Geology of The Society Islands
Pacific Ocean Formation
The Pacific Ocean’s crust is the oldest existing ocean floor on Earth. The ocean’s origins extend back to the Proterozoic, 750 Ma (million years ago), when the supercontinent Rodinia began rifting. The rift possibly resulted from a massive super plume of magma rising and breaching the continent (much like what is occurring in East Africa today). Over the next few hundred million years, the rift widened and formed the massive Panthlassic Ocean, which was the Pacific Ocean’s ancestor.
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Fast forwarding a few hundred million years to the Pliocene (5 Ma), Earth has seen the formation and destruction of Pangaea, but the Pacific Ocean still remains. The longevity of this ocean is due to the continual “crust factory” occurring along west of South America called the East Pacific Rise (EPR). The EPR is a mid-ocean ridge where the Pacific Plate is separating from the Cocos, Nazca, and Antarctic plates, which causes magma to rise to the seafloor’s surface, solidify, and create new oceanic crust. Accompanying the creation of new Pacific Ocean crust is the subduction and destruction of old crust which is occurring at the Marianas Subduction Zone on the northwestern side of the ocean. The oldest oceanic crust on Earth, 167 Ma Jurassic basalt, is currently being subducted here. Rifting on the southeastern and subduction on the northwestern edges of the Pacific plate causes the plate to have a relative northwestern motion, moving at about 50-133 mm/yr.
Island Formation
2. The hot spot theory works well except that in recent years, some chains have been found to have no age progression pattern. To explain this, a fracture theory has been employed, postulating that the mid-plate islands form from propagating fractures that develop from the twisting of the Pacific Plate as it moves. The fractures release pressure in the upper mantle and cause decompression melting, resulting in magma eruption along the fractures. As the plate continues to move, the fractures continually develop and form new islands.
Future research is needed to shed light on which theory is more correct. The Society Islands do, in fact, show an age progression, with the youngest island (Mehetia) to the southeast and the oldest (submerged seamounts) to the northwest. The age progression is observed throughout the other islands as well; Tahiti is around 1 Ma, Moorea is 1.5-2.0 Ma, Huahine is 2.0-3.0 Ma, Raiatea is 2.29-2.75 Ma, Tahaa is around 3 Ma, and Bora Bora is 3.1-3.5 Ma. The ages of the islands agree with the current rate of plate motion, showing an average movement of 110 mm/yr. Each island formed after about 1 million years of volcanism.
Several types of islands exist on the Pacific Plate. Some are formed from rising magma following subduction (Mariana Arc), some are uplifted reefs (Guam), and some are formed away from plate margins and have a debated origin (Society Islands, Galapagos, Hawaiian Islands). These particular islands generally form as chains, linearly progress in age, and are oriented in the same direction as the Pacific Plate’s motion. There are two major theories as to their formation:
1. The most popular theory is the hot spot theory. This idea asserts that there is a deep seated magma plume in either the core/mantle boundary (primary plumes) or at the base of the upper mantle transition zone which releases magma onto the seafloor. Over time and after repeated eruptions, the solidified lava builds upward, sometimes above the surface of the ocean to form islands (such as is currently occurring in Hawaii). As millions of years pass, the Pacific Plate moves over and away from the magma plume and a new island begins to form above the same magma plume but on a different part of the Pacific Plate. This theory explains the general trend of progressively older islands to the northwest, following plate motion.
Petrology
The dominant volcanic rock type on the islands is basalt. Basalt is the most abundant rock on Earth: a dark, extrusive rock (forms on the crust, not inside the Earth), rich in plagioclase and pyroxene. Basalt underlies all the oceans and is much denser (3.0 g/cm3) than continental crust (2.7 g/cm3) , which is why oceanic crust always subducts under continental crust. The basalt found on the Society Islands is tholeiitic, which causes it to have a high iron content. The iron in the rock is leached out over time by rain water, which mixes with organic materials to form the red mud seen throughout the islands. Several types of basalt are found on the islands, including ankaramite (basalt with large pyroxene and olivine phenocrysts), benmorite (basalt with small phenocrysts), and trachyphonolite (basalt with intermediate sized phenocrysts).
The proto-pacific ocean, Panthalassa.
Source: montesangiorgio.com
The Society Islands
Source: lib.utexas.edu
Source: discovergalapagos.org.uk
Basalt
Source: geology.com
Source: blogs.agu.org
Source: socratic.org
Basalt thin section in polarized light
Source: fineartamerica.com
Trachyphonolite
Source: flickr.com
Ankaramite thin section in polarized light
Source: soest.hawaii.edu
Ankaramite
Source: lithotheque.ens-lyon.fr
Island Evolution
While the Society Islands may have all had similar origins, they do not look the same. Tahiti is a large island with a small, narrow reef and lagoon, and Bora Bora has a huge lagoon with a large surrounding reef. A long while ago, Bora Bora looked very similar to Tahiti, and Bora Bora’s reef represents the outline of what the island used to be. The island looks so different now due to the basalt cooling and becoming more dense, which causes the island to subside into the ocean. The chemical conditions of the water around the islands is very conducive to coral growth, so as each island began to form, a fringing reef formed on the island’s edges, much like what is seen at Tahiti today. As the island subsided, the carbonate reef continued to grow upwards, forming a barrier reef like Bora Bora. Once the island has completely sunk, the reef becomes an atoll, and only the island’s outline is left. Tectonics add another formational element to the islands, and may either lift or sink them so suddenly that the reef cannot respond quick enough and dies, leaving only a seamount submerged in the water.
