Formation of Islands and Reefs in the Pacific

Volcanic Origin

The theory of plate tectonics proposes that the earth crust is broken up in large plates. The plate boundaries do not necessarily match the coastlines of continents. A plate can consist of continental crust, oceanic crust, or both. In most cases, continents are part of larger plates that extend for hundreds of miles offshore. Many plate boundaries are far out in the middle of the ocean. There are three types of plate boundaries: divergent, convergent, and transform.

• Divergent boundaries exist where plates move away from each other, pushed apart by heated, material moving upwards from the asthenosphere. Divergent boundries undersea are known as Seafloor Spreading Zones. An additional force involved in divergence may be the subduction of the heavier, older, and thicker crust at the opposite ends of each diverging plate. As the heavy edge sinks, it pulls the rest of the plate with it, away from the divergent boundary. Magma at the divergent boundary hardens, adding new crust to the edges of the separating plates. Scientists often refer to these as constructive boundaries, due to the construction of new material. Mid-ocean ridges, as the East Pacific Rise and the Galapagos Ridge, are examples of this type of boundary. These ridges frequently resemble submarine mountain ranges, portions of which are high enough to break the ocean’s surface (such as the Galapagos Islands).
• Convergent boundaries boundaries where two plates collide. When an oceanic plate, such as the Nazca Plate which moves eastwards under the southeastern Pacific Ocean, meets a continental edge such as South America, the denser and heavier oceanic crust is normally subducted and partially melted beneath the continental plate. At subduction zones, where one plate moves beneath the other, the subducted plate is dragged downwards into the earth's mantle until it reaches a depth where high temperatures partially melt the rock. The resulting magma then rises along vertical fissures and reaches the surface through a volcanic vent. Ocean trenches at the boundary of the plate and mountain chains on the continental plate often result. When fissures open up on the seafloor, volcanic islands form as a result, such as Japan, Philippines, Indonesia, New Zealand and most islands in the of the South Pacific.  Earthquakes can occur at these plate margins, shifting plates by up to 5 metres (about 15 feet) at once.
• At a transform boundary, plates move past each other in opposite directions. Little volcanic activity accompanies transform boundaries, but large, shallow earthquakes can occur. The San Andreas Fault in California, United States, is the most famous example of this type of boundary. The interaction of plates at a transform boundary does not normally lead to volcanic activity.

Hawaiian Ridge

As the islands move northwest and age, they erode and subside. Midway Islands, more than 2,600 kilometres (1,616 miles) to the northwest, are the furthest and oldest on the Hawaiian Ridge. They are about 20 million years old and barely above sea level.

The Hawaiian Ridge connects at Midway’s northwest end to the north-south Emperor Seamounts system, which disappears into the Kuril Trench off the Siberian peninsula of Kamchatka. The northernmost Emperor seamount is more than 70 million years old and may have originated from the present-day Hawaiian hot spot.

