Hans Hass

ATOLL RESEARCH BULUTIIT

No. g1

Central su-bsidence. A new theory of atoll formation by

Hans Hass

Issued by

THE PACIFIC SCIEIKE BOARD

National Academy of Sciences--National Research Council Washington, D. C.

D ~ c 3 .Ler 13, 1252

C e n t r a l subsidence. A new theory of a t o l l formation

1/

Hans Hass

I n t r o d u c t i o n

The Xarifa-Expedition t o t h e Maldives and Nicobars i n 1957/58 was an undertaking of t h e I n t e r n a t i o n a l I n s t i t u t e f o r Submarine Research a t Vaduz, Liechtenstein, a p r i v a t e foundation of mine. It was p a r t i a l l y sponsored By t h e Max Planck G e s e l l s c h a f t a t Goettingen and t h e Ministry of Culture of North Rhine-Westfalia. I had hoped t h a t t h e main p a r t of t h e c o s t s trould be c o n t r i b u t e d By t h e Deutsche Forschungsgemejnschaft;

u n f o r t u n a t e l y t h i s was not p o s s i b l e and I t h e r e f o r e covered t h e c o s t s by producing 26 t e l e v i s i o n programs f o r t h e BBC and German t e l e v i s i o n . It had been my i n t e n t i o n t o i n v e s t i g a t e t h e development of a t o l l formation, a s I hoped t h a t underwater i n v e s t i g a t i o n s i n t h e Maldives might perhaps throw l i g h t on t h i s s t i l l obscure problem. I n t h e circumstances, however, my time was almost e n t i r e l y absorbed by f i l m work and t h e management of t h e expedition. For t h i s reason t h e following arguments can only be s u b s t a n t i a t e d i n a very g e n e r a l way. A s I s e e no opportunity t o c a r r y on t h i s r e s e a r c h i n t h e n e a r f u t u r e , and as my theory may perhaps give o t h e r s a l e a d , I decided t o p u b l i s h t h e following s h o r t account from my book

"Expedition i n s Unbekannte" published by U l l s t e i n , 1961. This book w i l l e v e n t u a l l y be t r a n s l a t e d i n t o English and published by Hutchinson, London;

it a l s o includes a l i s t of t h e s c i e n t i f i c papers g i v i n g t h e r e s u l t s of t h i s expedition.

A t o l l formation

Damin considered t h a t some of t h e i s l a n d s surrounded by c o r a l

r e e f s had subsided i n t h e course of time u n t i l f i n a l l y they had disappeared completely beneath t h e surface of t h e sea, and t h a t a t t h e seme time t h e o r i g i n a l f r i n g i n g r e e f had g r o m higher and higher u n t i l it f i n a l l y remained a s a r i n g i n t h e sea.

On t h e o t h e r hand, Murray and o t h e r s have explained t h e formation of a t o l l s by a rise i n t h e ocean bed. I n t h i s way a r i s e of t h e bed t o w i t h i n perhaps 50 meters of t h e surface would B e followed by t h e o n s e t of

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Ergebnisse d e r Xarifa-Expedition 1957/58.

1nte6nationales I n s t i t u t e

flir

Submarine Forschung.

Note. We a r e p u b l i s h i n g t h i s paper i n s p i t e of t h e negative r e a c t i o n s of some of our advisors, not because it r e p r e s e n t s i n any way t h e conclusions of t h e P a c i f i c Science Board Coral A t o l l Program, o r of t h e e d i t o r s , bu.t because it i s t h e e d i t o r i a l p o l i c y of t h e A t o l l Research B u l l e t i n not t o suppress ideas t h a t a r i s e from f i e l d observation. It must be pointed o u t t h a t t h e observations suggest t h a t t h e s t r u c t u r e of Maldivian a t o l l s must be v e r y d i f f e r e n t from t h a t of t h e C e n t r a l P a c i f i c A t o l l s with which we are f a m i l i a r .

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-Eds

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grovth i n reef-building corals. A s conditions would be more favorable on the outer edge, there would be g r e a t e r growth there, and so a r i n g would be formed.

