Lava Lamp Physics and Earth's Crust

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Bubbles in a blue lava lamp.
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Lava lamps are aptly named -- the mesmerizing globby wax mimics the way pockets of molten rock rise buoyantly through the Earth's crust.

The lamps wouldn't be as much fun if the wax were neutrally buoyant, neither sinking nor rising. Yet Earth's system of plate tectonics may depend on such a layer of neutrally buoyant magma that likely exists beneath Earth's continents.

Most simple models of the Earth's interior present three concentric layers: crust, mantle, and core. Geologists call the layer of rigid rocks that extends from the surface of the Earth down to the top of the mantle the lithosphere. Just below the lithosphere is a layer called the asthenosphere, a slowly-flowing, not-quite-melted layer -- think slush.

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Geophysicists studying the lithosphere-asthenosphere boundary (let's just call it the LAB) noticed evidence that a thin layer of magma may separate the two layers. But some argued that because less-dense magma normally rises through rock, like blobs of wax in a lava lamp, such a layer couldn't remain stable for long.

Or could it?

A French team calculated how a melted magma would behave at the LAB if it were made of a rock called alkali basalt (similar to the basalt shown), likely present in the asthenosphere. At the pressures and temperatures expected at the LAB beneath continents, alkali basalt melts but doesn’t sink into the asthenosphere or rise into the lithosphere -- it's neutrally buoyant and could remain stable at the LAB, the team writes in Earth and Planetary Science Letters, which published their results.

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This magma layer likely helps rocky continents glide more easily over the asthenosphere, and may even explain the beginnings of plate tectonics on Earth.

The LAB was too hot for a stable magma layer to form until around three billion years ago, which is about the same time that plate tectonics fired up, shaping the Earth we know today.

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