Some terminology

• SeamountA seamount is a volcano that rises from the seabed but does not emerge above the surface of the water. Seamounts can occur alone or in a group. The most well-known chain of seamounts is the Emperor Seamounts of the North Pacific Ocean, which stretch northwards from the Midway Islands towards the Kamchatka Peninsula of Siberia.
• Guyot: A guyot is a flat-topped vulcano seamount. Named after a Swiss geologist, most guyots occur in the mid-Pacific Ocean, especially to the southwest of Hawaii. They have an average depth of 1,200 metres (3,937 feet) at their flat summits. Geological evidence, especially coral and wave-eroded sand, indicates that many guyots were once in shallow water. It is theorized that these seamounts rose above or to sea level, were planed off by erosion, and later submerged as the seafloor aged and became more dense.
• Islands: Most Pacific islands are a result of volcanic activities at plate boundaries in the Pacific Ocean, where two plates collide. The magma will built up through volcanic vents from seamounts to active vulcanoes reaching above sealevel. The shapes of volcanoes vary according to the types of particles thrown from the volcano during eruptions. Some vulcanoes can built up to an altitude of several kilometres in altitude. Strato-volcanoes are the highest and steepest volcanoes in the world. Shield volcanoes, on the other hand, are predominantly lava-based landforms that have gradual slopes and wide bases, because they release fluid lava slowly. These volcanoes can create huge landforms (Hawaii, New Zealand, Savaii). Some enormous, craterous basins called calderas, at the top of long-dormant or extinct volcanoes, form when a massive explosion forces the upper part of a volcano to collapse. Some of these calderas eventually fill with water, forming deep lakes, such as the picturesque Lake Taupo in New Zealand and on Tofua Island in Tonga. Volcanoes can be active, dormant, or extinct. Active volcanoes have erupted in a relatively recent period. Dormant volcanoes are those that have not erupted for many years, but have the potential to erupt again. The eruption that follows prolonged dormancy is usually violent (eg. the explosion in 1980 of Mount Saint Helens in the northwest US, after 123 years of inactivity and the massive eruption in 1991 of Mount Pinatubo, in the Philippines, came after six centuries of dormancy). Extinct volcanoes have not erupted in thousands of years and show no signs of doing so in the future. Most Islands of the South Pacific are extinct volcanoes.
• Atols: Atolls are rings of coral reef and small sandy islands that form around a lagoon. A lagoon is a shallow area of sea water that is nearly or completely separated from the sea by a land barrier such as a coral reef. Atols are most common in the tropical Pacific Ocean where whole groups of islands, such as the Marshall, Tuamotu, and Kiribati islands, are atoll chains. Based on a well-accepted theory developed by the English naturalist Charles Darwin (1809-1882), atolls form because of the subsidence of oceanic islands. When reefs first form around oceanic islands, they are in the shallow waters along the shore as a fringing reef, such as around the young islands of Hawaii or Tahiti. As the island erodes and subsides, the reef continues to grow upwards, forming an offshore barrier reef separated from the main island by a lagoon. Bora-Bora is an example of this: over millions of years, the volcanic core of the island has completely eroded and has subsided below sea level. The coral has continued to grow upwards, however, to create an atoll. Midway, Enewetak, Bikini, and Tarawa are other examples of atolls.
• Raised Atols: This type of atols are lifted up, where coral reefs have come above the water creating a spectaculair cave landscape.
• Coral reef: A coral reef is a ridge or mound composed of the chalky skeletons of former generations of coral animals, and serving as a platform for the living coral polyps. A polyp is the form of certain invertebrates, such as coral animals and sea anemones, which have tubular bodies capped with stinging tentacles. Reefs usually grow in the shallow, clear water of tropical oceans. The largest Coral Reef is the Great Barrier Reef, located in the Coral Sea off the northeast coast of Australia. Though its name suggests that it is a large single reef, it is actually a series of thousands of individual reefs and hundreds of small coral islands formed along the edge of the Queensland continental shelf. The Great Barrier Reef extends along the shelf edge and is the largest organic feature on Earth.

Reefs Millions of Years in the Making

Parts of the Great Barrier Reef in Australia may be as much as 20 million years old. Most of the reef, however, has built up since the last Ice Age, over the past 2.5 million years. The largest of its kind and one of the world's most impressive natural wonders, the reef extends south from Cape York Peninsula for 2,012 kilometres (about 1,250 miles). Conditions for coral growth are ideal, and the water is usually crystal clear. Red and blue-green algae near the surface give the reef a distinctive colour. Although much of it is protected in national parks, increasing tourism, oil exploration, and the mining of limestone nonetheless threaten the reef.

Coral reefs are formed by small animals that secrete a calcium carbonate skeleton. Millions of skeletons eventually form a reef. Unlike those which form around oceanic islands due to subsidence, the Great Barrier Reef was caused by rising sea levels after the last Ice Age. The corals contain algae in their tissue that require sunlight, making the reef grow upwards in response to rising sea levels. Corals also require warm sea water to thrive, preferably near 25°C (77°F) and the Great Barrier Reef is ideally located in a region where the warm South Pacific Equatorial Current feeds into the East Australian Current. Although coral reefs appear to be rocky and indestructible, they are complex and fragile ecosystems.