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Weathering of Cathedral

A cache by GCEdo Send Message to Owner Message this owner
Hidden : 11/11/2016
Difficulty:
3 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:




At the listed coordinates you will find displayed two building elements from Zagreb Cathedral. They are completely identical but one of them was exposed to weathering for long time and another one is brand new. They are both made from limestone, same as the entire Zagreb Cathedral. You can observe those two building elements and notice what weathering can do to rocks.

Weathering


In general, weathering is natural process that breaks down rocks into smaller pieces. There are two main types of weathering – mechanical and chemical. Mechanical weathering is caused by the elements of nature such as wind, water, and cold / heat. These elements physically break off pieces of rock. Chemical weathering includes various chemical reactions that break down rocks. Also, there are several important factors that affect the rate of weathering in specific environment.

Weathering is often confused with erosion, although they represent two different types of processes. Weathering occurs in place and no movement is involved in weathering. Rocks are broken down but they stay on their location. If rock fragment is moved from its location then the process is called erosion. Erosion involves movement of rock fragments by some flowing agent such as air, water or ice. So, if a rock fragment is loosened, chemically or mechanically, but stays put, it is weathering. Once the rock fragment starts moving, it is erosion.

Weathering
Source: Geologycafe.com

Mechanical Weathering


Mechanical weathering includes various processes that cause physical disintegration of exposed rock without any change in the chemical composition of the rock. There are several different types of mechanical weathering: thermal expansion, frost shattering, plant activity and abrasion.

Thermal expansion includes expansion and contraction of minerals within rocks under different temperatures. Since rocks contain different minerals with different rates of expansion and contraction when exposed to changes in temperature, rapid temperature changes (e.g. change between day and night) can create cracks within rocks and finally cause gradual breakdown of rock.

Frost shattering includes breakdown of rocks due to the expansion of ice. It happens when water enters into cracks that exist within rocks. When temperature drops water freezes and expands and initial small cracks become larger and eventually can break off portions of the rock.

Plant activity can also break down rocks. The roots of plants and trees can grow into cracks that exist within rocks. As roots grow in size, they widen the cracks and can cause pieces of the rock to fall away.

Abrasion includes the process of breakdown of rock by friction. With this process rocks are scraped away by other loose rock and mineral fragments that are carried by air or water. These loose fragments come in direct contact with the stationary rocks and cause gradual smoothing off the surface of rocks.

Mechanical vs. Chemical Weathering
Source: Slideplayer.com

Chemical Weathering


Chemical weathering includes process by which rocks are broken down by chemical reactions. In these processes, rocks are not only broken down into smaller pieces, they are also chemically changed. Types of chemical weathering processes are hydrolysis, oxidation, carbonation and exposure to acids.

Hydrolysis is process that involves chemical breakdown of material when it comes in contact with water. In case of chemical weathering of rocks, chemical reaction happens when rainwater comes in contact with minerals from rocks. As rainwater comes in contact with minerals they are chemically changed and that change weakens the rock.

Oxidation involves reaction of oxygen and minerals within rock. Rocks can be exposed to oxidation if they contain iron. When iron from rock reacts with oxygen it forms iron oxide. Since iron oxide is not very strong, when a rock gets oxidized, it is weakened and crumbles easily, allowing the rock to break down. Rocks that are exposed to oxidation look red because iron oxide is red in color.

Carbonation includes mixing of water with carbon dioxide and creation of carbonic acid. Carbonic acid comes into contact with rocks and dissolves them. One of the most vulnerable rocks to carbonation is limestone as it contains mineral calcite that is very soluble in carbonic acid. Carbonation is the main process responsible for the creation of caves.

Exposure to acids involves acids different from carbonic acid. In this process rocks are broken down when they come in contacts with such acids. One example of acids that cause chemical weathering of rocks includes acid rain. Acid rain becomes acidic by various pollutants in air and when such acid rain falls to the ground and comes in contact with rocks it break down the minerals within rocks. Another example includes lichens that grow on rocks. They also produce acid and that acid can break down minerals within rocks. Chemical weathering caused by lichens is very often near rivers and streams.

Rate of Weathering


There are several important factors that can affect the rate of weathering. They are: (1) exposure to the atmosphere, (2) composition of rock and (3) climate.

The degree of exposure of rocks to the atmosphere can significantly affect the rate of weathering. Those rocks that are covered by ground are less affected by weathering because they are less exposed to elements that cause weathering. Different rocks have different mineral composition and different minerals have different chemical composition. Some minerals are more reactive when mixed with water, oxygen or other elements and will weather more rapidly, while others will weather slowly. Also, some minerals are softer than others and they will be more affected by weathering than others. Climate affects the temperature and rainfall of a region. Both temperature and water are important elements in weathering. Warm, humid environments will have higher rates of weathering than cold, dry environments.

