Volcanic hotspots such as Ascension Island in the South Atlantic may have a surprising origin, indicates a report published on January 6 in Science.
Scientists have long thought that these islands were fueled by thermal plumes welling up from deep within the Earth’s mantle. However, when researchers compared the temperatures of volcanic hotspots and mid-ocean ridges around the world, they found that many of these so-called hotspots were actually rather cool.
Conditions at these sites might not be warm enough for the plumes to travel up from the deep mantle, says Carolina Lithgow-Bertelloni, a geophysicist at the University of California, Los Angeles and coauthor of the findings. Understanding how these enigmatic hotspots do form may provide important clues about geologic processes at work at shallower depths beneath Earth’s surface, she and her team concluded.
Most of the world’s volcanic activity occurs at the boundaries between tectonic plates, where underwater mountain ranges called mid-ocean ridges are found. “At these mid-ocean ridges the material that’s underneath the crust on which we live upwells and melts and forms volcanism, and as a result the plates separate and you create new ocean floor,” says Lithgow-Bertelloni. “It’s an expression of the large-scale motions inside the [planet’s] interior.”
However, in rare cases volcanoes are created by different processes and may pop up far from mid-ocean ridges. This second category includes the volcanic islands of Hawaii and the Galápagos. The most widely-accepted explanation is that superheated plumes rise from great depths, perhaps even from the boundary between the Earth’s rocky mantle and the core, at these hotspots. The plumes melt the surrounding rock to form magma, which eventually erupts from the surface. As the tectonic plate overlying the plume moves, a chain of volcanoes forms over time.
Scientists have found that the basaltic rock formed when lava cools at these sites has different chemical properties from basalt formed along mid-ocean ridges. “If they do come from hot material being brought up [from] deep in the interior, they give us a window into the chemistry of the interior which we have no other way of accessing, and also into the chemical evolution of the planet,” Lithgow-Bertelloni says.
The temperature of mid-ocean ridges should represent typical temperatures within the mantle, she says. In order to upwell, a plume must be somewhere between 100 to 150 degrees Celsius (212 to 302 degrees Fahrenheit) hotter than the surrounding rock. “They have to be hotter to be able to rise through the entire mantle, and therefore hotter than the mid-ocean ridge,” Lithgow-Bertelloni says.
Researchers have estimated that hotspot basalts melted at temperatures roughly 100 to 300 degrees Celsius (212 to 572 degrees Fahrenheit) higher than those at mid-ocean ridges. However, Lithgow-Bertelloni says, estimates for a given hotspot are often inconsistent and only reflect temperatures within the upper portion of the mantle, far above the depths at which plumes originate.
The key to determining temperatures as far as 600 kilometers (373 miles) beneath mid-ocean ridges and volcanic hotspots, she says, turned out to be earthquakes. Whenever an earthquake takes place, it releases huge amounts of energy in the form of seismic waves. The speed at which these waves travel and arrive at seismometers varies depending on the composition, depth, and temperature of the surrounding rock.
The researchers used a model that drew upon seismic measurements from around the globe to infer the temperature at all 46 known oceanic hotspots. The team also calculated that the average temperature at mid-ocean ridges was roughly 1,388 degrees Celsius (2,530 degrees Fahrenheit).
“What we found incredibly interesting and shocking was that most of the hotspots are really not that hot,” Lithgow-Bertelloni says.
She and her colleagues identified three distinct clusters of hotspots. When they zeroed in on 26 hotspots with well-documented plumes, the researchers determined that 12 were truly hot, with temperatures 155 degrees Celsius or more above that of mid-ocean ridges. However, 10 hotspots were merely warm, with temperatures 50 to 136 degrees Celsius (122 to 277 degrees Fahrenheit) hotter than mid-ocean ridges. An additional four were downright chilly, with temperatures similar to or cooler than mid-ocean ridges.
The toastiest cluster of hotspots included the volcanoes of Iceland, Samoa, Galápagos, and Hawaii. Warm hotspots were found at Bermuda and the Canary Islands, while cold hotspots were found at Cameroon, Ascension Island, and the Great Meteor, or New England, hotspot in the North Atlantic.
“We still have hot hotspots, and those hot hotspots reflect material that is deep and from an ancient domain, but there aren’t very many of them,” Lithgow-Bertelloni says. “The rest are different, which means we’re in new territory.”
One explanation is that cold and warmish hotspots may originate at relatively shallow depths within the mantle. “We also suggested that maybe these [deep] plumes started hot and then they got trapped in material that is cold,” Lithgow-Bertelloni says. “It doesn’t stop them altogether; they still are able to rise, but they cool down and rise more slowly.”
One source of uncertainty for the temperature estimates the researchers developed is that earthquakes and the seismometers that record their activity aren’t evenly distributed around the world. “So our coverage of the earth’s interior is not complete,” Lithgow-Bertelloni acknowledges.
Still, she says, the findings indicate that volcanic islands that look similar on the surface may have very different origins. As a next step, the researchers will explore how volcanic sites differ between the Pacific, Atlantic, and Indian Oceans.
“Those ridges are recording the ancient tectonic history…of the planet,” Lithgow-Bertelloni says. This history “could be revealed by the temperatures of the ridges, and the differences in temperature between the ridges and the hotspots in those ocean basins.”