and lava is molten rock on the surface of planet usually expelled by a volcano during an eruption. If the rock is formed from magma it is also called plutonic and if it is formed from lava it is called volcanic. The term “plutonic” came from the name of classical god of the underworld - Pluto, and the term “volcanic” came from the fact that the rock is formed from lava erupted from a volcano.

Igneous Rocks

Source: Wikipedia

Intrusive or Extrusive

Plutonic rocks are also called intrusive rocks. That means that magma is slowly pushed up from the mantle within the earth and it intrudes by force into any cracks or spaces it can find. On the other hand, volcanic rocks are also called extrusive rocks because they are formed when hot magma extrudes onto the surface of planet as lava. The process of cooling of hot magma deep below surface of planet lasts much longer than cooling of hot lava on the surface of planet. Since it lasts longer, the larger crystals can grow during the process of solidification of intrusive rocks when compared to extrusive rocks.

Phaneritic or Aphanitic

During the process of cooling and solidification of magma or lava, the process of crystallization or formation of crystals might or might not happen. So, that means that there are igneous rocks with crystals and igneous rocks without crystals. Those igneous rocks that are formed with crystals can be divided into those that have crystals large enough to be seen by the naked eye (they are also called phaneritic rocks), and those that have crystals too small to be seen by the naked eye (they are also called aphanitic rocks). Phaneritic rocks are also intrusive rocks, because they have more time to develop larger crystals during cooling of magma deep below surface of planet. Aphanitic rocks are also extrusive rocks because lava cools on surface of the planet and it happens much faster than below surface.

Fine, Medium or Coarse

Crystals in igneous rocks are seen in the form of grains of different colors and different sizes. Regarding their size, grains can be: (1) fine-grained (diameter of the grains is smaller than 1 millimeter), (2) medium-grained (diameter of the grains is between 1 and 5 millimeters), and (3) coarse-grained (diameter of the grains is larger than 5 millimeters). Intrusive or plutonic rocks usually have larger crystals because the process of cooling and solidification lasts much longer below the surface of planet. So, they are usually coarse-grained. Extrusive or volcanic rocks usually have smaller crystals because the process of cooling and solidification lasts much shorter on the surface of planet. So, they are usually fine-grained.

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Sources

Ovaj EarthCache Vas dovodi do biste dr. Franje Tuđmana. On je bio prvi predsjednik Republike Hrvatske, od 1990. do 1999. godine. Za potrebe ovog EarthCachea zanima nas postolje biste. Postolje je napravljeno od kamena koji je lijep primjerak stijene koja je kategorizirana kao magmatska, plutonska i intruzivna.

Plutonske ili vulkanske

Magmatske stijene su jedna od tri velike grupe stijena (ostale dvije grupe su sedimentne stijene i metamorfne stijene). Magmatske stijene nastaju hlađenjem magme ili lave i njihovim skrutnjavanjem. Iako su i magma i lava zapravo rastaljene stijene, magma je rastaljena stijena koja se još nalazi ispod površine planeta, dok lava predstavlja rastaljenu stijenu koja se nalazi na površini planeta i koja je najčešće izbačena iz unutrašnjosti planeta erupcijom vulkana. Ako je stijena nastala od magme tada se naziva plutonska stijena, a ako je nastala od lave tada se naziva vulkanska stijena. Termin “plutonski” je došao od imena rimskog boga podzemlja – Plutona. Termin “vulkanski” se koristi zbog činjenice da je ta vrsta stijena nastala od lave koja je izbačena vulkanskom erupcijom.

Magmatske stijene

Izvor: Wikipedia

Intruzivne ili ekstruzivne

Plutonske stijene se još nazivaju i intruzivne stijene. To znači da magma polagano pritišće prema gore iz zemljinog plašta i prodire silom u sve pukotine i slobodan prostor koji joj se nađe na putu. S druge strane, vulkanske stijene se još nazivaju i ekstruzivne stijene zato što nastaju kada magma izbije na površinu planeta u vidu lave. Proces hlađenja magme duboko ispod površine planeta traje puno duže od hlađenja lave na površini planeta. S obzirom da process traje duže, veći kristali se formiraju tijekom skrutnjavanja intruzivnih stijena u odnosu na ekstruzivne stijene.

Faneritične ili afanitične

Tijekom procesa hlađenja i skrutnjavanja magme ili lave može, ali i ne mora doći do procesa kristalizacije, tj. do formiranja kristala može, ali i ne mora doći. To znači da postoje magmatske stijene s kristalima i magmatske stijene bez kristala. Magmatske stijene koje su formirane s kristalima mogu se podijeliti na stijene koje imaju kristale koji su dovoljno veliki da ih se može vidjeti golim okom i one se nazivaju faneritične stijene, te na stijene kod kojih se kristali ne mogu vidjeti golim okom i one se nazivaju afanitične stijene. Faneritične stijene su ujedno i intruzivne stijene jer su imale dovoljno vremena za razvoj velikih kristala tijekom hlađenja magme duboko ispod površine planeta. Afanitične stijene su ujedno i ekstruzivne stijene jer se lava hladi na površini planeta i taj proces se događa puno brže nego ispod površine planeta.

Fino, srednje ili krupno

Kristali u magmatskim stijenama su vidljivi u obliku zrna različitih boja i veličina. S obzirom na njihovu veličinu, zrna mogu biti: (1) fino-zrnata (promjer zrna je manji od 1 mm), (2) srednje-zrnata (promjer zrna je između 1 i 5 mm), i (3) krupno-zrnata (promjer zrna je veći od 5 mm). Intruzivne ili plutonske stijene uobičajeno imaju veće kristale zato što process njihovog hlađenja i skrutnjavanja traje duže. Stoga su ove stijene najčešće krupno-zrnate. Ekstruzivne ili vulkanske stijene uobičajeno imaju manje kristale zato što process njihovog hlađenja i skrutnjavanja traje puno kraće. Stoga su ove stijene najčešće fino-zrnate.

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