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Hotspot Volcanism
A volcano is a rupture in the crust of the Earth from where gases, ashes and
molten rock material – magma – escape to the ground. A volcano is called an
active volcano if the materials mentioned are being released or have been
released out in the recent past. Pacific Ring of Fire has one of the maximum
numbers of active volcanoes in the world. Most of the volcanoes are found
underwater.
Hotspot Volcanism
Volcanoes are found where tectonic plates are diverging or converging. The vast
majority of earthquakes and volcanic eruptions occur near plate boundaries or
margins, but there are some exceptions. For example, the Hawaiian Islands, which
are entirely of volcanic origin, have formed in the middle of the Pacific Ocean
more than 3,200 km from the nearest plate boundary.
Hotspot Volcanism refers to this intra-plate volcanism, which describes a volcanic
activity that occurs within tectonic plates. The position of these hotspots on the
Earth’s surface is independent of tectonic plate boundaries.
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Hotspot volcanism is unique because of its occurrence. It does not occur at the
boundaries of Earth’s tectonic plates, where all major volcanic activity takes place.
Instead, it occurs within the plates at abnormally hot centres known as mantle
plumes.
Hotspot Volcano
What is a Hotspot?
“Hotspot” refers to an area in the Earth’s mantle from where hot plumes rise
upward, forming volcanoes on the overlying crust.
A hotspot is fed by a region deep within the Earth’s mantle from where these
mantle plumes rise through the process of convection.
The heat from mantle plumes facilitates the melting of rock at the base of
the lithosphere, where the brittle, upper portion of the mantle meets the
Earth’s crust.
High heat and lower pressure at the base of the lithosphere (tectonic plate)
facilitates melting of the rock.
This molten material (rock), called magma, rises through cracks in the crust and
erupts to form volcanoes.
Hot-spots are relatively fixed in comparison to the plates
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As the tectonic plate moves over the stationary hot spot, the volcanoes are
rafted away and new ones form in their place, resulting in chains of volcanoes,
such as the Hawaiian Islands.
Mantle Plumes
Hotspot volcanism occurs at abnormally hot centres known as the mantle
plume
A mantle plume is an upwelling of abnormally hot rock within the
Earth’s mantle, first proposed by Tuzo Wilson in 1963.
In 1971, geophysicist Jason Morgan further developed the hypothesis of
mantle plumes. In this hypothesis, convection in the mantle transports heat
from the core to the Earth’s surface in thermal diapirs.
These mantle plumes are almost like lava lamps, with a rising bulbous head fed
by a long, narrow tail that originates in the mantle.
As the plume head reaches the lithosphere, it spreads into a mushroom shape
that reaches roughly 500 to 1000 kilometres in diameter. These features are
called diapirs.
When the head of a plume encounters the base of the lithosphere, it undergoes
widespread decompression. As a result, melting takes place, and large volumes
of basalt magma are formed which, finds its way to the earth surface when an
explosion takes place.
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Layers of Earth
Mantle Plume
Distribution of Hotspots
Because of differing definitions of what a hot spot, there is also diverging opinions
about the numbers of hotspots in the world. Forty to fifty hotspots are thought to
exist around the world, although this number varies greatly. Major hot spots in
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the world include the Iceland hotspot, under the island of Iceland in the North
Atlantic, the Reunion hotspot, under the island of Reunion in the Indian Ocean
etc.
Distribution of Hotspot across the globe
Wilson’s hotspot theory
“Hotspot” theory was given by J. Tuzo Wilson, the Canadian geophysicist in
1963.
Wilson in his study found that in certain locations around the world, such as
Hawaii, volcanism has been active for very long periods of time.
Based on this he gave the idea of “Hotspots” referring to small, long-lasting, and
exceptionally hot regions which existed below the plates and provided a
localized source of high heat energy (mantle plumes) to sustain volcanism.
This led to a new hypothesis by Wilson that the distinctive linear shape of the
Emperor Seamount chain in the Hawaiian Islands resulted from the movement
of the Pacific Plate over a deep, stationary hotspot in the mantle, located below
the current position of the Hawaiian Island.
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Heat emerging from this hotspot produced a perennial source of magma by
partly melting the overriding Pacific Plate.
The magma, being lighter than the surrounding solid rock rises through the
interior of the earth to erupt onto the seafloor, forming an active seamount.
Over time, a large number of eruptions cause the seamount to grow until it
finally emerges above sea level forming an island volcano.
He suggested that continuing plate movement eventually carries the island
beyond the hotspot. Hence the source of magma is cutoff and volcanism ceases.
As one island volcano cease to exist, another develops over the hotspot, and
the cycle is repeated.
This process growth and death of volcano, over many millions of years, has left
a long trail of volcanic islands and seamounts across the Pacific Ocean floor.
According to Wilson’s hotspot theory, the farther the volcanoes of the Hawaiian
chain travel beyond the hotspot, the older and more eroded they get.
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Cycle of Volcanism
Interestingly, a volcano located above a hot spot does not erupt forever.
Attached to the tectonic plate below, the volcano moves and is eventually cut
off from the hot spot.
Without any source of heat, the volcano becomes extinct and cools. The plate
beneath the volcano (and above the hot spot) also cool.
This cooling causes the rock of the volcano and the tectonic plate to become
denser
The volcano and the plate gradually subside as they move away from the hot
spot.
As the volcano subsides below sea-level, the top is eroded flat by waves.
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In time, new and active volcanoes are developed over the hot spot, and this
cycle of volcanism goes on.
Even giant volcanoes, like Mauna Loa on Hawaii, will eventually disappear into
the ocean.
Hotspot Features
Seamounts – Volcanic activity at hot spots can create submarine mountains
known as seamounts. Depending on the amount of volcanic activity, seamounts
can rise hundreds or thousands of meters from the seafloor.
Chain of Islands – Hotspot seamounts that reach the surface of the water can
create entire chains of islands, such as the U.S. state of Hawaii.
Hot spots can also develop beneath continents, for example, The Yellowstone
hotspot, U.S.A
Geysers – Hotspots don’t always create volcanoes that spew rivers of lava.
Sometimes, the water and steam have erupted like a volcano from within the
earth surface due to heating up of the groundwater by the magma. These
eruptions are called geysers. A famous geyser is Old Faithful in Yellowstone
National Park.
Hotspot tracks – While a plume that feeds hot spot volcanoes remains
stationary relative to the mantle, the plate above it usually moves. The result is
that a chain of progressively older volcanoes is created on the overlying plate.
Pacific ocean has some of the best examples of such “hot spot tracks”.
Reunion Hotspot
The Reunion hotspot is a volcanic hotspot
Presently, it’s lying under the Island of Reunion in the Indian Ocean.
The hotspot is believed to have been active for over 66 million years.
About 66-68 million years ago present-day India was above the hot spot
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Deccan traps, a vast bed of basalt lava that covers part of central India is
thought to have been formed by a huge eruption of this hotspot 66 million
years ago.
The Laccadive Islands, the Maldives, and the Chagos
Archipelago are atolls, resting on tracks created by Reunion Hotspot.
As the plate moved in the northeastern direction more volcanic centres were
formed: the Mauritius Islands from 18-28 million years ago, the Mascarene
Plateau 40 million years ago, the Chagos Ridge 48 million years ago and the
Maldives from 55-60 million years ago.
The youngest volcanoes, Piton de la Fournaise and Piton des Neiges, were
formed in the last 5 million years. Piton de la Fournaise is one of the most active
volcanoes on the Earth surface.
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