Atolls

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Atolls

• Biogenic Islands in Oceanic Deserts

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Darwins Theory of Reef Development

• Requirements for Coral growth
• Light
• Salinity
• Water quality
• Water motion

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Light Penetration
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Reefs Do not Grow at River Mouths
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Reefs Require Vigorous Water Motion
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Darwins Subsidence Theory

• Corals grow on pre-existing volcanic platform
• Subsidence reduces the size of the island
• Coral growth keeps up with subsidence
• Eventually no trace left of the basalt foundation
• Atoll stage

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Alternative Hypothesis

• Sediments accumulate on deep rise
• Eventually sedimentary edifice reaches the photic
  zone
• Coral grows

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Darwin's final and very characteristic utterance
on the coral-reef controversy is found in a
letter which he wrote to Professor Alexander
Agassiz, May 5th, 1881 less than a year before
his death "If I am wrong, the sooner I am
knocked on the head and annihilated so much the
better. It still seems to me a marvellous thing
that there should not have been much, and
long-continued, subsidence in the beds of the
great oceans. I wish that some doubly rich
millionaire would take it into his head to have
borings made in some of the Pacific and Indian
atolls, and bring home cores for slicing from a
depth of 500 or 600 feet.
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Hypothesis test

• Drill through carbonate to basement
• Pelagic sediments?
• Basalt?
• Funafuti 500m
• Enewetak 2500m

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Enewetak
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Bore Hole at Enewetak
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Darwins Reef Types

• Fringing Reefs
• Patch Reefs
• Barrier Reefs
• Atolls

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Volcanic Island Basalt
Fringing reef
Barrier reef
Lagoon
Carbonates (Old Reef)
Atoll
Basalt Old volcanic structure
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Fringing Reef
Fringing Reef
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Fringing Reef at Coconut Island
Fringing Reef
Toyota Corolla
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Reef Crest
Back Reef
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Patch Reefs in Kaneohe Bay
Barrier reef
Patch Reefs
Fringing Reef
Back Reef
Golf Cart
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(Sunken) Floating City
Patch Reef
Fringing Reef
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Atolls

• In Hawaii we have seen the developmental series
  from high island to Atoll
• In many archipelagos especially in the Indian
  and Pacific Oceans no trace of the basalt
  foundation is visible

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Atoll Morphology

• Surrounding Reef
• Motu islets (sand or vegetated)
• Passes
• Reef structure (windward vs. leeward)
• Fore reef slope
• Reef crest
• Reef flat
• Back reef
• Lagoon
• Patch reefs

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Components of Atoll Structure

• Seaward slope
• Outer Reef Flat
• Motu
• Back Reef
• Lagoon

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Motu and reef flats
Motu
Lagoon
Reef flat
Windward reef
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Reef Crest
Motu
Reef Flat
Lagoon
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Pisonia forest
Beach Rock
Scaevola strand
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Pisonia forest Enewetak Atoll Marshall Islands
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Beach rock
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Raised reef Makatea
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Phosphate mining Naru Raised Atoll
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Spur Groove Zone
Wave Direction
Spur
Groove

• Diffuses wave energy
• Algal Ridge
• Slopes to outer reef
• and back reef

Algal Ridge
Back Reef
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