Volcanoes are usually found near the borders of tectonic plates that are violently either pushing or pulling at each other. Mysteriously, however, volcanoes sometimes erupt in the middle of these plates instead.
The culprits behind these outbursts might be giant pillars of hot molten rock known as mantle plumes, jets of magma rising up from near the Earth’s core to penetrate overlying material like a blowtorch.
Still, decades after mantle plumes were first proposed, controversy remains as to whether or not they exist.
The concept of mantle plumes began in 1963 with the enigma of the Hawaiian volcanoes, which dwell more than 3,200 km from the nearest plate boundary. Scientists think that as the Pacific plate slid over a “hot spot,” a line of volcanoes blossomed.
On a map of the Pacific basin, we can find three linear chains of volcanoes and submarine volcanoes (seamounts). Although separated by thousands of miles, the three linear chains are parallel to each other.
Of the three, the Hawaii-Emperor seamount chain is the most well known. For each chain, the islands become progressively younger to the southeast. The extreme southeast end of each chain is marked by active volcanoes.
In 1971, geophysicist W. Jason Morgan proposed that hot spots resulted from plumes of magma originating in the lower mantle near the Earth’s core at depths of more than 2,500 km. Researchers think these mantle plumes are shaped like mushrooms: narrow streams of molten rock topped with bulbous heads that buoyantly bob upward.
Importance of mantle plumes
Although plumes are secondary in terms of heat transport, they have probably played an important role in continental geology.
A new plume starts with a large spherical head that can cause uplift and start flood basalt volcanism, and may be responsible for regional-scale metamorphism or crustal melting and varying amounts of crustal extension.
Plume heads are followed by narrow tails that give rise to the familiar hot-spot tracks. The cumulative effect of processes associated with tail volcanism may also significantly affect continental crust.
Mantle plumes influence surface processes, including continental break-up and mass extinctions.
Plumes are thought to spread out laterally at the base of a continent, creating increased pressure that stretches the crust and results in uplift, fracturing, rifting, or flood basalts.
Mantle plumes are thought to be strong enough to induce rifting and the formation of plates.
The pressure creates a domed region that eventually splits in a three‐pronged pattern (triple junction or triple point).
The best example of a triple junction in the world is provided by the three faults marked by the Red Sea, the Gulf of Aden, and the inactive African Rift Valley. The rifting is separating the Arabian Peninsula from the African continent and is thought to be related to a mantle plume.
Other areas that are underlain by mantle plumes are the Hawaiian Islands (oceanic crust) and Yellowstone National Park (continental crust) in the United States.
The potential importance of mantle plumes may go well beyond explaining volcanism within plates. For example, the mantle plume that may lie under Réunion Island in the Indian Ocean has apparently burned a track of volcanic activity that reaches about 5,500 km northward to the Deccan Plateau region of what is now India. Catastrophic volcanism there 65 million years ago gushed lava across 1.5 million km2, potentially hastening the end of the age of dinosaurs.
Do mantle plumes really exist?
However, it remains hotly debated whether mantle plumes exist.
For example, Massachusetts Institute of Technology seismologist Qin Cao and her colleagues used seismic waves to image activity beneath Hawaii . Instead of finding a narrow mantle plume, they discovered that a giant thermal anomaly about 500–1,250 miles (800–2,000 km) wide located far west of the islands is apparently what feeds its volcanoes.
The seismologists suggest Hawaiian volcanoes are fueled by a vast pool of hot matter on top of the lower mantle, not at its bottom near Earth’s core by a deep mantle plume.
Some researchers suggest hot spots may form in ways besides mantle plumes, such as spreading or cracking within tectonic plates, or “superplumes” that reach up from near the core to the near the base of the upper mantle, where they then give rise to smaller plumes that rise to the surface.
Must read: Plate tectonics and its relation to the evolution of major landform features
For more information https://en.wikipedia.org/wiki/Mantle_plume
PRACTICE QUESTIONS
QUES 1 . Define mantle plume and explain its role in plate tectonics.
