(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Hawaii is the quintessential “hotspot.” The geologic definition of a “hotspot” is that region of the Earth beneath an area that has experienced high levels of active volcanism for an extended period of time.

The 5,800-km (3,200-mile) Hawaiian-Emperor Seamount chain is a testament to the longevity of the Hawaiian hotspot. Hotspots are fed by mantle plumes, a rising column of magma from the interior of the earth.

Besides Hawaii, other localities that owe their volcanism to a hotspot include Yellowstone, Iceland, the Galapagos, and the Reunion-Maldives islands.

Although it’s difficult to establish the dimensions of the Hawaii hotspot definitively, we can deduce its size from the area containing active volcanoes. Hualalai, Mauna Loa, Kilauea, and Loihi, a seamount southeast of Hawaii Island, have been active within the last few hundred years.

Loihi, the youngest volcano, must be near the leading edge of the hotspot. Correspondingly, the post-shield volcano Hualalai must be close to the trailing edge of the hotspot. The distance between the leading and trailing edges is approximately 90 km (56 mi) in a northwest to southeast direction.

What about the cross axis dimension? At the very least this dimension should include the summits of the active volcanoes and the dormant Mauna Kea.

If we draw a line between the summits of Kohala, Mauna Kea and Kilauea, parallel to the Pacific plate motion, and another line between Loihi, Mauna Loa, and Hualalai, the average distance between the two lines is approximately 30–35 km (18–22 mile). Therefore, an approximate minimum dimension for the hotspot is a 90 km by 35 km (56 mi by 22 mile) ellipse.

Coincidently, these two lines are referred to by geochemists as the Loa and Kea trends. Lava erupted from volcanoes along the Loa trend (Loihi, Mauna Loa, Hualalai and Kahoolawe, including other volcanoes up the island chain) is chemically different from lava erupted from volcanoes along the Kea trend (Kilauea, Mauna Kea, Kohala, and Haleakala).

Temperatures within the hotspot are not uniform and vary concentrically. The center represents the hottest portion of the hotspot or mantle plume. As you move outward, away from the center, the temperature gradually decreases toward the edge.

Where is the hottest part of the hotspot plume relative to the volcanoes on Hawaii Island? For this we rely on a geothermometer using the composition of lava and olivine, the common green mineral in Hawaiian lavas.

Using this approach, Mauna Loa takes the prize, because its lavas are hotter than those of neighboring volcanoes. The composition of Mauna Loa lavas suggest that they have been produced by greater amounts of melting of the hotspot source than lavas from the Kea volcanoes, Kilauea and Mauna Kea.

To further constrain the hotspot and the Loa and Kea trends, geochemists rely on elements such as the isotopes of lead (Pb), strontium (Sr), hafnium (Hf), neodymium (Nd), and helium (He). These elements and their isotopes provide clues to the structure and geometry of the hotspot or mantle plume.

Simple growth and evolution of Hawaiian volcanoes support a zoned hotspot. Most isotopes (Sr, Nd, Hf) support a concentric, thermally zoned mantle plume. However, lead isotopic data complicate the picture by inferring that the composition of the hotspot is asymmetrical.

Helium data, which is more abundant on the Loa trend, also supports an asymmetrical model and suggests that the magma supply is deep and hot.

But all data agree on the answer to the question — “Mirror, mirror on the wall, who’s the hottest of them all?” — and, for now, that answer is Mauna Loa.