Logging Tasks


To log this EarthCache answer the following questions:

1. Observe two displayed building elements and describe the differences between them.
2. Observe the surface of displayed building elements and damages caused by weathering. In your opinion, which type of weathering is the main cause of observed damages? Explain.
3. Can you find any traces of oxidation on displayed building elements? Explain.

Send your answers through my profile at geocaching.com (through e-mail or message center) and NOT in a log. You don't have to wait for me to approve your answers. Once you send your answers feel free to log this EarthCache as found. If there are problems with your answers, I will contact you.

Sources


1. Alecia M. Spooner, Geology for Dummies, Hoboken: Wiley Publishing, Inc., 2011.
2. Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa, Earth: An Introduction to Physical Geology (10th Edition), New York: Pearson, 2010.
3. Wikipedia.org
4. Geologycafe.com




Na izlistanim koordinatama pronaći ćete izložena dva građevna elementa sa Zagrebačke katedrale. Elementi su u potpunosti identični osim što je jedan dugo vremena izložen trošenju, dok je drugi element u potpunosti nov. Oba elementa su izrađena od vapnenca, kao i cijela katedrala. Promatranjem ova dva građevna elementa možete uočiti što proces trošenja može učiniti stijenama.

Trošenje


Općenito, trošenje je prirodni proces koji razbija stijene u manje komade. Postoje dvije glavne vrste trošenja – mehaničko i kemijsko. Mehaničko trošenje je uzrokovano prirodnim elementima poput vjetra, vode, te hladnoće / topline. Navedene pojave fizički otkidaju dijelove stijena. Kemijsko trošenje obuhvaća različite kemijske reakcije koje razbijaju stijene. Također, nekoliko je važnih faktora koji utječu na brzinu trošenja u određenom okruženju.

Trošenje se često zabunom poistovjećuje s erozijom, iako se radi o dvije različite vrste procesa. Trošenje se događa na mjestu i nikakvo kretanje nije uključeno u procese trošenja. Stijene se razbijaju, ali ostaju na svom mjestu. Ukoliko dolazi do pomicanja dijelova stijena s njihove početne lokacije, tada se radi o procesu erozije. Erozija podrazumijeva kretanje fragmenata stijena uz pomoć određenih pokretnih agenata poput zraka, vode ili leda. Prema tome, ukoliko je fragment stijene razbijen kemijskim ili mehaničkim putem, ali je još uvijek na svojoj početnoj lokaciji riječ je o trošenju. Ukoliko dođe do pomicanja fragmenata stijena s njihove početne lokacije, tada je riječ o eroziji.

Trošenje
Izvor: Geologycafe.com

Mehaničko trošenje


Mehaničko trošenje obuhvaća različite procese koji uzrokuju fizičko raspadanje stijena bez promjena u njihovom kemijskom sastavu. Nekoliko je različitih vrsta mehaničkog trošenja: termička ekspanzija, drobljenje ledom, aktivnost biljaka i abrazija.

Termička ekspanzija uključuje širenje i skupljanje minerala unutar stijena pod utjecajem promjene temperature. Stijene sadrže različite minerale koji imaju različite stope širenja i skupljanja pod utjecajem promjene temperature, te stoga nagle promjene temperature (npr. uslijed izmjene dana i noći) mogu izazvati pukotine u stijeni, a u konačnici i postupno raspadanje stijene.

Drobljenje ledom predstavlja raspadanje stijena do kojega dolazi uslijed širenja leda. Do njega dolazi kada voda uđe u pukotine u stijeni, te kada se zbog snižavanja temperature voda unutar stijene zaledi. Uslijed navedenoga dolazi do širenja leda, te početne male pukotine postaju sve veće, te u konačnici dolazi do odlamanja dijelova stijene.

Biljke mogu također uzrokovati odlamanje i raspadanje stijena. Korijenje biljaka i stabala može urasti u postojeće pukotine u stijenama. Kako korijenje raste, dolazi do povećavanja pukotina, te u konačnici može doći do odlamanja dijelova stijena.

Abrazija je proces raspadanja stijena do kojega dolazi uslijed trenja. Ovim procesom se komadići stijene otkidaju pod utjecajem drugih fragmenata stijena koi su nošeni vjetrom ili vodom. Fragmenti stijena koji su nošeni vjetrom ili vodom dolaze u izravan kontakt s nepokretnom stijenom i uzrokuju postupno zaglađivanje površine stijene.

