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174
ZEMLJA ISPOD OCEANA
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Neki glavni topografski oblici morskog dna
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Topografski profi preko sjevernog Atlantika
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Glavni fiziografski oblici u Atlantskom oceanu
• kontinentalni rub– kontinentalni šelf
– kontinentalna padina
– kontinentalni spust
• abisalna ravnica• podmorske planine• srednjeoceanski hrbat
– abisalna brda
– središnja riftna dolina
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Kontinentalni šelf
• široka ravnica koja se pruža od obalne crte do početke kontinentalne padine
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Kontinentalni šelf
• plitkovodniji okoliš pod utjecajem valova i plimskih struja, uglavnom prekriven pijeskom i muljem
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Kontinentalna padina
• strmija padina, najčešće prekrivena muljem, označava rub kontinentalnog šelfa
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Kontinentalna padina
• ispresijecana podmorskim kanjonima
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Tipičan pasivni kontinentalni rub
kontinentalni šelf
podmorski
kanjon kontinentalna
padinaspust
abisalnaravnica
turbiditne
struje
turbiditni
talozi
podmorska
lepeza
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Kontinentalni spust
• blago nagnuti pokrov sedimenta nastao odlaganjem pijeska i mulja uz bazu kontinentalne padine (dubina 2-3 km)
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Abisalna ravnica
• ravnica koja zauzima oceansko dno niže od kontinentalnog spusta, na dubini 4-6 km
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Abisalna ravnica
• sedimentacija nekoliko mm/god• najčešći sedimenti: sitnozrnasta glina, prašina donešena vjetrom i ljušture mikroskopskih organizama
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Abyssal hills
Kalcitna kompenzacijska dubina(CCD)
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Abisalna brda
Abyssalhills
Abyssalplain
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Topografski profil preko južnog Tihog oceana
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Tektonski aktivni rubovi Tihog
oceana
ocean-oceangranica ploče
ocean-kontinentgranica ploče
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akrecijska prizma
rov
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kontinentalni vulkanski pojas
akrecijska prizma
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Obalni krajolici su vrlo različiti, a ovise o:
• tektonskoj stabilnosti obalnog područja
• osobinama stijena i sedimenta na obali• dugotrajnoj promjeni razine mora• energiji valova• energiji morskih mijena
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• brzina vjetra• trajanje vjetra• površina koja je pod utjecajem
vjetra
Čimbenici koji nadziru energiju valova
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Orbitalni valovi potaknuti vjetrom
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Kretanje vala je pod utjecajem dubine vode i oblika obalne linije
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plitko-vodni
val
dubokovo-dni val
valna baza
valna baza
zona izdiza-nja vala
zona loma vala
plaža
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Što se događa kada dubina
vode postane manja od polovice duljine vala?
• dubokovodni val postaje plitkovodni val• val se usporava• val ulazi u zonu izdizanja vala
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Što se događa kada se val usporava (tj. u zoni izdizanja vala)?
• period vala ostaje isti• dužina vala se smanjuje• visina vala se povećava• može se pojaviti refrakcija vala!
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plitkovodni valdubokovodni val
valna baza
L 2
=
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Refrakcija vala:
valna baza
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refrakcija valova koncentriraenergiju prema rtovima uzrokujući pojačanu eroziju
refrakcija valova smanjujeenergiju u zaljevima uzrokujući pojačano taloženje
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Dužobalni drift
• prijenos sedimenta (npr. pijeska) paralelno obalnoj liniji, uglavnom u zoni izdizanja i mlata valova.
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Morske mjene su rezultat gravitacijskog privlačenja Mjeseca i
Sunca. Morske mjene nastale utjecajem mjeseca - lunarne morske mjene, a one nastale utjecajem Sunca - solarne morske mjene.
Spring mjene se pojavljaju kada su solarne i lunarne mjene u
fazi.
Neap mjene se pojavljaju kada solarne i lunarne mjene nisu u
fazi. Tvore 28-dnevne cikluse.
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Spring mjene:
Neap mjene:
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Intenzitet obalne erozije u SAD
U.S. Geological Survey
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Erozijska činjenica
• 30-50% svih obalnih struktura unutar 1.500 m od današnje obalne linije u Meksičkom zaljevu će biti erodirano u sljedećih 60 god.
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Pitanje: Što determinira je li neka plaža erozivna ili je stabilna?
Odgovor: Zaliha pijeska!
Što je zaliha pijeska?
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Kako sačuvati obalu od erozije?
• strukturni pristup– lukobrani
– molovi– nasipi– valobrani
• nestrukturni pristup: – nasipavanje plaža
– napuštanje/premještanje
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Lukobrani
• svrha:
– prikupljanje sedimenta s ciljem onemogućavanja ili smanjenja erozije plaže
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Pitanje: Što lukobrani, molovi, nasipi i valobrani imaju za posljedicu?
Odgovor: Svi uzrokuju eroziju niz struju!
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• dodavanje pijeska ili nekog drugog materijala s ciljem smanjenja erozije
• ... i ne uzrokuje eroziju niz struju
• ... ali, mora se periodički obnavljati!
Nasipavanje plaža
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Vremenske promjene barijernih otoka na području Cape Coda:
1830.-1987.
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Izotopi kisika bilježe brojne visoke i niske razine mora
(visoka razina)
(niska razina)
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