Passage V

Unlike most volcanoes, hot spot volcanoes (HSVs) develop far from tectonic plate boundaries. Two scientists discuss the origin and properties of HSVs.

Scientist 1

In the mantle beneath an HSV, at depths between 200 km and 400 km, hot magma rises toward Earth's surface in one large column called a mantle plume. The ascending magma causes earthquakes and creates networks of large fractures in crustal rocks. Propagation of these fracture networks enables magma to reach the surface more easily, which is why the frequency of eruptions at an HSV typically increases over time. Magma that does not breach Earth's surface will cool and eventually sink back down into the mantle.

HSVs erupt iron-rich lavas that are chemically similar to mantle rocks. Olivine and pyroxenes are the most abundant minerals in mantle rocks and in the lavas erupted at HSVs. The lavas at HSVs also retain a lot of water from the mantle. By weight, water vapor accounts for 75% of the total gas output at HSVs, while CO₂ accounts for only 10%-15%. All other gases combined never account for more than 10%-15% of the total gas output. Kilauea Caldera, in Hawaii, erupts this way.

Scientist 2

In certain places near the top of the mantle, at depths of less than 100 km, a high concentration of dissolved CO₂ allows mantle rocks to melt at lower temperatures than they normally would. This is how the magma that fuels an HSV forms. The magma then rises toward Earth's surface in small isolated bodies that melt through the entire thickness of the crust, sometimes causing small fractures (less than 1 km long) in crustal rocks. Each eruption at an HSV depletes some of the excess CO₂, which is why eruption frequency at an HSV will typically slow down over time.

HSVs erupt lavas in which feldspar is the most abundant mineral. These aluminum-rich lavas contain much less water than most mantle rocks. By weight, CO₂ and water vapor each account for 45% of the total gas output at HSVs. Carbon monoxide (CO) accounts for 6% of the total gas output, and all other gases combined account for only 4%. Mt. Erebus, in Antarctica, erupts this way.