Recent research has unveiled the existence of an ancient ghost plume lurking beneath Oman. This magma plume is trapped beneath a thick section of the Earth's crust and the upper mantle, which is the planet's middle layer. Due to its location, the material contained within the plume is unable to rise and trigger volcanic activity at the surface. While scientists are uncertain whether this plume has ever ignited eruptions, evidence indicates that it played a significant role in altering the trajectory of the Indian tectonic plate during its collision with the Eurasian plate tens of millions of years ago, as detailed in a new study.
The plume is situated beneath Oman's Salma Plateau (also referred to as Salmah or Selma), which reaches elevations of up to 6,600 feet (2,000 meters). According to study lead author Simone Pilia, a geophysicist and assistant professor at King Fahd University of Petroleum and Minerals in Saudi Arabia, the plateau likely formed as a result of the plume. However, some scientists also attribute the plateau's formation to the bending of the Earth's crust caused by the Makran subduction zone, located off the coasts of Pakistan and Iran. Pilia explained, "A plume is hot material that wants to rise, rise, rise — so it's underneath and it's pushing up, creating topography." Although the uplift at the Salma Plateau is relatively minor, it is still indicative of the plume's activity.
The discovery of the Dani plume was made possible through the analysis of seismic waves, which are sound waves that travel through the Earth at varying speeds depending on the chemical composition of the material they pass through. Oman boasts a dense network of stations recording seismic data, facilitating this groundbreaking research. Pilia affectionately named the plume "Dani" after his son.
The Dani plume represents the first clear instance of an amagmatic ghost plume — a term coined by the study's authors to describe mantle plumes that do not result in volcanic activity. These mantle plumes originate from the core-mantle boundary, approximately 1,800 miles (2,900 kilometers) beneath the Earth's surface. Typically, mantle plumes fuel volcanic eruptions through a process known as decompression melting as they ascend through the mantle and crust. While many mantle plumes trigger eruptions in oceanic plates, such as those in Hawaii, they rarely cause eruptions within continental plates due to the thicker crust and upper mantle.
Researchers previously assumed that the absence of volcanic activity from mantle plumes in continental plates indicated there were no plumes present. However, Pilia emphasized that "absence of evidence is not evidence of absence." The existence of the Dani plume proves that mantle plumes can exist without inducing volcanic activity. Pilia further speculates that numerous other ghost plumes remain undiscovered, particularly in regions like Africa, which sits above one of Earth's large low-shear-velocity provinces—massive blobs that extend from the core-mantle boundary and give rise to plumes.
The Salma Plateau is estimated to be around 40 million years old, suggesting that the Dani plume is at least as ancient. This timing coincides with the collision of the Indian and Eurasian plates, prompting researchers to investigate further. The collision occurred near present-day Oman before the two plates shifted northward to their current positions. Pilia and his colleagues meticulously reconstructed the trajectory of the Indian plate, discovering that it experienced a slight directional change between 40 million and 25 million years ago.
Pilia explained, "We made some other calculations and basically demonstrated that the shear stress produced by the plume was the reason for the change in the azimuth angle of the Indian plate." Although scientists have long understood that plumes can redirect tectonic plates, prior to the identification of the Dani plume, they had not linked this shift to a specific plume. While tectonic plates are in constant motion, plumes tend to remain stationary, allowing scientists to trace a plume's evolution through the evidence left on the tectonic plates as they move over it. Unfortunately, in the case of the Dani plume, the evidence has been obscured and erased by the Makran subduction zone, rendering it forever lost.