Mehaničko vs. kemijsko trošenje
Izvor: Slideplayer.com

Kemijsko trošenje


Kemijsko trošenje obuhvaća procese raspadanja stijena do kojih dolazi uslijed kemijskih reakcija. U tim procesima ne dolazi samo do raspadanja stijena nego i do promjena u njihovom kemijskom sastavu. Procesi kemijskog trošenja su: hidroliza, oksidacija, karbonizacija i izloženost kiselinama.

Hidroliza je proces kemijskog raspadanja stijene do kojega dolazi zbog kontakta s vodom. Do kemijske reakcije dolazi kada voda dođe u kontakt s mineralima iz stijene. Pri tom kontaktu, dolazi do kemijske promjene minerala u stijeni, te zbog navedene kemijske promjene dolazi do slabljenja i u konačnici raspadanja stijene.

Oksidacija obuhvaća reakciju kisika i minerala iz stijene. Stijene mogu biti izložene oksidaciji ako sadrže željezo. Kada željezo iz stijene reagira s kisikom nastaje željezni oksid. S obzirom da željezni oksid nije pretjerano čvrst, kada dođe do oksidacije čvrstoća stijene slabi te lakše dolazi do raspadanja stijene. Stijene koje su izložene oksidaciji su crvene boje jer je željezni oksid također crvene boje.

Karbonizacija uključuje miješanje vode s ugljičnim dioksidom, pri čemu nastaje ugljična kiselina. Ugljična kiselina dolazi u kontakt sa stijenama, te ih pri tome otapa. Jedna od stijena koja je najpodložnija karbonizaciji je vapnenac. Vapnenac sadrži mineral kalcita koji je izrazito topljiv u ugljičnoj kiselini. Karbonizacija je glavni proces stvaranja krškog reljefa i špilja.

Izloženost kiselinama obuhvaća procese s kiselinama različitim od ugljične kiseline. U ovim procesima stijene se raspadaju kada dođu u kontakt s kiselinama. Jedan primjer takvih kiselina koje uzrokuju kemijsko trošenje stijena su kisele kiše. Kisele kiše postaju kisele zbog izloženosti različitim zagađivačima zraka. Kada takve kiše dođu u kontakt sa stijenama dolazi do raspadanja minerala koji čine stijenu. Drugi primjer se odnosi na lišajeve koji rastu na stijenama. Lišajevi stvaraju kiseline, a te kiseline također uzrokuju raspadanje minerala od kojih se stijena sastoji. Do kemijskog trošenja uzrokovanog lišajevima vrločesto dolazi u blizini rijeka i potoka.

Brzina trošenja


Nekoliko je važnih faktora koji utječu na brzinu trošenja stijena. Oni su: (1) izloženost atmosferi, (2) sastav stijene i (3) klima.

Stupanj izloženosti stijena atmosferi može značajno utjecati na brzinu trošenja. Stijene koje su prekrivene zemljom su u manjoj mjeri podložne trošenju jer su manje izložene elementima koji uzrokuju trošenje. Različite stijene imaju različit mineralni sastav, a različiti minerali imaju različit kemijski sastav. Neki minerali u većoj mjeri reagiraju u kontaktu s vodom, kisikom ili drugim elementima, te će se uslijed toga brže trošiti, dok će se drugi minerali trošiti sporije. Također, neki minerali su mekši od drugih, te će i oni biti više pogođeni trošenjem od ostalih. Klima određuje temperaturu i razinu padalina na određenom području, a upravo su temperatura i voda bitni elementi koji određuju brzinu trošenja. U toplim i vlažnim područjima će trošenje stijena biti brže nego u hladnim i suhim područjima.

Zadaci za logiranje


Kako biste logirali ovaj EarthCache odgovorite na sljedeća pitanja:

1. Promotrite dva izložena građevna elementa i opišite razlike među njima.
2. Promotrite površinu izloženih građevnih elemenata. Prema Vašem mišljenju, koja vrsta trošenja je glavni uzrok uočenih oštećenja? Objasnite.
3. Možete li pronaći ikakve tragove oksidacije na izloženim građevnim elementima?

Odgovore pošaljite preko mog profila na geocaching.com, a NE u Vašem logu! Ne morate čekati na moje potvrđivanje Vaših odgovora. Jednom kada pošaljete Vaše odgovore slobodno logirajte pronalazak ovog EarthCachea. Ako Vaši odgovori budu pogrešni ja ću kontaktirati Vas.

Izvori


1. Alecia M. Spooner, Geology for Dummies, Hoboken: Wiley Publishing, Inc., 2011.
2. Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa, Earth: An Introduction to Physical Geology (10th Edition), New York: Pearson, 2010.
3. Wikipedia.org
4. Geologycafe.com

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