Magma Matters — Why Earth’s Still Hot After All These Years

A time-lapse video showing lava engulfing a car on Hawaii’s Big Island a few days ago

The images are incredible. Hissing lava heedlessly crossing roads, gardens, engulfing cars and setting homes on fire. All the result of a major and deadly eruption of the Kilauea volcano on Hawaii’s Big Island. It all began on May 1 when small cracks appeared on the volcano on May 1. A towering plume of pink smoke followed and then a magnitude 6.9 earthquake. Now a dozen fissures have opened up as chunky lava flows bulldoze through the community of Leilani Estates.

Lavas on the moon were thin and runny like this flow An earlier lava flow from Kilauea, Hawaii. The volcano’s lava flows are thin and runny similar to flows that occurred on the moon when it was more volcanically active eons ago. USGS

We feel powerless watching the lava flows have their way with human property and boundaries. Nature is so utterly uncaring, but how can you get angry? There’s no ill intent or will to destroy. It’s just Earth being Earth, terrible as the consequences might be.

Kīlauea is a shield volcano on the island of Hawaii, the largest and the southeastern-most of the Hawaiian islands. In the foreground, the vent of the volcano has erupted fluid lava in the crater at the peak of Kilauea. The peak behind the vapor column is Mauna Loa, a separate volcano. Kilauea is one of the world’s most active volcanoes. USGS

Hawaii and in particular, the Big Island, sits atop a volcanic hotspot, a narrow stream of hot mantle (made of magma or melted rock) that rises from the boundary between the planet’s molten core and the mantle, a thick layer of semi-plastic rock deep beneath the crust. Hawaii is located on a large plate of rock that floats atop the mantle called the Pacific Plate. As it moves to the west, magma rising from the hotspot punches through the plate, pouring out flow after flow of lava that cools and solidifies, forming a new island in the Hawaiian Island chain. That’s why Hawaii is a west-to-east succession of islands. Each was built while on the move over the magma plume.

This diagram illustrates the hotspot area of the crust in cross-section. As the Pacific Plate slides west (left), magma rises from the hotspot plume and dumps layer after layer of new lava atop the crust, building new islands in the Hawaiian chain. Joel E. Robinson / USGS

Hawaiian volcanoes aren’t the tall, explosive type but rather “shield volcanoes,” named for the convex but flattened shape of a warrior’s shield. They ooze highly fluid lavas of molten rock that flow like rivers in slow motion, covering everything in their path. Magma is the name given to molten rock stored in Earth’s crust; lava is magma that erupts at the surface. Temperatures of most magmas and lavas are between 1,300° F to 2,400° F (700 °C to 1300 °C), hot enough to melt copper and gold but not quite steel.

You might wonder about the source of heat that heats the Earth that melts the rock that causes all these troubles for crust-dwellers. There are three we know of. The primary is the residual heat left over from the formation of the Earth 4.6 billion years ago. Called primordial heat, it was generated when small bodies crashed and ultimately stuck together through gravity to form the early Earth. The energy released in those impacts heated materials to around 18,000° F (10,000° C) and remains with us today, locked in the deep mantle and core 4,000 miles (6,400 km) beneath our toes.

Earth has several different layers or nested spheres of different materials: crust, mantle and core. Residual heat from the planet’s formation plus fluid movement and radioactivity heat the planet’s interior and melt rock that sometimes breaks through onto the surface through volcanic eruptions like the one in Kilauea. NASA

A second is the flow of heavy, iron-rich material toward the core. That releases an additional ~3,000° F of heating. Then there’s the heat of radioactive decay from radioactive rocks within the crust and mantle. At this time, we don’t have a good handle yet on just how much it contributes to heating and melting of rock, but it’s a factor.

View of the Aristarchus, Herodotus and Schroeter’s Valley photographed from lunar orbit through the window of the Apollo 15 command module. Hot lava issuing from beneath the lunar crust once coursed between the banks of Schroeter’s Valley. NASA

Earth’s not alone.  Other planets, even moons, have molten cores and release lava … or at least once did before their cores cooled. Mars, the moon, Mercury Venus and Io (at Jupiter) comes to mind. Moons like Saturn’s Enceladus, Neptune’s Triton, and the dwarf planets Ceres and Pluto have volcanic activity based on liquid water or other fluids. Residual heat within or created by gravitational interactions with larger bodies, as in the case of Jupiter’s flexing and heating Io’s interior, keep these balls of rock lively.

Stay tuned to the volcano’s fits and starts via the U.S. Geological Survey here, or check their webcam here.

** Special note: I participated in a podcast recently with my colleagues at the Duluth News Tribune about the pleasure of looking up and my new book. We had a lot of fun. The podcast just came out today — click here to listen.

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