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
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,
- 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,
Indonesia, New Zealand and most islands in the of the South
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.
Islands have a different
(non-plate-tectonic) origin, formed over hot
spots in the earth's
surface far from plate boundaries. The Hawaiian
Islands are located atop a submarine feature called the
Hawaiian Ridge. The volcanic
ridge extends in a southeast-northwest direction for more than 3,000
kilometres (1,865 miles). The ridge is youngest in the southeast at the
site of the large island
which sits above an active hot
spot on the ocean
floor. As the Pacific
Plate moves northwest, it carries the Hawaiian chain with it.
When the large island of Hawaii moves away from the hot spot it will be
replaced with another island, Loihi,
an active seamount
about 1 kilometre (0.62 miles) below sea
level to the southeast.
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
The northernmost Emperor seamount is more than 70 million years old and
may have originated from the present-day Hawaiian hot spot.
|Below you find some terminology
explained about island formation in the Pacific.
- 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
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
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,
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
as a fringing reef, such as around the young islands of Hawaii
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,
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
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.