Fakarava atoll
VulcanSpirit
Richard & Alison Brunstrom
Sat 16 Jun 2012 17:11
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This place is a true atoll - a ring of coral reef
surrounding a lagoon. It's huge - the reef is 60km long by 20km wide (big, but
not even anywhere near the top 20 largest in the world). It was Charles Darwin
(him again) who brilliantly deduced the origin of coral reefs (see his lesser
known "The Structure and Distribution of Coral Reefs" of 1842) following his
trip in 'The Beagle'. that he did this before the advent of modern geology,
when almost nothing was known of vulcanology, plate tectonics etc is nothing
short of astounding. And he was right - his theory has stood the test of time.
In essence atolls are built on top of subsiding
volcanos. A volcano forms a tropical island and becomes extinct. Coral starts to
grow in the shallow water around it, and deposits aragonite (a form of calcium
carbonate, limestone) as it does so. The sheer weight of lava discharged by the
volcano, and then the limestone added by the coral as well, depresses the
earth's crust, causing the volcanic island to slowly sink back under the sea.
The coral however continues to grow, upwards, as the island goes down.
Eventually the island may become completely submerged, but if the growth rate of
coral (10-250mm per year) is greater than the rate of subsidence then an atoll
will form, building upon the original fringing reef. A shallow lagoon develops
in the centre as the rate of coral growth is slower there. Some reefs are very
thick (well over 160m at Midway Island for instance) and have been growing for
30 million years or more.
In the case of Fakarava the land has risen
again slightly, resulting in the reef rising above sea level (the highest
point is about 3m). The land has risen unevenly resulting in a series of islands
to the east and north while the southwestern side of the reef remains submerged.
The main island is L shaped, about 20km long but only about 300m(!) wide. Here
is the fringing reef on the east side:
 Subsequent to the atoll rising a new fringing reef
is growing with a tiny strip of lagoon behind it. This is low tide on a calmish
day with live coral exposed. The water just offshore is extremely deep and clear
with low nutrients, just what the corals like.
Below is the reef covering as the tide rises; the
tide is only about half a metre so the growing corals and algae get plenty of
sunlight.
 And here is the reef at low tide. Note that the
exposed reef, above low water mark, is extremely rough. The land is still rising
so reef has been exposed. The coral dies and the limestone they have deposited
dissolves in rain which is acidic leaving a very jagged surface. You do not want
to get washed ashore here.
 Here are the growing corals themselves. These are
the ones that can tolerate very shallow water and extreme sunlight - those in
deeper water are much bigger. Although they look very delicate these are
actually limestone structures in or on which the live corals reside (each polyp
is like a miniature sea anemone), so they are robust, heavy and in some cases
extremely sharp:
 The whole lot is mixed up with the reef-building
red alga Porolithon which cements the reef together by secreting
limestone sheets. The eventual structure is solid limestone with live
coral only on the very suface of the reef.
Here is one small pretty coral colony with its tips
just out of the water (it's about 15cm across):
 There are a few lovely animals living amongst the
coral in the inter-tidal zone. Here is a holothurian (sea cucumber) of unknown
(to me) designation. An echinoderm (relative of the starfish and sea urchins),
and thus with (disguised) pentaradial symmetry it is about 30cm long and lives
right at the tide line. It grips the substrate with its hundreds of
hydraulically controlled tube feet (they have no blood so use seawater) which
you can see as spikes along the body - actually they're soft. They have a mouth
at one end with five feet with 'fingers' on the end, also hydraulically
controlled, which they use to pick up sand grains and stuff into their mouths.
They live on the bacteria and algae coating the sand.
 And here is a small 'giant' clam, embedded in the
coral substrate. This one is about 20cm across, and they come in a range of
beautiful colours. You can see the row of primitive eyes lining each valve of
the open shell. They are filter feeders, straining plankton out of the water and
closing tight at the first sign of unusual movement (that's why primitive eyes
are good enough). Their design is so good that they're almost unchanged
over many tens even hundreds of millions of years.
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