Theories of how some volcanoes form may be too simple.

Outside Science According to a recent study, the hotspots that gave rise to volcanic islands like Hawaii and Iceland often prove to be unexpectedly cool. According to experts, these findings suggest that such hotspots are not always formed by huge streams of red-hot rock rising from near the Earth's core, as previously thought. Volcanoes formed by collisions between giant slabs of rock floating on the mantle layer between the Earth's core and crust are often located near the edges of tectonic plates. The volcanoes that form the so-called Ring of Fire on the Pacific Ocean are classic examples of this type of eruption. Occasionally, however, volcanoes in the center of tectonic plates also erupt. The sources of these hotspots could be mantle sources. Lumens are heated columns of rock that rise from the Earth's deep mantle and burn the overlying material like a blowtorch. According to geologists, chains of volcanic islands can form as tectonic plates move across such plumes. According to previous studies, volcanic hotspots are between 100 and 300 degrees Celsius hotter than mid-ocean ridges, where magma rises when tectonic plates separate beneath the sea. This supported the idea that hotspots were heated by material from near Earth's hot core and mid-ocean ridges were heated by cooler mantle rock. Scientists have since found that many hotspots are much cooler than previously thought, raising concerns about their origin. A seismologist from the College of Maryland, College Park, explained that "a significant portion of the hotspots do not fit the classic plume model." According to geologists, chains of volcanic islands can form as tectonic plates move across such plumes. According to previous studies, volcanic hotspots are between 100 and 300 degrees Celsius hotter than mid-ocean ridges, where magma rises when tectonic plates separate under the ocean. This supported the idea that hotspots were heated by material from near Earth's hot core and mid-ocean ridges were heated by cooler mantle rock. Scientists have since determined that many hotspots are significantly cooler than previously thought, raising concerns about their origin. A seismologist at the College of Maryland, College Park, who was not involved in the study, said that "a significant portion of the hotspots do not fit the classic plume model." The researchers examined the velocity of seismic waves flowing through the new study. To calculate temperatures at these locations, tectonic plates must move across the Earth's mantle beneath oceanic hotspots and ridges. (Cold rock allows seismic waves to propagate faster.) About 45% of the hotspots are hotter than the mid-ocean ridges by more than 155 °C. However, only about 40% of the hotspots are much hotter than the mid-ocean ridges, i.e., they have temperatures between 50 and 136 °C, so they are not buoyant enough to support active upwelling of rocks from the deep mantle. In addition, only 15% of the hotspots are particularly warm, with temperatures only 36 °C higher or lower than those of mid-ocean ridges. The scientists also examined the ratio of the rarer helium-3 to the more abundant helium-4 in their rocks to shed light on the origin of these different types of hotspots. (Helium-3 ions contain only one neutron per nucleus, while helium-4 nuclei have two neutrons per nucleus.) While helium from Earth's interior is richer in helium-3 and likely comes from reservoirs of ancient material that preserved the original ratio between these isotopes during Earth's earliest days, helium from Earth's crust is composed mainly of helium-4, which was formed from the degradation of uranium and other radioactive isotopes over time. Unlike the cold hotspots, the hot hotspots had a much larger ratio of helium-3 to helium-4, the researchers said. Although the traditional theory that hotspots originate from plumes rising from Earth's deep mantle could explain many of the known hotspots, including those under Hawaii, Iceland, Samoa, and Easter Island, it may actually be the case that few hotspots are actually caused by plumes from Earth's deep mantle.