8: Coral Reefs Flashcards
What are the 3 overarching requirements for coral reefs?
- Be able to secrete carbonate
- Suitable substrate (shallow areas and can tolerate high energy)
- Light (zooxanthellae photosynthesis)
Will corals be able to acclimatise to conditions expected under climate change?
Uncertain
What are the 4 states of coral reef environments?
Fringing reefs
Small shoals/platforms
Barrier Reefs
Atolls
Where do fringing reefs occur?
On the fringes of islands which defines its own boundaries
What are barrier reefs?
These are when the island is located within a deeper oceanic lagoon and so the reefs occur further out from the boundaries of the island and not necessarily the boundary of the island
What are atoll reefs? and when do they occur?
These are essentially coral reef islands. They occur when the island has subsided significantly below the surface and so the top of the once island now provides the substrate for the reefs to occur on and stick just at the surface where they then appear as an island
What was Charles Lyell’s interpretation of what atolls were?
areas where coral reef had occupied the tip of an underwater volcano
What did Reginald Daly believe?
That the geographical distribution of reefs would not have existed in marginal sea areas as they would have been too cold. He also proposed the glacial control hypothesis
What was Reginal Daly’s glacial control hypothesis?
Lowering of sea level would expose some coral reefs which would then be subject to increased wave erosion resulting in their destruction. As sea level fell again this process would continue. As sea level then rose again there would be the restoration of coral reefs. These falls/rises were all hypothesised to be consistent with the LGM etc
Why was the glacial control hypothesis incorrect?
Because we know that there has not been a coral reef growth of 120m to match the same rise in sea level since the LGM.
What is Darwin’s subsidence theory for the formation of coral reefs? 6 steps
(1) volcano forms over hotspot
(2) plate moves over from hotspot and in turn becomes inactive and subsides deeper
(3) becomes so deep that a fringing reef can occur on the volcano sides
(4) As movement continues it becomes a barrier reef
(5) then an atoll
(6) all before eventually that reef joins the tectonic plate falling in to the ocean trench and is lost to the mantle
What theory for coral reef formation came about from work in Tahiti?
The Tahiti Reef Cycle
What was the first stage of the Tahiti Reef Cycle?
volcano created under a hotspot has a coral reef grow just below the surface on its sides as a fringing reef. As sea level rises the reef can grow further up on the sides of the volcano. When they reach the surface they form a flat platform.
What was the second stage of the Tahiti Reef Cycle?
Sea level falls again and reefs start to grow on the first location where the fringing reefs first occurred. As sea level rises the reefs join them on growing on top of the reefs that grew on the volcano sides before. Again the reef platform that was created before now has the reef form around it as a new flat platform that is essentially a barrier reef
What was the third stage of the Tahiti Reef Cycle?
Process of second stage repeats but the volcano has now undergone so much subsidence over a long time period that the tip of the volcano now lies underwater and so the reef on the outer sections is now all that is above the surface. This means it has now formed a coral atoll.
