|
VOLCANOES OF THE PACIFIC
There are between 20 and 30 thousand islands in the Pacific Ocean. Those located below the Tropic of Cancer are traditionally grouped into three divisions: Melanesia, Micronesia and Polynesia.
Melanesia (black islands) includes New Guinea (which is the largest Pacific island), New Caledonia, Zenadh Kes (Torres Strait Islands), Vanuatu, Fiji and the Solomon Islands.
Micronesia (small islands) includes the Marianas, Guam, Wake Island, Palau, the Marshall Islands, Kiribati, Nauru and the Federated States of Micronesia. Most of these islands lie north of the equator.
Polynesia (many islands) includes New Zealand, the Hawaiian Islands, Rotuma, the midway Islands, Samoa, American Samoa, Tonga, Tuvalu, Wallis and Futuna, Tokelau, Niue, French Polynesia, Easter Island and, of course, the Cook Islands. This is the largest of the three areas.
Most of these islands are formed from volcanic activity located at edges, rifts and weakspots (also called hotspots) in the Pacific tectonic plate. The Pacific Plate, shown in pale yellow in the image to the right, is the largest of the planet's tectonic plates, and has an often violent interaction with the surrounding plates. To the north-east, it meets the North American Plate along the famous San Andreas Fault and to the west, it subducts under the Philippine Sea Plate forming the Mariana Trench, and touches the Indo-Australian Plate from Papua New Guinea, through Vanuatu (where Mt Yasur has been erupting for over 800 years) to New Zealand.
The Pacific Plate has one major hotspot that has formed the Hawaiian Islands. As the plate moves westwards over the hotspot, new volcanos are formed above the hotspot, which then erupt over a period of time and become dormant as they move away. There are nearly 100 volcanos in the Hawaiian archipelago, over a length of nearly 5000 kilometres, and ranging from 65 million years old to currently erupting, and newly forming under the sea.
While Hawaii may offer one of the clearest examples of this type of volcanic archipelago formation, there are numerous other examples across the Pacific. The islands that make up the Southern Group of the Cook Islands, along with islands of French Polynesia, is an example of an archipelago, called the Cook-Austral Volcanic Chain, formed by the Pacific Plate passing across not one but a number of smaller hotspots creating strings of volcanic islands and atolls. There are numerous other examples across the South Pacific.
These strings of islands form by volcanic activity occuring above and as a result of hotspots in the earth's crust. As the Pacific Plate moves, so does the volcano that has formed above the hotspot, and as the volcano therefore moves away from the hotspot, the volcanic activity ceases, and the island moves into a different phase of its lifespan, which we will discuss a little further on. Over time, the volcano will have moved some distance away from the hotspot and in time, depending on just how hot, or weak, the earth's crust is, another volcano will form above the hotspot. This repeating process results in a long string of islands typically in a straight or slightly curving line across the ocean, from youngest near the hotspot, to oldest furtherest away, and usually beyond as seamounts that have long since eroded below the surface of the ocean once more.
COOK-AUSTRAL VOLCANIC CHAIN
The Cook-Austral Volcanic Chain is located in the southern part of the Pacific Plate, in a region of anomalous shallow seafloor known as the South Pacific Superswell. The chain extends to the northwest for over 2200 kilometres from the Macdonald Seamount, an active submarine volcano, to the island of Aitutaki in the Cook Islands. The chain comprises 11 islands and 2 atolls with very little area above sea level, along with numerous extinct submarine volcanoes.
Although oriented roughly in the direction of the present Pacific Plate movement (11 centimetres per year in a WNW direction), the pattern of both the aerial and submarine volcanoes is rather complex. Two recent geophysical cruises, one at the southern end of the chain, and the other over the central and northern parts have revealed the complexity of the overlapping volcanism.
The figure above shows a shaded view of the seafloor topography for the Cook-Austral region. The map projection is made along the direction of present motion of the Pacific Plate (from right to left). The white lines correspond to crustal magnetic anomalies, and their corresponding ages are in white numerals. The age of the oceanic crust along the chain ranges from approximately 35 million to over 80 million years.
The morphology and geometry of the island groups suggest the existence of two distinct volcanic alignments (formed by at least 2 hotspots of volcanic activity). The Aitutaki-Mauke Islands group (Rimatara, Rurutu, Tubuai, Raivavae, and Président Thiers bank) form the northeast alignment. Rarotonga and Mangaia Islands, Neilson Bank, Rapa, Marotiri, and Macdonald Seamount, the only known active volcano, form the southwest branch.
Ages have been measured for almost all the islands, and seamounts in the Taukina and Ngatemato chains and in the northern Austral region. In the north of the Cook-Austral region, Rarotonga has a younger age of between 1.8 and 1.1 million years while at Aitutaki (located at the north western end of the northern chain), a 1.2 million year age coexists with an 8.5 million year old stage. In Rurutu (near the centre of the northern chain), two different volcanic stages have been identified, an old one at 12 million years of age, compatible with the progression in ages along the northeastern volcanic alignment, and a young one at 1.1 million years.
