Mantle plume

A mantle plume is a theory about how heat moves inside the Earth. Scientists think that hot material from deep within the Earth rises in a stream, like a bubble rising in water. When this hot material gets close to the surface, it can melt and create volcanoes. This idea helps explain why some places far from the edges of Earth's moving plates have lots of volcanic activity. Famous examples include Hawaii and Iceland, where big volcanoes form even though they are not near the places where plates push together. These rising streams of heat are also thought to be behind huge areas of old volcanic rock, like the Deccan and Siberian Traps. Studying mantle plumes helps scientists understand how Earth's surface changes over millions of years.

Concepts

Mantle plumes were first suggested by J. Tuzo Wilson in 1963 and later developed by W. Jason Morgan in 1971 and 1972. A mantle plume is thought to be a stream of very hot material that starts at the core-mantle boundary and moves upward through the Earth's mantle. Instead of a constant flow, these plumes are like a series of hot bubbles. When they reach the Earth's crust, they create areas called "hotspots." One example is the Hawaiian–Emperor seamount chain, where older volcanoes form a line leading away from a hotspot.

The idea is that these plumes help move heat from deep inside the Earth to the surface. There are two main ways this happens: one is the steady movement of tectonic plates, and the other is the occasional burst of heat from these plumes. Scientists studied this idea using small experiments with fluids, which helped them understand how these plumes might look—like mushrooms with a wide head and a thin stem. When a plume reaches the Earth's crust, it can melt and create large areas of volcanic rock, known as flood basalts. Examples include the Deccan Traps in India and the Siberian Traps in Asia.

The chemicals found in these volcanic rocks can tell scientists about the deep materials that helped form them. These rocks often come from deep in the Earth, where old ocean floors have sunk down. As these plumes rise, they can melt and create new volcanoes, often forming chains as the Earth's plates move over them. The Hawaiian Islands are a well-known example of this process.

The lower mantle and the core

The Earth has a special layer called the D″ layer at the bottom of the mantle, right above the core. Scientists think hot, rising columns of rock, called mantle plumes, might start here. These plumes might explain why some places on Earth, like hotspots, have lots of volcanic activity.

There are two big areas in the lower mantle, one under Africa and one under the central Pacific Ocean, where seismic waves move more slowly. These areas are called large low-shear-velocity provinces. Some believe that mantle plumes might rise from these areas. The slower seismic waves were once thought to mean these areas were hotter, but now we know it might be because of different chemicals there.

Evidence for the theory

Some common ways scientists support the idea of mantle plumes are by looking at volcanic chains, special gases, and how seismic waves move through the Earth.

Linear volcanic chains

One strong piece of evidence is the way volcanic islands line up in chains. For example, the Hawaiian-Emperor seamount chain shows a pattern that suggests a deep plume of hot rock rising from deep within the Earth. As the tectonic plate moves, new volcanoes form in a line. Other places with similar patterns include Réunion, the Chagos-Laccadive Ridge, the Louisville Ridge, the Ninety East Ridge, and Kerguelen, Tristan, and Yellowstone.

Noble gas and other isotopes

Main article: Helium-3

Special gases, like helium, can also show signs of mantle plumes. Helium-3 is a rare gas made during the Big Bang. It does not get added to Earth very often. Scientists find higher amounts of helium-3 at some volcanic hotspots, suggesting these areas tap into deep parts of the Earth where this gas is stored.

Geophysical anomalies

Scientists also look at how seismic waves move through the Earth. Hot plumes should change how these waves travel. By measuring these changes, scientists can create images of what lies below the surface. These images sometimes show areas that look like they could be plumes.

Geochemistry

Rocks from volcanic islands, like mid-ocean ridge basalt, have different chemical makeups. These differences help scientists understand what deep parts of the Earth might be melting to create these rocks.

Seismology

In 2015, scientists used data from big earthquakes to create detailed images of the Earth’s interior. These images showed large, hot plumes rising from deep within the Earth under many hotspots, such as Pitcairn, Macdonald, Samoa, Tahiti, Marquesas, Galapagos, Cape Verde, and the Canary hotspots. These plumes were much wider than expected, which opens new questions about how heat moves from the Earth’s core to its surface.

Suggested mantle plume locations

Scientists think mantle plumes might be behind something called flood basalts. These are huge, fast eruptions of lava that create big flat areas of rock, both on land and under the ocean. Examples include the Deccan Traps, the Siberian Traps, the Karoo-Ferrar flood basalts in Gondwana, and the Central Atlantic magmatic province (CAMP).

Many of these big lava eruptions happen when continents start to split apart. This fits with the idea that when material rises in a mantle plume, other material sinks down, which can cause the earth's surface to split.

Alternative hypotheses

There are two main ideas besides the mantle plume theory that try to explain some of Earth's volcanoes. These are the plate hypothesis and the impact hypothesis.

The plate hypothesis says that volcanoes happen because of movements in Earth's outer layers, especially where plates are moving apart. This idea suggests that volcanoes are not caused by deep, hot columns rising from deep within the Earth, but instead by processes closer to the surface connected to plate movements.

The impact hypothesis suggests that some volcanoes may be caused by very large impacts from space. These impacts can melt rock and create volcanoes, especially in areas far from where plates meet. This idea is still being studied and is different from the ideas about plates and mantle plumes.