Representation of the morphological diversity of Pacific tropical islands using GIS and a modular symbology
Initialy published in French for the 2009 Pacific Science Meeting in Tahiti http://www.psi2009.pf/
Geomorphologic diversity of tropical islands is seldom presented in a clear way. High islands and atoll are often opposed, making us forget that they have a common origin. We present a simplified classification and a modular symbology that gives a new vision of this variety and this continuity, accounting for logic continuity. Examples are given in French polynesia, with the possibility to extend this method to all the tropical pacific.
“The ring-formed reef of the lagoon-island is surmounted in the greater part of its length by linear islets. On the northern or leeward side there is an opening through which vessels can pass to the anchorage within. On entering, the scene was very curious and rather pretty; its beauty, however, entirely depended on the brilliancy of the surrounding colours. The shallow, clear, and still water of the lagoon, resting in its greater part on white sand, is, when illumined by a vertical sun, of the most vivid green. This brilliant expanse, several miles in width, is on all sides divided, either by a line of snow-white breakers from the dark heaving waters of the ocean, or from the blue vault of heaven by the strips of land, crowned by the level tops of the cocoa-nut trees. As a white cloud here and there affords a pleasing contrast with the azure sky, so in the lagoon bands of living coral darken the emerald green water.” Charles Darwin, The Voyage of the Beagle
The problem : the great geomorphological diversity of tropical islands
1) Volcanic construction mainly related to hot spot activity.
2) Erosion, cataclysmic landslides and land collapses.
3) Reef growth.
4) Tectonic plate drift and subsidence.
5) Sealevel or/and ocean floor rise and fall (climate variation, flexed lithosphere)
Large scale systems (islands) become scattered when seen at small scale leading to problems in using continuous cartographic representation.
Morphological continuity leads to difficult logic and pedagogic thematic classification. Is the usual opposition between high island and atoll really pedagogicaly justified as the accepted paradigm is geological continuity between them ?
– We propose a new morphological class “island-atoll” between high islands with barriere reef and atolls. This class is larger than “almost-atoll”: Island-atoll embodies high islands with several coral islets (motu) sometime linked to main island, whatever the height of the volcanic remain.
– GIS allow to overtake scale representation (zoom). We tried to define a symbology allowing an immediate understanding of geomorphological main features. Using modulary markers formed with imbricated symbols we can represent natural diversity and the concentric symbols are easily associated to the features of island systems.
Our cartography is intended to fit most needs from the humblest to the highest.
From 3D diagrams to synthetic symbols
Drawing design based on http://coraux.univ-reunion.fr/
A deep seamount is a relief rising above sea bottom usually erected by a volcano.
A shallow seamount is closer to sea surface than a deep seamount, 100 m below sea level.
High island with no reef
The volcano reaches the sea surface. It becomes a “high island”.
High island with fringing reef
When the fringing reef appears the island is getting old. The volcano is usually extinct
High island with barrier reef
High island with coral islets (atoll-island)
Erosion and subsidence lead to relief lowering. A channel appears between the reef and the island.
The reef becomes wider and coral islets (motu) with vegetation appear.
The aerial volcano has now disappeared. Reef growth compensates for erosion and subsidence. The annular coral reef is the only “emerged land” and it surrounds a lagoon. Passes when present permit exchange of water with the ocean.
When reef growth cannot compensate subsidence or sea level variation the atoll disappears under sea level and becomes a shallow seamount (guyot) and eventually a deep seamount.
Subsidence, erosion and co
Volcanism, subsidence and erosion are the main processes shaping the islands. Two others factors act on island morphology:
– Sea level variation (eustatism): some islands can be uplifted or flooded following changes of sea level related to global climate changes (e.g. quaternary glacial and interglacial periods).
– Raising of ocean floor: lithosphere may bend because of shallow magma or at trench approach or because of the load of a volcano.
Sunken islands go back to shallow seamount state and eventually deep seamount. Uplifted islands are
surrounded by typical fortresslike coral walls.
Our symbols are made from simple ideas
– concentric circles follow natural landshape :
– green ring for coral islets (motu),
– brown circle for volcanic island
– light blue ring for lagon
– gray ring for coral (with peaks for elevated parts)
– seamounts are smaller than islands
– active volcanos display a red star
Contrast between colors were choosen for easy reading.
I How to use this work
We present examples of use of our symbology in eastern polynesia with ArcView and GoogleEarth.