# A Historic Event: The Krakatoa Eruption and Its Unmatched Sound
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Chapter 1: Understanding Sound and Its Limits
To grasp the extraordinary phenomenon of the Krakatoa eruption, it's crucial to define sound itself. Sound is generated through variations in air pressure. For instance, when we speak or whisper, we cause air molecules to oscillate rapidly, altering the air pressure around us. The louder the sound, the more intense these fluctuations become, as was the case with Krakatoa, where the effects were felt far and wide.
However, there exists a threshold to sound intensity. Beyond a certain point, the pressure fluctuations create a vacuum, as low-pressure areas reach zero pressure. This limit is approximately 194 decibels in Earth's atmosphere. Beyond this level, sound does not merely travel through the air; it pushes the air itself, resulting in a shock wave (Aatish Bhatia for Nautilus, 2016).
To put this into perspective, a construction site with jackhammers produces around 100 decibels. Being near a jet engine exposes you to about 150 decibels, which exceeds human tolerance (around 130 decibels). Notably, an increase of 10 decibels is perceived as roughly double the loudness.
Event and Aftermath
On August 27, 1883, the Earth released a sound of unparalleled intensity. At 10 AM local time, Krakatoa, an Indonesian island nestled between Java and Sumatra, erupted, generating noise that could be heard 3,000 miles away on Rodrigues, an island in the Indian Ocean.
“[...] coming from the eastward, like the distant roar of heavy guns.”
What transpired? A volcanic eruption of catastrophic proportions obliterated the island, tearing Krakatoa apart. The materials expelled from the volcano reached speeds exceeding half a mile per second—over twice the speed of sound.
The aftermath was devastating. The British ship Norham Castle was anchored 40 miles from the eruption site.
“So violent are the explosions that the ear-drums of over half my crew have been shattered. My last thoughts are with my dear wife. I am convinced that the Day of Judgement has come.” — Captain’s log
Additionally, the shock wave created a powerful tsunami, claiming the lives of an estimated 36,000 to over 120,000 individuals in coastal Indonesian villages.
Pattern of a sound wave. Image by OpenClipart-Vectors on Pixabay.
As the sound propagated from the volcano, it traversed Australia and the Indian Ocean, with locals reporting “gunshot” sounds. Although it dissipated after 3,000 miles and could no longer be heard by humans, it was still detectable by instruments at over 50 weather stations worldwide. The wave radiated outward, colliding with itself and causing measurable spikes in air pressure for five days post-eruption, repeating approximately every 34 hours at affected locations.
India, England, and San Francisco all experienced noticeable increases in ocean waves coinciding with these air movements.
“It was a sound that could no longer be heard but that continued moving around the world, a phenomenon that people nicknamed ‘the great air-wave’.”
The Krakatoan Incident was truly extraordinary, producing a sound intensity that exceeded expectations. In modern measurements, the eruption registered over 172 decibels—100 miles from its source—creating a massive shock wave felt globally multiple times.
“This is so astonishingly loud, that it’s inching up against the limits of what we mean by ‘sound’.”
Chapter 2: Visualizing Volcanic Eruptions
This video illustrates the impact of volcanic eruptions on air pressure. As the eruption occurs, you can observe the shock wave traveling through the air, condensing water vapor into clouds. The couple filming was fortunate to be situated 4.4 kilometers (2.7 miles) away, allowing them time to react, in stark contrast to the Krakatoan eruption, which was heard 3,000 miles away.
Sources:
- http://nautil.us/issue/38/noise/the-sound-so-loud-that-it-circled-the-earth-four-times
- Judd, J.W., et al. The Eruption of Krakatoa, and Subsequent Phenomena Trübner & Company, (1888).
- Winchester, S. Krakatoa: The Day the World Exploded Penguin, London, United Kingdom (2004).
- Simkin, T. & Fiske, R.S. Krakatau, 1883, the Volcanic Eruption and Its Effects Smithsonian Institution Scholarly Press, Washington, D.C. (1983).