Putting Earth's history to sound

by Brian Fisher Johnson
Thursday, January 5, 2012

Geophysicist Dave Engebretson of Western Washington University in Bellingham has struggled with his eyesight since birth. But when his vision took a serious downturn in 1996 — today he has difficulty recognizing faces up close — Engebretson grasped for the world of sound. He has made a considerable hobby out of audifying scientific data — taking numbers from datasets and setting them to sound frequencies to create seconds-long clips at his home studio. The result ranges from a series of differently pitched explosions that fast-forward through the last 20,000 years of eruptions along the Cascade Mountains, to haphazard movements up and down the music scale that represent changes in the tides.

EARTH reporter Brian Fisher Johnson reviews a few of Engebretson’s favorite projects and what insights they provide into Earth’s history.

The Tune of the Tides (above): Engebretson had always thought tides would make a pretty good sound, so the first thing he tried to audify was a dataset of the tides along Washington’s San Juan Island. The first take sounded “pretty awful,” Engebretson says, partly because he didn’t “map” the data in actual notes and partly because of irregularities in the data that made for a rather rackety sound. The piece came out smoother once Engebretson got rid of the white noise and assigned notes to the data. By that point, Engebretson says, he gained a new appreciation for the nuances of tidal cycles, such as spring and neap tides.

Musical Magnetic Reversals: Although Earth’s magnetic field currently points toward the North Pole, the planet’s magnetic dipole flips direction every few hundred thousand years or so. Engebretson tracked the last 85 million years of these magnetic reversals, with higher pitches representing shorter polarities (a period of time when the direction of the magnetic field stays the same), and lower pitches longer ones. The project gave him new insight into a larger pattern of the magnetic field. “There seems to be a speed-up of magnetic reversals toward our time now,” he says. “I haven’t pursued it any further, but I think somebody will.”

Volcanic Chorus: Formed from rising magma as oceanic crust subducted under continental crust, the Cascade Mountains of the Pacific Northwest have had their fair share of volcanic eruptions. Based on the geologic record, Engebretson plotted out the last 20,000 years of dated eruptions to the sound of explosions, with short, gun-like explosions representing smaller eruptions and bellowing, cannon-like explosions representing larger eruptions. Engebretson then put the piece in stereo, dividing the sounds between the left speaker of his sound system (which focused on eruptions from the northern end of the Cascades) and the right speaker (which focused on eruptions from the southern end of the Cascades). The set-up makes listeners feel as if they are observing 20,000 years of Cascade eruptions from a boat out in the Pacific.

The final product gave Engebretson and his colleagues some insight into the geologic record of volcanism in the Cascades. The earliest 8,000 years or so are relatively quiet, he says, affirming the notion that the glaciers of that time erased most of the eruption record. Once the glaciers disappeared about 12,000 years ago, however, the piece sounds “like a war zone,” Engebretson says.

Earth Tides, in A Major: The oceans aren’t alone in experiencing tides, Engebretson says. Earth itself also experiences small, millimeter-sized tides, called Earth tides. “It’s not really the displacement in the water, it’s the displacement in the solid earth,” he says.

“And these Earth tides are predictable by finding the position of the moon and the sun relative to the Earth.” Using a dataset from NASA’s Jet Propulsion Laboratory in California that went back to A.D. 1600 and projected nearly 200 years into the future to 2200, Engebretson calculated the net gravitational force of the sun and the moon at particular intervals and then mapped them onto the A major scale. “It’s a song without a beginning or an end, but it does have movements in it,” Engebretson says. “It’s beautiful.”

The Cosmos on Key: Engebretson’s latest project involves taking the orbital periods of all eight planets in the solar system and turning them into corresponding frequencies: Planets that have short orbits like Mercury have higher frequencies, whereas more distant planets with longer orbits like Neptune have lower frequencies. Interestingly, it looks like all the frequencies lie close to the F sharp minor scale, Engebretson says, “so I’ve been working on why that might be.”