A t h i r d theory ( h l y ) i s based on the f a l l of the sea l e v e l during the l a s t Ice Age and the subsequent cooling of the water. This would have caused the death of the c o r a l s followed by wave erosion of the reefs, leaving large, bare platforms. When the sea rose again and the water grew warmer, new c o r a l growth would have begun, p r i n c i p a l l y on the outer edges, and s o the f a m i l i a r r i n g would have been formed.

An a e r i a l survey of the r e e f s i n the Maldives has suggested t o me quite a d i f f e r e n t theory. A l l stages of development can be seen here.

F i r s t l y , i s o l a t e d cone-like growths appeared and j u s t reached the surface.

There a l s o l a r g e r growths i n which the centers had sanded over, though c o r a l s t i l l appeard t o be growing strongly on the outer edges. Finally, there were the t y p i c a l r e e f rings surrounding b e a u t i f u l blue lagoons ( f i g s . 1-5). The depth of the lagoons seemed t o bear a d i r e c t r e l a t i o n s h i p t o the s i z e of the a t o l l : the l a r g e r the reef, the deeper the blue of the lagoon.

It occurred t o me t h a t the r i s i n g and extending r e e f s f i r s t became barren i n the middle and then subsided i n the center l i k e a piece of cake. The f a r t h e r they extended outwards the more they sank i n the middle.

Subsequent underwater investiga-Lions showed t h a t t h i s f i r s t impress- ion of an uninterrupted t r a n s i t i o n from a small c o r a l reef t o an a t o l l was not mistaken, and l e d me t o an observation vhich thrown new l i g h t on the o l d problem. I discovered, i n f a c t , t h a t the inner s t r u c t u r e of these r e e f s i s by no means a s firm and s o l i d as has been assmed. They a r e a c t u a l l y b u i l t by very d e l i c a t e , much branched corals, such a s Acropora and Echino ora which grow up over each other t o form a kind of loose s c a f d i g s . 6, 7). When the c o r a l dies, fragments break o f f and r o l l down t o form a slope of loose debris extending i n t o deep water a t an angle of about 45O ( f i g . 8).

A t a depth of about 18 meters, I t r i e d t o drive a tunnel i n t o the side of the reef, and found t h i s quite easy t o do, .even with my bare hands.

The s t r u c t u r e was, however, so loose and s h i f t i n g t h a t it soon became undermined and collapsed.

These f a c t s suggest a simple explanation t o account f o r the forma- t i o n of a t o l l s . A s t h e r e e f r i s e s and extends outwards on a l l s i d e s by growth and the accumulation of debris, the l a t t i c e work of c o r a l s which was a t first on the outside becomes p a r t of the inside of the reef and thus aubject t o the weight of the debris l y i n g above. I n the course of time, the calcareous material becomes b r i t t l e , possibly even r e - c r y s t a l - l i z e s , and thus causes the o r i g i n a l l y loose s t r u c t u r e t o collapse under the pressure from above. This process obviously bears some r e l a t i o n s h i p t o t h e age of the reef i t s e l f , a s one can see very c l e a r l y i n the Maldives.

With a r i n g diameter of about 2 Ian, t h e depth of t h e lagoon is between 10 and 20 meters; a diameter of 8 km gives a depth of up t o 40 m, a diameter of 24 km a depth of 70 m, and so on.

A s o t h e r observers have mentioned, t h e lagoon i t s e l f does not f i l l up again with c o r a l s because the envirorunental conditions t h e r e a r e un- favorable f o r t h e i r growth. We were a b l e t o follow t h i s general process very c l o s e l y i n t h e Maldives ( f i g s . 2-4). A s soon a s a r e e f reaches t h e surface and begins t o spread out i n a l l d i r e c t i o n s , a barren a r e a forms i n t h e middle, something l i k e a b a l d head. Here t h e r e i s a double r e l a t i o n - s h i p between cause and e f f e c t . The more favowably placed c o r a l s f l o u r i s h - ing on t h e o u t e r edge take a g r e a t e r share of the a v a i l a b l e oxygen and food, with t h e r e s u l t t h a t the c o r a l s growing i n the middle gradually p e r i s h and crumble, forming c o r a l sand. This sand i s then swept backwards and forwards by t h e t i d e s and so helps t o smother the surviving c o r a l s . I n this way a reef with a diameter of perhaps not more than 300 m w i l l become a miniature a t o l l with a c e n t e r growing pTogressively b a r e r and a t t h e sane time s t a r t i n g t o su.bside.