Aitutaki and Rurutu are interesting as they suggest an older existing volcano passing over a second hotspot causing new volcanic activity. In Rurutu's case, the 1993 discovery of the Arago Seamount, some 120-130km to its south east went a long way to answering that very question. Arago, aged at just 230 thousand years gives away the approximate present position of this newly identified hotspot. Rarotonga and Aitutaki, in the Cook Islands, appear to be located at the beginning of new volcanic chains.
The figure below shows the ages of the volcanos in the Cook-Austral region. The three hotspots identified (not including those near Aitutaki and Rarotonga for which little data has thus far been obtained) are tracked and the black numbers indicate time (in millions of years ago) along each track. The northernmost track starts at 6.5 million years ago for the last volcanic event known on this track. The active Macdonald and recently active Arago Seamounts are represented by black stars.
CORAL REEFS AND ISLAND EVOLUTION
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Corals are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, and are formed by polyps that live together in groups. The polyps secrete a hard carbonate exoskeleton which provides support and protection for the body of each polyp. Reefs grow best in warm, shallow, clear, sunny and agitated waters.
Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on Earth. They occupy less than one tenth of one percent of the world ocean surface, about half the area of France, yet they provide a home for twenty-five percent of all marine species, including fish, molluscs, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep water and cold water corals also exist on smaller scales in other areas.
Most coral reefs were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves. This means that most coral reefs are less than 10,000 years old. As coral reef communities were established on the shelves, they built reefs that grew upwards, keeping pace with the rise in sea level. Reefs that didn't keep pace could become drowned reefs, covered by so much water that there was insufficient light for further survival. Coral reefs are also found in the deep sea away from the continental shelves, around oceanic islands and as atolls. The vast majority of these ocean coral islands are volcanic in origin. The few exceptions have tectonic origins where plate movements have lifted the deep ocean floor to the surface.
In 1842, during the voyage of the HMS Beagle, Charles Darwin began to lay the foundations of his theory regarding the formation of atolls, that they are formed by the uplift and subsidence of the Earth's crust under the oceans. It starts with a fringing reef forming around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef, and ultimately an atoll reef.
Darwin predicted that underneath each lagoon would be a bed rock base, the remains of the original volcano. Subsequent drilling proved this correct. Darwin's theory followed from his understanding that coral polyps thrive in the clean seas of the tropics where the water is agitated, but can only live within a limited depth of water, starting just below low tide. Where the level of the underlying land stays the same, the corals grow around the coast to form what he called fringing reefs, and can eventually grow out from the shore to become a barrier reef.
A fringing reef can take ten thousand years to form, and an atoll can take up to 30 million years Where the land is rising, fringing reefs can grow around the coast, but coral raised above sea level dies and becomes white limestone. If the land subsides slowly, the fringing reefs keep pace by growing upwards on a base of dead coral, forming a barrier reef enclosing a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Barrier reefs and atolls don't usually form complete circles, but are broken in places by storms. Should the land subside too quickly or sea level rise too fast, the coral dies as it is below its habitable depth.
Darwin’s theory sets out a sequence of three stages (four if you include initial island formation) of island and coral reef evolution ultimately resulting in the formation of an atoll and these are shown in the figures below:
|
1. First there is a volcanic island which becomes extinct once the volcano moves away from the hotspot where it initially formed.
|
|
|
2. As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef.
|
|
3. As the subsidence continues the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside.
|
|
4. Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon.
|
THIS EARTHCACHE
Now that we have discussed the formation of Pacific volcanic islands, and their life cycle from brand new volcano through to their typical ulitmate form as a coral atoll, you need to perform some tasks in order to log the earthcache. Some of these tasks require you to send information to the cache owner while other tasks will require you to submit information when you log this find online. Please make sure you complete ALL tasks, as we do not enjoy denying people permission to log.
1. Please describe, in as much detail as possible, the terrain around the published coordinates, specifically to the east, and to the west. Also, please describe what this motu (island) is indeed made of. These descriptions must be sent to the cache owner and NOT included in your log.
2. Referring to the stages of island evolution mentioned in the cache description above, which stage is the island of Rarotonga currently in? You must justify your answer, and explain why you chose that particular stage. This answer must be sent to the cache owner and NOT included in your log.
3. Choose a piece of coral located at or very near the published coordinates. DO NOT REMOVE THE CORAL. Describe the shape and texture of the coral and optionally photograph it. If you choose not to photograph it, you must still provide a detailed description. Please include this optional photo and your description of the coral in your online log. Do not use someone else's photo or description.
4. This is an optional task, but we urge visitors to attempt it: Using the diagrams PROVIDED ON THIS CACHE PAGE, particularly the one showing the ages of the various volcanic islands and the tracking of the hotspots, estimate the current location of the hotspot that created the original Rurutu (12 million years ago) and Raivavae (6.5 million years ago). Give your estimated answer to the nearest whole degrees, S and W. This answer should be sent to the cache owner and NOT included in your log.
5. This is an optional task, but we urge visitors to attempt it: Perform some independent research and provide the cache owner examples of other pacific islands of volcanic origin that are in each of the other stages of their evolution, as per the stages mentioned above and in question 2. The results of your research should be sent to the cache owner and NOT included in your log.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
References:
Wikipedia - Atoll
Wikipedia - Plate Tectonics
The Cook-Austral Volcanic Chain
Wikipedia - Pacific Islands
|