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.
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.
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.
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
- 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.
- 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.
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 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.
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.
- 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.
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.
- 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.
- 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".
- 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
- 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.