The r a t e of subsidence i s probably influenced by the p a r t i c u l a r s o l i d i t y of the upper p a r t of t h e r e e f , t h e so-called reef f l a t . This firmness i s due c h i e f l y t o calcareous algae which l i v e only i n shallow water. They cover a l l t h e c o r a l fragments with a c r u s t and join them f i r m l y together so t h a t - - a s t h e reef extends--a s o l i d platform develops r e s t i n g on t h e porous mass below ( f i g . 10, A, B ) . Only when t h i s platform has reached a c e r t a i n s i z e and has become barren and sandy i n the c e n t e r does subsidence s e t i n ( f i g . 10, C, D). This suggests t h a t possibly t h e c e n t e r of t h e platform, when it becomes barren and covered with sand, somehow d i s i n t e g r a t e s and l o s e s some of i t s firmness.

Another p o i n t may a l s o play a p a r t here: the l a r g e r t h e a t o l l , t h e g r e a t e r the volume of water enclosed i n r e l a t i o n t o t h e circumference of t h e r e e f . It must be remembered t h a t whereas t h e c i r c u l a r r e e f merely extends i n one dimension by an increase i n circumference, t h e enclosed l a t e r increases by t h e square. More o u t l e t s thus become necessary t o allow t h e tidal water t o flow i n and o u t of t h e a t o l l . A t ebb t i d e t h e water i s trapped i n s i d e t h e a t o l l , and u n t i l it f i n a l l y manages t o flow out i t s l e v e l i s a l i t t l e higher than t h a t of t h e surround in^ s e a ( f i g - 11). This means t h a t f o r some time t h e r e i s a g r e a t e r pressure on t h e bed of t h e lagoon, and it i s q u i t e possible t h a t i n the course of thousands

Of years t h i s c o n s t a n t l y changing t i d a l pressure may influence the subsidence of the lagoon bed, by a kind of rhythmic massage.

I t may seem t h a t t h i s idea i s incompatible with t h e porous s t r u c t u r e of -the r e e f a s described above, b u t here again the firmness of t h e reef f l a t must be considered. The reef f l a t r e p r e s e n t s a more o r l e s s imper- vious bottom r e s t i n g - - a f t e r subsidence--like a cup on t o p of the porous s t r u c t u r e . From time t o time sudden t r o p i c a l rainstorms may a l s o increase t h e volume and weight of t h e trapped water. In t h i s way a downward

pressure i s exerted.

Only a s t h e a t o l l grows and channels appear i n t h e r i n g w i l l t h e conditions f o r c o r a l growth i n the lagoon again improve progressively.

Secondary f r i n g i n g r e e f s w i l l then form along the inner margin of t h e a t o l l , causing a considerably s t e e p e r slope ( f i g . 12, F), and secondary cone-like r e e f s w i l l grow up from t h e lagoon bed ( f i g . 12, R ) . This completes t h e chain of development which i s t y p i c a l of t h e Maldives. The

secondary lagoon r e e f s experience, of course, t h e same extension and subsequent c e n t r a l subsidence, and t h i s explains the presence of small r i n g s within t h e l a r g e r ones ( f i g s . 9 and 1 2 A).

The theory p u t forward here a l s o accounts f o r t h e formation of b a r r i e r r e e f s . I f a f r i n g i n g reef continues t o extend towards t h e sea,

then, by t h e same reasoning, t h e r e a r s e c t i o n gradually subsides and a deepening lagoon i s formed betveen the l a n d and t h e r e e f . The t h e o r i e s of upheaval and abrasion do n o t s u f f i c i e n t l y explain t h e f a c t t h a t the l a r g e r t h e a t o l l s are, the deeper they become, and a l s o t h e lagoons of b a r r i e r r e e f s become deeper a s the r e e f extends f a r t h e r out from t h e land. The p r e s e n t theory, on the o t h e r hand, gives an explanation of both these phenomena.

