Sinking atoll islands Flashcards
what is an Atoll?
An atoll is a ring-shaped coral reef, often surrounding a central lagoon, that forms around a submerged volcanic island in tropical ocean waters.
why are atolls significant?
habitats for marine life
home to human populations and rich culture
offer insight into geological and biological processes
1st theory of atoll formation?
Land Subsidence Theory (Darwin)
Explain Land Subsidence theory
coral reef begins to grow around edges of volcanic island in shallow water
volcanic island begins to sink due to subsidence
coral continues to grow upward creating a reef separated from the island by a lagoon
volcanic island becomes completely submerged below sea level
coral forms reef shaped ring with lagoon in the centre
what evidence was found to support Darwin’s theory?
Data from deep drilling on coral atolls in the pacific ocean
as depths of corals increased age of corals also increased
fossil corals dated back 50 million years
found underlying volcanic rock (basalt)
shallow corals found at deep depths
problem with Darwin’s theory?
Doesn’t consider rising sea levels or changes in sea levels
What is Daly’s glacial control theory?
alternative explanation for atoll formation
emphasised role of glacial cycles and sea level changes during the ice ages in shaping atolls
model highlights how fluctuations in global sea levels caused by glaciation and deglaciation impact coral reef growth and atoll formation
step one in Daly glacial model atoll formation process
during ice age sea levels lowered
many coral reefs exposed to the air and growth stopped because corals cannot survive out of water
step 2 in Daly model
post glacial sea level rise glaciers melted and reef platforms were submerged underwater
coral start growing again
step 3 Daly model
atoll forms
corals grow upward along the edges of the submerged reef platforms keeping pace with the rising sea levels
upward coral eventually form ring shaped structure with central lagoon in middle
what is the key driving force in Darwins theory?
tectonic activity leading to subsidence
what is the key driving force in Dalys theory?
sea level fluctuations
weakness of Daly glacial model?
no reference to subsidence doesn’t explain why atolls are located in clearly subsiding locations
Suggests this is unrelated
what does the Antecedent karst theory propose about atoll formation?
atolls form on pre-existing karst landscapes (limestone platforms) that were shaped by chemical weathering and erosion before coral reefs began to grow
what is a karst landscape?
a landscape formed by the chemical weathering of limestone through acidic water creating features like sinkholes, caves and depressions
How does a karst landscape form the foundation of an atoll?
when sea levels rise the karst features are flooded providing base for coral reefs to grow
what happens to the central depressions in a karst landscape?
they become the lagoons of the atoll as coral grows around the rims of the depressions
what role does sea level play in the antecedent karst theory?
at low sea levels during glacial periods allows karst landscapes to form
at high sea levels during interglacial periods - floods landscapes allow coral to form
what are the strengths of the antecedent karst theory?
explains atolls on non-volcanic limestone platforms
considers the role of sea level change
weakness of this model?
is usefull in karst landscapes but is not a universal model for all atolls
which theory suggests that atoll islands will dissapear?
Darwin Land subsidence theory
which theory suggests that atoll islands will survive?
Daly glacial model