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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175

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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176

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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177

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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178

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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179

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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180

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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181

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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182

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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184

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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185

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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186

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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187

Izotopi kisika bilježe brojne visoke i niske razine mora

(visoka razina)

(niska razina)

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