Few t h e o r i e s have accounted f o r t h e f a c t t h a t a t o l l s e x i s t only i n c e r t a i n t r o p i c a l areas. My answer t o t h i s problem i s simply t h a t t h e r e e f s a r e n o t everywhere b u i l t by the same kinds of coral.

The p a r t i c u l a r firmness and s o l i d i t y of the upper p a r t of t h e r e e f has, i n my view, l e d t o a misunderstanding. S c i e n t i s t s who have n o t had t h e opportunity to dive have been misled by t h i s s o l i d i t y of t h e r e e f f l a t t o assume t h a t i t s inner s t r u c t u r e was s i m i l a r l y firm. I n t h e Red Sea, where t h e more compact P o r i t e s c o r a l s p l a y a g r e a t e r p a r t i n the b u i l d i n g of t h e r e e f s , t h i s may w e l l be so, b u t t h i s is n o t so i n t h e case of t h e Maldive r e e f s , Here t o o t h e platform spreads out l i k e a s o l i d mass of concrete, b u t t h e foundation on which it r e s t s i s loose and porous. The f a c t t h a t it takes a long time before t h i s platform a c t u a l l y subsides i n t h e middle may be due t o s t a t i c f a c t o r s and t o a n a t u r a l d i s i n t e g r a t i o n of t h e m a t e r i a l i n the c e n t e r .

7hether t h i s theory of c e n t r a l subsidence s a t i s f a c t o r i l y accounts f o r every known r i n g formation i s another matter. A s Hoff'meister and Ladd have pointed out, it i s p o s s i b l e t h a t n o t all a t o l l s have been formed i n t h e same way. Some may have come about through t h e r i s e o r f a l l of t h e ocean bed, and o t h e r s through i c e age abrasion. However, amongst t h e various p o s s i b i l i t i e s I f e e l t h a t t h e present theory is e n t i t l e d t o some consideration, p a r t i c u l a r l y a s it has i n f a c t t h e advantage t h a t it does n o t r e l y on e i t h e r a r i s e o r a subsidence i n t h e ocean bed o r on a f a l l

i n t h e s e a l e v e l , b u t explains t h e o r i g i n of both a t o l l s and b a r r i e r r e e f s from t h e normal processes of c o r a l growth.

Summary: Reefs b u i l t mainly by s l e n d e r branching c o r a l s have a porous and unstable inner s t r ~ ~ c t u r e . This causes t h e more extensive r e e f f l a t s t o subside i n t h e center. Such c e n t r a l subsidence. may p o s s i b l y be f u r t h e r influenced by r e - c r y s t a l l i z a t i o n of t h e underlying material, by t h e d i s i n t e g r a t i o n o f t h e m a t e r i a l of t h e center of t h e r e e f f l a t i t s e l f , and by the cumulative e f f e c t of changes i n water pressure i n t h e lagoon caused by t h e t i d e s and occasional rainstorms.

Figure 9

F i g . 1 0 : Schemotrc s e c t i o n s t h r o u g h a r e e f g r n d u a l l y t u r n m g i n t o a n a t o l l . A ) a c o n e - l i k e r e e f grows toward t h e s u r f a c e . U) R e a c h i n g t h e s u r f a c e , t h e r e e f f o r m s a s o l i d r e e f f l a t . C) Reef and r e e f f l a t e x t e n d o u t w a r d s , t h e c e n t e r o f t h e p l a t f o r m ~ e t s b n r r e n and c o v e r e d w i t h s a n d . D) The r e e f h a s f u r t h e r i n c r e a s e d i n s i z e nnd t h e p l a t f o r m s u b s i d e s u t t h e c e n t e r .

1 = l i v i n g c o r a l b = b a r r e n )urea p a r t l y c o v e r e d w i t h s a n d p = p o r o u s s t r u c t u r e s = s a n d and g r o v e l

r = r e e f f l a t ^c = c e n t r a l p o r t o f t h e p l a t f o r m which p o s s i b l y d i s i n t e g r a t e s