Land subsidence theory

A description of the subsidence theory regarding the formation of coral reefs

1. THEORIES OF ORIGIN OF CORAL REEFS
Dr. MeenakshiPrasad
Assistant Professor
P.G. Deptt. of Geography
MagadhUniversity
BodhGaya
Source : googleimages

2. Disclaimer
The material for the presentation has been
compiled from various books and online sources
& it is for general information & educational
purpose only. While the author makes an
endeavor to keep the information up to date and
correct she makes no representation of any kind
about the completeness and accuracy of the
material. The information shared through this
presentation should be used for educational
purposes only.

3. Content
•Introduction
•Theory
•Evidence in favourof
the theory
•Criticisms
Source : googleimages

4. Introduction
•There is no dispute about the origin of fringing
coral reefs, but the base of barrier reefs and atolls
is sometimes at such a depth where it is not
possible for the coral polyps to survive. To solve
this problem, some theories have been given
about the origin of coral reefs, out of which three
theories have special recognition. These are –
•Land Subsidence Theory by Darwin (1837 & 1842)
•Stand Still Theory by Murray &
•Glacial Control Theory by Daly

5. •The most widely recognised& accepted amongst the
three theories is the Land Subsidence Theory. It is
followed by the Glacial Control Theory & Stand Still
Theory respectively.
Source : googleimages

6. Land Subsidence Theory of Darwin
•The theory of Land
Subsidence is the most
recognized theory related
to the origin of coral reefs
as it explains the origin of
most coral reefs.
•Darwin first presented
this theory in 1837 AD
and again in 1842 AD,
based on some evidences
he presented a revised
form of it.
DARWIN
Source : googleimages

7. •The basis of Darwin's
study is Bikini Island
which is an island in the
Marshall group of Islands.
•In later years this theory
was endorsed by Dana
(1885), Davis (1928), and
Menord(1964).
•Davis has described it in
his book ‘The Coral Reef
Problem’.
Source : googleimages

8. •The theory of land subsidence is based on the belief that
the development of coral reefs is associated with earth-
movement & land subsidence. First the fringing coral reef
was developed, then the subsidence of the landmass
resulted in a change of sea-level and it turned into a bariier
reef and then finally in to an atoll.
•According to Darwin coral polyps develop in the shallow
waters along the east coasts of continents in the tropical
zone because the trade winds here preserve ideal
temperature and salinity conditions for their growth.
•The place or island along which the coral reef is formed is
not stable. It gradually subsides and this subsidence causes
the growth of coral reefs of different shape and thickness.

9. •According to Darwin, all types of coral reefs begin as
fringing reefs. As long as there is no difference
between the relative floor of the land and the sea,
the reef keeps developing in this form.
•This is followed by subsidence in the landmass,
which causes the coral reef to sink into deep water.
•In such a state, living coral organisms constantly rise
on top of each other due to which the fringing reef
grows upwards & outwards. This increases the
thickness of the coral reef and also increases the
depth of the lagoon.

10. •The coral organisms on the outer edge of the reef,
grow more rapidly and grow upwards because they
receive food in sufficient quantity.
•Their growth in the interior of the reef towards the
coast stops because they lack food.
•As a result, only the outer part of the coral reef is able
to keep the action of rising upward with the action of
subsidence and the inner part of the fringing reef is
submerged in the sea water.
•As a result, the width of the lagoon increases and the
fringing reef moves away from the coast and becomes
a barrier reef.

11. •If the coral reef develops along the coast of an island
then due the continuation of the subsidence process
the island finally completely submerges and only a ring
shaped reef with a lagoon in between remains, which
is known as an atoll.
•Thus, according to Darwin, fringing reefs, barrier reefs
and atolls are the successive stages of development of
coral reefs.
•This theory explains the presence of older age
limestone with depth, depth of coral structure and
deepening of the lagoon with time.

12. Source : googleimages

13. •Based on this theory, it is possible to interpret
thousands of meters thick coral structure. Otherwise
coral creatures cannot grow at a depth of 1000 m.
•The thickest coral reef is found in Bikini Island (1300
m). It is located in Central Pacific ocean.

14. Evidence in favour of this theory
•The most evidence in Darwin's favour was gathered
by Davis. Davis has confirmed this theory based on
the thickness of several islands in the Pacific Ocean.
Again Davis has also reported that the shoreline is
highly eroded in the areas of barrier reefs and atolls.
The formation of creeks and deep seas here confirms
the land subsidence.
•Several evidences are found in favour of the theory
of land subsidence which are as follows -

15. (i)The shallowness of the lagoon remains unchanged,
indicating subsidence. If the land had remained
stable, the lagoon would have been filled due to
the continuous deposition of sediments.
(ii)The absence of cliffs along the coasts having coral
reefs exhibits subsidence of land as cliffs are found
along the stable coast.
(iii)There is a complete absence of coral reefs along
the parts of the Pacific Ocean, where uplifted
beaches are found.

16. Criticisms of the theory
•Between 1950 and 1960, petroleum exploration took
place on several islands in the Pacific Ocean, which
proved to be the basis for the development of
knowledge related to coral reefs. But some islands do
not confirm the subsidence theory. The main ones are
Solomon and Palau islands. There is no evidence of
land subsidence on these islands but the depth of the
coral structure along these islands is more than 200
m.
•There are some other criticisms of the theory as well -

17. (i)If fringing reefs, barrier reefs & atolls are
successive stages of development of coral reefs
then both fringing and barrier reefs should not
occur along the same island together. But on the
basis of new discoveries, many such evidences are
found. For example -In Nairaiisland of Fiji, both
types of coral reefs are found together. This is
contrary to the theory of subsidence.
(ii)Many coral reefs are composed of the structure of
same age. The structure in Palau and Solomon
Islands in particular are not explained.

18. (iii)At a few places coral reefs are also found along the
rising/uplifting island (Aldabra island in western
Indian ocean). They refute this theory.
(iv)According to this theory, atolls should always be
extremely deep, but sometimes atolls are also found
at low depths.
Despite these exceptions, this theory is widely
recognized.

19. References
•Sharma, R.C. & Vatal, M. : Oceanography for
Geographers, ChaitanyaPublishing House, Allahabad,
1995
•Singh. S : Physical Geography, PrayagPustakBhawan,
Allahabad,2012

20. Source : googleimages