Why does Crane Beach have purple sound?

Crane Beach belongs to the Trustees of Reservations and is part of the historic Crane Estate. The property includes Crane Castle, miles of shoreline, and over 5 miles of marked trails through the dunes at Castle Neck and Steep Hill Beach, open year-round.

Sandy Tilton explained at Storm Surge Public Forum that the purple sand we often see on top of the light-colored sand at Crane Beach and Plum Island is garnet sand.

“The mineral garnet comes to us via the streams & rivers from the White Mountains. Water & wind erode the rock & it is carried in the waters here until it meets the ocean & gets deposited on our beaches! As the storms erode our dunes & beaches, lots of garnet sand is exposed. It is heavier than the white quartz sand, so doesn’t wash away as easily. In the spring & summer, the winds blow the white quartz sand & easily covers the garnet again….look in the eroded edges of the dunes…you can often see the layers of purple & white there….pretty cool!!”

Eroding dunes reveal layers of garnet on Plum Island & Cranes Beach. Photo courtesy of Sandy Tilton.

JEOL is a global supplier of electron microscopes, ion beam instruments, mass spectrometers and NMR spectrometers. After some of their employees found purple sand at Plum Island, they analyzed it using an optical microscope, a scanning electron microscopes (SEM) and an energy dispersive X-Ray spectrometer (EDS).

“At first look under the optical microscope, the granules of sand appeared like scattered jewels of many colors; predominantly glassy pink angular grains, with smaller quantities of milky white rounded grains, clear angular grains, black grains (some magnetic and some not), and even the occasional green. When large amounts of fine-grained pink is intermixed with a smaller number of darker grains and dampened by rain or sea water the human eye will “see” the sand as a much darker pink to almost purple. The two most common pink minerals are rose quartz, which is found only in a few isolated pegmatite deposits in NH & southern Maine, and the solid solution series of almandine and pyrope garnet, which is quite common in the Seacoast area from the abundance of metamorphic rocks called mica schist and from contact metamorphism.”

The electron microscope provided a scientific breakdown:

• The pink-purple color comes from the abundance of almandine-pyrope garnet.
• The white grains are K-feldspar (potassium alumino-silicates) and quartz.
• The black nonmagnetic grains were a mix of a pyroxene called augite and a mix of ilmenite and hematite which are the magnetic components.
• The green was confirmed to be epidote.

Read more at JEOL USA

But why does the sand at Crane Beach sometimes squeak?

Manchester by the Sea is famous for its “Singing Beach” but occasionally in the summer the sand at Crane Beach also squeaks beneath our feet. Wikipedia provides some answers:

“On some beaches around the world, dry sand will make a singing, squeaking, whistling, or screaming sound if a person scuffs or shuffles their feet with sufficient force. It has been found that quartz sand will do this if the grains are very well-rounded and highly spherical. It is believed by some that the sand grains must be of similar size, so the sand must be well sorted by the actions of wind and waves, and that the grains should be close to spherical and have matter-free surfaces. The “singing” sound is then believed to be produced by shear as each layer of sand grains slides over the layer beneath it. The similarity in size, the uniformity, and the cleanness mean that grains move up and down in unison over the layer of grains below them. Even small amounts of pollution on the sand grains reduces the friction enough to silence the sand.

Others believe that the sound is produced by the friction of grain against grain that have been coated with dried salt, in a way that is analogous to the way that the rosin on the bow produces sounds from a violin string. It has also been speculated that thin layers of gas trapped and released between the grains act as “percussive cushions” capable of vibration, and so produce the tones heard.

Water also influences the effect. Wet sands are usually silent because the grains stick together instead of sliding past each other, but small amounts of water can actually raise the pitch of the sounds produced. The most common part of the beach on which to hear singing sand is the dry upper beach above the normal high tide line, but singing has been reported on the lower beach near the low tide line as well.”

“Singing sand” has been reported throughout the world, including a number of beaches along North America’s Atlantic coast.

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