Sunday, April 12, 2009

Hawaii hotspot theory: the moving pencil writes...

The latest in the complex geophysics of tectonic plates and hotspots, which has direct application to the Hawaiian-Emperor chain of islands and seamounts, is that both can move.

(Image: The Hawaiian Emperor Seamount Chain, with the Hawaiian Islands at lower right and the bend clearly visible, roughly halfway to Japan. The arc at the top, where the Emperor Seamounts terminate, is the northern edge of the Pacific Plate. Credit: NOAA.)

Here's a simplified analogy.

If you repeatedly tap a pencil on a piece of paper, it makes a dot.

If you drag the paper as the pencil is tapping, it makes a line of dots.

If you start moving the pencil as the paper is being dragged, the line of dots changes direction.

There's been lots of speculation about how that works on the earth's surface. In this case, the piece of paper is the Pacific Plate—the vast chunk of the Earth's crust that underlies most of the Pacific Ocean. The pencil is a hotspot, which repeatedly pokes through the plate to form volcanoes.

Current theory is that the Pacific Plate is adrift, moving slowly to the northwest, with islands popping through as it goes over the hotspot. The nascent island Loihi and the Big Island are being formed now, fed directly by the hot spot. Maui is a million or so years old, O'ahu two or three million and Kaua'i five million years and 300 miles from the hotspot.

On it goes in roughly a straight line out to beyond Kure Atoll, where the Hawaiian archipelago takes a distinct northward turn, and it becomes the Emperor Seamounts. There's long been talk about how the change in direction took place, but most folks have settled on a change in the direction of the Pacific Plate's movement.

The newest theory is presented in Science: “The Bent Hawaiian-Emperor Hotspot Track: Inheriting the Mantle Wind,” by University of Rochester geophysicist John Tarduno, and fellow scientists Hans-Peter Bunge, Norman Sleep and Ulrich Hansen. You can find a rundown at

The upshot is that they argue that the hotspot was moving south as the Pacific Plate moved northwest, and the bend is the result of the hotspot dramatically slowing down. In the pencil-paper analogy, as the paper is being dragged by its upper left corner, the tapping pencil is being moved down, and then stops and continues tapping in place.

The science of why the hotspot moved are beyond the scope of this piece, but it's not just a theory; there is science beyind it.

One piece of compelling evidence is that geological testing can indicate how far north a volcanic rock was formed. If a hotspot were stationary, then all the rocks of the Hawaiian Emperor chain would have signs of being formed at the latitude where Loihi is now being formed.

They don't. The oldest seamounts in the Emperors were formed farther north. Tarduno, quoted in ScienceDaily, says: "The only way to account for these findings is if the hotspot itself was moving south.”

There's some background on hotspot theory at

The science paper: John Tarduno, Hans-Peter Bunge, Norm Sleep, and Ulrich Hansen. The Bent Hawaiian-Emperor Hotspot Track: Inheriting the Mantle Wind. Science, 2009; 324 (5923): 50 DOI: 10.1126/science.1161256.

© Jan TenBruggencate 2009

1 comment:

Steve Jordan said...

Thanks for this great post!